Attachment 3
Sonoma County Multijurisdictional
Hazard Mitigation Plan Update 2021
Volume 1—Area-Wide Elements
October 2021
PREPARED FOR PREPARED BY
Permit Sonoma Tetra Tech
2550 Ventura Avenue 1999 Harrison Street Phone: 510.302.6300 Santa Rosa, California 95403 Suite 500 Fax: 510.433.0830 Oakland, CA 94612 www.tetratech.com
Tetra Tech Project #103S6990
2021-10-19_SonomaCountyHMP_Vol1_Adoption Draft.docx
Contents
CONTENTS
Executive Summary ............................................................................................................. xix PART 1— PLANNING PROCESS AND COMMUNITY PROFILE
1. Introduction to Hazard Mitigation Planning ................................................................................................ 1-1
1.1 Why Prepare This Plan? ...................................................................................................................... 1-1
1.1.1 The Big Picture .......................................................................................................................... 1-1
1.1.2 Purposes for Planning ................................................................................................................ 1-1
1.2 Who Will Benefit From This Plan?..................................................................................................... 1-2
1.3 Contents of This Plan .......................................................................................................................... 1-2
2. Plan Update—What Has Changed............................................................................................................... 2-1
2.1 The Previous Plan................................................................................................................................ 2-1
2.2 Why Update?....................................................................................................................................... 2-1
2.2.1 Federal Eligibility ...................................................................................................................... 2-1
2.2.2 Changes in Development ........................................................................................................... 2-1
2.2.3 Multi-Jurisdictional Planning..................................................................................................... 2-2
2.3 The Updated Plan—What Is Different? .............................................................................................. 2-2
3. Plan Update Approach ................................................................................................................................. 3-1
3.1 Funding................................................................................................................................................ 3-1
3.2 Defining Stakeholders ......................................................................................................................... 3-1
3.3 Formation of the Core Planning Team ................................................................................................ 3-1
3.4 Establishment of the Planning Partnership.......................................................................................... 3-2
3.5 Defining the Planning Area................................................................................................................. 3-2
3.6 The Steering Committee...................................................................................................................... 3-3
3.7 Coordination with Stakeholders and Agencies.................................................................................... 3-3
3.8 Review of Existing Programs.............................................................................................................. 3-5
3.9 Public Involvement.............................................................................................................................. 3-6
3.9.1 Strategy ...................................................................................................................................... 3-6
3.9.2 Public Involvement Results ..................................................................................................... 3-10
3.10 Plan Development Chronology/Milestones..................................................................................... 3-11
4. Sonoma County Profile................................................................................................................................ 4-1
4.1 Geographic Overview.......................................................................................................................... 4-1
4.2 Historical Overview............................................................................................................................. 4-3
4.3 Major Past Hazard Events ................................................................................................................... 4-4
4.4 Physical Setting ................................................................................................................................... 4-5
4.4.1 Topology and Surface Waters.................................................................................................... 4-5
4.4.2 Soils ........................................................................................................................................... 4-6
4.4.3 Climate....................................................................................................................................... 4-6
4.5 Sensitive Resources............................................................................................................................. 4-8
4.5.1 Cultural Resources..................................................................................................................... 4-8
4.5.2 Scenic Resources ....................................................................................................................... 4-8
4.5.3 Natural Resources ...................................................................................................................... 4-9
4.6 Development Profile............................................................................................................................ 4-9 v
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements
4.6.1 Land Ownership and Use........................................................................................................... 4-9
4.6.2 Building Count, Occupancy Class and Estimated Replacement Value ................................... 4-10
4.6.3 Critical Facilities...................................................................................................................... 4-10
4.6.4 Development Trends................................................................................................................ 4-12
4.7 Demographics.................................................................................................................................... 4-15
4.7.1 Total Population Estimates ...................................................................................................... 4-15
4.7.2 Age Distribution....................................................................................................................... 4-16
4.7.3 Race, National Origin, and Language...................................................................................... 4-18
4.7.4 Individuals with Disabilities or with Access and Functional Needs........................................ 4-18
4.8 Economy............................................................................................................................................ 4-19
4.8.1 Income ..................................................................................................................................... 4-19
4.8.2 Industry, Businesses, and Institutions...................................................................................... 4-20
4.8.3 Employment Trends and Occupations ..................................................................................... 4-20
5. Regulations and Programs ........................................................................................................................... 5-1
5.1 Relevant Federal and State Agencies, Programs and Regulations ...................................................... 5-1
5.2 Local Plans, Reports and Codes .......................................................................................................... 5-1
5.3 Local Capability Assessment .............................................................................................................. 5-6
5.3.1 Legal and Regulatory Capabilities............................................................................................. 5-7
5.3.2 Fiscal Capabilities...................................................................................................................... 5-7
5.3.3 Administrative and Technical Capabilities................................................................................ 5-7
5.3.4 NFIP Compliance....................................................................................................................... 5-7
5.3.5 Public Outreach Capability........................................................................................................ 5-7
5.3.6 Participation in Other Programs................................................................................................. 5-7
5.3.7 Development and Permitting Capability.................................................................................... 5-8
5.3.8 Adaptive Capacity...................................................................................................................... 5-8
5.3.9 Integration Opportunity ............................................................................................................. 5-8 PART 2— RISK ASSESSMENT
6. Identified Hazards of Concern and Risk Assessment Methodology............................................................ 6-1
6.1 Identified Hazards of Concern............................................................................................................. 6-1
6.2 Risk Assessment Tools........................................................................................................................ 6-2
6.2.1 Mapping..................................................................................................................................... 6-2
6.2.2 Hazus ......................................................................................................................................... 6-2
6.3 Risk Assessment Approach ................................................................................................................. 6-3
6.3.1 Hazard Profile Development...................................................................................................... 6-3
6.3.2 Exposure and Vulnerability ....................................................................................................... 6-3
6.4 Sources of Data Used in Modeling and exposure analysis.................................................................. 6-4
6.4.1 Building and Cost Data.............................................................................................................. 6-4
6.4.2 Hazus Data Inputs...................................................................................................................... 6-4
6.4.3 Other Local Hazard Data ........................................................................................................... 6-5
6.4.4 Data Source Summary ............................................................................................................... 6-5
6.5 Limitations........................................................................................................................................... 6-7
7. Dam Failure ................................................................................................................................................. 7-1
7.1 General Background............................................................................................................................ 7-1
7.1.1 Definition and Classification of Dams....................................................................................... 7-1
7.1.2 Causes of Dam Failure............................................................................................................... 7-1 vi
Contents
7.1.3 Planning Requirements .............................................................................................................. 7-2
7.2 Hazard Profile...................................................................................................................................... 7-3
7.2.1 Past Events................................................................................................................................. 7-3
7.2.2 Location ..................................................................................................................................... 7-3
7.2.3 Frequency................................................................................................................................... 7-6
7.2.4 Severity ...................................................................................................................................... 7-6
7.2.5 Warning Time ............................................................................................................................ 7-8
7.2.6 Secondary Hazards..................................................................................................................... 7-8
7.3 Exposure.............................................................................................................................................. 7-9
7.3.1 Population .................................................................................................................................. 7-9
7.3.2 Property...................................................................................................................................... 7-9
7.3.3 Critical Facilities...................................................................................................................... 7-10
7.3.4 Environment............................................................................................................................. 7-10
7.4 Vulnerability...................................................................................................................................... 7-11
7.4.1 Population ................................................................................................................................ 7-11
7.4.2 Property.................................................................................................................................... 7-12
7.4.3 Critical Facilities...................................................................................................................... 7-12
7.4.4 Environment............................................................................................................................. 7-12
7.5 Future Trends in Development.......................................................................................................... 7-13
7.6 Scenario............................................................................................................................................. 7-13
7.7 Issues ................................................................................................................................................. 7-14
8. Drought ........................................................................................................................................................ 8-1
8.1 General Background............................................................................................................................ 8-1
8.1.1 Monitoring and Categorizing Drought....................................................................................... 8-1
8.1.2 Drought Impacts......................................................................................................................... 8-4
8.1.3 California Drought Response..................................................................................................... 8-4
8.2 Hazard Profile...................................................................................................................................... 8-5
8.2.1 Local Water Use and Supply ..................................................................................................... 8-5
8.2.2 Past Events................................................................................................................................. 8-8
8.2.3 Location ................................................................................................................................... 8-10
8.2.4 Frequency................................................................................................................................. 8-10
8.2.5 Severity .................................................................................................................................... 8-10
8.2.6 Warning Time .......................................................................................................................... 8-12
8.2.7 Secondary Hazards................................................................................................................... 8-12
8.3 Exposure............................................................................................................................................ 8-12
8.4 Vulnerability...................................................................................................................................... 8-12
8.4.1 Population ................................................................................................................................ 8-12
8.4.2 Property.................................................................................................................................... 8-13
8.4.3 Critical Facilities...................................................................................................................... 8-13
8.4.4 Environment............................................................................................................................. 8-13
8.5 Future Trends in Development.......................................................................................................... 8-14
8.6 Scenario............................................................................................................................................. 8-14
8.7 Issues ................................................................................................................................................. 8-14
9. Earthquake ................................................................................................................................................... 9-1
9.1 General Background............................................................................................................................ 9-1
9.1.1 Earthquake Location .................................................................................................................. 9-1 vii
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements
9.1.2 Earthquake Geology................................................................................................................... 9-1
9.1.3 Earthquake-Related Hazards...................................................................................................... 9-2
9.1.4 Earthquake Classifications......................................................................................................... 9-2
9.1.5 Ground Motion........................................................................................................................... 9-4
9.1.6 USGS Earthquake Mapping Programs ...................................................................................... 9-4
9.1.7 Liquefaction and Soil Types ...................................................................................................... 9-5
9.2 Hazard Profile...................................................................................................................................... 9-6
9.2.1 Past Events................................................................................................................................. 9-6
9.2.2 Location ..................................................................................................................................... 9-7
9.2.3 Frequency................................................................................................................................... 9-9
9.2.4 Severity .................................................................................................................................... 9-12
9.2.5 Warning Time .......................................................................................................................... 9-12
9.2.6 Secondary Hazards................................................................................................................... 9-12
9.3 Exposure............................................................................................................................................ 9-13
9.3.1 Population ................................................................................................................................ 9-13
9.3.2 Property.................................................................................................................................... 9-13
9.3.3 Critical Facilities...................................................................................................................... 9-13
9.3.4 Environment............................................................................................................................. 9-15
9.4 Vulnerability...................................................................................................................................... 9-15
9.4.1 Population ................................................................................................................................ 9-15
9.4.2 Property.................................................................................................................................... 9-15
9.4.3 Critical Facilities...................................................................................................................... 9-21
9.4.4 Environment............................................................................................................................. 9-27
9.5 Future Trends in Development.......................................................................................................... 9-27
9.6 Scenario............................................................................................................................................. 9-27
9.7 Issues ................................................................................................................................................. 9-27
10. Flooding................................................................................................................................................... 10-1
10.1 General Background........................................................................................................................ 10-1
10.1.1 Types of Floodplains in the Planning Area............................................................................ 10-1
10.1.2 FEMA Regulatory Flood Zones............................................................................................. 10-2
10.1.3 Floodplain Ecosystems and Beneficial Functions ................................................................. 10-3
10.1.4 Effects of Human Activities................................................................................................... 10-4
10.2 Hazard Profile.................................................................................................................................. 10-5
10.2.4 Past Events........................................................................................................................... 10-10
10.2.5 Location ............................................................................................................................... 10-11
10.2.6 Frequency............................................................................................................................. 10-11
10.2.7 Severity ................................................................................................................................ 10-15
10.2.8 Warning Time ...................................................................................................................... 10-15
10.2.9 Secondary Hazards............................................................................................................... 10-16
10.3 Exposure........................................................................................................................................ 10-17
10.3.1 Population ............................................................................................................................ 10-17
10.3.2 Property................................................................................................................................ 10-17
10.3.3 Critical Facilities.................................................................................................................. 10-17
10.3.4 Environment......................................................................................................................... 10-21
10.4 Vulnerability.................................................................................................................................. 10-21
10.4.1 Population ............................................................................................................................ 10-21 viii
Contents
10.4.2 Property................................................................................................................................ 10-21
10.4.3 Critical Facilities.................................................................................................................. 10-21
10.4.4 Environment......................................................................................................................... 10-23
10.5 Future Trends in Development...................................................................................................... 10-23
10.6 Scenario......................................................................................................................................... 10-24
10.7 Issues ............................................................................................................................................. 10-24
11. Landslide/Mass Movement...................................................................................................................... 11-1
11.1 General Background........................................................................................................................ 11-1
11.1.1 Mass Movement Types.......................................................................................................... 11-1
11.1.2 Factors Causing Mass Movements ........................................................................................ 11-1
11.2 Hazard Profile.................................................................................................................................. 11-2
11.2.1 Past Events............................................................................................................................. 11-2
11.2.2 Location ................................................................................................................................. 11-3
11.2.3 Frequency............................................................................................................................... 11-5
11.2.4 Severity .................................................................................................................................. 11-5
11.2.5 Warning Time ........................................................................................................................ 11-5
11.2.6 Secondary Hazards................................................................................................................. 11-6
11.3 Exposure.......................................................................................................................................... 11-6
11.3.1 Population .............................................................................................................................. 11-6
11.3.2 Property.................................................................................................................................. 11-7
11.3.3 Critical Facilities.................................................................................................................... 11-8
11.3.4 Environment........................................................................................................................... 11-8
11.4 Vulnerability.................................................................................................................................... 11-9
11.4.1 Population .............................................................................................................................. 11-9
11.4.2 Property................................................................................................................................ 11-10
11.4.3 Critical Facilities.................................................................................................................. 11-10
11.4.4 Environment......................................................................................................................... 11-10
11.5 Future Trends in Development...................................................................................................... 11-11
11.6 Scenario......................................................................................................................................... 11-11
11.7 Issues ............................................................................................................................................. 11-12
12. Sea Level Rise.......................................................................................................................................... 12-1
12.1 General Background........................................................................................................................ 12-1
12.2 Hazard Profile.................................................................................................................................. 12-1
12.2.1 Previous Assessments ............................................................................................................ 12-2
12.2.2 Location ................................................................................................................................. 12-3
12.2.3 Frequency............................................................................................................................... 12-3 Severity ............................................................................................................................................. 12-6
12.3 Exposure.......................................................................................................................................... 12-6
12.3.1 Population .............................................................................................................................. 12-6
12.3.2 Property.................................................................................................................................. 12-8
12.3.3 Critical Facilities.................................................................................................................... 12-8
12.3.4 Environment........................................................................................................................... 12-8
12.4 Vulnerability.................................................................................................................................... 12-9
12.5 Future Trends in Development...................................................................................................... 12-10
12.6 Scenario......................................................................................................................................... 12-10
12.7 Issues ............................................................................................................................................. 12-10 ix
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements
13. Severe Weather ........................................................................................................................................ 13-1
13.1 General Background........................................................................................................................ 13-1
13.1.1 Thunderstorms, Lightning and Hail....................................................................................... 13-1
13.1.2 Damaging Winds ................................................................................................................... 13-1
13.1.3 Extreme Heat ......................................................................................................................... 13-2
13.1.4 Public Safety Power Shutoff Events ...................................................................................... 13-3
13.2 Hazard Profile.................................................................................................................................. 13-3
13.2.1 Past Events............................................................................................................................. 13-3
13.2.2 Location ................................................................................................................................. 13-5
13.2.3 Frequency............................................................................................................................... 13-5
13.2.4 Severity .................................................................................................................................. 13-5
13.2.5 Warning Time ........................................................................................................................ 13-5
13.2.6 Secondary Hazards................................................................................................................. 13-5
13.3 Exposure.......................................................................................................................................... 13-6
13.4 Vulnerability.................................................................................................................................... 13-6
13.4.1 Population .............................................................................................................................. 13-6
13.4.2 Property.................................................................................................................................. 13-6
13.4.3 Critical Facilities.................................................................................................................... 13-7
13.4.4 Environment........................................................................................................................... 13-7
13.5 Future Trends in Development........................................................................................................ 13-7
13.6 Scenario........................................................................................................................................... 13-7
13.7 issues ............................................................................................................................................... 13-8
14. Tsunami.................................................................................................................................................... 14-1
14.1 General Background........................................................................................................................ 14-1
14.2 Hazard Profile.................................................................................................................................. 14-2
14.2.1 Past Events............................................................................................................................. 14-2
14.2.2 Location ................................................................................................................................. 14-3
14.2.3 Frequency............................................................................................................................... 14-3
14.2.4 Severity .................................................................................................................................. 14-5
14.2.5 Warning Time ........................................................................................................................ 14-5
14.2.6 Secondary Hazards................................................................................................................. 14-6
14.3 Exposure.......................................................................................................................................... 14-7
14.3.1 Population .............................................................................................................................. 14-7
14.3.2 Property.................................................................................................................................. 14-8
14.3.3 Critical Facilities.................................................................................................................... 14-8
14.3.4 Environment......................................................................................................................... 14-10
14.4 Vulnerability.................................................................................................................................. 14-10
14.5 Future Trends in Development...................................................................................................... 14-10
14.6 Scenario......................................................................................................................................... 14-11
14.7 Issues ............................................................................................................................................. 14-12
15. Wildfire.................................................................................................................................................... 15-1
15.1 General Background........................................................................................................................ 15-1
15.1.1 Factors Affecting Fire Behavior ............................................................................................ 15-1
15.1.2 Secondary Hazards................................................................................................................. 15-1
15.2 Hazard Profile.................................................................................................................................. 15-2
15.2.1 Wildfire Factors for the Planning area................................................................................... 15-2 x
Contents
15.2.2 Past Events............................................................................................................................. 15-3
15.2.3 Location ................................................................................................................................. 15-8
15.2.4 Frequency............................................................................................................................... 15-8
15.2.5 Severity .................................................................................................................................. 15-8
15.2.6 Warning Time ...................................................................................................................... 15-11
15.3 Exposure........................................................................................................................................ 15-11
15.3.1 Population ............................................................................................................................ 15-11
15.3.2 Property................................................................................................................................ 15-12
15.3.3 Critical Facilities.................................................................................................................. 15-12
15.3.4 Environment......................................................................................................................... 15-14
15.4 Vulnerability.................................................................................................................................. 15-15
15.4.1 Population ............................................................................................................................ 15-15
15.4.2 Property................................................................................................................................ 15-15
15.4.3 Critical Facilities.................................................................................................................. 15-15
15.4.4 Environment......................................................................................................................... 15-16
15.5 Future Trends in Development...................................................................................................... 15-16
15.6 Scenario......................................................................................................................................... 15-17
15.7 Issues ............................................................................................................................................. 15-18
16. Climate Change........................................................................................................................................ 16-1
16.1 General Background........................................................................................................................ 16-1
16.1.1 What is Climate Change?....................................................................................................... 16-1
16.1.2 How Climate Change Affects Hazard Mitigation.................................................................. 16-1
16.1.3 Current Indicators of Climate Change ................................................................................... 16-2
16.1.4 Projected Future Impacts ....................................................................................................... 16-5
16.1.5 Responses to Climate Change................................................................................................ 16-7
16.2 Sonoma County Efforts to Address Climate Change ...................................................................... 16-7
16.2.1 A Roadmap for Climate Resilience in Sonoma County......................................................... 16-7
16.2.2 Sonoma County Regional Climate Protection Authority....................................................... 16-8
16.3 Vulnerability Assessment— Hazards of Concern........................................................................... 16-9
16.3.1 Dam Failure ........................................................................................................................... 16-9
16.3.2 Drought ................................................................................................................................ 16-10
16.3.3 Earthquake ........................................................................................................................... 16-12
16.3.4 Flood .................................................................................................................................... 16-12
16.3.5 Landslide/Mass Movements ................................................................................................ 16-13
16.3.6 Sea Level Rise...................................................................................................................... 16-14
16.3.7 Severe Weather .................................................................................................................... 16-14
16.3.8 Tsunami ............................................................................................................................... 16-15
16.3.9 Wildfire................................................................................................................................ 16-15
16.4 Issues ............................................................................................................................................. 16-16
17. Hazards of Interest ................................................................................................................................... 17-1
17.1 Public Health Emergency................................................................................................................ 17-1
17.2 Terrorism......................................................................................................................................... 17-2
17.2.1 Defining Terrorism ................................................................................................................ 17-2
17.2.2 Cyberterrorism ....................................................................................................................... 17-2
17.2.3 Addressing Terrorism ............................................................................................................ 17-3
17.3 Cyber-Attack ................................................................................................................................... 17-4 xi
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements
18. Risk Rating............................................................................................................................................... 18-1
18.1 Probability of Occurrence................................................................................................................ 18-1
18.2 Impact.............................................................................................................................................. 18-2
18.3 Risk Rating...................................................................................................................................... 18-3 PART 3— MITIGATION STRATEGY
19. Mission Statement, Goals, and Objectives............................................................................................... 19-1
19.1 Mission Statement ........................................................................................................................... 19-1
19.2 Goals................................................................................................................................................ 19-1
19.3 Objectives........................................................................................................................................ 19-2
20. Mitigation Best Practices and Adaptive Capacity.................................................................................... 20-1
20.1 Mitigation Best Practices................................................................................................................. 20-1
20.2 Adaptive Capacity ......................................................................................................................... 20-10
21. Area-Wide Action Plan............................................................................................................................ 21-1
21.1 Recommended Mitigation Actions.................................................................................................. 21-1
21.2 Action Plan Prioritization................................................................................................................ 21-1
21.2.1 Benefit/Cost Review .............................................................................................................. 21-2
21.2.2 Implementation Priority......................................................................................................... 21-3
21.2.3 Grant Pursuit Priority............................................................................................................. 21-3
21.2.4 Prioritization Summary for Mitigation Actions..................................................................... 21-3
21.3 Classification of Mitigation Actions................................................................................................ 21-4
22. Plan Adoption and Maintenance.............................................................................................................. 22-1
22.1 Plan Adoption.................................................................................................................................. 22-1
22.2 Action Plan Implementation............................................................................................................ 22-1
22.3 Plan Maintenance Strategy.............................................................................................................. 22-1
22.3.1 Plan Monitoring ..................................................................................................................... 22-2
22.3.2 Progress Reporting................................................................................................................. 22-3
22.3.3 Plan Evaluation ...................................................................................................................... 22-4
22.3.4 Incorporation into Other Planning Mechanisms .................................................................... 22-4
22.3.5 Grant Monitoring and Coordination ...................................................................................... 22-5
22.3.6 Plan Update............................................................................................................................ 22-5
22.3.7 Continuing Public Participation............................................................................................. 22-6 References
Appendices
Appendix A. Public Involvement Materials
Appendix B. Summary of Federal and State Agencies, Programs and Regulations Appendix C. Mapping Methods and Data Sources
Appendix D. Risk Assessment Results
Appendix E. Peak Riverine Discharges in the Planning Area Appendix F. FEMA Approval and Planning Partner Adoption Resolutions Appendix G. Progress Report Template
xii
Contents
Tables
Table ES-1. Area-Wide Hazard Mitigation Actions ............................................................................................ xxiv Table 2-1. Plan Changes Crosswalk....................................................................................................................... 2-3 Table 3-1. Hazard Mitigation Planning Partners.................................................................................................... 3-2 Table 3-2. Steering Committee Members............................................................................................................... 3-4 Table 3-3. Summary of Public Outreach Events .................................................................................................. 3-11 Table 3-4. Plan Development Chronology/Milestones ........................................................................................ 3-11 Table 4-1. Historical Sonoma County Natural Hazard Events............................................................................... 4-4 Table 4-2. Planning Area Building Counts by Occupancy Class......................................................................... 4-11 Table 4-3. Estimated Replacement Value of Planning Area Buildings................................................................ 4-11 Table 4-4. Planning Area Critical Facilities ......................................................................................................... 4-12 Table 4-5. Population Growth Data...................................................................................................................... 4-16 Table 4-6. Top Employers for the Planning Area ................................................................................................ 4-21 Table 5-1. Summary of Relevant Federal Agencies, Programs and Regulations................................................... 5-2 Table 5-2. Summary of Relevant State Agencies, Programs and Regulations....................................................... 5-3 Table 6-1. Hazus Model Data Documentation ....................................................................................................... 6-6 Table 7-1. High-Hazard Dams in the Planning Area or with Inundation Areas Extending into Planning Area .... 7-4 Table 7-2. State of California Downstream Hazard Potential Classification ......................................................... 7-8 Table 7-3. Exposed Property in Evaluated Dam Failure Inundation Area ........................................................... 7-10 Table 7-4. Estimated Dam Failure Impacts on Population................................................................................... 7-12 Table 7-5. Estimated Impact of a Dam Failure in the Planning Area................................................................... 7-12 Table 8-1. State Drought Management Program.................................................................................................... 8-5 Table 9-1. Mercalli Scale and Peak Ground Acceleration Comparison................................................................. 9-3 Table 9-2. NEHRP Soil Classification System ...................................................................................................... 9-6 Table 9-3. Earthquakes Modeled for Risk Assessment........................................................................................ 9-15 Table 9-4. Estimated Earthquake Impact on Persons........................................................................................... 9-15 Table 9-5. Age of Housing Units in Planning Area ............................................................................................. 9-21 Table 9-6. Estimated Impact of Earthquake Scenario Events in the Planning Area ............................................ 9-21 Table 10-1. Flood Insurance Statistics ................................................................................................................. 10-5 Table 10-2. Levees Shown on FIRM for Sonoma County................................................................................... 10-6 Table 10-3. CRS Status of Participating Jurisdictions ......................................................................................... 10-6 Table 10-4. Repetitive Loss Properties in Sonoma County.................................................................................. 10-8 Table 10-5. Declared Sonoma County Flood Disaster Events ........................................................................... 10-11 Table 10-6. Peak Riverine Discharges on the Petaluma and Russian Rivers..................................................... 10-14 Table 10-7. Summary of Still-Water Elevations Along San Pablo Bay............................................................. 10-15 Table 10-8. Regional Storm Surge Water Elevations......................................................................................... 10-15 Table 10-9. Damage and Estimated Losses from Recent Floods in Sonoma County ........................................ 10-16 Table 10-10. Exposed Population in Evaluated Flood Hazard Zones ................................................................ 10-17 Table 10-11. Exposed Property in Evaluated Flood Hazard Zones.................................................................... 10-18 Table 10-12. Estimated Flood Impacts on Residents ......................................................................................... 10-21 xiii
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Table 10-13. Estimated Impact of a Flood Event in the Planning Area............................................................. 10-21 Table 11-1. Landslide/Mass Movement Events in Sonoma County .................................................................... 11-2 Table 11-2. Exposed Population in Mapped Landslide Hazard Zones ................................................................ 11-6 Table 11-3. Exposed Property in Mapped Landslide Hazard Zones.................................................................... 11-7 Table 11-4. Loss Estimation for Mass Movement ............................................................................................. 11-10 Table 12-1. Estimated Population Exposure for Sea Level Rise.......................................................................... 12-7 Table 12-2. Estimated Property Exposure for Sea Level Rise ............................................................................. 12-8 Table 13-1. Beaufort Wind Chart......................................................................................................................... 13-2 Table 13-2. Operational Enhanced Fujita Scale ................................................................................................... 13-2 Table 13-3. Recorded Past Severe Weather Events in the Planning Area............................................................ 13-3 Table 14-1. Exposed Property in the Tsunami Inundation Zone.......................................................................... 14-8 Table 15-1. Sonoma County Fires Since 1964..................................................................................................... 15-4 Table 15-2. CAL FIRE Destructive Wildfire Statistics, 1939-2020 .................................................................... 15-5 Table 15-3. Exposed Population in Mapped Relative Fire Hazard Zones.......................................................... 15-12 Table 15-4. Exposed Property in Mapped Relative Fire Hazard Zones............................................................. 15-13 Table 15-5. Loss Estimates for Wildfire Hazard................................................................................................ 15-15 Table 16-1. Historical and Future Projections for Climate Information in Sonoma County................................ 16-6 Table 17-1. Common Mechanisms for Cyber Attacks ......................................................................................... 17-5 Table 18-1. Probability of Hazards....................................................................................................................... 18-2 Table 18-2. Impact on People, Property and the Economy from Hazards ........................................................... 18-3 Table 18-3. Hazard Risk Rating ........................................................................................................................... 18-3 Table 19-1. Objectives for the Hazard Mitigation Plan........................................................................................ 19-2 Table 20-1. Alternatives to Mitigate the Dam Failure Hazard ............................................................................. 20-2 Table-20-2. Alternatives to Mitigate the Drought Hazard.................................................................................... 20-3 Table-20-3. Alternatives to Mitigate the Earthquake Hazard............................................................................... 20-4 Table-20-4. Alternatives to Mitigate the Flooding Hazard .................................................................................. 20-5 Table-20-5. Alternatives to Mitigate the Landslide/Mass Movement Hazard ..................................................... 20-6 Table-20-6. Alternatives to Mitigate the Severe Weather Hazard ....................................................................... 20-7 Table 20-7. Alternatives to Mitigate the Tsunami Hazard ................................................................................... 20-8 Table-20-8. Alternatives to Mitigate the Wildfire Hazard ................................................................................... 20-9 Table-21-1. Action Plan—Countywide Mitigation Initiatives ............................................................................. 21-2 Table 21-2. Prioritization of Area-Wide Mitigation Actions ............................................................................... 21-3 Table 21-3. Analysis of Mitigation Actions ......................................................................................................... 21-4 Table 22-1. Plan Maintenance Matrix .................................................................................................................. 22-2 Figures
Figure 3-1. Sample Page from Hazard Mitigation Plan Web Site.......................................................................... 3-7 Figure 3-2. Example Story Map Data Page............................................................................................................ 3-8 Figure 3-3. Sample Page from Survey Distributed to the Public ........................................................................... 3-9 xiv
Contents
Figure 4-1. Sonoma County Planning Area............................................................................................................ 4-2 Figure 4-2. Chief Characteristics of Land within Sonoma County ...................................................................... 4-10 Figure 4-3. Critical Facilities, 1 of 2 .................................................................................................................... 4-13 Figure 4-4. Critical Facilities, 2 of 2 .................................................................................................................... 4-14 Figure 4-5. California and Sonoma County Historical Population Growth Rates................................................ 4-16 Figure 4-6. Planning Area Age Distribution ........................................................................................................ 4-17 Figure 4-7. Planning Area Race Distribution ....................................................................................................... 4-19 Figure 4-8. Industry in the Planning Area ............................................................................................................ 4-22 Figure 4-9. California and Sonoma County Unemployment Rate ....................................................................... 4-22 Figure 7-1. Locations of Dams in Sonoma County................................................................................................ 7-5 Figure 7-2. Dam Inundation Area Used for Risk Assessment................................................................................ 7-7 Figure 7-3. Socially Vulnerable Populations in Dam Failure Inundation Zone Census Blocks............................. 7-9 Figure 7-4. Building Occupancy Class Distribution in the Dam Failure Inundation Zone .................................. 7-10 Figure 7-5. Critical Facilities in Dam Failure Inundation Zones and Countywide .............................................. 7-11 Figure 7-6. Estimated Damage to Critical Facilities from Dam Failure............................................................... 7-13 Figure 8-1. Standard National Drought and Precipitation Indices ......................................................................... 8-3 Figure 8-2. Russian River Watershed and the Sonoma Water Supply System ...................................................... 8-6 Figure 8-3. Groundwater Availability in Sonoma County ..................................................................................... 8-9 Figure 8-4. Percent of Sonoma County Affected by Each USDM Rating, 2000 – 2020 ..................................... 8-11 Figure 9-1. Peak Acceleration (%g) with 2% Probability of Exceedance in 50 Years .......................................... 9-5 Figure 9-2. Mapped Faults in Sonoma County....................................................................................................... 9-8 Figure 9-3. NEHRP Soil Class ............................................................................................................................. 9-10 Figure 9-4. Liquefaction Susceptibility................................................................................................................ 9-11 Figure 9-5. Peak Horizontal Acceleration (%g) with 10% Probability of Exceedance in 50 Years .................... 9-12 Figure 9-6. Socially Vulnerable Populations Living on NEHRP D or E Soils Census Blocks............................ 9-13 Figure 9-7. Critical Facilities Constructed on NEHRP Type D and E Soils, and Countywide............................ 9-14 Figure 9-8. 100-Year Probabilistic Earthquake.................................................................................................... 9-16 Figure 9-9. Hayward M7.57 Earthquake Scenario............................................................................................... 9-17 Figure 9-10. Maacama M7.55 Earthquake Scenario ............................................................................................ 9-18 Figure 9-11. Rodgers Creek – Healdsburg M7.19 Earthquake Scenario ............................................................. 9-19 Figure 9-12. San Andreas M8.04 Earthquake Scenario ....................................................................................... 9-20 Figure 9-13. Critical Facility Damage Potential, 100-Year Probabilistic Earthquake ......................................... 9-22 Figure 9-14. Critical Facility Damage Potential, Hayward Fault Scenario.......................................................... 9-22 Figure 9-15. Critical Facility Damage Potential, Maacama Fault Scenario......................................................... 9-23 Figure 9-16. Critical Facility Damage Potential, Rodgers Creek – Healdsburg Fault Scenario .......................... 9-23 Figure 9-17. Critical Facility Damage Potential, San Andreas Scenario.............................................................. 9-24 Figure 9-18. Critical Facility Functionality, 100-Year Probabilistic Earthquake................................................. 9-24 Figure 9-19. Critical Facility Functionality, Hayward Fault Scenario ................................................................. 9-25 Figure 9-20. Critical Facility Functionality, Maacama Fault Scenario ................................................................ 9-25 Figure 9-21. Critical Facility Functionality, Rodgers Creek – Healdsburg Fault Scenario.................................. 9-26 Figure 9-22. Critical Facility Functionality, San Andreas Fault Scenario............................................................ 9-26 Figure 10-1. Typical Transect Schematic............................................................................................................. 10-4 Figure 10-2. FEMA FIRM Flood Hazard Areas ................................................................................................ 10-12 Figure 10-3. Sonoma County Flood Awareness Areas ...................................................................................... 10-13 xv
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Figure 10-4. Socially Vulnerable Populations in Flood Zone Census Blocks ................................................... 10-18 Figure 10-5. Building Occupancy Classes in the Mapped Flood Zones ............................................................ 10-19 Figure 10-6. Critical Facilities in Mapped Flood Hazard Areas and Countywide ............................................. 10-20 Figure 10-7. Estimated Damage to Critical Facilities from 1% Annual Chance Flood ..................................... 10-22 Figure 10-8. Estimated Damage to Critical Facilities from 0.2% Annual Chance Flood .................................. 10-22 Figure 11-1. Susceptibility to Deep-Seated Landslides........................................................................................ 11-4 Figure 11-2. Socially Vulnerable Populations in High and Very High Landslide Risk Zones Census Blocks.... 11-7 Figure 11-3. Building Occupancy Classes in the Mapped Landslide Hazard Zones ........................................... 11-8 Figure 11-4. Critical Facilities in Mapped Landslide Susceptibility Classes and Countywide............................ 11-9 Figure 12-1. Estimated Inundation Area for 200-Centimeter Sea Level Rise with No Storm ............................. 12-4 Figure 12-2. Estimated Inundation Area for 200-Centimeter Sea Level Rise with a 100-Year Storm Event...... 12-5 Figure 12-3. Medium Projection for Sonoma County Sea Level Rise Above 1992 Baseline.............................. 12-6 Figure 12-4. Socially Vulnerable Populations in 200-cm SLR + 100-Yr Inundation Zone Census Blocks ........ 12-7 Figure 12-5. Building Occupancy Classes in the Mapped Flood Zones .............................................................. 12-8 Figure 12-6. Critical Facilities in Sea-Level Rise Inundation Areas.................................................................... 12-9 Figure 14-1. Common Sources of Tsunamis........................................................................................................ 14-1 Figure 14-2. Runup Distance and Height in Relation to the Datum and Shoreline.............................................. 14-2 Figure 14-3. 1964 Alaska Earthquake Tsunami Event......................................................................................... 14-3 Figure 14-4. Tsunami Inundation Zones .............................................................................................................. 14-4 Figure 14-5. Potential Tsunami Travel Times in the Pacific Ocean, in Hours..................................................... 14-7 Figure 14-6. Socially Vulnerable Populations in the Mapped Tsunami Inundation Zone Census Blocks........... 14-8 Figure 14-7. Structures in the Tsunami Inundation Zone, by Building Occupancy Class ................................... 14-9 Figure 14-8. Critical Facilities in Mapped Tsunami Inundation Zone and Countywide...................................... 14-9 Figure 14-9. 1700 Cascadia Subduction zone Earthquake Tsunami Event........................................................ 14-11 Figure 15-1. Relative Fire Hazard Zones ............................................................................................................. 15-9 Figure 15-2. Acres and Structures burned, 1964 - 2020..................................................................................... 15-10 Figure 15-3. Socially Vulnerable Populations in High and Very High Fire Hazard Zones Census Blocks....... 15-12 Figure 15-4. Structures in the High or Very High Relative Fire Hazard Zones, by Land Use Type ................. 15-13 Figure 15-5. Critical Facilities in Mapped Relative Fire Hazard Zones and Countywide ................................. 15-14 Figure 16-1. Global Carbon Dioxide Concentrations Over Time ........................................................................ 16-2 Figure 18-1. Hazard Risk Rating.......................................................................................................................... 18-4 xvi
Acknowledgments
ACKNOWLEDGMENTS
Sonoma County Consultants (Tetra Tech)
• Lisa Hulette, Permit Sonoma, Grants Manager
• Domenica Giovannini, Permit Sonoma, Policy Manager
• Shelly Bianchi-Williamson, Permit Sonoma, GIS
Supervisor
• Gary Helfrich, Permit Sonoma, Planner III
• Bradley Dunn, Permit Sonoma, PIO
• Steve Mosiurak, Fire Prevention, Interim Fire Marshall • Chris Godley, Emergency Management, Director
• Hunter McLaughlin, Public Works, Engineer
Stakeholders
• Rob Flaner, Project Manager
• Bart Spencer, Project Lead Planner
• Carol Baumann, Risk Assessment
Lead
• Des Alexander, Profiling Lead
• Cindy Rolli, Public Outreach (Story-
map) lead
• Dan Portman, Technical Editor
• Kimberly Jordan, Town of Windsor, Planner III
• Shari Meads, City of Santa Rosa, City Planner
• Katherine Duran, City of Cotati, Administrative Analyst • Sheri Emerson, Sonoma County Ag & Open Space District, Program Manager • Mollie Assay, Sonoma County Water Agency
• Marshall Turbeville, Northern Sonoma County Fire Protection District, Chief • Scott Westrope, Santa Rosa City Fire, Deputy Chief
• Ben Nicholls, Cal FIRE, Division Chief
• Sarah Newkirk, The Nature Conservancy, Senior Project Director • Lisa Micheli, PhD, Pepperwood Preserve, President & CEO • Marc Chadler, Community Development Commission, Env Compliance Manager • Robert Cantu, Western Builders Association
• Karissa Kruse, Sonoma County Winegrowers Association, Executive Director Special Acknowledgments
The development of this plan would not have been possible without the dedication and commitment to this process by the Hazard Mitigation Plan Steering Committee. The dedication of the volunteers on this committee to allocate their time to this process is greatly appreciated. Also, the citizens of Sonoma County are commended for their participation in the plan outreach activities. Their involvement sets the course for successful implementation of this plan.
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Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements
ACRONYMS/ABBREVIATIONS
AB—Assembly Bill
ADA—American with Disabilities Act
ASDSO—Association of State Dam Safety Officials
BLM—Bureau of Land Management
Cal EMA—California Emergency Management
Agency
CAL FIRE—California Department of Forestry and
Fire Protection
Cal OES—California Governor’s Office of Emergency
Services
Caltrans—California Department of Transportation
CCR—California Code of Regulations
CDBG-DR—Community Development Block Grant
Disaster Recovery
CEQA—California Environmental Quality Act
CFA—California Fire Alliance
CFR—Code of Federal Regulations
CRS—Community Rating System
CWA—Clean Water Act
DFIRM—Digital Flood Insurance Rate Maps
DMA —Disaster Mitigation Act
DWR—Department of Water Resources (California)
EAP—Emergency Action Plan
EPA—U.S. Environmental Protection Agency
EOC—emergency operations center
ESA—Endangered Species Act
EWP—Emergency Watershed Protection
FEMA—Federal Emergency Management Agency
FERC—Federal Energy Regulatory Commission
FIRM—Flood Insurance Rate Map
GIS—Geographic Information System
Hazus—Hazards, United States
HIFLD—Homeland Infrastructure Foundation-Level
Data
HMGP—Hazard Mitigation Grant Program
IA—Individual Assistance
IBC—International Building Code
IPCC—Intergovernmental Panel on Climate Change
LRA—Local Responsibility Areas
mph—Miles per hour
Mw—Moment Magnitude Scale
NASA—National Aeronautics and Space
Administration
NEHRP—National Earthquake Hazards Reduction
Program
NFIP—National Flood Insurance Program
NIMS—National Incident Management System
NOAA—National Oceanic and Atmospheric
Administration
NWS—National Weather Service
PA—Public Assistance
PGA—Peak Ground Acceleration
RCPA— Regional Climate Protection Authority
SEMS—Standardized Emergency Management
Systems
SFHA—special flood hazard area
SLR—sea-level rise
SRA—State Responsibility Areas
SRL—severe repetitive loss
USDA—U.S. Department of Agriculture
USDM—U.S. Drought Monitor
USGS—U.S. Geological Survey
WUI—wildland/urban interface
xviii
EXECUTIVE SUMMARY
Hazard mitigation is the use of long-term and short-term policies, programs, projects, and other activities to alleviate the death, injury, and property damage that can result from a disaster. Sonoma County developed an updated hazard mitigation plan in partnership with the following local governments within the county:
• Town of Windsor • County of Sonoma
• City of Sonoma • Northern Sonoma County Fire Protection • City of Cotati District
• City of Santa Rosa • Rancho Adobe Fire
• Gold Ridge Resource Conservation District • North Sonoma Coast Fire Protection District • Sonoma Resource Conservation District • Timber Cove Fire Protection District • Sonoma County Agricultural Preservation & • Cloverdale Fire Protection District Open Space District • Sonoma County Fire Protection District The hazard mitigation plan defines measures to reduce risks from natural disasters in the Sonoma County Operational Area, which consists of the entire county, including unincorporated areas, incorporated cities, and special purpose districts. The plan complies with federal and state hazard mitigation planning requirements to establish eligibility for funding under Federal Emergency Management Agency (FEMA) grant programs for all planning partners. It updates the County’s previous plan, the 2016 Sonoma County Operational Area Hazard Mitigation Plan.
PLAN DEVELOPMENT APPROACH
Organization
A core planning team consisting of a contract consultant and Sonoma County staff was assembled to facilitate this plan update. A planning partnership was formed by engaging eligible local governments and making sure they understood their expectations for compliance under the updated plan. A steering committee was assembled to oversee the plan update, consisting of both governmental and non-governmental stakeholders within the planning area. Coordination with other local, state, and federal agencies involved in hazard mitigation occurred throughout the plan update process. Organization efforts included a review of the County’s 2016 hazard mitigation plan, the California statewide hazard mitigation plan, and existing programs that may support hazard mitigation actions.
Public Outreach
The planning team implemented a multi-media public involvement strategy utilizing the outreach capabilities of the planning partnership that was approved by the Steering Committee. The strategy included public meetings, a hazard mitigation survey, a project website, the use of social media, and multiple media releases. xix
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Plan Document Development
The planning team and Steering Committee assembled a document to meet federal hazard mitigation planning requirements for all partners. The updated plan contains two volumes. Volume 1 contains components that apply to all partners and the broader planning area. Volume 2 contains all components that are jurisdiction-specific.
Each planning partner has a dedicated annex in Volume 2.
Adoption
Once pre-adoption approval has been granted by the California Office of Emergency Services and FEMA Region IX, the final adoption phase will begin. Each planning partner will individually adopt the updated plan.
RISK ASSESSMENT
Risk assessment is the process of measuring the potential loss of life resulting from natural hazards, as well as personal injury, economic injury, and property damage, in order to determine the vulnerability of people, buildings, and infrastructure. For this update, risk assessment models were enhanced with new data and technologies. The Steering Committee used the risk assessment to rate risk and to gauge the potential impacts of each hazard of concern in the planning area. The risk assessment included the following:
• Hazard identification and profiling
• Assessment of the impact of hazards on physical, social, and economic assets • Identification of particular areas of vulnerability
• Estimates of the cost of potential damage.
Based on the risk assessment, hazards were rated for the risk they pose to the overall planning area. Figure ES-1 shows the resulting scores and ratings for the entire Sonoma County planning area. Each planning partner also rated hazards for its own area. Figure ES-2 summarizes how the 14 participating planning partners rated each hazard. The results indicate the following general patterns:
• Almost all planning partners rated earthquake and wildfire in the highest risk category. • Flooding, severe weather, and landslide/mass movement are the hazards most commonly rated in the medium risk category by individual planning partners, though landslide/mass movement was rated in the highest risk category countywide.
• Sea-level rise and tsunami are the hazards most commonly rated as lowest risk by individual planning partners, though sea-level rise was rated as a medium risk countywide. xx
Executive Summary
4 Earthquake
36 Landslide/ Mass
Movement
42 Wildfire
36 Dam Failure
24 Flood
18 Sea-level rise
18 Severe Weather
182
3 Highest
Risk
Medium
Risk
Drought
12 Tsunami
51 Lowest
Risk
Values shown are risk rating. Larger circles indicate higher rating. 0 0 1 2 3 4 5 6 7 8 9
Figure ES-1. Countywide Hazard Risk Rating
Number of Parters Assigning This Rating to This Hazard
14 12
10 8
6 4
2 0
Highest Risk Medium Risk 12
Lowest Risk Not Ranked 11
10 10
9 9
6 6
5 4
3 3 3 3 3
2 2 2 2 2 2
1 1 1
0 0 0 0 0 0 0 0
13 1
0 0
Figure ES-2. Summary of Risk Rating for Individual Planning Partners 10 xxi
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements
MITIGATION GOALS AND OBJECTIVES
The Steering Committee reviewed and made minor updates to the guiding principle, goals, and objectives from the 2016 Sonoma County Operational Area Hazard Mitigation Plan. The following guiding principle guided the Steering Committee and planning partners in selecting actions contained in this plan update:
Create a resilient Sonoma County for the whole community.
Goals
The Steering Committee and planning partners established the following goals for the plan update:
1. Protect people and minimize loss of life, injury, and social impacts
2. Minimize potential for loss of property, economic and social impacts, and displacement due to hazards.
3. Minimize potential for environmental impacts and consider a broad-range of mitigation solutions, including nature-based solutions where feasible.
4. Communicate natural hazard risk to the whole community within Sonoma County.
5. Support and inform the development of relevant mitigation policies and programs.
6. Promote an adaptive and resilient Sonoma County that proactively anticipates the future impacts from hazards within the county.
7. Pursue the development and implementation of long-term, cost-effective, and environmentally sound mitigation projects.
8. Enhance the capability/capacity of the Sonoma County planning area to prepare, respond and recover from the impact of natural hazards.
The effectiveness of a mitigation strategy is assessed by determining how well these goals are achieved.
Objectives
Each selected objective meets multiple goals, serving as a stand-alone measurement of the effectiveness of a mitigation action, rather than as a subset of a goal. The objectives also are used to help establish priorities. The objectives are as follows:
1. Incorporate mitigation best management measures into plans, codes, and other regulatory standards for the private sector, nonprofit agencies, and community-based organizations within the planning area.
2. Maintain established partnerships in the identification and implementation of mitigation measures in the Sonoma County Planning area.
3. Retrofit, purchase, mitigate or relocate structures in high hazard areas, with an emphasis on those subject to repetitive damages.
4. Promote and implement hazard mitigation plans and projects that are consistent with state, regional, and local climate action and adaptation goals, policies, and programs.
5. Improve and expand systems that provide warning and emergency communications to the whole community.
6. Increase resilience and capabilities of community lifelines. xxii
Executive Summary
7. Prevent (or discourage) new development in hazardous areas or ensure that if building occurs in high-risk areas that it is done in such a way as to minimize risk
8. At the local government level, continually improve understanding of the location and potential impacts of natural hazards, utilizing the best available data and science
9. Consider the impacts of natural hazards in all planning mechanisms that address current and future land uses within the planning area
10. Minimize adverse impacts from flood risk on vulnerable communities.
11. Through the enforcement of relevant federal, State and local regulations, sustain life and property protection measures for all communities and structures located in the Sonoma County Planning area.
12. All cities, the County, special districts, and tribal organizations will develop, adopt, and implement local hazard mitigation principles that may be integrated with local comprehensive plan safety elements, Community Wildfire Protection Plans, floodplain management plans, facilities master plans, and other local planning initiatives.
MITIGATION ACTION PLAN
The planning partners selected mitigation actions to work toward achieving the goals set forth in this plan update.
Mitigation actions presented in this update are activities designed to reduce or eliminate losses resulting from natural hazards. The update process resulted in the identification of 309 mitigation actions for implementation by individual planning partners, as presented in Volume 2 of this plan. In addition, the Steering Committee and planning partners identified countywide actions benefiting the whole partnership, as listed in Table ES-4.
IMPLEMENTATION
The Steering Committee developed a plan implementation and maintenance strategy that includes grant monitoring and coordination, a strategy for continued public involvement, a commitment to plan integration with other relevant plans and programs, and a recommitment from the planning partnership to actively monitoring and evaluating the plan over the five-year performance period.
Full implementation of the recommendations of this plan will require time and resources. The measure of the plan’s success will be its ability to adapt to changing conditions. Sonoma County and its planning partners will assume responsibility for adopting the recommendations of this plan and committing resources toward implementation. The framework established by this plan commits all planning partners to pursue actions when the benefits of a project exceed its costs. The planning partnership developed this plan with extensive public input, and public support of the actions identified in this plan will help ensure the plan’s success. xxiii
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Table ES-1. Area-Wide Hazard Mitigation Actions
Action Number and Description
Priority for
Implementation
Priority for
Pursuing
Grants
CW-1—The County will pursue an “Information Sharing Access Agreement” with FEMA allowing the County to readily access FEMA repetitive loss data for the entire county as needed at a level of detail to study and analyze repetitive flood loss problems in the county High N/A
CW-2—Continue to maintain a county-wide hazard mitigation website and the associated Story- Map that will store the hazard mitigation plan and provide the public an opportunity to monitor plan implementation progress. Each planning partner can support this initiative by including an initiative in its action plan of creating a link to the County hazard mitigation website.
High N/A
CW-3—Leverage public outreach partnering capabilities in the planning area (such as CERT) to promote a uniform and consistent message on the importance of proactive hazard mitigation.
High N/A
CW-4—Continue to update hazard mapping with best available data and science as it evolves, within the capabilities of the planning partnership. Support FEMA’s RiskMAP initiative.
High Medium
CW-5—Retain a steering committee as a working body over time to monitor progress of the hazard mitigation plan, provide technical assistance to planning partners, manage data, and oversee the update of the plan according to schedule. This body will continue to operate under the ground rules established at its inception.
High N/A
CW-6—Strive to capture time-sensitive, perishable data—such as high-water marks, extent and location of hazard, and loss information—following hazard events to support future updates to the risk assessment as well as other plans and programs that utilize hazard extent and location data
High N/A
CW-7—Utilize viable and relevant information, data and tools (Hazus models) developed as part of the update to the risk assessment of this plan update to support training and exercise of the County’s preparedness, response and recovery programs
High N/A
xxiv
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021 PART 1—PLANNING PROCESS AND
COMMUNITY PROFILE
1. INTRODUCTION TO HAZARD MITIGATION PLANNING
1.1 WHY PREPARE THIS PLAN?
1.1.1 The Big Picture
Hazard mitigation is defined as any action taken to reduce or alleviate the loss of life, personal injury, and property damage that can result from a disaster. It involves long- and short-term actions implemented before, during and after disasters. Hazard mitigation activities include planning efforts, policy changes, programs, studies, improvement projects, and other steps to reduce the impacts of hazards.
The federal Disaster Mitigation Act (DMA) of 2000 emphasizes planning for disasters before they occur. The DMA requires state and local governments to develop hazard mitigation plans as a condition for federal disaster grant assistance. Regulations developed to fulfill the DMA’s requirements are included in Title 44 of the Code of Federal Regulations (44 CFR).
The responsibility for hazard mitigation lies with many, including private property owners, commercial interests, and local, state and federal governments. The DMA encourages cooperation among state and local authorities in pre-disaster planning. The enhanced planning network called for by the DMA helps local governments to articulate accurate needs for mitigation, resulting in faster allocation of funding and more cost-effective risk- reduction projects.
The DMA also promotes sustainability in hazard mitigation. To be sustainable, hazard mitigation needs to incorporate sound management of natural resources and address hazards and mitigation in the largest possible social and economic context.
1.1.2 Purposes for Planning
Sonoma County prepared a hazard mitigation plan in compliance with the DMA in 2006 and has updated the plan every five years since then, most recently in 2016. This Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021 fulfills the ongoing update requirement.
In preparing this update, Sonoma County has partnered with local cities and special-purpose districts. One of the benefits of such multi-jurisdictional planning is the ability to pool resources and eliminate redundant activities within a planning area that has uniform risk exposure and vulnerabilities. The Federal Emergency Management Agency (FEMA) encourages multi-jurisdictional planning under its guidance for the DMA. The recommendations of this update were selected because they best meet the needs of all the planning partners and their citizens. 1-1
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements The 2021 Update will help guide and coordinate mitigation activities throughout the planning area. It was developed to meet the following objectives:
• Meet or exceed requirements of the DMA.
• Enable all planning partners to continue using federal grant funding to reduce risk through mitigation. • Meet the needs of each planning partner as well as state and federal requirements. • Create a risk assessment of local hazards of concern. • Meet the planning requirements of FEMA’s Community Rating System (CRS), allowing eligible planning partners to consider participation in the CRS program.
• Coordinate existing plans and programs so that high-priority projects to mitigate possible disaster impacts are funded and implemented.
1.2 WHO WILL BENEFIT FROM THIS PLAN?
All citizens and businesses of Sonoma County are the ultimate beneficiaries of this hazard mitigation plan. The plan reduces risk for those who live in, work in, and visit the planning area. It provides a viable planning framework for all foreseeable natural hazards. Participation in development of the plan by key stakeholders helped ensure that outcomes will be mutually beneficial. The resources and background information in the plan are applicable across the planning area, and the plan’s goals and recommendations can lay groundwork for the development and implementation of local mitigation activities and partnerships.
1.3 CONTENTS OF THIS PLAN
This plan has been set up in two volumes so that elements that are jurisdiction-specific can easily be distinguished from those that apply to the whole planning area:
• Volume 1—Volume 1 includes all federally required elements of a disaster mitigation plan that apply to the entire planning area. This includes the description of the planning process, public involvement strategy, goals and objectives, planning area hazard risk assessment, planning area mitigation actions, and a plan maintenance strategy.
• Volume 2—Volume 2 includes all federally required jurisdiction-specific elements, in annexes for each participating jurisdiction. It includes a description of the participation requirements established by the Steering Committee, as well as instructions and templates that the partners used to complete their annexes. Volume 2 also includes “linkage” procedures for eligible jurisdictions that did not participate in development of this plan but wish to adopt it in the future.
Both volumes include elements required under federal guidelines. Where sections of this plan address specific DMA requirements, the CFR section number in which the requirement is found is cited..
The following appendices provided at the end of Volume 1 include information or explanations to support the main content of the plan:
• Appendix A—Public involvement information used in preparation of this update • Appendix B—A summary of federal and state programs and regulations relevant to hazard mitigation • Appendix C—A description of data sources and methods used for mapping hazard areas in the county 1-2
1. Introduction to Hazard Mitigation Planning
• Appendix D—Quantitative results from risk assessment modeling for individual cities and Sonoma County supervisorial districts
• Appendix E—Peak stream flow levels used for Sonoma County Flood Insurance Study • Appendix F—Plan adoption resolutions from planning partners • Appendix G—Progress report template
All planning partners will adopt Volume 1 in its entirety and at least the following parts of Volume 2: Part 1; each partner’s jurisdiction-specific annex; and the appendices. 1-3
2. PLAN UPDATE—WHAT HAS CHANGED
2.1 THE PREVIOUS PLAN
The 2016 update of the Sonoma County Hazard Mitigation Plan, like the 2011 plan before it, focused on earthquake, flood, wildland fire, and landslide hazards as these were considered to be the greatest risk to the county based on past disaster events, future probabilities, and degree of vulnerability. The 2016 Plan also addressed secondary and tertiary hazards such as winter storms, coastal erosion and bluff failure, tsunamis, and post-fire erosion. The 2016 update discussed the implications that climate change may have on hazard trends in Sonoma County and included an expanded discussion of sea level rise and drought. Based on its assessment of these hazards, the 2016 plan recommended 60 mitigation actions that address the following hazards of concern:
• Flood
• Earthquake
• Wildland fire
• Landslide
• Sea level rise
The Sonoma County Board of Supervisors adopted the plan (Resolution No. 17-0168) on April 25, 2017. FEMA issued approval of the plan on May 4, 2017.
2.2 WHY UPDATE?
2.2.1 Federal Eligibility
Under 44 CFR, hazard mitigation plans must present a schedule for monitoring, evaluating, and updating the plan.
This provides an opportunity to reevaluate recommendations, monitor the impacts of actions that have been accomplished, and determine if there is a need to change the focus of mitigation strategies. A jurisdiction covered by a plan that has expired is not able to pursue elements of federal funding for which a current hazard mitigation plan is a prerequisite.
2.2.2 Changes in Development
Hazard mitigation plan updates must be revised to reflect changes in development within the planning area during the previous performance period of the plan (44 CFR Section 201.6(d)(3)). The plan must describe changes in development in hazard-prone areas that increased or decreased vulnerability for each jurisdiction since the last plan was approved. If no changes in development impacted the jurisdiction’s overall vulnerability, plan updates 2-1
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements may validate the information in the previously approved plan. The intent of this requirement is to ensure that the mitigation strategy continues to address the risk and vulnerability of existing and potential development and takes into consideration possible future conditions that could impact vulnerability.
The planning area experienced a 1.55-percent increase in population between 2010 and 2020, an average growth rate of 1.05 percent per year. Sonoma County and its incorporated cities have general plans that govern land-use decisions and policymaking, as well as building codes and specialty ordinances based on state and federal mandates. This plan update assumes that some new development triggered by increased population occurred in hazard areas. Because all such new development would have been regulated pursuant to local programs and codes, it is assumed that vulnerability did not increase even if exposure did. More detailed information on the types and location of new construction over the last five years is available in the city and County annexes in Volume 2 of this plan.
Since the scope and scale of this plan update was significantly different than that of the 2016 plan (multi- jurisdictional vs. single-jurisdictional) and different data and methodologies were applied to community profiling and risk assessment, it was not possible to compare growth demographic data between the two plans. Therefore, this plan update will be treated as the baseline for growth trend comparative analyses for future plan updates that will include the following metrics:
• Change in population over the performance period
• Change in general building stock within the planning area over the performance period • Change in the value of the general building stock over the performance period • Change in land-use over the performance period
2.2.3 Multi-Jurisdictional Planning
Unlike the current update, the original Sonoma County hazard mitigation plan and previous updates were prepared for the County only. No municipalities or special purpose districts participated in those earlier versions.
This was driven by the fact that most of the eligible local governments within the planning area had developed their own plans or did not see a need for one.
During the performance period of the 2016 plan, extensive outreach within the operational area was performed by Permit Sonoma as well as Sonoma County Emergency Management on the net benefits of working together as a multi-jurisdictional partnership through a collaborative effort. This was spurred by response efforts to the numerous catastrophic wildfires that impacted the operational area during the performance period. Thus, a multi- jurisdictional partnership was formed. While this 2021 plan update does not include all eligible local governments within the Sonoma County Operational Area, it does represent an enhanced effort in collaboration and coordination with these entities.
2.3 THE UPDATED PLAN—WHAT IS DIFFERENT?
The updated plan differs from the initial plan in a variety of ways. Table 2-1 indicates the major changes between the two plans as they relate to 44 CFR planning requirements. 2-2
2. Plan Update—What Has Changed
Table 2-1. Plan Changes Crosswalk
44 CFR Requirement Previous Plan Updated Plan
§201.6(b): In order to develop a more The planning process for the 2016 plan is The scope and scale of the plan was comprehensive approach to reducing the summarized in the introduction section of the plan. changed from a single jurisdiction scope effects of natural disasters, the planning The plan update was facilitated through an to a multi-jurisdictional scope. This plan process shall include: Oversight Committee made up predominately of update process built upon the successes (1) An opportunity for the public to County staff. The Oversight Committee developed of the 2016 planning effort using an comment on the plan during the a public outreach strategy in accordance with approach tied more closely to maximizing drafting stage and prior to plan FEMA guidelines. This strategy includes credit potential under FEMA’s Community approval; coordinated messaging through a variety of Rating System (CRS) program. This (2) An opportunity for neighboring mediums to ensure the whole community is included:
communities, local and regional informed of the Plan and initiatives. Risks were re- • Plan update was facilitated through a agencies involved in hazard
mitigation activities, and agencies
that have the authority to regulate
development, as well as businesses,
academia and other private and non-
profit interests to be involved in the
planning process; and
(3) Review and incorporation, if
appropriate, of existing plans,
studies, reports, and technical
information.
evaluated using updated data. The Mitigations
strategies were reviewed and updated as needed.
The actions were prioritized according to the same
methodology as was used on the 2011 plan.
stakeholder Steering Committee
made up of planning partners and
other key stakeholders
• A two-phase public outreach strategy
deployed to gauge the public’s
perception of risk early in the
process, and an opportunity to
comment on the draft plan late in the
process.
• A strategy for agency coordination
and inclusion in the plan update
process
• A comprehensive core capability
assessment process designed to
identify existing core capabilities that
can support or enhance the
outcomes of this plan.
• All of this is documented in Volume
1, Chapter 3 of the plan update
§201.6(c)(2): The plan shall include a risk The Plan includes a detailed risk assessment of 4 Volume 1 Part 2 presents a risk assessment that provides the factual identified hazards of concern (Seismic, Flood, assessment of 9 hazards of concern. basis for activities proposed in the Wildfire and Landslide). Each hazard of concern These identified hazards were expanded strategy to reduce losses from identified was profiled to provide the following information: from the 2016 plan to include Dam hazards. Local risk assessments must • General description of the hazard including Failure, Drought, Severe Weather, Sea- provide sufficient information to enable type, location, and extent of all hazards that level Rise, and Tsunami as stand-alone the jurisdiction to identify and prioritize impact each jurisdiction within the planning hazard profiles. The hazards are profiled appropriate mitigation actions to reduce area. as they impact the Sonoma County losses from identified hazards. • Discussion of destructive characteristics from planning area. Hazard profiles are a national and local perspective standardized for each hazard of concern, • Information on historical occurrences of so that there is uniformity in the hazard events discussion of each hazard and the
• The probability of future hazard occurrence information provided can support rating of risk for each jurisdiction. Other hazards of
interest were qualitatively assessed to
develop a more complete picture of the
hazards facing the planning area. The
planning area for this update was
expanded from that utilized in the 2016
plan to include the entire Sonoma County
operational area.
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Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements 44 CFR Requirement Previous Plan Updated Plan
§201.6(c)(2)(i): [The risk assessment
shall include a] description of the …
location and extent of all natural hazards
that can affect the jurisdiction. The plan
shall include information on previous
occurrences of hazard events and on the
probability of future hazard events.
Each hazard of concern was profiled to include:
• Hazard Description
• Hazard History
• Future Potential
• Exposure and Vulnerability
• Impact and Loss Estimates
• Plans
• Codes and Regulations
• Mitigation Programs and Activities
• Financial Resources
Note that the scope of these profiles was for the
unincorporated areas of the county only.
Volume 1 Part 2 presents a risk
assessment of each hazard of concern.
Each chapter includes the following
components:
• Hazard profile, including maps of
extent and location, historical
occurrences, frequency, severity,
and warning time.
• Secondary hazards
• Exposure of people, property, critical
facilities and environment.
• Vulnerability of people, property,
critical facilities and environment.
• Future trends in development
• Scenarios
• Issues
§201.6(c)(2)(ii): [The risk assessment
shall include a] description of the
jurisdiction’s vulnerability to the hazards
described in paragraph (c)(2)(i). This
description shall include an overall
summary of each hazard and its impact
on the community
Each hazard profile included a vulnerability
component. For the Seismic hazard, Hazus was
utilized following the level I protocol. For the other
hazards, the loss estimation (vulnerability) was
more qualitative with and emphasis on exposure.
Vulnerability was assessed for all hazards
of concern. The Hazus risk assessment
platform (V 4.2) was used for the dam
failure, earthquake, flood, sea level rise
and tsunami hazards. These were Level 2
(user defined) analyses using city and
County data. Site-specific data on County-
identified critical facilities were entered
into the Hazus model. Hazus outputs were
generated for other hazards by applying
an estimated damage function to an asset
inventory extracted from Hazus. The risk
assessment methodology for this plan
update is described in Part 2, Chapter 6 of
this volume
§201.6(c)(2)(ii): [The risk assessment]
must also address National Flood
Insurance Program insured structures
that have been repetitively damaged
floods
The Flood Hazard chapter of the plan included a
profile on repetitive loss properties within the
unincorporated areas of the county.
Volume 1, Part 2, Chapter 10, Section
10.5.2 of the plan includes a
comprehensive analysis of repetitive loss
areas that includes an inventory of the
number and types of structures in the
repetitive loss area. Repetitive loss areas
are delineated, causes of repetitive
flooding are cited, and these areas are
reflected on maps. This analysis includes
all repetitive loss properties within the
county.
§201.6©(2)(ii)(A): The plan should
describe vulnerability in terms of the
types and numbers of existing and future
buildings, infrastructure, and critical
facilities located in the identified hazard
area.
The 2016 plan does include building exposure
counts for each hazard assessed, but it
emphasizes County-owned structures and does
not include details on the General Building stock.
Focus of this analysis is on the unincorporated
areas of the county.
The current update used Hazus to model
impacts from dam failure, earthquake,
flood, sea-level rise and tsunami. A
complete inventory of the numbers and
types of buildings exposed was generated
for each hazard of concern. Critical
facilities were defined for the planning
area, and these facilities were inventoried
by exposure. Each hazard chapter
provides a discussion on future
development trends.
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2. Plan Update—What Has Changed
44 CFR Requirement Previous Plan Updated Plan
§201.6(c)(2)(ii)(B): [The plan should
describe vulnerability in terms of an]
estimate of the potential dollar losses to
vulnerable structures identified in
paragraph (c)(2)(i)(A) and a description of
the methodology used to prepare the
estimate.
With the exception for the Earthquake hazard, the
2016 plan does not estimate losses. The focus for
this component of the plan is on exposure. The
earthquake hazard does include Hazus modeling
that includes loss estimation.
Dollar loss estimates were generated for
all hazards of concern. These estimates
were generated by Hazus for the dam
failure, earthquake, flood and Tsunami.
For the other hazards, loss estimates
were generated by applying a regionally
relevant damage function to the exposed
inventory. In all cases, a damage function
was applied to an asset inventory. The
asset inventory was the same for all
hazards and was generated in Hazus.
§201.6(c)(2)(ii)(C): [The plan should
describe vulnerability in terms of]
providing a general description of land
uses and development trends within the
community so that mitigation options can
be considered in future land use
decisions.
The hazard profiles do address land uses within
the unincorporated areas of the county. This was
done by looking at the County land-use
designations in each hazard area.
There is a discussion of the overall land
use within the planning area, and a spatial
analysis of land use was performed for
hazards with a clearly defined extent and
location. There is a discussion on future
development trends as they pertain to
each hazard of concern. This discussion
looks predominantly at the existing land
use and the current regulatory
environment that dictates this land use.
§201.6(c)(3): The plan shall include a
mitigation strategy that provides the
jurisdiction’s blueprint for reducing the
potential losses identified in the risk
assessment, based on existing
authorities, policies, programs and
resources, and its ability to expand on
and improve these existing tools.
The 2016 plan includes mitigation actions
developed to minimize the county’s vulnerabilities
to natural hazards. It sets forth the measures the
County will pursue as part of 2016 – 2021
performance period. This strategy
Was derived from:
• The in-depth consideration of the county’s
existing hazard vulnerabilities and
• The state and County goals and objectives to
protect public health and safety, reducing
injury, damage, and disruption from disaster
events.
The plan contains a mission statement,
goals, objectives and actions. The actions
are jurisdiction specific and strive to meet
multiple objectives. The objectives of this
plan are broad but measurable. All
objectives meet multiple goals and stand
alone as components of the plan. Each
planning partner was asked to complete a
capability assessment that looks at its
regulatory, technical, and financial
capabilities.
§201.6(c)(3)(i): [The hazard mitigation
strategy shall include a] description of
mitigation goals to reduce or avoid long-
term vulnerabilities to the identified
hazards.
The 2016 plan identified 4 goals and 6 objectives
for the plan. These goals and objectives were
consistent with goals and objectives within the
County General Plan.
A mission statement, eight goals, and 12
objectives are described in Part 3 of this
volume. Goals were adapted from those in
the 2016 plan. Objectives were identified
that meet multiple goals and were used to
help establish priorities for the action
items identified in the plan. The objectives
are measurable components of the plan
and are the basis for identifying and
prioritizing multi-objective actions.
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Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements 44 CFR Requirement Previous Plan Updated Plan
§201.6(c)(3)(ii): [The mitigation strategy
shall include a] section that identifies and
analyzes a comprehensive range of
specific mitigation actions and projects
being considered to reduce the effects of
each hazard, with particular emphasis on
new and existing buildings and
infrastructure.
The 2016 plan identifies a range of mitigation
actions under each of the 6 identified objectives for
the plan. Actions are identified for each hazard of
concern as well as “multi-hazard actions. Actions
identified could be applied to both new or existing
structures, or both.
Volume 1, Part 3 includes a catalog of
mitigation best management practices that
was developed through a facilitated
process. This catalog identifies actions
that manipulate the hazard, reduce
exposure to the hazard, reduce
vulnerability, or increase mitigation
capability. The catalog segregates actions
by scale of implementation. A table in
each planning partner’s action plan
analyzes each action by mitigation type to
illustrate the range of actions selected.
§201.6(c)(3)(ii): [The mitigation strategy]
must also address the jurisdiction’s
participation in the National Flood
Insurance Program, and continued
compliance with the program’s
requirements, as appropriate.
The 2016 Plan addresses the NFIP in the context
of the mapping available within the planning area.
The Plan does address the NFIP as a financial
resource available to mitigate the impacts of the
flood hazard that does profile the Unincorporated
County’s participation in the NFIP. The Plan did
include a mitigation action for the County to initiate
an application to the CRS program.
The plan addresses the National Flood
Insurance Program and the participation
status of all cities within the county. Each
municipal planning partner has profiled
their NFIP status in their annex in volume
2 of the plan All municipal planning
partners that participate in the NFIP
identified an action stating their
commitment to maintain compliance and
good standing under the program.
§201.6(c)(3)(iii): [The mitigation strategy
shall describe] how the actions identified
in Section (c)(3)(ii) will be prioritized,
implemented, and administered by the
local jurisdiction. Prioritization shall
include a special emphasis on the extent
to which benefits are maximized
according to a cost benefit review of the
proposed projects and their associated
costs.
Multiple factors were considered to determine the
mitigation priorities for the implementation period.
Criteria considered included:
• Greatest potential for protecting life and
property in areas of highest risk or
vulnerability,
• The amount of vulnerability and the frequency
of potential hazard occurrence,
• Greatest potential to help assure critical
County infrastructure, structures and
government services remain functional
following a disaster,
• Cost/benefit assessments or considerations
where available,
• Compatibility with goals and objectives in the
County General Plan Public Safety Element
and Hazard Mitigation Plan,
• Degree to which mitigation strategies help
reduce repetitive flood loss properties and or
help assure continued compliance with the
NFIP,
• Compatibility with goals, and funding priorities
of the State Hazard Mitigation Plan, the
California Earthquake Loss Reduction Plan
and the State Flood Hazard Mitigation Plan
• Achievability of social acceptance, technical
feasibility, administrative, political, legal,
economic and environmental considerations.
Each recommended action was prioritized
using a qualitative methodology that
looked at the objectives the project will
meet, the timeline for completion, how the
project will be funded, the impact of the
project, the benefits of the project and the
costs of the project. This prioritization
scheme is detailed in the introduction to
Volume 2 of this plan.
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2. Plan Update—What Has Changed
44 CFR Requirement Previous Plan Updated Plan
§201.6(c)(4)(i): [The plan maintenance
process shall include a] section
describing the method and schedule of
monitoring, evaluating, and updating the
mitigation plan within a five-year cycle.
The 2016 plan includes a chapter on plan
maintenance that calls for:
• Ensure the mitigation strategy is implemented
according to the plan.
• Provide the foundation for ongoing mitigation
programs.
• Standardize long-term monitoring of hazard-
related activities.
• Integrate mitigation principles into day-to-day
operations throughout the county.
• Maintain momentum through continued
engagement and accountability.
Volume 1, Part 3 details a plan
maintenance strategy for monitoring,
evaluating, and updating the mitigation
plan within a five-year cycle, that includes
annual progress reporting.
§201.6(c)(4)(ii): [The plan shall include a]
process by which local governments
incorporate the requirements of the
mitigation plan into other planning
mechanisms such as comprehensive or
capital improvement plans, when
appropriate.
The plan maintenance strategy for the 2016 plan
includes a goal to; “Integrate mitigation principles
into day-to-day operations throughout the county”.
Volume 1, Part 3 details
recommendations for incorporating the
plan into other planning mechanisms,
such as:
• Comprehensive plan
• Emergency response plan
• Capital improvement programs
• Municipal code
Specific current and future plan and
program integration activities are detailed
in each participating jurisdiction’s annex in
Volume 2.
§201.6(c)(4)(iii): [The plan maintenance
process shall include a] discussion on
how the community will continue public
participation in the plan maintenance
process.
The plan maintenance strategy for the 2016 does
not directly define a process for continued public
participation.
Volume 1, Part 3 details a comprehensive
strategy for continuing public involvement.
§201.6(c)(5): [The local hazard mitigation
plan shall include] documentation that the
plan has been formally adopted by the
governing body of the jurisdiction
requesting approval of the plan (e.g., City
Council, County Commission, Tribal
Council).
Adoption documentation and the FEMA approval
letter were provided in the plan as appendix.
Volume 1, Appendix E includes all
supporting documentation for adoption of
the plan by all planning partners
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3. PLAN UPDATE APPROACH
3.1 FUNDING
This planning effort was supplemented by a FEMA Hazard Mitigation Assistance grant (DR-4344-0054-P).
Permit Sonoma, Sonoma County’s land use planning and permitting agency, was the applicant agent for the grant.
The grant covered 75 percent of the cost of developing the plan; planning partners covered the balance through in- kind matching.
3.2 DEFINING STAKEHOLDERS
At the beginning of the planning process, the planning team identified stakeholders to engage during the update of the hazard mitigation plan. For this process, “stakeholder” was defined as any person or public or private entity that owns or operates facilities that would benefit from the mitigation actions of this plan, and/or has an authority or capability to support mitigation actions identified by this plan. Stakeholders were separated into two categories:
• Participatory Stakeholders—Stakeholders that actively participated in the planning process as planning partners or members of the Steering Committee.
• Coordinating Stakeholders—Stakeholders that were not able to commit to actively participating in the process as a participatory stakeholder but were kept apprised of plan development milestones or were able to provide data that was used in the plan development.
3.3 FORMATION OF THE CORE PLANNING TEAM
Permit Sonoma contracted with Tetra Tech, Inc. to assist with development, update, and implementation of the plan. The Tetra Tech project manager managed the overall plan development; Tetra Tech’s lead planner was tasked with interacting with the Permit Sonoma grant manager. A core planning team was formed to lead the planning effort, made up of the following members:
• Lisa Hulette, Permit Sonoma, Grants Manager
• Domenica Giovannini, Permit Sonoma, Policy Manager
• Shelly Bianchi-Williamson, Permit Sonoma, GIS Supervisor • Rob Flaner, Tetra Tech, Project Manager
• Bart Spencer, Tetra Tech, Project Lead Planner
• Carol Baumann, Tetra Tech, Risk Assessment Lead
• Des Alexander, Tetra Tech, Hazard Profiling Lead
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Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements
3.4 ESTABLISHMENT OF THE PLANNING PARTNERSHIP
Sonoma County opened this planning effort to all eligible local governments within the planning area. The planning team made a presentation at a stakeholder kickoff meeting on June 2, 2020, to introduce the mitigation planning process and solicit planning partners. Key meeting objectives were as follows:
• Provide an overview of the Disaster Mitigation Act
• Describe the reasons for a plan
• Outline the hazard mitigation plan update- work plan
• Outline planning partner expectations
• Seek commitment to the planning partnership
• Seek volunteers for the Steering Committee
Each jurisdiction wishing to join the planning partnership was asked to provide a letter of intent to participate that designated a point of contact for the jurisdiction and confirmed the jurisdiction’s commitment to the process and understanding of expectations. Linkage procedures have been established (see Volume 2 of this plan) for any jurisdiction wishing to link to this hazard mitigation plan in the future. The planning partners covered under this plan are shown in Table 3-1.
Table 3-1. Hazard Mitigation Planning Partners
Jurisdiction Point of Contact Title
County of Sonoma Lisa Hulette Department Program Manager City of Cotati Damien O’Bid City Manager
City of Santa Rosa Shari Meads City Planner
City of Sonoma Wayne Wirick Development Services Director Town of Windsor Kim Jordan Planner III
Cloverdale Fire Protection District Jason Jenkins Chief North Sonoma Coast Fire Protection District Bonnie Plakos Chief Northern Sonoma County Fire Protection District Marshall Turbeville Chief Rancho Adobe Fire Protection District Andy Taylor Chief Sonoma Valley Fire District Trevor Smith Fire Marshal
Timber Cove Fire Protection District Erich Lynn Chief
Gold Ridge Resource Conservation District Brittany Heck Executive Director Sonoma Resource Conservation District Valerie Minton Executive Director Sonoma County Agricultural Preservation & Open Space District Sheri Emerson Stewardship Manager
3.5 DEFINING THE PLANNING AREA
The planning area was defined to consist of the unincorporated county, incorporated cities, and special purpose districts within the geographical boundary of Sonoma County. All partners to this plan have jurisdictional authority within this planning area. A map showing the geographic boundary of the defined planning area for this plan update is provided in Chapter 4, along with a description of planning area characteristics. 3-2
3. Plan Update Approach
3.6 THE STEERING COMMITTEE
Hazard mitigation planning enhances collaboration among diverse parties who can be affected by hazard losses. A key element of the public engagement strategy for this plan update was the formation of a stakeholder steering committee to oversee all phases of the update. The members of this committee included planning partner representatives, citizens, and other stakeholders from within the planning area. The planning team assembled a list of candidates representing interests within the planning area that could have recommendations for the plan or be impacted by its recommendations. The planning partners confirmed a committee at the kickoff meeting. Table 3-2 lists the Steering Committee members and their designated alternates.
Leadership roles and ground rules were established during the Steering Committee’s first meeting. The Steering Committee then met about every other month as needed throughout the course of the plan’s development. The planning team facilitated each Steering Committee meeting, which addressed a set of objectives based on an established work plan. The Steering Committee met nine times from June 2020 through June 2021. Meeting summaries and attendance logs are provided in Appendix A to this volume. All Steering Committee meetings were open to the public and were advertised as such on the hazard mitigation plan website. Agendas were posted to the website prior to each scheduled Steering Committee meeting, and meeting summaries were posted to the hazard mitigation plan website following their approval by the Steering Committee.
3.7 COORDINATION WITH STAKEHOLDERS AND AGENCIES
Opportunities for involvement in the planning process must be provided to neighboring communities, local and regional agencies involved in hazard mitigation, agencies with authority to regulate development, businesses, academia, and other private and nonprofit interests (44 CFR, Section 201.6(b)(2)). Agency coordination for this plan was accomplished as follows:
• Steering Committee Involvement—Agency representatives were invited to participate on the Steering Committee.
• Agency Notification—The following agencies were invited to participate in the plan development process from the beginning and were kept apprised of plan development milestones:
American Red Cross-Northern California Coastal Region California Department of Water Resources, California State National Flood Insurance Program Coordinator
California Office of Emergency Services, Emergency Services Coordinator FEMA Region IX, Lead Community Planner
U.S. Geological Survey, Science Advisor
California Department of Transportation, Director-District 1 Bureau of Land Management, Tribal Relations
California Department of Forestry and Fire Protection, Resource Management Division Cloverdale Rancheria
Dry Creek Rancheria
Federated Indians of Graton Rancheria
Kashia Pomo Stewarts Point Rancheria
Lytton Rancheria
Middletown Rancheria
Mishewal Wappo Tribe
Torres Martinez Desert Cahuila Indians
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Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Table 3-2. Steering Committee Members
Name Title Jurisdiction/Agency
PRIMARY MEMBERS
Lisa Hulette (committee
chair)
Project Manager County of Sonoma, Permit Sonoma
Gary Helfrich Planner County of Sonoma, Permit Sonoma
Shelley Bianchi-Williamson GIS Supervisor County of Sonoma, Permit Sonoma Richard Diaz Deputy Emergency Coordinator County of Sonoma, Emergency Management Hunter McLaughlin Engineer County of Sonoma, Public Works Domenica Giovannini PIO County of Sonoma, Permit Sonoma Kimberly Jordan Planner Town of Windsor
Shari Meads City Planner City of Santa Rosa
Katherine Duran Administrative Analyst City of Cotati
Karen Gaffney Program Manager Sonoma County Agricultural Preservation & Open Space District
Mollie Asay Grants & Funded Projects Sonoma Water
Marshall Turbeville District Fire Chief Northern Sonoma County Fire District Scott Westrope Deputy Fire Chief Santa Rosa City Fire
Ben Nicholls Division Chief CAL Fire
Sarah Newkirk Senior Project Director The Nature Conservancy Lisa Micheli President & CEO Pepperwood Preserve
Kirsten Larsen Environmental Compliance Manager Community Development Commission Robert Cantu President/Chair, Construction Coalition Steering Committee Western Builders Karissa Kruse Executive Director Sonoma County Winegrowers
DESIGNATED ALTERNATES
John Mack Natural Resource Manager County of Sonoma, Permit Sonoma Cecily Condon Supervising Planner County of Sonoma, Permit Sonoma Caerleon Safford Fire Inspector County of Sonoma, Fire Prevention Lisa Figueroa Deputy Emergency Services Coordinator County of Sonoma, Emergency Management Adrianne Garayalde Department Analyst County of Sonoma, Public Works Mickie Tagle Senior Management Analyst Town of Windsor
Amy Lyle Supervising Planner City of Santa Rosa
Damian O’Bid City Manager City of Cotati
Sheri Emerson Stewardship Program Manager Sonoma County Agricultural Preservation & Open Space District
Steve Hancock Emergency Preparedness & Response Manager Sonoma Water Shane Vargas Fire Captain CalFire
Chase Beckman Fire Captain CalFire
Shane Galvez Fire Captain CalFire
Elizabeth Forsburg Associate Director The Nature Conservancy Mark Chandler Housing Rehabilitation Specialist Community Development Commission Kate Piontek VP of Operations & Sustainability Sonoma County Winegrowers 3-4
3. Plan Update Approach
These agencies received meeting announcements, meeting agendas, and meeting minutes by e-mail throughout the plan development process and were provided the option to attend meetings. Some agencies supported the effort by attending meetings or providing feedback on issues. • Pre-Adoption Review—All the agencies listed above were provided an opportunity to review and comment on this plan, primarily through the hazard mitigation plan website (see Section 3.9). All were sent an e-mail message informing them that draft portions of the plan were available for review. Upon completion of a public comment period, a complete draft plan was sent to the California Office of Emergency Services for a pre-adoption review to ensure program compliance.
3.8 REVIEW OF EXISTING PROGRAMS
Hazard mitigation planning must include review and incorporation, if appropriate, of existing plans, studies, reports and technical information (44 CFR, Section 201.6(b)(3)). Chapter 5 of this plan provides a review of laws and ordinances in effect within the planning area that can affect hazard mitigation actions. In addition, the following programs can affect mitigation within the planning area:
• California Fire Code
• California Fire Alliance
• 2019 California Building Code
• California State Hazard Mitigation Forum
• Local capital improvement programs
• Local emergency operations plans
• Local general plans
• Local tribal hazard mitigation plans
• Housing elements of general plans
• Safety elements of general plans
• Local zoning ordinances
• Local coastal program policies.
• Sonoma County Operational Area Emergency Operations Plan (2014)—This is an emergency support function-based plan that directs emergency response actions in the planning area • Sonoma County General Plan 2020 (adopted by resolution, September 2008; amended by resolution, August 2016)—This plan directs land use policy in Sonoma County • Sonoma County Local Coastal Plan, Bodega Bay Focused Vulnerability Assessment and Adaptation Strategies (September 2019)
• Sonoma County Five-Year Strategic Plan (March 2021)—This plan includes the goals that will shape the County’s priorities over the next five years. It includes Climate Action and Resiliency and Resilient Infrastructure.
• Regional Climate Protection Authority, Regional Climate Action Plan (2016) • Sonoma County Community Wildfire Protection Plan (2016) 3-5
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Assessments of all planning partners’ regulatory, technical, and financial capabilities to implement hazard mitigation actions are presented in the individual jurisdiction-specific annexes in Volume 2. Many of these relevant plans, studies and regulations are cited in the capability assessments.
3.9 PUBLIC INVOLVEMENT
Broad public participation in the planning process helps ensure that diverse points of view about local needs are considered and addressed. The public must have opportunities to comment on disaster mitigation plans during the drafting stages and prior to plan approval (44 CFR, Section 201.6(b)(1). The Community Rating System expands on these requirements by making CRS credits available for optional public involvement activities. For this plan update, “public” has been defined as the general public within the Sonoma County planning area. This includes, but is not limited to:
• Residents
• Tribal members
• Tourists
• Employers within the planning area
• Employees within the planning area
• Students (primary and secondary education levels).
3.9.1 Strategy
The strategy for involving the public in this plan emphasized the following elements:
• Include members of the public on the Steering Committee. • Use a survey to determine if the public’s perception of risk and support of hazard mitigation has changed since the initial planning process.
• Attempt to reach as many planning area citizens as possible using multiple media. • Identify and involve planning area stakeholders.
Stakeholders and the Steering Committee
Stakeholders are the individuals, agencies and jurisdictions that have a vested interest in the recommendations of the hazard mitigation plan, including all planning partners. The effort to include stakeholders in this process included stakeholder participation on the Steering Committee. In addition to planning partners and those represented on the steering committee, the planning team invited all potential stakeholders listed in Section 3.7 to actively participate in the plan update process.
Internet
At the beginning of the planning process, a website was created to keep the public posted on plan development milestones and to solicit relevant input (see Figure 3-1). The site’s address was publicized in all press releases, mailings, surveys, and public meetings (https://sonomacounty.ca.gov/PRMD/Long-Range-Plans/Hazard- Mitigation-Update/).
3-6
3. Plan Update Approach
Figure 3-1. Sample Page from Hazard Mitigation Plan Web Site Each planning partner established a link to this site on its own agency website. Information on the plan development process, the Steering Committee, a plan survey, and drafts of the plan was made available to the public on the site throughout the process. Sonoma County intends to keep a website active after the plan’s completion to keep the public informed about successful mitigation projects and future plan updates.
Story Map
A “Story Map” was created, using ESRI Story Map software, to communicate the variety and severity of hazard risks facing Sonoma County (see Figure 3-2). The applicability of the Story Map goes beyond the life of the hazard mitigation plan update, meaning that it will remain with the County (on its own ESRI account) and continue as a template to support visual and data-based communication about the range of hazards relevant in the planning area. New and revised data can be loaded into the platform in the future to compare hazard risk with any other spatial data set (i.e. soft story structure inventory, social vulnerability data, etc.).
During the update process, the Story Map was released to the public and promoted through social media and the project website. It included risk assessment results for all relevant hazards, an interactive hazard mapping tool, and a report function to produce comprehensive hazard exposure summaries for any given property, block, or defined area. The Story Map expanded opportunities for public outreach and the ways in which members of the public could interact with hazard data as the hazard mitigation plan update was underway. 3-7
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Figure 3-2. Example Story Map Data Page
Survey
A hazard mitigation plan survey (see Figure 3-3) was developed by the planning team with guidance from the Steering Committee. The survey was used to gauge household preparedness for natural hazards and the level of knowledge of tools and techniques that assist in reducing risk and loss from natural hazards. This survey was designed to help identify areas vulnerable to one or more natural hazards. The answers to its 42 questions helped guide the Steering Committee in selecting goals, objectives and mitigation strategies. The survey was made available on the hazard mitigation plan website and advertised throughout the course of the planning process.
During the course of this planning process, 691 completed surveys were submitted. The complete survey and a summary of its findings can be found in Appendix A of this volume.
Public Outreach
The public outreach process for this plan update consisted of general outreach information during various partner meetings and events. A virtual public meeting was held on February 15, 2021 to present the Story Map and describe the hazard mitigation plan update process. The draft plan was made available to the public for comment during a noticed, two-week period, June 14 – 28, 2021. A virtual public meeting was held on the evening of July 21, 2021. The format of the meeting was a short overview of the planning process, plan content and how to comment, followed by breakout rooms sponsored by each municipal planning partner to allow for jurisdiction- specific public comment opportunities. The meeting gave the public an opportunity to comment on the draft plan update prior to its submittal to the California Governor’s Office of Emergency Services (Cal OES) and FEMA.
The principle avenue for public comment on the draft plan was the website established for this plan update. 3-8
3. Plan Update Approach
Figure 3-3. Sample Page from Survey Distributed to the Public 3-9
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements In order to engage non-English-speakers in the outreach, all social media posts and the agendas and minutes for Steering Committee meetings were posted in English and Spanish, and a translator was available during the virtual meetings to interpret the presentation and discussion into Spanish. The translator service was advertised in meeting announcements and as each meeting was underway. Spanish-speaking participants could take advantage of this service by joining a break-out room of the virtual meeting. No participants chose to use this service.
3.9.2 Public Involvement Results
Survey
Detailed analysis of the survey findings is presented in Appendix A; a summary is as follows:
• 691 surveys were completed.
• Surveys were received from each planning partner.
• Survey respondents ranked the wildland fire as the hazard of greatest concern, followed by climate change, drought, earthquake, severe weather, sea-level rise, and flood. • 81 percent of respondents reported having experienced an evacuation, over 76 percent reported having experienced a wildland fire, and more than 70 percent reported having experienced drought. • Most respondents (75.71 percent) felt that personal experience with one or more natural hazards/disasters provided useful hazard and disaster preparedness information to the public, followed by emergency preparedness information from government sources (federal, state, or local) (75.39 percent). • Most respondents (39.81 percent) stated that they could survive for 4 to 7 days following a natural hazard event based on their preparedness. 8 to 15 days (24.01 percent), 1 to 3 days (19.43 percent), and 16 days or more (15.80 percent) were the next most common responses. Only 0.95 percent of respondents stated they would survive 0 days.
Survey results were provided to the Steering Committee for use in support of confirming the guiding principle, goals, objectives and county-wide actions for this plan update. Additionally, the survey results were included in the toolkit provided to each planning partner through the jurisdictional annex process described in Volume 2.
Each planning partner was able to use the survey results to help identify actions as follows:
• Gauge the public’s perception of risk and identify what citizens are concerned about. • Identify the best ways to communicate with the public. • Determine the level of public support for different mitigation strategies. • Understand the public’s willingness to invest in hazard mitigation.
Public Outreach Events
The public involvement strategy used for this plan update introduced the concept of mitigation to the public and provided the Steering Committee with feedback to use in developing the plan. All citizens of the planning area were provided ample opportunities to provide comment during all phases of this plan update process. Details of attendance and comments received from the public outreach events are summarized in Table 3-3. 3-10
3. Plan Update Approach
Table 3-3. Summary of Public Outreach Events
Date Location Number of Attendees
2/25/2021 Virtual Public Meeting (Story Map Presentation) 109 Presentations at Meetings of Community Organizations
4/7/2021 Windsor Senior Citizens Advisory Committee 24
5/10/2021 Bodega Bay Community Emergency Response Team 16 5/11/2021 Oakmont Citizens Advisory Committee 92
5/11/2021 Cotati City Council Meeting 56
5/12/2021 Town of Windsor Parks and Recreation 44
5/12/2021 Mark West Community Advisory Committee 72
5/17/2021 North County Citizens Organized to Prepare for Emergency ~25 5/20/2021 California Wildfire Recovery Roundtable ~70
5/25/2021 Springs Citizens Organized to Prepare for Emergency ~50 5/25/2021 Town of Windsor Planning Commission 17
6/14 – 28/2021 Open Public Comment Period N/A
7/21 Virtual Public meeting to present the draft plan and to allow opportunity for public comment 49 Total ~624
3.10 PLAN DEVELOPMENT CHRONOLOGY/MILESTONES
Table 3-4 summarizes important milestones in the plan update process.
Table 3-4. Plan Development Chronology/Milestones
Date Event Description Attendance
2020
4/21 Organize Resources County selects Tetra Tech as its technical assistance contractor to facilitate the plan update process.
N/A
4/23 Organize Resources Sonoma County Board of Commissioners approves contract with Tetra Tech and authorizes the notice to proceed on work for the update.
N/A
6/2 Project Kickoff Meeting • Review work plan
• Discuss planning partner expectations
• Organize Steering Committee
• Risk assessment data needs
• Discuss public involvement strategy
• Homework: review prior plan and state plan
24 6/26 Core Planning Team Meeting
#1
• Finalize planning partnership roster
• Discuss risk assessment data needs
• Finalize agenda for Steering Committee meeting #1
6 7/20 Public Outreach • Hazard mitigation plan website adapted for information on 2021 plan update process.
N/A
7/23 Steering Committee Meeting #1 • Homework report out: Review Sonoma County chapter of state hazard mitigation plan
• Project overview
• Role of the Steering Committee
• Introduce Phase 1 jurisdictional annex process
• Confirm ground rules and charter
• Discuss public outreach strategy
25 3-11
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Date Event Description Attendance
7/23 Steering Committee Debrief • Detail wrap-up from 1st Steering Committee meeting • Discuss future meeting interactivity
• Discuss facilitation
• GIS needs
4 Aug. Sonoma County Wildfires • Sonoma County wildfires caused postponing of core planning team and Steering Committee meetings
9/10 Core Planning Team Meeting
#2
• Phase 1 instructions and forms
• GIS meeting
• Inclusion of tribal communities in planning process
• Goals and Objectives
• Mission Statement
• Agenda for next Steering Committee Meeting
7 9/21 Core Planning Team Meeting
#3
• GIS Roundtable
• Review existing and pending hazard data sources
• Identify data gaps
13 9/24 Steering Committee Meeting #2 • Confirm Steering Committee rules, mission statement • Review homework: previous County hazard mitigation plan and State hazard mitigation plan
• Confirm hazards of concern
• Public involvement strategy
25 10/15 Core Planning Team Meeting
#4
• GIS Roundtable
• Focused on flood/tsunami/sea level rise risk assessment scenarios 13 10/21 Core Planning Team Meeting
#5
• Clarification on Phase 1 annex process
• Discuss critical facilities
6 10/22 Steering Committee Meeting #3 • Planning Process • Community Lifelines framework
• Jurisdictional Annex Process
• Public Involvement Strategy
24 11/5 Core Planning Team Meeting
#6
• GIS Roundtable
• Dam Failure, Earthquakes, and Landslide/Mass Movement data needs 13 11/19 Phase II Annex Meeting • Overview of Phase 2 process • Critical facilities discussion
11 11/19 Steering Committee Meeting #4 • Finalize goals and objectives • Survey review
• Public engagement
19 12/8 Core Planning Team Meeting
#7
• Story Map
• Phase 2 Annex Status
• Survey release
6 11/19 Public Outreach • Hazard mitigation plan survey released N/A 11/19 Planning Process • Workshop for planning partners to work together to complete Phase 2 of the jurisdictional annex process. Remote technical support provided by Tetra tech
12 12/17 Steering Committee Meeting #5 • Confirm objectives • Overview of Phase 3 process
• Survey status
• Public engagement
21 3-12
3. Plan Update Approach
Date Event Description Attendance
2021
1/11 Core Planning Team Meeting
#8
• Phase 2 annexes
• Story Map
• Survey
6 1/19 Core Planning Team Meeting
#9
• Phase 2 jurisdictional annex status report
• Phase 3 workshops
• Hazard analysis status
• Survey status and responses
8 1/28 Steering Committee Meeting #6 • Phase 2 jurisdictional annex process status • Phase 3 workshop schedule
• Development of action items
• Public engagement updates
24 2/11 Core Planning Team Meeting
#10
• Discuss March public meeting 6
2/23 Public Meeting Practice Run • Discuss zoom protocol • Show Story Map to Sonoma County core planning team
6 2/25 Steering Committee Meeting #7 • Plan progress report • Upcoming Phase 3 workshops
• Discuss plan maintenance strategy
• Discuss public meeting
18 2/25 1st Public Meeting • Discuss hazard mitigation plan process and plan • Show Sonoma County Story Map to the public
109
4/22 Steering Committee Meeting #8 • Discuss plan progress • Draft plan release date
• Public comment period
• Next public meeting date
20 5/12 Core Planning Team Meeting
#11
• Public comment process 6
4/27 – 5/25 Public Outreach Meeting with community groups to provide information about hazard mitigation plan
~650
6/1 – 6/24 Stakeholder Draft Review Tetra Tech sent draft hazard mitigation plan and corresponding annexes to Steering Committee, planning partners, and core planning team N/A
7/12 Public Outreach Opening of the 2-week public comment period N/A 7/21 Public Outreach Virtual Public Meeting to present the Draft Plan 49 7/30 Public Outreach Closure of the 2-week public comment period N/A 8/6 Plan Submittal Pre-adoption review draft of the plan submitted to Cal OES. N/A 9/27 APA Approval Pending Adoption (APA) provided by FEMA N/A 10/26 Adoption Adoption window opens for planning partnership N/A TBD Approval Final Plan approval issued by FEMA region IX N/A 3-13
4. SONOMA COUNTY PROFILE
4.1 GEOGRAPHIC OVERVIEW
Sonoma County, the most northerly of the nine counties in the San Francisco Bay Region, is located along the Pacific coastline about 40 miles north of San Francisco and the Golden Gate Bridge. At just over 1,500 square miles, it is the largest of the nine Bay Area counties. Sonoma County is bordered by the Pacific Ocean on the west, Marin County and San Pablo Bay to the south, Solano, Napa and Lake Counties to the east, and Mendocino County to the north (see Figure 4-1).
The major population centers in Sonoma County are the incorporated cities of Cloverdale, Cotati, Healdsburg, Petaluma, Rohnert Park, Santa Rosa, Sebastopol, Sonoma, and Windsor. Unincorporated communities include Annapolis, Bodega, Bodega Bay, Cazadero, Duncans Mills, Forestville, Fort Ross, Geyserville, Glen Ellen, Graton, Guerneville, Kenwood, Jenner, Monte Rio, Occidental, Salmon Creek, and The Sea Ranch. Santa Rosa, centrally located in the county, is the county seat and most populous city.
U.S. Highway 101 is the main highway in the county, running north to south through the county’s center.
Highway 1 follows the coastline along most of the county’s western boundary. Other major roadways are State Highways 12, 37, 116, 121 and 128. Airports include Charles M. Schulz–Sonoma County Airport in Santa Rosa and the Sonoma Valley Airport and Sonoma Skypark in the City of Sonoma. Sonoma County Transit buses run countywide. The SMART Train (Sonoma–Marin Area Rail Transit) carries passengers from the Charles M.
Schulz–Sonoma County Airport to Larkspur in Marin County; future extensions as far north as Cloverdale are planned.
The Sonoma County Board of Supervisors sits as the governing board of Sonoma County and of various special jurisdictions such as the Sonoma County Water Agency, the Northern Sonoma County Air Pollution Control District, the Agricultural Preservation and Open Space District, County Sanitation Districts, and the Community Development Commission. The Board is composed of five supervisors elected from supervisorial districts for four year terms. The risk analysis for this hazard mitigation plan assessed risk both countywide and for each supervisorial district. The boundaries of these districts are included on Figure 4-1. 4-1
Cotati
Sonoma
Windsor
Healdsburg
Sebastopol
Rohnert Park
Santa
Rosa
Cloverdale
Petaluma
¬«12
¬«116
¬«
116
¬«
37 ¬«
121
¬«
12 ¬«
128
¬«1
¬«1
£¤101
£¤
101
1st
Supervisorial
District
2nd
Supervisorial
District
3rd
Supervisorial
District
4th
Supervisorial
District
5th
Supervisorial
District
Figure 4-1. Sonoma County Planning Area
±
0 10 5
Miles
Data Sources: Sonoma Co.
Cities
County Boundary
Supervisorial District Boundaries
Highways
4. Sonoma County Profile
4.2 HISTORICAL OVERVIEW
Early peoples began to settle in Sonoma Valley roughly 12,000 years ago, attracted by fertile soil, water, game, fish, wild oats, berries, acorns, and other natural resources. The name Sonoma may have derived from an indigenous word for “many moons,” or may come from “noma,” a Miyakmah word for town (Sonomavalley.com 2021). Eventually, these early resident numbered about 5,000 people across a number of tribes:
• Miwoks along the coast
• Wintuns, Wapo and Miyakmahs in the north near the Mayacamas Mountain Range • Pomos in the lower valley
• Koskiwok near the edge of San Pablo Bay
• Patwins in the southeast corner
These early peoples lived in long, multi-family grass- and tule-thatched huts with communal cooking areas. Life focused on gathering and preparing food and tribal celebrations—religious and otherwise. Their activities included trade between tribes and clearing land (by burning) to expose game.
In 1812, Russians established the short-lived Fort Ross along the coast north of the Russian River. In the early 19th century, Spanish explorers and missionaries arrived, looking for land and converts and hoping to set up a bulwark against the Russians, who had advanced down the coast from the north. In 1823, Franciscan missionaries established the mission San Francisco Solano de Sonoma. The Mission regime was harsh, and a rebellion in 1826 caused mission founder Fr. Jose Altamira to flee Sonoma Valley. A memorial outside Sonoma’s restored mission bears the names of the native people who died there.
Sonoma became the county’s first town, a pueblo, under General Mariano Vallejo. Sections of the county were transformed into land-grant ranchos, such as Vallejo’s holdings that extended from today’s Petaluma to the town of Sonoma (County of Sonoma n.d.). The continual encroachment of European and American settlers overwhelmed the native population. By the late 1800s, indigenous tribes had all but vanished as a society. Many died from smallpox and measles; others were sent north to reservations or absorbed into the burgeoning new pueblo of Sonoma.
Sonoma County was one of the original counties when California achieved statehood on September 9, 1850. The county seat of Sonoma County was moved to Santa Rosa in 1854 (City of Sonoma 2017). After statehood, logging along the coast hills, cattle ranching, wheat and potato farming, and the early development of the wine industry supported the sparsely settled county. During the 1860s to the 1890s, Petaluma, at the head of navigation on the Petaluma Creek, enjoyed rapid economic growth that fueled the construction of its downtown.
Later, the railroads facilitated the movement of goods and people, leading to the establishment of processing plants and factories along the rail lines. Around the turn of the century, the Russian River developed as a vacation resort, a destination for those in the San Francisco Bay Area. During this time, Santa Rosa saw an increase in population and importance as the center of finance and county government. Until World War II, the poultry industry, the processing of local fruit, and the production of hops sustained the economy throughout the county. In 1935, Sonoma County ranked tenth in the nation in overall agricultural production.
Today the southwestern part of the county continues to support cattle grazing and dairy farms. Toward the north, many of the ranches and orchards have been replaced with acres of vineyards and thriving winery operations. 4-3
’
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Over the years many of the poultry farms, fruit growers, and dairy operations have relocated to the Central Valley.
In their place, small specialty farms and ranches now operate sustainable and organic endeavors. Dotting the countryside throughout the county are modern residences where rural homesteads used to be. The Russian River area still caters to vacationers. The cities along the freeway continue to expand to provide housing and services with new subdivisions, business parks, and strip-mall shopping centers.
4.3 MAJOR PAST HAZARD EVENTS
Presidential disaster declarations are typically issued for hazard events that cause more damage than state and local governments can handle without assistance from the federal government, although no specific dollar loss threshold has been established for these declarations. A presidential disaster declaration makes federal recovery programs available to help public entities, businesses, and individuals. Some disaster assistance programs are partially matched by state programs. Review of presidential disaster declarations helps establish the probability of reoccurrence for each hazard and identify targets for risk reduction. Table 4-1 shows the declared disasters that have affected Sonoma County through 2020 (records date back to 1964).
Table 4-1. Historical Sonoma County Natural Hazard Events Year Event Name Dates
County EOCa
Activated
Governor s
Declaration
Presidential
Declaration
1964 Heavy Rains and Flooding Dec. 24 X
1969 Severe Storms, Flooding Jan. 26 X
1981-1982 Severe Storms, Flood, Mudslides, High Tide Dec. 19 – Jan. 8 X 1983 Coastal Storms, Floods, Slides, Tornadoes Jan. 21 – Mar. 30 X 1986 Severe Storms, Flooding Feb. 12 – Mar. 10 X
1990-1991 Freeze of ‘91 Dec. 90–Feb. 91 X X
1993 Flood of ‘93 Jan. 20-25 X X X
1994 Fishing Emergency May–Sep. X X
1995 Flood of ‘95, Part 1 Jan. 8-31 X X X
1995 Flood of ‘95, Part II Mar. 7-15 X X X
1995 December Winter Storm Dec. 11-12 X
1996 February Winter Storm Feb. 4-5 X
1996 Cavedale Fire Jul. 31–Aug. 20 X
1996 Jenner Sandbarrier Jul. 31–Aug. 20
1996 Porter Creek Fire Oct. 27-28 X
1996-1997 New Year’s Flood Dec. 30, 1996–Jan. 4, 1997 X X X 1997 Superbowl Flood Jan. 25 X
1998-2000 Flood of ‘98/ Rio Nido Debris Flow Feb. 2, 1998–Jan. 4, 2000 X X X 1999 February Winter Storm Feb. 8-10 X
2002-2003 December Winter Storms Dec. 17, 02–Apr. 8, 03 2004 Geysers Fire Sept. 3-8 X
2005-2006 New Year’s Floods Dec. 31, 05–Jan. 3, 06 X X X 2006 Late Spring Storms Mar. 29-Apr. 16 X X
2007 SF Oil Spill Nov. 7 X
2009 H1N1 Influenza Pandemic Apr.-May
2011 Great Tohoku Tsunami Mar. 11 X X X
4-4
’
4. Sonoma County Profile
Year Event Name Dates
County EOCa
Activated
Governor s
Declaration
Presidential
Declaration
2012 Holiday Decoration Flood Dec. 2 X
2013 Lopez Protests Oct. 29 and Nov. 5 X
2014-2016 Drought Feb. 25 X
2014 South Napa Earthquake Aug. 24 X X X
2014 December Winter Storm Dec. 11-12 X
2015 Valley Fire Sep. 12-25 X X X
2017 Severe Winter Storms, Flooding, and Mudslides Jan. 3-12 X X 2017 Severe Winter Storms, Flooding, Mudslides Feb. 1-23 X X 2017 LNU Complex Fires October X
2017 Wildfires Oct. 8-31 X
2018 PG&E Power Shutoff October X
2019 Severe Winter Storms, Flooding, Landslides,
Mudslides
Feb. 24 – Mar. 1 X X
2019 PG&E Power Shutoff October X
2019 Kincade Fire Oct. 23 – Nov. 7 X X
2020 COVID-19 Pandemic Jan. 20 – present X X X
2020 Wildfires Aug. 14 – Sept. 26 X
2020 Wildfires Sept. 4 – Nov. 17 X
a. EOC = Emergency Operations Center
Sources: Sonoma County Department of Emergency Management, www.gov.ca.gov, www.fema.gov/disaster
4.4 PHYSICAL SETTING
4.4.1 Topology and Surface Waters
The broad, flat Santa Rosa Plain, which lies between the Sonoma Mountains on the east and low coastal hills on the west, contains the cities of Santa Rosa, Rohnert Park, and Cotati. The sparsely settled coastal area of the county includes redwood and mixed conifer forests in the north and rolling oak woodland, dairy lands, and coastal prairies in the south. The Mayacamas Range forms the eastern boundary of the county. The Mayacamas and Sonoma Mountain ranges enclose the Sonoma Valley or “Valley of the Moon,” which extends from near Santa Rosa southeast to the City of Sonoma and San Pablo Bay. In the north, the Mayacamas Range and Mendocino Highlands surround the farming regions of Alexander and Dry Creek Valleys. In the far northeast, the remote interior of the Mayacamas Range contains the Geysers geothermal steam field.
Topography in the county is varied and includes mountainous areas, rolling hills and broad flat river valleys, and bay flats. The valleys and foothills are predominantly in agricultural uses with some urbanized areas and with a dense population. The county contains numerous watersheds, but the Russian River is the largest and most significant, draining over 1,485 square miles as it flows south from Mendocino County to the Pacific Ocean. The Russian River is the primary water supply and a key attraction to many communities along its banks. The Petaluma River connects to San Pablo Bay and thence to the San Francisco Bay in the south. Lake Sonoma is a dam-created reservoir on Dry Creek in the northwest part of the county. 4-5
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Sonoma County is on the coast of the Pacific Ocean, north of San Francisco Bay. Santa Rosa lies in the county’s central valley near the junction of the Mantazas and Santa Rosa Creeks, which flow to the west from hills that surround a large central valley (U.S. Soil Conservation Service 1972).
In general, the northern half of the county is made up of small, rugged mountains that begin at the coast and rise to an elevation of 3,500 to 4,400 feet. The Russian River flows from Mendocino County in a southeasterly direction through the north-central half of Sonoma County and then turns west a few miles south of Healdsburg.
Eventually, after passing through the large resort and recreational areas surrounding Guerneville and Monte Rio, this river empties into the Pacific Ocean.
The western part of the southern half of Sonoma County generally is low, rolling grassy hills at an elevation of 500 to 600 feet. The cities of Petaluma and Sonoma are in long narrow valleys in the southwestern and southeastern parts of the county, respectively. East of the Sonoma Plains and on both sides of the Sonoma Valley are grass-covered hills that rise to about 2,000 feet. Tidal flats reclaimed from the San Pablo Bay are at the lower ends of Sonoma and Petaluma Valleys and the Petaluma plains area.
4.4.2 Soils
Sonoma County has over a dozen unique soil associations (U.S. Soil Conservation Service 1972). Table 4-2 lists each association, along with descriptions and percent of soil occupation.
4.4.3 Climate
Sonoma County’s Mediterranean climate is characterized by a summer dry season followed by a winter rainy season, generally extending from November to April. Rainfall varies throughout the county from 70 to 20 inches annually in the north central and the southeastern sections of the county. The quantity of rainfall in the county increases with elevation, with the greatest precipitation occurring over the highest ridges. The valleys, where most of the water users are located, receive considerably less rainfall with some areas averaging just over 20 inches of precipitation annually.
In the Russian River Watershed, approximately 93 percent of the annual precipitation normally falls during the wet season, October to May, with a large percentage of the rainfall typically occurring during three or four major winter storms. These major storms often come in the form of an atmospheric river, the horizontal transport of large amounts of water vapor through the atmosphere along a narrow corridor. Although brief, atmospheric rivers can produce 30 to 50 percent of the region’s annual precipitation in a matter of a few days.
Except for areas immediately along the coast, the weather from May through October is generally warm and dry during the day, with peak summer day temperatures of 80° to 100° F, and relative humidity ranging between 20 and 35 percent. Gradient winds are generally out of the south/southwest at 5 to 10 mph, strengthening to 10 to 15 mph in late afternoon and diminishing by dark. Strong and dry northeast “Santa Ana” or “Foehn” winds often occur in the fall months.
Coastal onshore flow, often accompanied by fog, frequently prevails after sunset, allowing for good nighttime relative humidity recovery in the warm inland areas. In the inland valleys, fog usually dissipates by 11:00 am. Fog in the county usually is seen at elevations between 1,000 and 1,500 feet. Elevations above this often do not experience fog or receive the same nighttime cooling and moisture recovery as lower elevations. 4-6
4. Sonoma County Profile
Table 4-2. Sonoma County Soil Types
Soil
Association Soil Description
Portion of
County Distribution of Soil Types
Clear Lake-
Reyes
Poorly drained, nearly level to gently sloping clays to clay loams; in basins and on tidal flats
6% 50% Clear Lake soils, 40% Reyes soils, 10% Wright /
Yolo soils
Haire-Diablo Moderately well drained and well drained, gently sloping to steep fine sandy loams to clays; on terraces and uplands
4% 45% Haire soils, 45% Diablo soils, 10% Arbuckle /
Clear Lake / Raynor / Zamora soils
Huichica-
Wright-
Zamora
Somewhat poorly drained to well drained, nearly level
to strongly sloping loams to silty clay loams; on low
bench terraces and alluvial fans
6% 35% Huichica soils, 30% Wright soils, 25% Zamora
soils, 10% Clear Lake / Yolo / Pajaro / Cole / Cortina
soils
Pajaro Somewhat poorly drained, nearly level to gently
sloping fine sandy loams to clay loams; on low
terraces and flood plains
1% 90% Pajaro soils, 10% Blucher / Goldridge /
Steinbeck / Los Osos soils
Yolo-Cortina
Pleasanton
Well-drained to excessively drained, nearly level to
moderately sloping very gravelly sandy loams to clay
loams; on flood plains, alluvial fans, and low terraces 3% 60% Yolo soils, 15% Cortina soils, 15% Pleasanton
soils, 10% Arbuckle / Manzanita / Pajaro / Positas /
Zamora soils
Spreckels-
Felta
Well-drained, gently sloping to very steep very
gravelly loams to clay loams; on mountain foothills
and on high terraces
4% 50% Spreckels soils, 40% Felta soils, 10% Laniger /
Toomes / Guenoc soils
Yorkville-
Suther
Moderately well drained, moderately sloping to very
steep loams and clay loams; on uplands
8% 40% Yorkville, 40% Suther soils, 20% Hugo /
Josephine / Laughlin soils
Goulding-
Toomes-
Guenoc
Well-drained, gently sloping to very steep clay loams
to loams; on uplands
8% 70% Goulding soils, 10% Toomes soils, 10% Guenoc
soils, 10% Boomer / Henneke / Josephine / Red Hill /
Spreckels / Supan soils
Kidd-
Forward-
Cohasset
Somewhat excessively drained and well-drained,
moderately sloping to very steep gravelly and stony
loams; on uplands
2% 30% Kidd soils, 30% Forward soils, 20% Cohasset
soils, 20% Laniger / Red Hill / Spreckels / Supan soils and Rock land
Los Gatos-
Henneke-
Maymen
Well-drained to excessively drained, moderately
sloping to very steep loams, gravelly loams, and
gravelly sandy loams; on mountains
75 50% Los Gatos soils, 20% Henneke soils, 20%
Maymen soils, 10% Boomer / Huse / Hugo /
Josephine / Montara soils
Hugo-
Josephine-
Laughlin
Well-drained, gently sloping to very steep gravelly
loams and loams; on mountains
33% 55% Hugo soils, 20% Josephine soils, 15% Laughlin
soils, 10% Boomer / Hely / Maymen / Sites / Suther /
Yorkville soils
Steinbeck-
Los Osos
Moderately well drained and well drained, gently
sloping to steep loams and clay loams; on uplands
6% 65% Steinbeck soils, 25% Los Osos soils, 10% Cotati
/ Diablo/ Goldridge soils and Kneeland sandy variant
Goldridge-
Cotati-
Sebastopol
Moderately well drained and well drained, gently
sloping to steep fine sandy loams and sandy loams;
on coastal terraces and uplands
6% 60% Goldridge soils, 20% Cotati soils, 10%
Sebastopol soils, 10% Clear Lake / Diablo / Steinbeck
soils
Kneeland-
Rohnerville-
Kinman
Well drained and moderately well drained, nearly level
to steep loams to clay loams; on coastal benches,
terraces, and uplands
3% 30% Kneeland soils, 25% Rohnerville soils, 25%
Kinman soils, 20% Baywood / Laughlin / Los Osos /
Noyo / Yorkville soils
Empire-
Caspar-
Mendocino
Well drained and moderately well drained, strongly
sloping to steep sandy loams to sandy clay loams; on
coastal uplands and terraces
3% 35% Empire soils, 30% Caspar soils, 20% Mendocino
soils, 15% Goldridge / Hugo / Josephine soils
Source: (U.S. Soil Conservation Service 1972)
4-7
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements
4.5 SENSITIVE RESOURCES
Sonoma County boasts scenic vistas, fertile agricultural lands, impressive redwood forests, a sizeable meandering river, and 50 miles of rocky coastline. In addition to these natural resources, there are cultural landscapes that illustrate the county’s historic past with a broad array of properties that mirror the passage of time.
4.5.1 Cultural Resources
Historic building surveys for the coastal, Sebastopol, Healdsburg, and Sonoma Valley areas provide an inventory of Sonoma County’s historic resources, some of which may be threatened by development or by a lack of maintenance. With reference to residential, commercial, and industrial architecture, many of the towns still retain excellent examples of both high style and vernacular building examples from the 19th and early 20th centuries.
Archaeological sites provide information on the history and culture of the county’s earliest residents. Heritage and landmark trees enhance the quality of the environment and have historical significance (County of Sonoma 2013).
4.5.2 Scenic Resources
Coastal bluffs, vineyards, San Pablo Bay, the Laguna de Santa Rosa, and other landscapes are of special importance to Sonoma County. Preservation of these scenic resources is important to the quality of life of county residents and the tourists and agricultural economy. Other features such as the Mayacamas and Sonoma Mountains provide scenic backdrops to communities. As the county urbanizes, the openness of these areas provides important visual relief from urban densities. These landscapes have little capacity to absorb development without significant visual impact. Major Scenic Landscape Units have been identified as follows (County of Sonoma 2013):
• The Coast—The Sonoma coast is a scenic resource vital to the county. Three basic types of landscapes are included, the flat terraces south of the Russian River, the hillier terraces from Fort Ross northward, and the cliffs and landslide areas in between.
• Oat Valley—Oat Valley and the hillsides above it provide the scenic northern entrance to the county near Cloverdale.
• Alexander and Dry Creek Valleys—Protection of these agricultural valleys’ scenic beauty is not only important from an aesthetic standpoint, but also from an economic one as agricultural marketing is closely tied to the area’s scenic image. The hills along Highway 101 and above the valley floor are particularly sensitive.
• Hills East of Windsor—These hills provide a scenic backdrop to the Santa Rosa Plain. North of Windsor the area extends into the plain and adjoins the low, rolling hills that form part of the Windsor-Healdsburg Community Separator.
• Eastside Road—This area of rolling hills is an important transition between the community of Windsor and the rich agricultural and mineral resource areas of the Russian River Valley. • River Road—This area provides a variety of landscapes, including valleys planted in vineyards, orchard covered hillsides, and redwood groves adjacent to the Russian River. • Laguna de Santa Rosa—This area consists primarily of the scenic lowlands and floodplain around the Laguna de Santa Rosa marsh, swamp, and riparian forest. It also includes hills between Forestville, Sebastopol, and Meacham Hill. It defines the eastern boundary of Sebastopol and associated rural residential development.
4-8
4. Sonoma County Profile
• Bennett Valley—Bennett Mountain forms a scenic backdrop from Bennett Valley Road. This area defines Santa Rosa’s southeastern boundary and abuts Annadel State Park. • Highway 116—The view corridor along Highway 116 contains unique views of orchards, redwood groves, and the Russian River. This area also defines the community boundaries of Forestville, Guerneville, and Monte Rio and their adjacent rural residential development. • Atascadero Creek—This area consists primarily of the lowlands and floodplains along Atascadero Creek and the hills along Occidental Road. The area defines the western boundary of Sebastopol and its adjacent rural residential development, separates Sebastopol and Graton, and creates a visual connection to the Laguna de Santa Rosa.
• Coleman Valley—The Coleman Valley Road area contains unique views of forests, canyons, grazing lands, and the ocean.
• Sonoma Mountains—These are highly valuable scenic lands, clearly defining the eastern edge of the Santa Rosa Plain between Petaluma and Sonoma.
• Hills South of Petaluma—The open grassy hillsides and ridgelines of the area are extremely sensitive.
Located at the Marin County border, this area serves as a gateway to the county. • Sonoma Valley/Mayacamas Mountains—Included in this area are the Sonoma-Napa Mountains that provide a backdrop to the valley and agricultural areas bordering the valley. These areas define the boundaries of the urban and rural communities and are very sensitive because of their small size and the unobstructed view of them from roads and adjoining urban areas. • South Sonoma Mountains—These hillsides are an important part of the south county landscape with a simple landform, minimal vegetation, and a clear widespread viewing area. Pasture and forage lands along the Highway 37 corridor are included to preserve views of the San Pablo Bay.
4.5.3 Natural Resources
Sonoma County’s varied natural landscapes range from the marine environments of the coast to the forests, woodlands and grasslands of the Coast Range to the vernal pools and freshwater marshes of the Santa Rosa Plain and other valley floors to the extensive marshlands along San Pablo Bay. Areas of natural vegetation support many native plant and animal species and encompass habitat for special status species, wetlands, and sensitive natural communities. The vegetative cover also helps reduce surface runoff, protect water quality, maintain air quality, retain soil, increase recharge, and maintain stream channels. These areas together create a varied natural environment important to the quality of life and the unique character of the county. The background and policies below are separated into a Biotic Habitat Areas section that addresses protection of several types of biotic habitat in the county and a section that focuses on one type of habitat, the Riparian Corridor.
4.6 DEVELOPMENT PROFILE
4.6.1 Land Ownership and Use
The Sonoma County General Plan and Land Use Maps govern the types of land uses and development that may occur in different areas of the unincorporated county. Figure 4-2 indicates the breakdown of land use as of the 2008 Land Use Element of the General Plan; that element is scheduled to be updated in 2021. Current land use policies promote city and community centered growth, and limit new development to levels consistent with adequate infrastructure and services, including public safety considerations. 4-9
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Source: (County of Sonoma 2017)
Land Extensive Agriculture
19.0%
Land Intensive
Agriculture
7.4%
Diverse Agriculture
0.04%
Resources and
Rural Residential
7.4%
Urban Residential
0.8%
Commercial
0.3%
Industrial
0.2%
Public/Quasi Public
6.0%
Rural Development
51.8%
Figure 4-2. Chief Characteristics of Land within Sonoma County Approximately half of the county is in rugged rural areas with limited access. Most of the development in these areas is limited to open space and timber/natural resource production.
4.6.2 Building Count, Occupancy Class and Estimated Replacement Value Table 4-2 presents planning area building counts by building occupancy class. Table 4-3 summarizes estimated replacement value for building structures and contents combined.
4.6.3 Critical Facilities
Critical facilities are those that are essential to the health and welfare of the population. These become especially important after any hazard event. Also included are facilities that hold or carry significant amounts of hazardous materials with a potential to impact public health and welfare during a hazard event. The risk assessment for each hazard in this plan discusses that hazard’s potential impact on critical facilities. 4-10
4. Sonoma County Profile
Table 4-2. Planning Area Building Counts by Occupancy Class Number of Buildings
Residential Commercial Industrial Agricultural Religion Government Education Total Cloverdale 2,914 166 16 20 8 22 12 3,158
Cotati 2,450 149 34 7 5 27 10 2,682
Healdsburg 4,047 346 46 50 15 32 16 4,552
Petaluma 18,275 999 137 39 25 91 43 19,609
Rohnert Park 11,284 387 40 14 14 37 14 11,790
Santa Rosa 50,372 2,396 142 136 75 308 118 53,547
Sebastopol 2,489 285 8 9 12 21 8 2,832
Sonoma 4,109 397 12 22 14 44 7 4,605
Windsor 8,017 272 55 36 9 43 12 8,444
Unincorporated
1st Supervisorial Dist.
2nd Supervisorial Dist.
3rd Supervisorial Dist.
4th Supervisorial Dist.
5th Supervisorial Dist.
12,473
5,684
745
6,601
21,736
447
217
106
439
805
48 25
17 144
63 2,045
1,487
97 3,705
4,475
19 16
1 24
71 92
79 17
105
356
17 21
3 26
59 15,141
7,529
986
11,044
27,565
Total 151,196 7,411 787 12,142 308 1,274 366 173,484
Table 4-3. Estimated Replacement Value of Planning Area Buildings Jurisdiction Estimated Total Replacement Value (Structure and Contents) a Cloverdale $2,499,664,593
Cotati $2,163,132,258
Healdsburg $4,803,401,892
Petaluma $18,679,915,783
Rohnert Park $9,749,459,659
Santa Rosa $44,098,486,212
Sebastopol $2,676,395,901
Sonoma $3,658,235,342
Windsor $6,407,101,168
Unincorporated $123,838,778,174
Total $218,574,570,981
For some hazards, potential damage to critical facilities was estimated using the Hazards U.S. (Hazus) computer model developed by FEMA. For this reason, the list of critical facilities was categorized using categories that are defined in the Hazus model:
• Safety and Security—Law Enforcement/Security, Search and Rescue, Fire Services, Government Service, Responder Safety, and Imminent Hazard Mitigation • Food, Water and Sheltering—Evacuations, Schools, Food/Potable Water, Shelter, Durable Goods, Water Infrastructure, and Agriculture
• Health and Medical—Medical Care/Hospitals: Patient Movement, Public Health, Fatality Management, Health Care, and Supply Chain
4-11
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements • Energy—Power (Grid), Temporary Power and Fuel
• Communications—Infrastructure, Alerts, Warnings, Messages, 911 and Dispatch, Responder Communications and Financial Services
• Transportation—Highway/Roadway, Mass Transit, Railway, Aviation, Maritime and Pipeline • Hazardous Materials—Facilities, Hazardous Debris, Pollutants and Contaminants Table 4-4 summarizes the number of critical facilities by Hazus-defined category, based on the best data available on critical facilities at the time of this plan update. The County and its planning partners consider this information to be subject to change as new information about critical facilities become available during the performance period for this plan. Due to the sensitivity of this information, a detailed list of facilities is not provided. General locations of critical facilities in the planning area are shown in Figure 4-3 and Figure 4-4.
Table 4-4. Planning Area Critical Facilities
Number of Facilities
Communications Energy
Food, Water,
Shelter
Hazardous
Material
Health &
Medical
Safety &
Security Transportation Total
Cloverdale 2 1 5 29 4 15 2 58
Cotati 1 0 0 45 4 17 1 68
Healdsburg 10 0 5 45 10 20 2 92
Petaluma 20 2 11 128 22 70 6 259
Rohnert Park 10 2 4 191 9 34 1 251
Santa Rosa 55 3 37 354 93 209 5 756
Sebastopol 7 0 3 27 13 19 0 69
Sonoma 11 1 3 65 11 19 0 110
Windsor 6 0 6 105 9 16 1 143
Unincorporated
1st Supervisorial Dist.
2nd Supervisorial Dist.
3rd Supervisorial Dist.
4th Supervisorial Dist.
5th Supervisorial Dist.
25 7
3 17
17 5
8 1
44 7
11 7
1 16
22 431
340
65 703
581
10 4
1 10
19 45
31 5
48 102
63 45
1 89
169
590
442
77 927
917
Total 191 74 131 3,109 219 650 385 4,759
4.6.4 Development Trends
The municipal planning partners have adopted general plans that govern land use decision and policy making for their jurisdictions. Decisions on land use will be governed by these programs. This plan will work together with these programs to support wise land use in the future by providing vital information on the risk associated with natural hazards in the planning area. All municipal planning partners will incorporate this hazard mitigation plan update in their general plans by reference. This will ensure that future development trends can be informed by the information on risk and vulnerability to natural hazards identified in this plan. County land use policies help reduce the potential impact of new development on hazard vulnerability within the unincorporated areas of Sonoma County.
4-12
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Cotati
Sonoma
Windsor
Healdsburg
Sebastopol
Rohnert Park
Santa
Rosa
Cloverdale
Petaluma ¬«12
¬«116
¬«
116
¬«
37 ¬«
121
¬«
12 ¬«
128
¬«1
¬«1
£¤101
£¤
101
Figure 4-3. Critical Facilities, 1 of 2
±
0 105
Miles
Data Sources: Sonoma Co.,
CA ENERGY, CA WRCB,
CALTRANS, DHS HIFLD
!. Food, Water, Shelter
!. Health and Medical
!. Safety and Security
Cities
County Boundary
Highways
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Cotati
Sonoma
Windsor
Healdsburg
Sebastopol
Rohnert Park
Santa
Rosa
Cloverdale
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116
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121
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Figure 4-4. Critical Facilities, 2 of 2
±
0 105
Miles
Data Sources: Sonoma Co.,
CA ENERGY, CA WRCB,
CALTRANS, DHS HIFLD
!. Communication
!. Energy
!. Transportation
!. Hazardous Material
Cities
County Boundary
Highways
4. Sonoma County Profile
Sonoma County was in the process of updating its General Plan at the time of this hazard mitigation plan update.
The following development trends from the previous General Plan (Sonoma County General Plan 2020) are likely to change when the General Plan update is completed:
• The number of housing units increased by 21,419 units between 2000 and 2010, with 16.6 percent of the increase in unincorporated county areas.
• Household growth was projected to grow about 38,490 units between 2000 and 2020 period, an average growth rate of 1,920 households per year.
• About 80 percent of 2000 – 2020 projected growth was expected to occur within city urban service areas, with the remainder in unincorporated areas outside of the cities.
4.7 DEMOGRAPHICS
The vulnerability of people and groups to hazard events is dynamic, varying with physical location as well as economic, social, geographic, demographic, cultural, institutional, governance, and environmental factors. The impacts of a hazard event on individuals and communities can depend on factors such as wealth, education, race, ethnicity, religion, gender, age, access and functional needs, and health status. The capacity to anticipate, cope with, and adapt to a hazard is an important factor of vulnerability (Cardona et al. 2012). These factors often overlap spatially, so spatial analysis to locate areas where there are higher concentrations of people experiencing different vulnerabilities can help to extend focused public outreach, education, and resources to these residents.
Understanding communities’ makeup and demographic changes over time is important to making decisions that may impact these communities future, such as land used decisions that affect housing, industry, stores, public facilities and services, and transportation.
4.7.1 Total Population Estimates
Current Population
Sonoma County is the 17th largest of California’s 58 counties. The U.S. Census Bureau estimates the population at 494,336 as of 2019.
Historical Population Trends
Population changes are useful socio-economic indicators. A growing population can indicate a growing economy, and a decreasing population may signify economic decline. Figure 4-5 shows the population growth trend in Sonoma County from 1960 to 2019 compared to that of the State of California. Since the 1960s, the county has seen slow and declining growth rates (less than 5 percent per decade); the state growth rate has declined to about 7 percent over the 9-year period from 2000 to 2010.
Table 4-5 shows the population of incorporated municipalities and the combined unincorporated areas in Sonoma County from 2000 to 2018. The portion of the planning area’s residents living outside incorporated areas has gradually decreased over that period, changing from about 32.7 percent in 2000 to about 28.3 percent in 2018.
Overall growth in the incorporated areas from 2000 to 2018 was approximately 4 percent. 4-15
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Source: California Department of Finance
0%
5%
10%
15%
20%
25%
30%
1960-1970 1970-1980 1980-1990 1990-2000 2000-2010 2010-2019 10-Year Growth Rate (%)
Sonoma County
California
Figure 4-5. California and Sonoma County Historical Population Growth Rates Table 4-5. Population Growth Data
Population
April 1, 2000 April 1, 2010 January 1, 2015 January 1, 2016 January 1, 2017 January 1, 2018 Cloverdale 6,831 8,618 8,893 8,927 8,988 9,134
Cotati 6,471 7,265 7,371 7,376 7,453 7,716
Healdsburg 10,915 11,254 11,707 11,734 11,757 12,061
Petaluma 54,550 57,941 60,953 61,488 61,657 62,708
Rohnert Park 42,236 40,794 42,325 42,586 42,490 43,598
Santa Rosa 147,595 167,815 175,693 176,937 178,064 178,488 Sebastopol 7,774 7,379 7,593 7,609 7,624 7,786
Sonoma 9,275 10,648 10,906 10,929 11,072 11,390
Windsor 22,744 26,801 27,364 27,445 27,492 28,060
Unincorporated 150,223 145,363 147,278 147,444 148,016 142,391 Total 458,614 483,878 500,083 502,475 504,613 503,332
Source: California Department of Finance
Projected Future Population
According to population projections by the California Department of Finance, Sonoma County’s population should decrease to 485,017 by 2040. This represents a 3.8 percent decrease from the 2018 population.
4.7.2 Age Distribution
Although advanced age by itself does not create vulnerability to hazards, certain problems that are more common in old age can increase vulnerability. They include decreased strength, poor tolerance of physical activity, functional limitations, and decreased sensory awareness. The severity of the impact of disasters on older people 4-16
4. Sonoma County Profile
depends on the specific characteristics of the elderly and their environments, the type and severity of the hazard, disaster management systems, and interactions between all of these (Pan American Health Organization 2012).
Children are particularly vulnerable during natural disasters and experience increased problems regarding their physical health, mental health, and learning after exposure. Compared to adults, children suffer more severe physical effects from disasters because they breathe more air per pound of their weight, have thinner skin, are at greater risk in cases of fluid loss, and are more likely to lose body heat. Disasters also can harm children indirectly. When a disaster affects parents and other adults (such as teachers), children’s care, protection, and support systems are eroded. Beyond the immediate trauma and harm caused by natural disaster exposure, children also may suffer longer-term physical, psychological, and educational deficits (Society for Research in Child Development 2020).
The overall age distribution for the planning area is illustrated in Figure 4-6. Based on U.S. Census data,
19.6 percent of the planning area’s population is 65 years or older, and 22.2 percent of the population is 19 years or younger. According to U.S. Census data, 6.7 percent of the over-65 years population have incomes below the poverty level. Of children under 18 years, 13.9 percent live below the poverty level.
Source: American Fact Finder, American Community Survey Age
85 years and over
75 to 84 years
65 to 74 years
60 to 64 years
55 to 59 years
45 to 54 years
35 to 44 years
25 to 34 years
20 to 24 years
15 to 19 years
10 to 14 years
5 to 9 years
Under 5 years
24,591
27,576
28,649
30,274
27,975
31,079
63,735
63,949
36,261
36,355
60,222
25,828
11980
0 10,000 20,000 30,000 40,000 50,000 60,000 70,000
Number of People
Figure 4-6. Planning Area Age Distribution
4-17
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements About U.S. Census Data on Race and
National Origin
The U.S. Census Bureau collects race data
in accordance with guidelines provided by
the U.S. Office of Management and Budget,
and these data are based on self-
identification. The racial categories included
in the census questionnaire generally reflect
a social definition of race recognized in the
Unites States, not an attempt to define race
biologically, anthropologically, or
genetically. The categories of the race item
include racial and national origin or
sociocultural groups. People may choose to
report more than one race to indicate their
racial mixture, such as “American Indian”
and “White.” People who identify their origin
as Hispanic, Latino, or Spanish may be of
any race.
The concept of race is separate from the
concept of Hispanic origin. Percentages for
the various race categories add to 100
percent, and should not be combined with
the percent Hispanic.
Source: U.S. Census 2021a
4.7.3 Race, National Origin, and Language
Research shows that communities of color are less likely to be involved in pre-disaster planning and experience higher mortality rates during a disaster event. Higher proportions of communities of color live below the poverty line than the white population, so these communities have fewer resources to prepare for disasters in advance or recover afterwards.
The U.S. Census Bureau’s 2019 American Community Survey reports the following data on race and national origin in Sonoma County:
• The racial composition of the planning area is
predominantly white, at about 74.8 percent. This
percentage has fallen steadily since 1980, when the white population made up 89.1 percent of the county total (MTC and ABAG, 2021).
• The largest racial categories in the 2019 Census data other than white, as self-reported by respondents, are “some
other race” at 12.9 percent and “two or more races” at 5.4 percent.
• The planning area has 26.7 percent Hispanic or Latino population across all races, which has risen steadily from a percentage of only 6.9 percent in 1980 (MTC and
ABAG, 2021).
Figure 4-7 shows the 2019 racial distribution in the planning area; results may differ in the newest U.S. Census data, which was still being processed at the time this plan was developed.
The planning area has a 16.9 percent foreign-born population. The most spoken language in the county other than English is Spanish. The census estimates 52.3 percent of the residents speak English “less than very well.”
4.7.4 Individuals with Disabilities or with Access and Functional Needs People with disabilities are more likely than the general population to have difficulty responding to a disaster.
Local government is the first level of response to assist these individuals, and coordination of efforts to meet their access and functional needs is paramount to life safety efforts. It is important for emergency managers to distinguish between functional and medical needs to plan for incidents that require evacuation and sheltering.
Knowing the percentage of population with a disability will allow emergency management personnel and first responders to have personnel available to provide services needed by those with access and functional needs. The last full U.S. Census (from 2010) estimated that one in five Americans with live with disabilities in the United States. According to U.S. Census data, 30.5 percent of the over-65 population in the planning area has disabilities of some kind, as well as 7.2 percent of those under 65. 4-18
4. Sonoma County Profile
Source: (U.S. Census Bureau 2021)
Note: Chart excludes Hispanic/Latino,
which is considered an ethnicity rather than White a race. Across all races, the County population 75% is 26.7% Hispanic/Latino
African American
1.7%
American Indian
/Alaskan Native
0.9%
Asian
4.1%
Native Hawaiian
and Other Pacific Other
12.9%
Two or More Races
5.4%
Islander
0.3%
Figure 4-7. Planning Area Race Distribution
4.8 ECONOMY
Over 25,000 large and small businesses call Sonoma County home. Business startups are discovering Sonoma County as an ideal place to launch and grow, with some of the most affordable housing costs in the San Francisco Bay Area and a more competitive cost-of-doing-business. Nerdwallet.com recently ranked the cities of Healdsburg and Sonoma in the top 20 places to start a business. Sonoma County has also been recognized as one of the top 10 places for supporting minority-owned businesses and Latino entrepreneurs (County of Sonoma 2021).
4.8.1 Income
In the United States, individual households generally use private resources to prepare for, respond to and recover from disasters to some extent. Households living in poverty have less access to time and resources to plan for and respond to hazards. Low-income households also typically occupy more poorly built and inadequately maintained housing. Mobile or modular homes, for example, are more susceptible to damage than other types of housing. In urban areas, families living in poverty often live in older houses and apartments, which are more likely to be made of a building type that is susceptible to earthquake damage. Residents below the poverty level are less likely to have insurance to compensate for losses from natural disasters, and federal aid is designed to restore property to owners, not renters (Howell and Elliott 2018). Personal household economics also significantly impact people’s decisions on evacuation.
Based on U.S. Census Bureau estimates, per capita income in the planning area in 2018 was $39,929, and the median income of all households was $76,753. It is estimated that 18.8 percent of households receive an income between $100,000 and $149,999 per year and 10.6 percent of household incomes are above $150,000 annually. 4-19
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements The Census estimates that 9.3 percent of all families in the planning area have incomes below the poverty level.
To analyze the hazard exposure of socially vulnerable populations, the risk assessment in this hazard mitigation plan identified low-income households as follows:
• County data for 2021 show “low income” (defined as 80 percent of the area median income) as $74,450 for two-person households and $83,750 for three-person households (County of Sonoma 2021). • The U.S. Census shows an average of 2.6 persons per household in Sonoma County • The Hazus model used for risk assessment allows analysis of households by income at four levels:
$50,000, $60,000, $75,000, or $100,000.
• The $75,000 household income level was chosen as closest to the low-income level for the average Sonoma County household size.
4.8.2 Industry, Businesses, and Institutions
Table 4-6 lists the top employers in the planning area in 2021 as identified by the California Employment Development Department. Figure 4-8 shows the breakdown of employment by industry type in the planning area, according to the State of California Employment Development Department.
4.8.3 Employment Trends and Occupations
The U.S. Census estimates a civilian labor force in Sonoma County of 268,068. According to the American Community Survey, about 65 percent of the planning area’s working-age population (16 and over) is in the labor force. Figure 4-9 compares California’s and Sonoma County’s unemployment trends from 2010 through July 2021. The county and state rates both declined steadily after the 2008-2009 recession until the COVID pandemic in spring of 2020. Although both rates have fallen sharply since the pandemic peak, neither has yet returned to pre-pandemic levels.
4-20
4. Sonoma County Profile
Table 4-6. Top Employers for the Planning Area
Employer Name Location Industry
1,000-4,999 Employees
Aabalat Fine & Rare Wines Petaluma Wineries
Medtronic Inc Santa Rosa Surgical Instruments-Manufacturers Santa Rosa Memorial Hospital Santa Rosa Hospitals
Sonoma Developmental Ctr Eldridge Hospitals
US Coast Guard Petaluma Federal Government-National Security 500 - 999 Employees
Amys Kitchen Santa Rosa Frozen Food Processors
Fairmont Sonoma Msn Inn & Spa Sonoma Hotels & Motels
Kaiser Permanente Santa Rosa Santa Rosa Hospitals
Protransport-1 Cotati Transportation Services
Scoe Employee Ctr Santa Rosa County Government-Education Programs Sonoma County Dept-Fire Santa Rosa Fire Departments
Sonoma County Sheriff Santa Rosa Government Offices-County Sutter Santa Rosa Regl Hosp Santa Rosa Hospitals
250 - 499 Employees
Army National Guard Recruiter Santa Rosa Government Offices-State First Security Svc Rohnert Park Security Guard & Patrol Service Flex Products Inc Santa Rosa Coatings-Vacuum Deposition Freeman Toyota Santa Rosa Automobile Dealers-Used Cars
Ghilotti Construction Santa Rosa Excavating Contractors La Torilla Factory Santa Rosa Factory Outlets
Macy's Santa Rosa Department Stores
Petaluma City Passports Petaluma Government Offices-City/Village & Township Petaluma Valley Hospital Petaluma Hospitals
Press Democrat Santa Rosa Newspapers
Santa Rosa Police Dept Santa Rosa Police Departments
Walmart Windsor Department Stores
Source: California Employment Development Department 2021 (using data from America's Labor Market Information System (ALMIS) Employer Database, 2021 1st Edition)
4-21
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Sales, office
administrative
21.9%
Farming, fishing,
forestry, construction
10.5%
Manufacturing,
Service, healthcare, transportation, material
public safety, food prep moving
19.9% 10.4%
Management,
business, science and
arts, education
37.3%
Figure 4-8. Industry in the Planning Area
Source: (Federal Reserve Bank of St. Louis 2021)
18 2
4 6
8 10
12 14
16 Unemployment Rate (%)
0 2010-01 2011-01 2012-01 2013-01 2014-01 2015-01 2016-01 2017-01 2018-01 2019-01 2020-01 2021-01 California Sonoma County
Figure 4-9. California and Sonoma County Unemployment Rate 4-22
5. REGULATIONS AND PROGRAMS
Existing regulations, agencies and programs at the federal, state, and local level can support or impact hazard mitigation actions identified in this plan. Hazard mitigation plans are required to include a review and incorporation, if appropriate, of existing plans, studies, reports, and technical information as part of the planning process (44 CFR, Section 201.6(b)(3)). Information presented in this section can be used to review local capabilities to implement the action plan this hazard mitigation plan presents. Individual review by each planning partner of existing local plans, studies, reports, and technical information is presented in the annexes in Volume 2.
5.1 RELEVANT FEDERAL AND STATE AGENCIES, PROGRAMS AND
REGULATIONS
State and federal regulations and programs that need to be considered in hazard mitigation are constantly evolving. For this plan, a review was performed to determined which regulations and programs are currently most relevant to hazard mitigation planning. The findings are summarized in Table 5-1 and Table 5-2. Short descriptions of each program are provided in Appendix B.
5.2 LOCAL PLANS, REPORTS AND CODES
Plans, reports, and other technical information were identified and provided directly by participating jurisdictions and stakeholders or were identified through independent research by the planning consultant. These documents were reviewed to identify the following:
• Existing jurisdictional capabilities.
• Needs and opportunities to develop or enhance capabilities, which may be identified within the local mitigation strategies.
• Mitigation-related goals or objectives considered during the development of the overall goals and objectives.
• Proposed, in-progress, or potential mitigation projects, actions and initiatives to be incorporated into the updated jurisdictional mitigation strategies.
5-1
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Table 5-1. Summary of Relevant Federal Agencies, Programs and Regulations Agency, Program or Regulation
Hazard Mitigation
Area Affected Relevance
Americans with Disabilities Act Action Plan
Implementation
FEMA hazard mitigation project grant applications require full compliance with applicable federal acts.
Bureau of Land Management Wildfire Hazard The Bureau funds and coordinates wildfire management programs and structural fire management and prevention on its lands.
Civil Rights Act of 1964 Action Plan
Implementation
FEMA hazard mitigation project grant applications require full compliance with applicable federal acts.
Clean Water Act Action Plan
Implementation
FEMA hazard mitigation project grant applications require full compliance with applicable federal acts.
Community Development Block
Grant Disaster Resilience
Program
Action Plan Funding This is a potential alternative source of funding for actions identified in this plan.
Community Rating System Flood Hazard This voluntary program encourages floodplain management activities that exceed the minimum National Flood Insurance Program requirements.
Disaster Mitigation Act Hazard Mitigation
Planning
This is the current federal legislation addressing hazard mitigation planning.
Emergency Relief for Federally
Owned Roads Program
Action Plan Funding This is a possible funding source for actions identified in this plan.
Emergency Watershed Program Action Plan Funding This is a possible funding source for actions identified in this plan.
Endangered Species Act Action Plan
Implementation
FEMA hazard mitigation project grant applications require full compliance with applicable federal acts.
Federal Energy Regulatory
Commission Dam Safety
Program
Dam Failure Hazard This program cooperates with a large number of federal and state agencies to ensure and promote dam safety.
Federal Wildfire Management
Policy and Healthy Forests
Restoration Act
Wildfire Hazard These documents mandate community-based collaboration to reduce risks from wildfire.
National Dam Safety Act
National Environmental Policy
Act
Dam Failure Hazard
Action Plan
Implementation
This act requires a periodic engineering analysis of most dams in the country FEMA hazard mitigation project grant applications require full compliance with applicable federal acts.
National Fire Plan (2001) Wildfire Hazard This plan calls for joint risk reduction planning and implementation by federal, state and local agencies.
National Flood Insurance
Program
Flood Hazard This program makes federally backed flood insurance available to homeowners, renters, and business owners in exchange for communities enacting floodplain regulations
National Incident Management
System
Action Plan
Development
Adoption of this system for government, nongovernmental organizations, and the private sector to work together to manage incidents involving hazards is a prerequisite for federal preparedness grants and awards Presidential Executive Order
11988 (Floodplain Management)
Flood Hazard This order requires federal agencies to avoid long and short-term adverse impacts associated with modification of floodplains
Presidential Executive Order
11990 (Protection of Wetlands)
Action Plan
Implementation
FEMA hazard mitigation project grant applications require full compliance with applicable presidential executive orders.
U.S. Army Corps of Engineers
Dam Safety Program
Dam Failure Hazard This program is responsible for safety inspections of dams that meet size and storage limitations specified in the National Dam Safety Act.
U.S. Army Corps of Engineers
Flood Hazard Management
Flood Hazard, Action
Plan Implementation,
Action Plan Funding
The Corps of Engineers offers multiple funding and technical assistance programs available for flood hazard mitigation actions
5-2
5. Regulations and Programs
Agency, Program or Regulation
Hazard Mitigation
Area Affected Relevance
U.S. Fire Administration Wildfire Hazard This agency provides leadership, advocacy, coordination, and support for fire agencies and organizations.
U.S. Fish and Wildlife Service Wildfire Hazard This service’s fire management strategy employs prescribed fire throughout the National Wildlife Refuge System to maintain ecological communities.
Table 5-2. Summary of Relevant State Agencies, Programs and Regulations Agency, Program or Regulation
Hazard Mitigation
Area Affected Relevance
AB 9: Fire safety: Wildfires: Fire Wildfire Hazard Establishes the Regional Forest and Fire Capacity Program to support Adapted Communities regional leadership to build local and regional capacity and develop, prioritize, and implement strategies and projects that create fire adapted communities and landscapes by improving watershed health, forest health, community wildfire preparedness, and fire resilience.
AB 32: The California Global
Warming Solutions Act
Action Plan
Development
Establishes a state goal of reducing greenhouse gas emissions to 1990 levels by 2020
AB 38: Fire safety: Low-Cost Wildfire Hazard Directs the California Natural Resources Agency to review the regional Retrofits: Regional Capacity capacity of each county that contains a very high fire hazard severity Review: Wildfire Mitigation zone and establishes a comprehensive wildfire mitigation and assistance program.
AB 70: Flood Liability Flood Hazard A city or county may be required to partially compensate for property damage caused by a flood if it unreasonably approves new development in areas protected by a state flood control project
AB 162: Flood Planning Flood Hazard Cities and counties must address flood-related matters in the land use, conservation, and safety and housing elements of their general plans.
AB 267: California Environmental
Quality Act: Exemption: Prescribed
Fire, Thinning, and Fuel Reduction
Projects.
Wildfire Hazard Extends to January 1, 2026, the exemption from requirements of the California Environmental Quality Act for prescribed fire, thinning, or fuel reduction projects on federal lands to reduce the risk of high-severity wildfire that had been reviewed under the National Environmental Policy Act.
AB 380: Forestry: Priority Fuel
Reduction Projects
Wildfire Hazard Requires the Department of Forestry and Fire Protection to identify priority fuel reduction projects annually and exempts the identified priority fuel reduction projects from certain legal requirements.
AB 431: Forestry: Timber
Harvesting Plans: Defensible
Space: Exemptions
Wildfire Hazard Extends to January 1, 2026, the exemption from a requirement to complete a timber harvest plan for maintaining defensible space between 150 feet and 300 feet from a habitable structure.
AB 497: Forestry and Fire
Protection: Local Assistance Grant
Program: Fire Prevention
Activities: Street and Road
Vegetation Management
Wildfire Hazard Appropriates funds for local assistance grants for fire prevention activities with priority for projects that that manage vegetation along streets and roads to prevent the ignition of wildfire.
AB 575: Civil Liability: Prescribed
Burning Activities: Gross
Negligence
Wildfire Hazard Provides that a private entity engaging in a prescribed burning activity that is supervised by a person certified as burn boss is liable for damages to a third party only if the prescribed burning activity was carried out in a grossly negligent manner.
AB 642: Wildfires Wildfire Hazard This bill is an omnibus fire prevention bill that makes various changes to support cultural and prescribed fire, including the creation of a Cultural Burning Liaison at the Department of Forestry and Fire Protection, and requires a proposal for creating a prescribed fire training center. 5-3
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Agency, Program or Regulation
Hazard Mitigation
Area Affected Relevance
AB 747: General Plans—Safety
Element
Hazard Mitigation
Planning
The safety elements of cities’ and counties’ general plans must address evacuation routes and include any new information on flood and fire hazards and climate adaptation and resiliency strategies.
AB 800: Wildfires: local general
plans: safety elements: fire hazard
severity zones.
Wildfire Hazard This Bill has provisions for wildfire hazard mapping and applications for that mapping in the Safety elements General plans within the state.
AB 1255: Fire prevention:
Department of Forestry and Fire
Protection: Grant Programs
Wildfire Hazard Requires the Natural Resources Agency to develop a guidance document that describes goals, approaches, opportunities, and best practices in each region of the state for ecologically appropriate, habitat-specific fire risk reduction. Requires consultation with counties related to the Department of Forestry and Fire Protection’s local fire prevention grant program.
AB 1295: Residential development
Agreements: Very High-Risk Fire
Areas
Wildfire Hazard Prohibits the legislative body of a city or county from entering into a residential development agreement for property in a very high fire risk area as designated by a local agency or a fire hazard severity zone classified by the director of CAL FIRE.
AB 1439: Property Insurance
Discounts
Wildfire Hazard Requires residential or commercial property insurance policies to include a discount if a local government where the insured property is located funds a local wildfire protection or mitigation program.
AB 1500: Safe Drinking Water,
Wildfire Prevention, Drought
Preparation, Flood Protection,
Extreme Heat Mitigation, and
Workforce Development Bond Act
of 2022.
Drought, Flood,
Extreme Heat and
Wildfire Hazards
If approved by the voters, would authorize the issuance of bonds to finance projects for safe drinking water, wildfire prevention, drought preparation, flood protection, extreme heat mitigation, and workforce development programs.
AB 2140: General Plans—Safety
Element
Hazard Mitigation
Planning
This bill enables state and federal disaster assistance and mitigation funding to communities with compliant hazard mitigation plans.
AB 2800: Climate Change—
Infrastructure Planning
Action Plan
Development
This act requires state agencies to take into account the impacts of climate change when developing state infrastructure.
Alquist-Priolo Earthquake Fault
Zoning Act
Earthquake Hazard This act restricts construction of buildings used for human occupancy on the surface trace of active faults.
California Coastal Management
Program
Flood, Landslide/Mass
Movement, Tsunami
and Wildfire Hazards
This program requires coastal communities to prepare coastal plans and requires that new development minimize risks to life and property in areas of high geologic, flood, and fire hazard.
Board of Forestry and Fire
Protection Fire Safe Regulations
Wildfire Hazard The Fire Safe Regulations set the floor for fire safety standards for perimeters and access to residential, commercial, and industrial building construction.
California Department of Forestry
and Fire Protection (CAL FIRE)
Wildfire Hazard CAL FIRE has responsibility for wildfires in areas that are not under the jurisdiction of the Forest Service or a local fire organization.
California Department of Parks and
Recreation
Wildfire Hazard State Parks Resources Management Division has wildfire protection resources available to suppress fires on State Park lands.
California Department of Water
Resources
Flood Hazard This state department is the state coordinating agency for floodplain management.
California Division of Safety of
Dams
Dam Failure Hazard This division monitors the dam safety program at the state level and maintains a working list of dams in the state.
California Environmental Quality
Act
Action Plan
Implementation
This act establishes a protocol of analysis and public disclosure of the potential environmental impacts of development projects. Any project action identified in this plan will seek full California Environmental Quality Act compliance upon implementation.
5-4
5. Regulations and Programs
Agency, Program or Regulation
Hazard Mitigation
Area Affected Relevance
California Fire Alliance Wildfire Hazard The alliance works with communities at risk from wildfires to facilitate the development of community fire loss mitigation plans.
California Fire Plan Wildfire Hazard This plan’s goal is to reduce costs and losses from wildfire through pre - fire management and through successful initial response.
California Fire Safe Council Wildfire Hazard This council facilitates the distribution of National Fire Plan grants for wildfire risk reduction and education.
California Fire Service and Rescue
Emergency Mutual Aid Plan
Wildfire Hazard This plan provides guidance and procedures for agencies developing emergency operations plans, as well as training and technical support.
California General Planning Law Hazard Mitigation
Planning
This law requires every county and city to adopt a comprehensive long- range plan for community development, and related laws call for integration of hazard mitigation plans with general plans.
California Multi-Hazard Mitigation
Plan
Hazard Mitigation
Planning
Local hazard mitigation plans must be consistent with their state’s hazard mitigation plan.
California Residential Mitigation
Program
Earthquake Hazard This program helps homeowners with seismic retrofits to lessen the potential for damage to their houses during an earthquake.
California State Building Code Action Plan
Implementation
Local communities must adopt and enforce building codes, which include measures to improve buildings’ ability to withstand hazard events.
Disadvantaged and Low-Income
Communities Investments
Action Plan Funding This is a potential source of funding for actions located in disadvantaged or low-income communities.
Division of the State Architect’s AB
300 List of Seismically At-Risk
Schools
Earthquake Hazard,
Action Plan
Development
The Division of the State Architect recommends that local school districts conduct detailed seismic evaluations of seismically at-risk schools identified in the inventory that was required by AB 300.
Governor’s Executive Order S-13-
08 (Climate Impacts)
Action Plan
Implementation
This order includes guidance on planning for sea level rise in designated coastal and floodplain areas for new projects.
Office of the State Fire Marshal Wildfire Hazard This office has a wide variety of fire safety and training responsibilities.
Senate Bill 12: Local government:
planning and zoning: wildfires.
Wildfire Hazard Requires the safety element to be reviewed and updated as necessary to include a comprehensive retrofit strategy to reduce the risk of property loss and damage during wildfires. Requires the planning agency to submit the adopted strategy to the Office of Planning and Research for inclusion into a central clearinghouse.
Senate Bill 92: Dam Emergency
Action Plans; Public Resources
Portion of Biennial Budget Bill
Dam Failure Hazard This bill requires dams (except for low-risk dams) to have emergency action plans that are updated every 10 years and inundation maps updated every 10 years, or sooner if specific circumstances change.
Senate Bill 97: Guidelines for
Greenhouse Gas Emissions
Action Plan
Implementation
This bill establishes that greenhouse gas emissions and the effects of greenhouse gas emissions are appropriate subjects for California Environmental Quality Act analysis.
Senate Bill 99: General Plans:
Safety Element: Emergency
Evacuation Routes
Action Plan
Implementation
This bill requires the safety element must include information to identify residential developments in hazard areas that do not have at least two emergency evacuation routes.
Senate Bill 182: Local Government:
Planning and Zoning: Wildfires
Wildfire Hazard This bill made a number of changes to state law regarding planning for and permitting development in areas designated as very high fire risk areas.
Senate Bill 379: General Plans:
Safety Element—Climate
Adaptation
Action Plan
Implementation
This bill requires cities and counties to include climate adaptation and resiliency strategies in the safety element of their general plans.
Senate Bill 1000: General Plan
Amendments—Safety and
Environmental Justice Elements
Action Plan
Implementation
Under this bill, review and revision of general plan safety elements are required to address only flooding and fires (not climate adaptation and resilience), and environmental justice is required to be included in general plans.
5-5
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Agency, Program or Regulation
Hazard Mitigation
Area Affected Relevance
Senate Bill 1241: General Plans:
Safety Element—Fire Hazard
Impacts
Wildfire Hazard This bill requires cities and counties to make findings regarding available fire protection and suppression services before approving a tentative map or parcel map.
Standardized Emergency
Management System
Action Plan
Implementation
Local governments must use this system to be eligible for state funding of response-related personnel costs.
Western Governors Association
Ten-Year Comprehensive Strategy
Wildfire Hazard This strategy implementation plan prepared by federal and Western state agencies outlines measures to restore fire-adapted ecosystems and reduce hazardous fuels.
The following local regulations, codes, ordinances, and plans were reviewed to develop complementary and mutually supportive goals, objectives, and mitigation strategies that are consistent across local and regional planning and regulatory mechanisms:
• General plans (land use, housing, safety, and open space elements) • Building codes
• Zoning and subdivision ordinances
• NFIP flood damage prevention ordinances
• Stormwater management plans
• Emergency management and response plans
• Land use and open space plans
• Climate action plans.
• Community wildfire protection plans
• Tribal hazard mitigation plans.
5.3 LOCAL CAPABILITY ASSESSMENT
All participating jurisdictions compiled an inventory and analysis of existing authorities and capabilities called a “capability assessment.” A capability assessment creates an inventory of a jurisdiction’s mission, programs and policies, and evaluates its capacity to carry them out. This assessment identifies potential gaps in the jurisdiction’s capabilities.
The planning partnership views all core jurisdictional capabilities as fully adaptable to meet a jurisdiction’s needs.
Every code can be amended, and every plan can be updated. Such adaptability is itself considered to be an overarching capability. If the capability assessment identified an opportunity to add a missing core capability or expand an existing one, then doing so has been selected as an action in the jurisdiction’s action plan, which is included in the individual annexes presented in Volume 2 of this plan.
Capability assessments for each planning partner are presented in the jurisdictional annexes in Volume 2. The sections below describe the specific capabilities evaluated under the assessment. 5-6
5. Regulations and Programs
5.3.1 Legal and Regulatory Capabilities
Jurisdictions can develop policies and programs and to implement rules and regulations to protect and serve residents. Local policies are typically identified in a variety of community plans, implemented via a local ordinance, and enforced through a governmental body.
Jurisdictions regulate land use through the adoption and enforcement of zoning, subdivision, and land development ordinances, building codes, building permit ordinances, floodplain, and stormwater management ordinances. When effectively prepared and administered, these regulations can lead to hazard mitigation.
5.3.2 Fiscal Capabilities
Assessing a jurisdiction’s fiscal capability provides an understanding of the ability to fulfill the financial needs associated with hazard mitigation projects. This assessment identifies both outside resources, such as grant- funding eligibility, and local jurisdictional authority to generate internal financial capability, such as through impact fees.
5.3.3 Administrative and Technical Capabilities
Legal, regulatory, and fiscal capabilities provide the backbone for successfully developing a mitigation strategy; however, without appropriate personnel, the strategy may not be implemented. Administrative and technical capabilities focus on the availability of personnel resources responsible for implementing all the facets of hazard mitigation. These resources include technical experts, such as engineers and scientists, as well as personnel with capabilities that may be found in multiple departments, such as grant writers.
5.3.4 NFIP Compliance
Flooding is the costliest natural hazard in the United States and, with the promulgation of recent federal regulation, homeowners throughout the country are experiencing increasingly high flood insurance premiums.
Community participation in the NFIP opens up opportunity for additional grant funding associated specifically with flooding issues. Assessment of the jurisdiction’s current NFIP status and compliance provides planners with a greater understanding of the local flood management program, opportunities for improvement, and available grant funding opportunities.
5.3.5 Public Outreach Capability
Regular engagement with the public on issues regarding hazard mitigation provides an opportunity to directly interface with community members. Assessing this outreach and education capability illustrates the connection between the government and community members, which opens a two-way dialogue that can result in a more resilient community based on education and public engagement.
5.3.6 Participation in Other Programs
Other programs, such as the Community Rating System, Storm/Tsunami Ready, and Firewise USA, can enhance a jurisdiction’s ability to mitigate, prepare for, and respond to natural hazards. These programs indicate a jurisdiction’s desire to go beyond minimum requirements set forth by local, state and federal regulations in order to create a more resilient community. These programs complement each other by focusing on communication, 5-7
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements mitigation, and community preparedness to save lives and minimize the impact of natural hazards on a community.
5.3.7 Development and Permitting Capability
Identifying previous and future development trends is achieved through a comprehensive review of permitting since completion of the previous plan and in anticipation of future development. Tracking previous and future growth in potential hazard areas provides an overview of increased exposure to a hazard within a community.
5.3.8 Adaptive Capacity
An adaptive capacity assessment evaluates a jurisdiction’s ability to anticipate impacts from future conditions. By looking at public support, technical adaptive capacity, and other factors, jurisdictions identify their core capability for resilience against issues such as sea level rise. The adaptive capacity assessment provides jurisdictions with an opportunity to identify areas for improvement by ranking their capacity high, medium or low.
5.3.9 Integration Opportunity
The assessment looked for opportunities to integrate this mitigation plan with the legal/regulatory capabilities identified. Capabilities were identified as integration opportunities if they can support or enhance the actions identified in this plan or be supported or enhanced by components of this plan. Planning partners considered actions to implement this integration as described in their jurisdictional annexes. 5-8
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021 PART 2—RISK ASSESSMENT
6. IDENTIFIED HAZARDS OF CONCERN AND RISK
ASSESSMENT METHODOLOGY
The risk assessments in this plan describe the risks associated with each identified hazard of concern. The following steps were used to define the risk of each hazard:
• Identify and profile each hazard—The following information is given for each hazard:
A summary of past events that have impacted the planning area Geographic areas most affected by the hazard
Event frequency estimates
Severity descriptions
Warning time likely to be available for response.
• Determine exposure to each hazard—Exposure was assessed by overlaying hazard maps with an inventory of structures, facilities, and systems to decide which of them would be exposed to each hazard. • Assess the vulnerability of exposed facilities—Vulnerability of exposed structures and infrastructure was evaluated by interpreting the probability of occurrence of each event and assessing structures, facilities, and systems that are exposed to each hazard. Tools such as GIS and Hazus were used for this assessment for the dam failure, earthquake, flood, and tsunami hazards. Outputs similar to those from Hazus were generated for other hazards, using data generated through GIS.
The risk assessments performed for this plan evaluated risk countywide and for individual incorporated areas.
6.1 IDENTIFIED HAZARDS OF CONCERN
The Steering Committee considered the full range of natural hazards that could affect the planning area and then listed hazards that present the greatest concern. The process incorporated a review of state and local hazard planning documents as well as information on the frequency of, magnitude of, and costs associated with hazards that have struck the planning area or could do so. Anecdotal information regarding natural hazards and the perceived vulnerability of the planning area’s assets to them was also used. Based on the review, this plan addresses the following hazards of concern (presented in alphabetical order; the order of listing does not indicate the hazards’ relative severity):
• Dam failure
• Drought
• Earthquake
• Flooding
• Landslide/mass movement
6-1
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements • Sea level rise
• Severe weather
• Tsunami
• Wildfire
The hazard mitigation plan includes a discussion of climate change, but it is not treated as a stand-alone hazard.
Instead, a review is provided on the ways in which climate change could affect the planning area’s exposure and vulnerability to the other identified hazards of concern.
An additional chapter provides a profile of “hazards of concern,” defined as hazards that may impact the planning area but whose risk is difficult to quantify due to a lack of data or well-established assessment parameters. That chapter provides a profile of these hazards but does not assess them to the same level of detail as the primary hazards of concern. The hazards of interest are not included in the risk rating for this plan.
6.2 RISK ASSESSMENT TOOLS
6.2.1 Mapping
National, state, county, and city databases were reviewed to locate available spatially based data relevant to this planning effort. Maps were produced using geographic information system (GIS) software to show the spatial extent and location of hazards when such datasets were available. These maps are included in the hazard profile chapters of this document and the jurisdiction-specific annexes in Volume 2. Details regarding the data sources and methodologies employed in these mapping efforts is located in Appendix C.
6.2.2 Hazus
Overview
In 1997, FEMA developed the standardized Hazards U.S., or Hazus, model to estimate losses caused by earthquakes and identify areas that face the highest risk and potential for loss. Hazus was later expanded into a multi-hazard methodology with new models for estimating potential losses from hurricanes, floods, and tsunamis.
Hazus is a GIS-based software program used to support risk assessments, mitigation planning, and emergency planning and response. It provides a wide range of inventory data, such as demographics, building stock, community lifelines, and multiple models to estimate potential losses from natural disasters. The program maps and displays hazard data and the results of damage and economic loss estimates for buildings and infrastructure.
Its advantages include the following:
• Provides a consistent methodology for assessing risk across geographic and political entities. • Provides a way to save data so that it can readily be updated as population, inventory, and other factors change and as mitigation planning efforts evolve.
• Facilitates the review of mitigation plans because it helps to ensure that FEMA methodologies are incorporated.
• Supports grant applications by calculating benefits using FEMA definitions and terminology. • Produces hazard data and loss estimates that can be used in communication with local stakeholders. 6-2
6. Identified Hazards of Concern and Risk Assessment Methodology • Is administered by the local government and can be used to manage and update a hazard mitigation plan throughout its implementation.
Levels of Detail for Evaluation
Hazus provides default data for inventory and hazards; this default data can be supplemented with local data to provide a more refined analysis. The model can carry out three levels of analysis, depending on the format and level of detail of information about the planning area:
• Level 1—All of the information needed to produce an estimate of losses is included in the software’s default data. This data is derived from national databases and describes in general terms the characteristic parameters of the planning area.
• Level 2—More accurate estimates of losses require more detailed information about the planning area. To produce Level 2 estimates of losses, detailed information is required about local geology, hydrology, hydraulics and building inventory, as well as data about utilities and critical facilities. This information is needed in a GIS format.
• Level 3—This level of analysis generates the most accurate estimate of losses. It requires detailed engineering and geotechnical information to customize it for the planning area.
6.3 RISK ASSESSMENT APPROACH
6.3.1 Hazard Profile Development
Hazard profiles were developed through web-based research and review of previously developed reports and plans, including community general plans and state and local hazard mitigation plans. Frequency and severity indicators include past events and the expert opinions of geologists, emergency management specialists, and others.
6.3.2 Exposure and Vulnerability
Dam Failure, Earthquake, and Flood
Community exposure and vulnerability to the following hazards were evaluated using Hazus:
• Dam Failure and Flood—A Level 2 user-defined analysis was performed for general building stock and for community lifelines using the flood module. Current mapping for the planning area was used to delineate hazard areas for flood and dam failure, and estimate potential losses. To estimate damage that would result from these inundation-based hazards, Hazus uses pre-defined relationships between water depth at a structure and resulting damage, with damage given as a percent of total replacement value.
Curves defining these relationships have been developed for damage to structures and for damage to typical contents within a structure. By inputting inundation depth data and known property replacement cost values, dollar-value estimates of damage were generated. • Earthquake—A Level 2 analysis was performed to assess earthquake exposure and vulnerability for four scenario events and one probabilistic event:
A Magnitude-7.57 event on the Hayward Fault with an epicenter 16 miles southeast of Petaluma. A Magnitude-7.55 event on the Maacama Fault with an epicenter 26 miles north-northwest of Cloverdale.
6-3
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements A Magnitude-7.19 event on the Rodgers Creek and Healdsburg Faults with an epicenter 3 miles north-northeast of Santa Rosa.
A Magnitude-8.04 event on the San Andreas Fault with an epicenter 16 miles west of Sebastopol. The standard Hazus 100-year probabilistic event.
Landslide/Mass Movement, Sea Level Rise, Severe Weather, Tsunami, and Wildfire Historical datasets were not adequate to model future losses for most of the hazards of concern. However, areas and inventory susceptible to some of the hazards of concern were mapped by other means to evaluate exposure. A qualitative analysis was conducted for other hazards using the best available data and professional judgment.
Drought
The risk assessment methodologies used for this update focus on damage to structures. Because drought does not impact structures, the risk assessment for this hazard was more limited and qualitative than the assessment for the other hazards of concern.
6.4 SOURCES OF DATA USED IN MODELING AND EXPOSURE ANALYSIS
6.4.1 Building and Cost Data
Replacement cost is the cost to replace the entire structure with one of equal quality and utility. Replacement cost is based on industry-standard cost-estimation models published in the 2020 RS Means Square Foot Costs. It is calculated using the RS Means square foot cost for a structure, which is based on the Hazus occupancy class (i.e., multi-family residential or commercial retail trade), multiplied by the square footage of the structure from the tax assessor data. The construction class and number of stories for single-family residential structures also factor into determining the square foot costs.
Replacement cost values and detailed structure information derived from parcel and tax assessor data provided by Sonoma County were loaded into Hazus. When available, an updated inventory was used in place of the Hazus defaults for community lifelines.
6.4.2 Hazus Data Inputs
The following hazard datasets were used for the Hazus Level 2 analysis conducted for the risk assessment:
• Flood—Flood hazards areas were delineated using a combination of the effective Digital Flood Insurance Rate Map (DFIRM), the preliminary FIRM, and the County’s Russian River flood stage inundation areas.
Potential losses for the FEMA 1-percent-annual-chance and 0.2-percent-annual-chance (100- and 500- year) flood events, and the equivalent stages in the Russian River data, were estimated. The 47-foot flood stage and the 51-foot flood stage, both as measured at the Guerneville stream gage, were considered equivalent to the FEMA 1-percent-annual chance and 0.2-percent-annual-chance events respectively.
Using the DFIRM and preliminary FIRM floodplain boundaries and base flood elevation information, and the County’s 3-foot digital elevation model (DEM), flood depth grids were generated and integrated into the Hazus model. The flood depth grids, included as part of the Russian River inundation areas were included as part of the County’s original flood stage dataset. • Dam Failure—Dam failure inundation area boundaries data for Annadel No. 1, Cook No. 2, Delta Pond, Dutcher Creek, Fern Lake, Foothill Regulating Park, Foss Creek North Area, Lagunita, Lake Helen, 6-4
6. Identified Hazards of Concern and Risk Assessment Methodology Lytton, Mallacomes, Matanzas Creek, Merlo, Middle Fork Brush Creek, Piner Creek, Santa Rosa Creek Reservoir, and Suttenfield were provided by the California Department of Water Resources. Associated inundation depth grid data were also provided for all dams except Delta Pond. Inundation area boundaries data for Azalea, Fountaingrove, Lake Ralphine, and Warm Springs Dam provided by the County. Depth grids for Delta Pond and the dams provided by the County were created using the inundation area boundaries and the 3-foot DEM data. The individual dam depth grids were combined using the maximum depth where the dam inundation areas overlapped, and the combined depth grid was integrated into the Hazus model.
• Earthquake—Earthquake ShakeMaps and probabilistic data prepared by the U.S. Geological Survey (USGS) were used for the analysis of this hazard. A National Earthquake Hazard Reduction Program (NEHRP) soils map from the California Department of Conservation, the USGS’s liquefaction susceptibility data, and susceptibility to deep-seated landslide data from the California Geological Survey were also integrated into the Hazus model.
6.4.3 Other Local Hazard Data
Locally relevant information on hazards was gathered from a variety of sources. Frequency and severity indicators include past events and the expert opinions of geologists, emergency management specialists, and others. Data sources for specific hazards were as follows:
• Flood—Additional areas called “flood awareness areas” were delineated using the County’s “functional riparian channels and floodplains” data. The mapped areas overlap with the FEMA and Russian River data in some areas but also include streams not mapped by FEMA. • Landslide/Mass Movement—Susceptibility to deep-seated landslides data were provided by the California Geological Survey. Areas categorized as very high and high susceptibility (categories X, XI, VIII, and VII) were used in the exposure analysis.
• Sea Level Rise—USGS’s Coastal Storm Modeling System sea level rise data were provided by Our Coast Our Future. Sea level rises of 200 cm (no storm) and 200 cm with a 100-year storm were used for the exposure analysis.
• Severe Weather—No GIS format severe weather area datasets were identified for Sonoma County. • Tsunami—Tsunami inundation area data were provided by the California Department of Conservation. • Wildfire—Sonoma County Wildfire Hazard Index data were provided by Sonoma County. Areas categorized as very high and high relative hazard were used in the exposure analysis.
6.4.4 Data Source Summary
Table 6-1 summarizes the data sources used for the risk assessment for this plan. 6-5
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Table 6-1. Hazus Model Data Documentation
Data Source Date Format
Property parcel data including building
information (use code, square footage, year
built)
Sonoma County 2020 Digital (GIS)
Building footprints (Sonoma County
Vegetation Mapping & LiDAR Program)
Sonoma County Unknown Digital (GIS)
Building replacement (square foot) costs RS Means 2020 Digital (pdf) Sonoma County Supervisorial District
boundaries
Sonoma County Downloaded 2021 Digital (GIS)
Population data FEMA Hazus version 4.2 SP03 2010 Digital (GIS and tabular)
CA State dam breach inundation maps
(inundation boundaries and depth grids)
California Department of Water Resources 2018-20 Digital (GIS) Dam inundation areas Sonoma County Unknown Digital (GIS) ShakeMap—Hayward RC+HN+HS M7.57 USGS 2017 Digital (GIS) ShakeMap—Maacama M7.55 USGS 2017 Digital (GIS)
ShakeMap—Rodgers Creek-Healdsburg
M7.19
USGS 2017 Digital (GIS)
ShakeMap—San Andreas
SAO+SAN+SAP+SAS M8.04
USGS 2017 Digital (GIS)
NEHRP soils (VsMapV3_Geology) California Department of Conservation 2015 Digital (GIS) Liquefaction susceptibility USGS (provided by Sonoma County) 2006 Digital (GIS) Digital Flood Insurance Rate Map (DFIRM)—
Sonoma County effective 3/7/2017, latest
Letter of Map Revision effective 6/19/2020
FEMA 2020 Digital (GIS)
Preliminary FIRM—Sonoma County dated
5/15/2020
FEMA 2020 Digital (GIS)
Russian River Flood Modeling (inundation
boundaries and depth grids)
Sonoma County 2017 Digital (GIS)
Functional Riparian Channels and
Floodplains
Sonoma County Agricultural Preservation & Open
Space District
2020 Digital (GIS)
Susceptibility to deep-seated landslides California Geological Survey 2011 Digital (GIS) USGS Coastal Storm Modeling System
(v. 2.0, v. 2.1, and v. 2.2) sea level rise data
Our Coast Our Future 2019 Digital (GIS)
Tsunami Inundation Map for Emergency
Planning
California Emergency Management Agency,
California Geological Survey, and University of
Southern California—Tsunami Research Center
2009 Digital (GIS)
Sonoma County Wildfire Hazard Index Permit Sonoma, Sonoma County, Fire Safe Sonoma, Tukman Geospatial, Digital Mapping Solutions,
Wildland Res Mgt
2021 Digital (GIS)
Sonoma Veg Map LiDAR Hydro Flattened
Bare Earth DEM (3-foot resolution)
Sonoma County 2013 Digital (GIS)
Critical Facilities
Police stations Sonoma County (original ArcGIS Online) Provided 2020 Digital (GIS) Fire stations Sonoma County Provided 2020 Digital (GIS) Public lands (city, county, state buildings) Sonoma County Provided 2020 Digital (GIS) County buildings Sonoma County Provided 2020 Digital (GIS) County leased buildings Sonoma County Provided 2020 Digital (GIS) Post offices Sonoma County (original ArcGIS Online) Provided 2020 Digital (GIS) 6-6
6. Identified Hazards of Concern and Risk Assessment Methodology Data Source Date Format
Public schools Sonoma County Provided 2020 Digital (GIS) Private schools Sonoma County Provided 2020 Digital (GIS) College and university parcels Sonoma County Provided 2020 Digital (GIS) Emergency shelters Sonoma County Provided 2020 Digital (GIS) Licensed and certified healthcare facilities Sonoma County (original ArcGIS Online) Provided 2020 Digital (GIS) Convalescent hospital parcels Sonoma County Provided 2020 Digital (GIS) Animal hospital parcels Sonoma County Provided 2020 Digital (GIS) Emergency Medical Services stations Sonoma County Provided 2020 Digital (GIS) Geothermal power plants Sonoma County Provided 2020 Digital (GIS) Hydroelectric power plants Sonoma County Provided 2020 Digital (GIS) Electric substations Sonoma County (original ArcGIS Online) Provided 2020 Digital (GIS) Sonoma-Marin Area Rail Transit stations Sonoma County Provided 2020 Digital (GIS) Airports Sonoma County Provided 2020 Digital (GIS)
Bus transit stops Sonoma County Provided 2020 Digital (GIS) Bridges Sonoma County Provided 2020 Digital (GIS)
Hazmat facilities Sonoma County Provided 2020 Digital (GIS) Certified Unified Program Agency
facilities
Sonoma County Provided 2020 Digital (GIS)
Natural gas stations California Energy Commission Downloaded 2018 Digital (GIS) Power plants California Energy Commission Downloaded 2020 Digital (GIS) Wastewater treatment plants California Water Resources Control Board Downloaded 2020 Digital (GIS) Hospital heliports California Department of Transportation Downloaded 2020 Digital (GIS) Local Emergency Operations Centers Homeland Infrastructure Foundation-Level Data (HIFLD)
Downloaded 2020 Digital (GIS)
Veterans Health Administration medical
facilities
HIFLD Downloaded 2020 Digital (GIS)
FM transmission towers HIFLD Downloaded 2020 Digital (GIS) AM transmission towers HIFLD Downloaded 2020 Digital (GIS) TV digital station transmitters HIFLD Downloaded 2020 Digital (GIS) TV analog station transmitters HIFLD Downloaded 2020 Digital (GIS) FDIC insured banks HIFLD Downloaded 2020 Digital (GIS)
Port facilities HIFLD Downloaded 2020 Digital (GIS)
6.5 LIMITATIONS
Loss estimates, exposure assessments and hazard-specific vulnerability evaluations rely on the best available data and methodologies. Uncertainties are inherent in any loss estimation methodology and arise in part from incomplete scientific knowledge concerning natural hazards and their effects on the built environment.
Uncertainties also result from the following:
• Approximations and simplifications necessary to conduct a study • Incomplete or outdated inventory, demographic or economic parameter data • The unique nature, geographic extent, and severity of each hazard • Mitigation measures already employed
• The amount of advance notice residents have to prepare for a specific hazard event. 6-7
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements These factors can affect loss estimates by a factor of two or more. Therefore, potential exposure and loss estimates are approximate and should be used only to understand relative risk. Over the long term, the planning partners will collect additional data to assist in estimating potential losses associated with other hazards. 6-8
7. DAM FAILURE
7.1 GENERAL BACKGROUND
7.1.1 Definition and Classification of Dams
A dam is an artificial barrier that can store water, wastewater, or liquid-borne materials for many reasons—flood control, human water supply, irrigation, livestock water supply, energy generation, containment of mine tailings, recreation, or pollution control. Many dams fulfill a combination of these functions. They are an important resource in the United States (ASDSO, 2013). In California, dams are regulated by the State of California Division of Safety of Dams. Additional regulatory oversight of dams is cited in Chapter 5 and described in Appendix B.
The California Water Code (Division 3) defines a dam as any artificial barrier, together with appurtenant works, that does or may impound or divert water, and that meets either of the following conditions:
• Is 25 feet or more in height from the natural bed of the stream or watercourse at the downstream toe of the barrier (or from the lowest elevation of the outside limit of the barrier if it is not across a stream channel or watercourse) to the maximum possible water storage elevation • Has an impounding capacity of 50 acre-feet or more
Dams can be classified according to their purpose, the construction material or methods used, their slope or cross- section, the way they resist the force of the water pressure, or the means used for controlling seepage. Materials used to construct dams include earth, rock, tailings from mining or milling, concrete, masonry, steel, timber, plastic, rubber, and combinations of these.
7.1.2 Causes of Dam Failure
Partial or full failure of dams has the potential to cause massive destruction to the ecosystems and communities located downstream. Partial or full failure can occur as a result of one or a combination of the following reasons (FEMA, 2015):
• Overtopping caused by floods that exceed the dam capacity (inadequate spillway capacity) • Prolonged periods of rainfall and flooding
• Deliberate acts of sabotage (terrorism)
• Structural failure of materials used in dam construction • Movement and/or failure of the foundation supporting the dam • Settlement and cracking of concrete or embankment dams 7-1
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements • Piping and internal erosion of soil in embankment dams • Inadequate or negligent operation, maintenance, and upkeep • Failure of upstream dams on the same waterway
• Earthquake (liquefaction/landslides).
Many dam failures in the United States have been secondary results of other disasters. The most common causes are earthquakes, landslides, extreme storms, equipment malfunction, structural damage, foundation failures, and sabotage. Poor construction, lack of maintenance and repair, and deficient operational procedures are preventable or correctable by a program of regular inspections. Terrorism and vandalism are serious concerns that all operators of public facilities must plan for; these threats are under continuous review by public safety agencies.
7.1.3 Planning Requirements
State of California
All dams whose inundation areas may impact the planning area have emergency action plans (EAPs) on file. The EAPs must include the following (Cal OES, 2018):
• Emergency notification flow charts
• Information on a four-step response process
• Description of agencies’ roles and actions in response to an emergency incident • Description of actions to be taken in advance of an emergency • Inundation maps
• Additional information such as revision records and distribution lists.
After the EAPs are approved by the state, the law requires dam owners to send the approved EAPs to relevant stakeholders. Local public agencies can then adopt emergency procedures that incorporate the information in the EAP in a manner that conforms to local needs and includes methods and procedures for alerting and warning the public and other response and preparedness related items (State of California, 2018).
Federal Energy Regulatory Commission
Dams that fall under the jurisdiction of the Federal Energy Regulatory Commission (FERC) also have specified planning requirements. FERC has the largest dam safety program in the United States. It cooperates with many federal and state agencies to ensure and promote dam safety and, more recently, homeland security. FERC requires licensees to prepare emergency action plans and conducts training sessions on how to develop and test these plans. The plans are designed to serve as an early warning system if there is a potential for, or a sudden release of water from, a dam failure or accident to the dam. The plans include operational procedures that may be used, such as reducing reservoir levels and reducing downstream flows and procedures for notifying affected residents and agencies responsible for emergency management. These plans are frequently updated and tested to ensure that in emergency situations everyone knows what to do, thus saving lives and minimizing property damage.
7-2
7. Dam Failure
7.2 HAZARD PROFILE
7.2.1 Past Events
No known failures have occurred on dams that impact Sonoma County. However, according to the 2013 State of California Multi-Hazard Mitigation Plan, there have been nine failures of federally regulated dams elsewhere in the state since 1950. Overtopping caused two of the nine dam failures in the state, and the others were caused by seepage or leaks. The most catastrophic event was the failure of the St. Francis Dam in Los Angeles County, which failed in 1928 and killed an estimated 450 people.
The state’s most recent dam emergency occurred in February 2017 when the Oroville Dam in Butte County was on the verge of overflow. The dam’s concrete spillway was damaged by erosion and a massive hole developed.
The auxiliary spillway was used to prevent overtopping of the dam, and it experienced erosion problems also.
Evacuation orders were issued in advance of a potential large uncontrolled release of water from Lake Oroville, but such a release did not occur. After this incident, state officials ordered that flood-control spillways be re-inspected on 93 California dams with potential geologic, structural or performance issues that could jeopardize their ability to safely pass a flood event.
7.2.2 Location
List of High-Hazard Dams
According to the Army Corps of Engineers’ National Inventory of Dams, there are 65 dams that are in the planning area or have inundation areas that extend into the planning area. Table 7-1 lists basic data about the 21 dams that are rated as high or extremely high risk. The locations of each dam are shown on Figure 7-1.
Warm Springs Dam
The largest dam in Sonoma County is the Warm Springs Dam that impounds Dry Creek and provides water supply and flood management. The dam was built in 1982 and forms Lake Sonoma Reservoir, which has a total storage capacity of 381,000 acre-feet: a water supply pool of 245,000 acre-feet and a flood pool of 136,000 acre- feet. The dam is owned by the U.S. Army Corps of Engineers. Water supply releases from this dam are controlled by the Sonoma County Water Agency. Water released for flood control is managed by the Corps (County of Sonoma 2017).
The Corps assessed the seismic integrity of the Warms Springs Dam in 2006. The assessment did not report any potential failure modes but did analyze the consequences of a failure. The occurrence of a seepage, overtopping, or a seismic event that could lead to a total breach of the dam could not entirely be ruled out due to the lack of updated studies. A breach could occur within a relatively short period of time, perhaps a few hours. The Corps categorized this dam as Dam Safety Action Class IV (considered marginally safe). This rating is based on probability of failure and on the population, residential and commercial structures, roads, farmland, bridge, and utilities downstream that could be damaged. The Corps plans to conduct additional safety assessment of the dam and appurtenant structures using the most recent information on seismic and flood conditions.
The Warm Springs Dam Failure Response Plan outlines the procedures and policy for potential failure of the Warm Springs Dam and possible impacts in the north central portion of the county. The plan identifies inundation areas, warning and evacuation procedures, and emergency contacts. 7-3
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Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Table 7-1. High-Hazard Dams in the Planning Area or with Inundation Areas Extending into Planning Area Name
Water
Course Owner
Year
Built
Hazard
Rating
Dam
Type
Crest
Length
(feet)
Height
(feet)
Storage
Capacity
(acre feet)
Condition
Assessment
Fern Lake Mill Cr. Trib. Sonoma Developmental Center 1921 High Earth 300 40 241 Satisfactory Suttenfield Sonoma
Creek
Sonoma Developmental Center 1938 Extremely
High
Earth 965 76 600 Satisfactory
Warm Springsa Dry Creek Corps of Engineers San
Francisco District
1982 High Earth 3000 319 130 n/a
Annadel No. 1 Spring Creek California Department of Parks and Recreation
1956 Extremely
High
Earth 400 67 395 Poor
Towibalyla Franz Cr.
Trib.
Kendall Jackson Wine Estates,
LTD
1962 High Earth 525 51 376 Satisfactory
Mallacomes Foote Creek Rancho Mallacomes 1951 High Earth 940 57 225 Satisfactory Piner Creek Paulin Creek Sonoma County Water Agency 1962 High Earth 205 28 172 Satisfactory Middle Fork
Brush Creek
Middle Fk.
Brush Cr.
Sonoma County Water Agency 1961 High Earth 1100 37 138 Satisfactory Matanzas Creek Matanzas
Creek
Sonoma County Water Agency 1963 Extremely
High
Earth 685 95 1,500 Satisfactory
Santa Rosa Creek
Reservoir
Santa Rosa
Cr. Trib.
Sonoma County Water Agency 1963 Extremely
High
Earth 1950 37 3,550 Satisfactory
Lake Ralphine Santa Rosa
Cr. Trib.
City of Santa Rosa 1882 Extremely
High
Earth 700 35 387 Satisfactory
Fountaingrove Mark West
Cr. Trib.
City of Santa Rosa 1953 Extremely
High
Earth 500 38 427 Satisfactory
Lagunita Windsor Cr.
Trib.
Private Entity 1954 Extremely
High
Earth 308 49 133 Satisfactory
Azalea N. Fk. Lancel
Cr.
Silver Eagle Ranch, LLC 1955 High Earth 140 44 85 Satisfactory Lytton Russian R.
Trib.
Lytton Rancheria of California 1956 High Earth 275 34 410 Satisfactory Lowe Franz Cr.
Trib.
Ferrari-Carano Vineyards and
Winery, LLC
1959 High Earth 550 30 108 Satisfactory
Bosch No. 2 Windsor Cr.
Trib.
Private Entity 1962 High Earth 230 55 37 Satisfactory
Foothill
Regulating Park
Windsor Cr.
Trib.
County of Sonoma Regional
Park
1963 High
Earth
274 51 109 Satisfactory
The Hill Ranch Santa Rosa
Cr.
Private Entity 1955 High Earth 202 49 160 Satisfactory
Merlo Fall Creek Private Entity 1982 High Earth 210 74 930 Satisfactory La Crema Winery Jackson Family Wines High Earth 1600 32 103 Fair a. The Warm Springs Dam is a federal dam, not under the jurisdiction of the State of California. Data taken from U.S. Army Corps of Engineers’ National Inventory of Dams
Source: California Department of Water Resources, Division of Safety of Dams, 2020 7-4
7. Dam Failure
Figure 7-1. Locations of Dams in Sonoma County
Coyote Valley Dam
The Coyote Valley Dam in Mendocino County regulates the northern Russian River and was built in 1959. It forms the Lake Mendocino Reservoir, which holds 122,400 acre-feet. Failure of this dam could affect areas in Sonoma County. This dam is owned by the Corps of Engineers. Water releases are controlled in a manner similar to the approach for the Warm Springs Dam (County of Sonoma 2017).
The Corps assessed the seismic integrity of the Coyote Valley Dam in 2005 and categorized it as Dam Safety Action Class III. Dams in Class III, for confirmed and unconfirmed dam safety issues, are significantly inadequate or have moderate to high risk based on the combination of life or economic consequences and probability of failure. The assessment found that the following are the most likely modes of failure, in order of decreasing risk:
• Seepage along the conduit leading to the formation of piping, which can quickly progress to rapid breaching of the embankment.
• Tunnel/Conduit joint failure caused by significant displacements of the shells during both the operating base earthquake and maximum design earthquake, and intake tower stability and embankment/foundation liquefaction failures during a maximum design earthquake. • The dam will be overtopped during the probable maximum flood event and will result in erosion of the crest and downstream slope leading to a complete breach. 7-5
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements The Corps plans to conduct additional safety assessment of the dam and appurtenant structures using the most recent information on seismic and flood conditions.
Other Dams in the County
There are 63 smaller dams throughout Sonoma County that are regulated by the Division of Safety of Dams. All are small and generally used for agricultural, drinking water, or stormwater management purposes. Failure of one of these dams could pose a significant threat to limited areas of the county. A dam inundation contingency plan for each dam is filed with the County.
Inundation Mapping
A key element for EAPs required for dams in California is a map defining the potential downstream inundation should the dam fail. For this risk assessment, digital data suitable for a quantitative assessment of dam failure risk was available for all high hazard dams listed in Table 7-1. To perform the risk assessment’s evaluation of exposure and vulnerability, the dam failure inundation areas were combined into a single hazard area shown in Figure 7-2.
7.2.3 Frequency
Dam failure events are infrequent and usually coincide with or follow events such as earthquakes, landslides and excessive rainfall and snowmelt. Although the recent Oroville event raised public concern about dam failure, the probability of such failures remains low in today’s regulatory environment. No recorded failures have occurred on dams that impact the planning area, so no estimate of frequency or probability of future occurrence can be developed based on the historical record.
All dams face a “residual risk” of failure, which represents the risk that conditions may exceed those for which the dam was designed. For example, dams may be designed to withstand a probable maximum precipitation, defined as “theoretically, the greatest depth of precipitation for a given duration that is physically possible over a given storm area at a particular geographical location at a certain time of the year” (Hansen 1982). The chance of occurrence of a precipitation event of a greater magnitude than that represents residual risk for such dams. This in turn represents a theoretical probability of future occurrence for a dam failure event, though the probability of an event exceeding the assumed maximum is not generally calculated as part of dam design.
7.2.4 Severity
Dam failure can be catastrophic to all life and property downstream. California’s Division of Safety of Dams has developed a hazard potential classification system for state-jurisdiction dams, as shown on Table 7-2. This system is modified from federal guidelines, which recommend three-tier classification. The California system adds a fourth hazard classification of “extremely high.” Dams classified as extremely high hazard may impact highly populated areas or critical infrastructure or have short evacuation warning times (California Division of Safety of Dams, 2017). All dams listed in Table 7-1 are classified as high hazard in this system. 7-6
Cotati
Sonoma
Windsor
Healdsburg
Sebastopol
Rohnert Park
Santa
Rosa
Cloverdale
Petaluma ¬«12
¬«116
¬«
116
¬«
37 ¬«
121
¬«
12 ¬«
128
¬«1
¬«1
£¤101
£¤
101
Figure 7-2. Dam Failure Inundation Area Used for Risk Assessment ±
0 10 5
Miles
Data Sources: Sonoma Co.,
CA DWR
Inundation Area
Cities
County Boundary
Highways
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Hazard Category
Table 7-2. State of California Downstream Hazard Potential Classification Direct Loss of Life Economic, Environmental, and Lifeline Losses Low None expected Low and principally limited to dam owner’s property Significant None expected Yes
High Probable (one or more expected) Yes, but not necessary for this classification Extremely High Considerable Yes, major impacts to critical infrastructure or property Source: California Division of Safety of Dams, 2017
7.2.5 Warning Time
Advance Warning of Failure
Warning time for dam failure varies depending on the cause of the failure. Events of extreme precipitation or massive snowmelt can be predicted in advance, so evacuations can be planned with sufficient time. In the event of a structural failure due to earthquake, there may be no or limited warning time. The USGS Earthquake Hazards Program has several dam-safety related earthquake programs, including dam-specific earthquake monitoring programs in California to help monitor safety concerns following seismic events.
Time for Failure to Occur
The process of the dam failure affects warning time. Earthen dams do not tend to fail completely or instantaneously. Once a breach is initiated, discharging water erodes the breach until either the reservoir water is depleted, or the breach resists further erosion. Concrete gravity dams also tend to have a partial breach as one or more monolith sections are forced apart by escaping water. The time of breach formation ranges from a few minutes to a few hours.
Time After Failure Before Downstream Areas Are Affected The number of people to be alerted and evacuated in the event of impending dam failure can vary widely. There may be few people along the river in winter, when only permanent residents are apt to be present; but there may be many people in summer, when seasonal cabins are occupied and there are visitors along all the rivers.
Another factor that must be considered is the initial flow in the river when the failure occurs. The initial flow is normally very low on all the rivers from May through October. During the winter, the initial flow is much higher and at times may even be equal to or greater than flood stage. This wide variation in initial flow has a significant impact on the areas that must be evacuated.
If the Warm Springs Dam failed, portions of the communities of Healdsburg, Windsor, Santa Rosa, Sebastopol, and Guerneville, as well as some rural population areas in the floodplain immediately downstream of the dam, would be within the 1- to 24-hour flood wave travel time bracket. Half of the rural population immediately downstream of the dam are within a 15-minute flood wave travel time and all are within a 1-hour flood wave travel time (County of Sonoma 2017).
7.2.6 Secondary Hazards
Dam failure can cause landslides, bank erosion, and destruction of downstream habitat. Dam failure may worsen the severity of a drought by releasing water that might have been used as a potable water source. 7-8
7. Dam Failure
7.3 EXPOSURE
A quantitative assessment of exposure to the dam failure hazard was conducted using the inundation mapping (see Figure 7-2) and the asset inventory developed for this plan. Detailed results by jurisdiction are included in Appendix D; countywide summaries are provided below.
7.3.1 Population
The estimated total population living in the evaluated dam failure inundation zone is 72,953 (15.0 percent of the total planning area population).
Socially vulnerable populations exposed to the dam failure hazard were estimated based on data for the Census- defined blocks that lie at least partially within the mapped dam failure inundation zone. Because many of those Census blocks extend outside the inundation zone, the estimates are greater than the actual exposed populations, but they provide reasonable relative data for use in mitigation planning. Figure 7-3 summarizes the estimated exposure of socially vulnerable populations.
Under Age 16
44,189
(23.2% of
Total) Over Age 65
23,060
(12.1% of
Total)
Total
190,624
0 20,000
40,000
60,000
80,000
100,000
120,000
140,000
160,000
180,000
200,000 Number of People in Hazard Area
Low Income
44,810
(63.2% of
Total)
Total
70,877
0 10,000
20,000
30,000
40,000
50,000
60,000
70,000
80,000 Number of Households in Hazard Area
See Section 4.8.1 for the definition of “low income” used in this analysis Figure 7-3. Socially Vulnerable Populations in Dam Failure Inundation Zone Census Blocks
7.3.2 Property
Table 7-3 summarizes the estimated property exposure in the evaluated dam failure inundation area. Figure 7-4 shows the Hazus-defined occupancy class of all buildings in the combined dam failure inundation area. These occupancy classes provide an indication of land use within the mapped hazard area. Some land uses are more vulnerable to dam failure inundation, such as single-family homes, while others are less vulnerable, such as agricultural land or parks.
7-9
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Table 7-3. Exposed Property in Evaluated Dam Failure Inundation Area Acres of inundation area 119,638
Number of Buildings Exposed 28,064
Value of Exposed Structures $20,123,003,848
Value of Exposed Contents $17,118,450,390
Total Exposed Property Value $37,241,454,238
Total Exposed Value as % of Planning Area Total 17.0%
Residential
23,564
84%
Commercial
2,054 Industrial
158Agriculture Religion
55 Government
440
2%
Education
83 0%
1,710 7%1%0% 6%
Figure 7-4. Building Occupancy Class Distribution in the Dam Failure Inundation Zone
7.3.3 Critical Facilities
Figure 7-5 shows critical facilities located in the dam inundation zone by facility type and river system. The total count of critical facilities in the dam failure inundation zone (1,059) represents 22 percent of the planning area total of 4,759. Significant facilities included in the mapped inundation zone include the following:
• 2 water treatment facilities • 12 fire stations
• 12 wastewater treatment facilities • 4 police stations • 660 hazardous material sites • 49 schools
• 3 hospitals • 111 road bridges
7.3.4 Environment
All natural features and wildlife in the dam inundation zone are at risk from the dam failure hazard. 7-10
7. Dam Failure
Facility Category
Transportation
Safety & Security
Health & Medical
Hazardous Material
Food, Water, Shelter
Energy
Communications
191
74 131
3,109
219
650
385
48 10
34 660
61 131
115
Combined Dam Failure Inundation Area
Planning Area Total
0 500 1,000 1,500 2,000 2,500 3,000 3,500
Number of Facilities in Identified Area
Figure 7-5. Critical Facilities in Dam Failure Inundation Zones and Countywide
7.4 VULNERABILITY
The vulnerability of people, property, and critical facilities was evaluated for the combined dam inundation area.
Detailed results by jurisdiction are included in Appendix D; countywide summaries are provided below.
7.4.1 Population
Vulnerable populations are all populations downstream from dam failures that are incapable of escaping the area before floodwaters arrive. This population includes the elderly and young who may be unable to get themselves out of the inundation area. The vulnerable population also includes those who would not have adequate warning from a television, radio emergency warning system, siren, or cell phone alert. Impacts on persons and households for the five dams chosen for further analysis were estimated for each event through the Level 2 Hazus analysis.
Table 7-4 summarizes the results.
7-11
-
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Table 7-4. Estimated Dam Failure Impacts on Population
Number of Displaced Residents 44,359
Number of Residents Requiring Short Term Shelter 3,157
7.4.2 Property
Vulnerable properties are those closest to the dam inundation zone. These properties would experience the largest, most destructive surge of water. Low-lying areas are also vulnerable since they are where the dam waters would collect. Properties in the dam inundation zone that are built to National Flood Insurance Program (NFIP) minimum construction standards may have some level of protection against dam inundation, depending on the velocity and elevation of the inundation waters. These properties also are more likely to have flood insurance.
Table 7-5 summarizes the loss estimates for dam failure.
Table 7-5. Estimated Impact of a Dam Failure in the Planning Area Number of Buildings Impacted 25,402
Estimated Loss, Structures $7,546,212,770
Estimated Loss, Contents $7,923,438,235
Estimated Loss, Total $15,469,651,005
Total Loss as % of Total Replacement Value 7.1%
7.4.3 Critical Facilities
Hazus estimated damage to critical facilities in the dam failure inundation zones as summarized in Figure 7-6.
Typical vulnerabilities of these facilities include the following:
• Transportation routes are vulnerable to dam inundation and have the potential to be wiped out, creating isolation issues and significant disruption to travel. Those that are most vulnerable are those that are already in poor condition and would not be able to withstand a large water surge. • Utilities such as overhead power lines, cable and phone lines in the inundation zone could also be vulnerable. If phone lines were lost, significant communication issues may occur in the planning area due to limited cell phone reception in many areas.
• In addition, emergency response would be hindered due to the loss of transportation routes as well as some protective-function facilities in the safety and security category located in the inundation zone.
Recovery time to restore many critical functions after an event may be lengthy.
7.4.4 Environment
The environment would be vulnerable to a number of risks in the event of dam failure. The inundation could introduce foreign elements into local waterways, resulting in destruction of downstream habitat and detrimental effects on many species of animals, especially endangered species such as the tidewater goby. 7-12
7. Dam Failure
Average Damage as % of Total Value
100
80 60
40 20
0 % Damage to Structure % Damage to Contents Number of Facilities Affected 617
75.0 70.2
62.0 59.4 58.2 58.8
48.8 40.6
37.2 32.6 30.7 30.9 31.4
114 108
3830 31
8 2.2
Safety and Food, Water and Health and Energy Communications Transportation Hazardous Security Sheltering Medical Material
700
600
500
400
300
200
100
0 Number of Facilities Affected
Figure 7-6. Estimated Damage to Critical Facilities from Dam Failure
7.5 FUTURE TRENDS IN DEVELOPMENT
Land use in the planning area will be directed by general plans adopted under state law. The safety elements of the general plans establish standards and plans for the protection of the community from hazards. Dam failure is currently not addressed as a stand-alone hazard in the safety elements, but flooding is. Municipalities participating in this plan have established comprehensive policies regarding sound land use in identified flood hazard areas.
Any structures that are built in the dam failure inundation area outside of the regulated floodplain will not be subject to floodplain management codes and standards. These structures would be more vulnerable than those constructed with floodplain codes and standards. Flood-related policies in the general plans will help to reduce the risk associated with dam failure for all future development in the planning area.
7.6 SCENARIO
In a worst-case scenario, an earthquake could lead to liquefaction of the ground soils where the dams that impact the planning area are located, causing the dams to fail. This could occur without warning in the middle of the night when downstream residents are asleep and unprepared to evacuate. A human-caused failure such as a terrorist attack also could trigger a catastrophic failure of one of the dams. 7-13
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements
7.7 ISSUES
The most significant issue associated with dam failure involves the properties and populations in the inundation zone. Flooding as a result of a dam failure would significantly impact these areas. There is often limited warning time for dam failures, which are frequently associated with other natural hazard events such as earthquakes, landslides, or severe weather. Important issues associated with the dam failure hazard include the following:
• Dam infrastructure may require repair and improvement to withstand climate change impacts, such as changing in the timing and intensity of rain events.
• Structures located in the dam inundation zone may be located outside of special flood hazard areas, meaning that they are not constructed to withstand floodwaters and are less likely to be covered by flood insurance. Even structures that have been designed with flood hazards in mind may not be able to withstand the height and velocity of flow from a dam failure event. • California law requires that a property’s location in a dam inundation area be disclosed to a seller if the seller or the seller’s agent has knowledge of the property’s location within the hazard area or if the local jurisdiction has compiled a list of parcels that are in the inundation area and has posted at the offices of the county recorder, county assessor, and county planning agency a notice that identifies the location of the list. It is unknown if this list has been compiled for the planning area. • The concept of residual risk associated with structural flood control projects should be considered in the design of capital projects and the application of land use regulations. • Federally regulated dams have an adequate level of oversight and sophistication in the development of emergency action plans for public notification in the unlikely event of failure. However, the protocol for notification of downstream citizens of imminent failure needs to be tied to local emergency response planning.
7-14
8. DROUGHT
8.1 GENERAL BACKGROUND
Drought is a significant decrease in water supply relative to what is needed to sufficiently meet typical demand in each location. It is a normal phase in the Mediterranean climate cycle, originating from a deficiency of precipitation over an extended period, usually a season or more. This leads to a water shortage for some activity, group, or environmental sector. Drought is generally defined based on four ways of measuring it (National Drought Mitigation Center, 2021):
• Meteorological drought—Based on measurements such as precipitation deficit compared to normal or expected precipitation. Anomalies of precipitation and temperature may last from several months to several decades. How long they last depend on interactions between the atmosphere and the oceans, soil moisture and land surface processes, topography, internal dynamics, and the accumulated influence of global weather systems.
• Agricultural drought—Based on impacts due to reduced precipitation and water supply (e.g., crop loss, herd culling, etc.)
• Hydrological drought—Based on measurements of stream flows, groundwater, and reservoir levels relative to normal conditions
• Socioeconomic drought—Based on direct and indirect socio-economic impacts on society and the economy. Socioeconomic drought occurs when the demand for an economic good exceeds supply as a result of a weather-related shortfall in water supply. If a community has stored enough water to meet its needs in the event of a shortage of rainfall, then it may not experience socioeconomic drought even though its geographic area experiences meteorological drought.
8.1.1 Monitoring and Categorizing Drought
NOAA Drought Indices
The National Oceanic and Atmospheric Administration (NOAA) has developed several indices to measure the impacts and severity of meteorological, agricultural, and hydrological drought and to map their extent and locations:
• The Crop Moisture Index measures short-term drought weekly to assess impacts on agriculture. • The Palmer Z Index measures short-term drought on a monthly scale. • The Palmer Drought Severity Index is based on long-term weather patterns. The intensity of drought in a given month is dependent on current weather plus the cumulative patterns of previous months. Weather patterns can change quickly, and the Palmer Drought Severity Index can respond fairly rapidly. 8-1
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements • The Palmer Hydrological Drought Index quantifies hydrological effects (reservoir levels, groundwater levels, etc.), which take longer to develop and last longer. This index responds more slowly to changing conditions than the Palmer Drought Index.
• The Standardized Precipitation Index considers only precipitation. A value of zero indicates the median precipitation amount; the index is negative for drought and positive for wet conditions. The Standardized Precipitation Index is computed for time scales ranging from one month to 24 months.
Each of these indices is meaningful for different sectors of society and the economy. For example an urbanized areas that uses water from reservoirs would be sensitive to hydrological drought characterized by the Palmer Hydrological Drought Index, while unirrigated grazing land would be sensitive to meteorological drought characterized by the Crop Moisture Index. Maps of these indices show drought conditions nationwide at a given point in time. They are not necessarily indicators of any given area’s long-term susceptibility to drought. Recent examples of these maps are shown on Figure 8-1.
U.S. Drought Monitor
The U.S. Drought Monitor (USDM) is a map that is updated weekly to show the location and intensity of drought across the country. The USDM uses a five-category system (USDM, 2021):
• D0—Abnormally Dry
Short-term dryness slowing planting, growth of crops
Some lingering water deficits
Pastures or crops not fully recovered
• D1—Moderate Drought
Some damage to crops, pastures
Some water shortages developing
Voluntary water-use restrictions requested
• D2—Severe Drought
Crop or pasture loss likely
Water shortages common
Water restrictions imposed
• D3—Extreme Drought
Major crop/pasture losses
Widespread water shortages or restrictions
• D4—Exceptional Drought
Exceptional and widespread crop/pasture losses
Shortages of water creating water emergencies
The USDM categories show experts’ assessments of conditions related to drought. These experts check variables including temperature, soil moisture, stream flow, water levels in reservoirs and lakes, snow cover, and meltwater runoff. They also check whether areas are showing drought impacts such as water shortages and business interruptions. Associated statistics show what proportion of various geographic areas are in each category of dryness or drought, and how many people are affected. U.S. Drought Monitor data go back to 2000. 8-2
8. Drought
Sources: National Weather Service, 2021; National Centers for Environmental Information, 2021a, 2021b, 2021c, 2021d Crop Moisture Index (Week Ending February 06, 2020) Palmer Z Index Short -Term Drought Conditions (January 2021) Palmer Hydrological Drought Index (January 2021) Palmer Drought Severity Index (January 2021) Standardized Precipitation Index (24-Months Ending December 2020) Figure 8-1. Standard National Drought and Precipitation Indices 8-3
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements
8.1.2 Drought Impacts
Drought can have a widespread impact on the environment and the economy, although it typically does not result in loss of life or damage to structures, as do other natural disasters. The National Drought Mitigation Center uses three categories to describe likely drought impacts:
• Economic Impacts—These impacts of drought cost people (or businesses) money. Farmers’ crops are destroyed; low water supply necessitates spending on irrigation system modifications, drilling of new wells, and/or trucking in water; water-related businesses (such as sales of boats and fishing equipment) may experience reduced revenue.
• Environmental Impacts—Plants and animals depend on water. When a drought occurs, their food supply can shrink, and their habitat can be damaged.
• Social Impacts—Social impacts include public safety, health, conflicts between people when there is not enough water to go around, and changes in lifestyle.
The demand that society places on water systems and supplies—such as expanding populations, irrigation, and environmental needs—contributes to drought impacts. Drought can lead to difficult decisions regarding the allocation of water, as well as stringent water use restrictions, water quality problems, and inadequate water supplies for fire suppression. There are also issues such as growing conflicts between agricultural uses of surface water and in-stream uses, surface water and groundwater interrelationships, and the effects of growing water demand on uses of water.
Vulnerability of an activity to drought depends on its water demand and the water supplies available to meet the demand. The impacts of drought vary between sectors of the community in both timing and severity:
• Water supply—The water supply sector encompasses urban and rural drinking water systems that are affected when a drought depletes surface and groundwater supplies due to reduced runoff and recharge from precipitation.
• Agriculture and commerce—Impacts on agriculture and associated commerce include the reduction of crop yield and livestock sizes due to insufficient water supply for crop irrigation and maintenance of ground cover for grazing.
• Environment, public health, and safety—Impacts on the environmental, public health, and safety sectors include wildfires that are both detrimental to the forest ecosystem and hazardous to the public.
The impacts also includes the desiccation of streams, resulting in the reduction of in-stream habitats for native species.
8.1.3 California Drought Response
During critically dry years, the California State Water Resources Control Board can mandate conservation by water users and agencies to address statewide water shortages. Table 8-1 lists State Drought Management Program stages mandated to water right holders.
8-4
8. Drought
Table 8-1. State Drought Management Program
Drought Stage State Mandated Customer Demand Reduction Rate Impacts Stage 0 or 1 <10% Normal rates
Stage 2 10 to 15% Normal rates; Drought surcharge
Stage 3 15 to 20% Normal rates; Drought surcharge
Stage 4 >20% Normal rates, Drought surcharge
8.2 HAZARD PROFILE
8.2.1 Local Water Use and Supply
Sonoma County has two principal sources of water for residential, commercial, industrial, and agricultural use:
the Russian River and groundwater. Additional water sources include diversions from small streams and springs and numerous reservoirs. Major users of Russian River water in Sonoma County are the cities of Cloverdale and Healdsburg, numerous individual diverters along the main stem of the Russian River and Dry Creek, and the Sonoma County Water Agency (Sonoma Water). About 73 percent of Sonoma County residents live in cities served by public water systems. Most residents of the unincorporated rural areas are outside urban service areas and are dependent on individual onsite wells or small-scale shared water supply systems.
Sonoma Water
Infrastructure
Sonoma Water serves the urbanized areas of Sonoma County and northern Marin County with water from the Russian River. The agency’s extensive water supply infrastructure generally mitigates the effects of short‐term dry periods for most water users (see Figure 8-2):
• Two major reservoirs regulate flow on the Russian River:
Sonoma Water and the Mendocino County Russian River Flood Control and Water Conservation Improvement District have water right permits authorizing storage up to the design capacity of 122,500 acre-feet per year in the Lake Mendocino reservoir (Coyote Valley Dam) in Mendocino County. Sonoma Water controls releases from the water supply pool in Lake Mendocino. However, the Corps manages flood control releases when the water level exceeds the top of the water supply pool elevation.
The Warm Springs Dam impounds water on Dry creek in Sonoma County forming Lake Sonoma, which has a total water supply capacity of 245,000 acre-feet (Sonoma Water, 2021a). Sonoma Water controls water supply releases from Lake Sonoma and the Corps manages flood control releases. • Sonoma Water diverts water from the Russian River near Forestville and conveys the water via its transmission system (including diversion facilities, treatment facilities, aqueducts, pipelines, water storage tanks, and booster pump stations) to its customers.
8-5
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Source: (Sonoma Water n.d.)
Figure 8-2. Russian River Watershed and the Sonoma Water Supply System 8-6
8. Drought
Planning Efforts
In accordance with California’s Urban Water Management Planning Act, Sonoma Water is updating its Urban Water Management Plan and Water Shortage Contingency Plan. The updated plan will describe the following (Sonoma Water 2021):
• Existing water supplies and transmission system facilities • Projected water demand in Sonoma Water’s service area over the next 25 years • Projected water supplies available to Sonoma Water over the next 25 years, the reliability of that supply, and general schedules for water supply projects
• Climate change impacts on the water supply
• Energy intensity
• Current and planned water conservation activities
• A comparison of water supply and water demand over the next 25 years under different hydrological assumptions (normal year, single dry year, multiple dry years).
Some municipalities in Sonoma County receive water deliveries from Sonoma Water and augment these with local supply, such as municipal groundwater supply wells. Most municipalities are currently updating their urban water management plans and water storage contingency plans to estimate projected water demand and water supply to ensure adequate drinking water is available for all users.
Sonoma Water’s 2018 Water Supply Strategies Action Plan, a regional planning document to increase water supply system reliability, reported the following progress on water efficiency goals and new plans for the future (Sonoma Water 2018).
• Groundwater Sustainability Plans are being developed. • Two Stormwater Resource Plans increase the region’s ability to leverage grant funding and prioritize multi-beneficial projects.
• Continued development of forecast-informed reservoir operation strategies • Following a multi-year drought in which water demands were significantly reduced, the region committed to extending the Sonoma Marin Saving Water Partnership memorandum or understanding. • Advanced quantitative precipitation information from radar units that can provide critical information on location, timing, and intensity of precipitation throughout the Bay Area. • Increased coordination with Lake Mendocino water users including Potter Valley Project relicensing activities.
• Climate adaptation planning to identify strategies to address climate risks and vulnerabilities to ensure an increased understanding of water supply reliability impacts.
Other Public Water Systems
In addition to Sonoma Water, municipalities and water districts provide water to customers throughout Sonoma County. About two dozen such systems serve 1,000 customers or more. The City of Santa Rosa serves nearly 180,000 customers. The cities of Petaluma, Rohnert Park, Healdsburg, and Sonoma, and the Town of Windsor, serve between 10,000 and 60,000 customers. Some of the public systems in the county are entirely reliant on 8-7
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements groundwater; others have a mix of sources. Some purchase Russian River water through Sonoma Water or have separate surface water rights.
Small Water Systems
About 70 small water systems (defined as having between 5 to 14 service connections) supply water in Sonoma County, serving campgrounds, small commercial establishments, mobile home parks, isolated rural residences and subdivisions, and small unincorporated communities. Permitting, inspecting, and monitoring are conducted through County Department of Health Services, Division of Environmental Health. The vulnerability of these systems varies, depending on their location, water supply, and available storage and demand (County of Sonoma 2017).
Private Wells
Private wells are vulnerable wherever limited recharge and excessive withdrawals lead to a decline in groundwater levels. Groundwater availability and aquifer conditions vary widely in the county, but shallow wells and wells in upland areas may be particularly vulnerable. Private wells considered most vulnerable to drought are located in the marginal groundwater areas (Class 3 and 4 on Figure 8-3), though groundwater levels can be adversely affected in major (Class 1) groundwater basins as well (County of Sonoma 2017).
Surface Water
Users who are reliant on surface water are most vulnerable to drought. This includes water right holders along the upper Russian River, and small reservoirs throughout the County that are reliant on each season’s rainfall to fill the reservoir. The west Petaluma area has been severely impacted when ponds and reservoirs do not fill.
8.2.2 Past Events
Periods of Drought in California
The following sections describe prolonged droughts in California that have impacted the planning area. 2012 to 2017 Drought
California’s last drought set several records for the state. The period from 2012 to 2014 ranked as the driest three consecutive years for statewide precipitation. Calendar year 2014 set new records for statewide average temperatures and for record-low water allocations from the State Water Project and the federal Central Valley Project. Calendar year 2013 set minimum annual precipitation records for many communities. Detailed executive orders and regulations addressed water conservation and management. The statewide drought emergency was lifted in April 2017. In Sonoma County, the Board of Supervisors proclaimed a local emergency due to drought conditions on February 25, 2014. That proclamation covered all of Sonoma County, including all nine cities and special districts and was continued until March 1, 2016. This proclamation was guided by mandatory state emergency conservation regulations issued to all water providers in California (County of Sonoma 2017). 8-8
8. Drought
Source: Sonoma County 2021b
Figure 8-3. Groundwater Availability in Sonoma County
2007 to 2009 Drought
The state proclaimed a statewide drought emergency on June 4, 2008 after spring 2008 was the driest spring on record, with low snowmelt runoff. On February 27, 2009, the state proclaimed a state of emergency for the entire state as severe drought continued. The largest court-ordered water restriction in state history (at the time) was imposed. In Sonoma County, abnormally dry to extreme drought conditions occurred from April 2007 to January 2012. At the height of drought severity in 2009, Lake Sonoma was 74 percent full. Lake Mendocino was 38 percent full in 2009. After two dry years, and only 7.28 inches of rain—compared to 15.5 inches in 2008—the area was listed by the U.S. Drought Monitor as in a D2 (severe drought) condition (County of Sonoma 2017). 1987 to 1992 Drought
California received precipitation well below average levels for four consecutive years. While the Central Coast was most affected, the Sierra Nevada range in Northern California and the Central Valley counties were also affected. During this drought, only 56 percent of average runoff for the Sacramento Valley was received. In 1991, 8-9
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements the State Water Project sharply decreased deliveries to water suppliers. By February 1991, all 58 counties in California were experiencing drought. Urban areas as well as agricultural areas were impacted. 1976 to 1977 Drought
California had a severe drought due to lack of precipitation in the winters of 1976 and 1977. 1977 was the driest period on record in California at that time,. The cumulative impact led to widespread water shortages and severe water conservation measures statewide. Over $2.6 billion in crop damage was recorded in 31 counties. FEMA declared a drought emergency (Declaration 3023-EM) on January 20, 1977 for 58 California counties. In Sonoma County, the Russian River saw only 6 percent of its normal runoff in 1977. The reduction of flow from this water source significantly impacted communities throughout Sonoma, Marin, and Mendocino counties. The Sonoma Water Board of Directors proclaimed an emergency for the Russian River Water Supply in February 1976 and the County Board of Supervisors proclaimed a local emergency in July 1976. The response required implementation of water conservation measures and construction of emergency wells to augment supplies (County of Sonoma 2017).
Agriculture-Related Drought Disasters
The U.S. Department of Agriculture (USDA) Farm Service Agency provides assistance for agriculture-related losses resulting from drought, flood, fire, freeze, tornadoes, pest infestation, and other natural disasters. The U.S.
Secretary of Agriculture is authorized to designate counties as disaster areas to make emergency loans to producers suffering losses in those counties and in counties that are contiguous to them. Between 2012 and 2020, the period for which data is available, Sonoma County was included in drought-related USDA declarations in 2012, 2013, 2014, 2015, 2016, 2017, and 2020 (USDA Farm Services Agency, 2021).
8.2.3 Location
Drought is a regional phenomenon that has the potential to impact the entire planning area. A drought affects all aspects of the environment and the community simultaneously and has the potential to directly or indirectly impact every person in the planning area as well as adversely affect the local economy.
8.2.4 Frequency
Drought has a high probability of occurrence in the planning area. From January 2000 to May 2020, some part of Sonoma County experienced a USDM rating of D1 or higher in 370 out of 1,065 weeks—slightly more than one out of every three weeks (see Figure 8-4). Sonoma County has also been included in USDA drought disaster declarations seven times since 2012. Historical drought data for the planning area indicate there have been four significant multi-year droughts in the last 40 years (1980 to 2020), amounting to a severe drought every 10 to 11 years on average.
8.2.5 Severity
The severity of any given drought depends on many factors. Driving factors are the amount and timing of precipitation, duration of below average rainfall, and the size and location of the affected area. The longer the duration of the drought and the larger the area impacted, the more severe the potential impacts. 8-10
8. Drought
Source: U.S. Drought Monitor, 2020
100
90 80
70 60
50 40
30 20
10 0
% of County Area in Drought Category, by
Week
D2 D3 D4D1
Figure 8-4. Percent of Sonoma County Affected by Each USDM Rating, 2000 – 2020 U.S. Drought Monitor Ratings
Sonoma County has a history of severe droughts. As shown in Figure 8-4, at least part of the county has experienced extreme (D3) or exceptional (D4) droughts more than once since 2000.
Drought Impact Reporter
The National Drought Mitigation Center developed the Drought Impact Reporter in response to the need for a national drought impact database for the United States. Information comes from a variety of sources: on-line, drought-related news stories and scientific publications, members of the public who visit the website and submit a drought-related impact for their region, members of the media, and staff of government agencies. The database is being populated beginning with the most recent impacts and working backward in time.
The Drought Impact Reporter indicates 149 impacts from drought that specifically affected Sonoma County from January 2010 through December 2019 (Drought Impact Reporter, 2020). Most (88.5 percent) are based on media reports. The following are the reported numbers of impacts by category (some incidents are assigned to more than one impact category):
• Agriculture—43
• Business and Industry—11
• Energy—6
• Fire—18
• Plants and Wildlife—49
• Relief, Response, and Restrictions—70
• Society and Public Health—54
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Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements • Tourism and Recreation—13
• Water Supply and Quality—82
Potential Agricultural Impact
The agricultural industry’s dependency upon water for production and processing makes it vulnerable to drought conditions. For example, the 2014 Sonoma County Crop Report indicated that field crops saw significant damage due to drought. Many of the crops produced one third of their normal yield; volunteer hay yield was 37 percent of the five-year average and grain oats produced just 33 percent. Pasture and rangeland were severely impacted, with yields of 38 percent and 26 percent of normal. The 2019 Sonoma County Crop Report indicated record rainfall in the winter and growth in the value of nursery product due to landscape and lawn replacement efforts. Since this nursery increase was largely of drought-resistant plants, decreased water availability remains an issue of concern.
8.2.6 Warning Time
Predicting drought depends on the ability to forecast precipitation and temperature. Only generalized warning can take place due to the numerous variables that scientists have not pieced together well enough to make accurate and precise predictions.
Determination of when drought begins is based on impacts on water users and assessments of available water supply, including water stored in reservoirs or groundwater basins. Different water agencies have different criteria for defining drought. Some issue drought watch or drought warning announcements.
8.2.7 Secondary Hazards
The secondary hazard most associated with drought is wildfire. A prolonged lack of precipitation dries out vegetation, which becomes increasingly susceptible to ignition as the duration of the drought extends. In addition, lack of sufficient water resources can stress trees and other vegetation, making them more vulnerable to infestation from pests, which in turn, can make them more vulnerable to ignition. Millions of board feet of timber have been lost, and in many cases erosion occurred, which caused serious damage to aquatic life, irrigation, and power production by heavy silting of streams, reservoirs, and rivers.
8.3 EXPOSURE
All people, property and environments in the planning area would be exposed to some degree to the impacts of moderate to extreme drought conditions.
8.4 VULNERABILITY
8.4.1 Population
The entire population of the county is vulnerable to drought events. Drought can affect people’s health and safety, including health problems related to low water flows, poor water quality, or dust. Droughts can also lead to loss of human life. Other possible impacts include recreational risks; effects on air quality; diminished living conditions related to energy, air quality, and hygiene; compromised food and nutrition; and increased incidence of illness and disease (Centers for Disease Control and Prevention, 2012). A secondary, indirect impact from drought is an 8-12
8. Drought
increase in wildfire risk. The vulnerability of the planning area population to the wildfire risk is discussed in Chapter 15.
8.4.2 Property
No structures will be directly affected by drought conditions, though some structures may become vulnerable to wildfires, which are more likely following years of drought. Droughts can have significant impacts on other types of property such as landscaped areas and economically important natural resources. Drought causes the most significant economic impacts on industries that use water or depend on water for their business, most notably agriculture and related sectors (forestry, fisheries, and waterborne activities), power plants (including geothermal power production), and oil refineries. In addition to losses in yields in crop and livestock production, drought is associated with increased insect infestations, plant diseases, and wind erosion. Drought can lead to other losses because so many sectors are affected - losses that include reduced income for farmers and reduced business for retailers and others who provide goods and services to farmers. This leads to unemployment, increased credit risk for financial institutions, capital shortfalls, and loss of tax revenue. Prices for food, energy, and other products may also increase as supplies decrease.
8.4.3 Critical Facilities
Critical facilities as defined for this plan will continue to be operational during a drought. Critical facility features such as landscaping may not be maintained due to limited water resources, but the risk to critical facility core functions is low.
8.4.4 Environment
Groundwater and Streams
Drought generally does not affect groundwater sources as quickly as surface water supplies, but groundwater supplies generally take longer to recover. Reduced precipitation during a drought means that groundwater supplies are not replenished at a normal rate. This can lead to a reduction in groundwater levels and problems such as reduced pumping capacity or wells going dry. Shallow wells are more susceptible than deep wells.
Reduced replenishment of groundwater affects streams, especially during the summer when there is little or no precipitation. Reduced groundwater levels mean that even less water will enter streams when stream flows are lowest. Where stream flows are reduced, development that relies on surface water may seek to establish new groundwater wells, which could further increase groundwater depletion.
Other Potential Losses
Environmental losses from drought are associated with damage to plants, animals, wildlife habitat, and air and water quality; forest and range fires; degradation of landscape quality; loss of biodiversity; and soil erosion. Some of the effects are short-term and conditions quickly return to normal following the end of the drought. Other environmental effects linger for some time or may even become permanent. Although environmental losses are difficult to quantify, growing public awareness and concern for environmental quality has forced public officials to focus greater attention and resources on these effects. The following are potential impacts of drought:
• Wildlife habitat may be degraded through the loss of wetlands, lakes and vegetation. The degradation of landscape quality, including increased soil erosion, may lead to a more permanent loss of biological productivity.
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Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements • Drought conditions greatly increase the likelihood of wildfires, a major threat to timber resources, structures, and other property.
• Water shortages and severe drought conditions would have a significant impact on Native American tribes’ way of life in fishing and farming subsistence. • Scenic resources in the county are vulnerable to the increased likelihood of wildfires associated with droughts.
• Drying up or dying off of forests could reduce ecological and eco-tourist values. • Shortage of water supply can have significant economic impacts. • Drought conditions often are associated with harmful algal blooms—specifically cyanobacteria that can cause severe illness and death in mammals.
8.5 FUTURE TRENDS IN DEVELOPMENT
All municipal planning partners in this effort have established general plans that include policies directing land use. They also have adopted local urban water management plans and water shortage contingency plans dealing with issues of long-term water supply planning and the protection of water resources. These plans provide the capability at the local municipal level to protect future development from the impacts of drought. All planning partners reviewed these plans under the capability assessments performed for this effort. Deficiencies identified by these reviews can be identified as mitigation actions to increase the capability to deal with future trends in development. In addition, water providers in the planning area have plans and programs in place to balance competing needs for water resources within the planning area.
Sonoma Water and its water contractors participate in the Sonoma Marin Water Saving Partnership to develop, fund, and implement water conservation to ensure careful use of water as a precious resource. Measures to promote water conservation and provide incentives for investment in long-term water savings are developed, funded, and implemented.
8.6 SCENARIO
A multi-year drought that impacts the entire west or the State of California is the worst-case scenario for the planning area. In the past, such droughts and the wildfires and floods that followed them have caused extensive damage to natural systems. If another severe drought occurs before these systems have a chance to recover, it could exacerbate the stress already placed on existing planning area water resources.
8.7 ISSUES
The planning team has identified the following drought-related issues:
• The promotion of additional demand management and water conservation efforts even during non- drought periods should be encouraged.
• The planning area should plan for frequent droughts or multi-year droughts that can limit the ability to successfully recover from one drought and prepare for the next—particularly considering the longevity of the 2012 to 2017 drought.
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8. Drought
• Water planning should consider impacts of additional drawn downs on groundwater supplies as pressure on surface water increases during drought.
• Drought in the county will increase and expand fire-prone areas and adversely affect the timber economy. • With the possibility of climate change, drought may become a more pervasive issue due to warming trends and wider fluctuations in precipitation patterns. The probability of drought frequencies and durations may increase.
• Alternative water supplies or increased use of recycled water will need to be identified and developed, as well as alternative strategies to allocate and distribute existing water sources. • Groundwater recharge techniques can be used to increase available water in storage and stabilize the groundwater supply.
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9. EARTHQUAKE
9.1 GENERAL BACKGROUND
An earthquake is the vibration of the earth’s surface following a release of energy in the earth’s crust. This energy can be generated by a sudden dislocation of the crust or by a volcanic eruption. Most destructive quakes are caused by dislocations of the crust. The crust may first bend and then, when the stress exceeds the strength of the rocks, break and snap to a new position. In the process of breaking, vibrations called “seismic waves” are generated. These waves travel outward from the source of the earthquake at varying speeds.
9.1.1 Earthquake Location
The location of an earthquake is commonly described by its focal depth and the geographic position of its epicenter. The focal depth of an earthquake is the depth from the Earth’s surface to the region where an earthquake’s energy originates (the focus or hypocenter). The epicenter of an earthquake is the point on the Earth’s surface directly above the hypocenter.
9.1.2 Earthquake Geology
Tectonic Plates
The Earth’s crust, which is the rigid outermost shell of the planet, is broken into seven or eight major tectonic plates (depending on how they are defined) and many minor plates. Where the plates meet, they move in one of three ways along their mutual boundary: convergent (two plates moving together), divergent (two plates moving apart), or transform (two plates moving parallel to one another). Earthquakes, volcanic activity, mountain- building, and oceanic trench formation occur along plate boundaries. Subduction is a geological process that takes place at convergent boundaries of tectonic plate, in which one plate moves under another. Regions where this process occurs are known as subduction zones, and they have the potential to generate highly damaging earthquakes.
California is seismically active because of movement of the North American Plate, east of the San Andreas Fault, and the Pacific Plate to the west, which includes the state’s coastal communities. The transform (parallel) movement of these tectonic plates against one another creates stresses that build as the rocks are gradually deformed. The rock deformation, or strain, is stored in the rocks as elastic strain energy. When the strength of the rock is exceeded, rupture occurs along a fault. The rocks on opposite sides of the fault slide past each other as they spring back into a more relaxed position. The strain energy is released partly as heat and partly as elastic waves called seismic waves. The passage of these seismic waves produces the ground shaking in earthquakes. 9-1
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Faults
Geologists have found that earthquakes reoccur along faults, which are zones of weakness in the earth’s crust.
When a fault experiences an earthquake, there is no guarantee that all the stress has been relieved. Another earthquake can still occur. In fact, relieving stress along one part of a fault may increase it in another part.
Faults are more likely to have future earthquakes on them if they have more rapid rates of movement, have had recent earthquakes along them, experience greater total displacements, and are aligned so that movement can relieve the accumulating tectonic stresses. Geologists classify faults by their relative hazards. “Active” faults, which represent the highest hazard, are those that have ruptured to the ground surface during the Holocene period (about the last 11,000 years). “Potentially active” faults are those that displaced layers of rock from the Quaternary period (the last 1,800,000 years) (California Department of Conservation, 2003).
Determining if a fault is “active” or “potentially active” depends on geologic evidence, which may not be available for every fault. Most of the seismic hazards are associated with well-known active faults. However, inactive faults or concealed faults (referred to as “blind-thrust” faults), where no displacements have been recorded, also have the potential to reactivate or experience displacement along a branch sometime in the future.
An example of a fault zone that has been reactivated is the Foothills Fault Zone. The zone was considered inactive until evidence of an earthquake (approximately 1.6 million years ago) was found near Spenceville, California.
Then, in 1975, an earthquake occurred on another branch of the zone near Oroville, California (now known as the Cleveland Hills Fault). The State Division of Mines and Geology indicates that increased earthquake activity throughout California may cause tectonic movement along currently inactive fault systems.
9.1.3 Earthquake-Related Hazards
According to the U.S. Geological Survey (USGS) Earthquake Hazards Program, an earthquake hazard is anything associated with an earthquake that may affect resident’s normal activities. This includes the following:
• Surface Faulting—Displacement that reaches the earth’s surface during slip along a fault. Commonly occurs with shallow earthquakes, those with an epicenter less than 20 kilometers. • Ground Motion (shaking)—The movement of the earth’s surface from earthquakes or explosions.
Ground motion or shaking is produced by waves that are generated by sudden slip on a fault or sudden pressure at the explosive source and travel through the earth and along its surface. • Mass Movement—A movement of surface material down a slope. • Liquefaction—A process by which water-saturated sediment temporarily loses strength and acts as a fluid. Earthquake shaking can cause this effect.
• Tectonic Deformation—A change in the original shape of a material due to stress and strain. • Tsunami—A sea wave of local or distant origin that results from large-scale seafloor displacements associated with large earthquakes, major submarine slides, or violent underwater volcanic eruptions.
9.1.4 Earthquake Classifications
Earthquakes are typically classified in one of two ways: By the amount of energy released, measured as magnitude; or by the impact on people and structures, measured as intensity. 9-2
9. Earthquake
Magnitude
An earthquake’s magnitude is a measure of the energy released at the source of the earthquake. Magnitude is commonly expressed by ratings on the moment magnitude scale (Mw), the most common scale used today (USGS, 2017). This scale is based on the total moment release of the earthquake (the product of the distance a fault moved and the force required to move it). The scale is as follows:
• Great—Mw > 8
• Major—Mw = 7.0 – 7.9
• Strong—Mw = 6.0 – 6.9
• Moderate—Mw = 5.0 – 5.9
• Light—Mw = 4.0 – 4.9
• Minor—Mw = 3.0 – 3.9
• Micro—Mw < 3
Intensity
The most used intensity scale is the modified Mercalli intensity scale. Ratings of the scale as well as the perceived shaking and damage potential for structures are shown in Table 9-1. The modified Mercalli intensity scale is generally represented visually using shake maps, which show the expected ground shaking at any given location produced by an earthquake with a specified magnitude and epicenter. An earthquake has only one magnitude and one epicenter, but it produces a range of ground shaking at sites throughout the region, depending on the distance from the earthquake, the rock and soil conditions at sites, and variations in the propagation of seismic waves from the earthquake due to complexities in the structure of the earth’s crust. A shake map shows the variation of ground shaking in a region immediately following significant earthquakes (for technical information about shake maps see USGS, 2018).
Table 9-1. Mercalli Scale and Peak Ground Acceleration Comparison Modified Potential Structure Damage Estimated PGAa
Mercalli Scale Perceived Shaking Resistant Buildings Vulnerable Buildings (%g) I Not Felt None None <0.17%
II-III Weak None None 0.17% - 1.4%
IV Light None None 1.4% - 3.9%
V Moderate Very Light Light 3.9% - 9.2%
VI Strong Light Moderate 9.2% - 18%
VII Very Strong Moderate Moderate/Heavy 18% - 34%
VIII Severe Moderate/Heavy Heavy 34% - 65%
IX Violent Heavy Very Heavy 65% - 124%
X – XII Extreme Very Heavy Very Heavy >124%
a. PGA = peak ground acceleration. Measured in percent of g, where g is the acceleration of gravity Sources: USGS, 2008; USGS, 2010
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Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements
9.1.5 Ground Motion
Earthquake hazard assessment is based on expected ground motion. During an earthquake when the ground is shaking, it also experiences acceleration. The peak acceleration is the largest increase in velocity recorded by a particular station during an earthquake. Estimates are developed of the annual probability that certain ground motion accelerations will be exceeded; the annual probabilities can then be summed over a time period of interest.
The most commonly mapped ground motion parameters are horizontal and vertical peak ground accelerations (PGA) for a given soil type. PGA is a measure of how hard the earth shakes, or accelerates, in a given geographic area. Instruments called accelerographs record levels of ground motion due to earthquakes at stations throughout a region. PGA is measured in g (the acceleration due to gravity) or expressed as a percent acceleration force of gravity (%g). These readings are recorded by state and federal agencies that monitor and predict seismic activity.
Maps of PGA values form the basis of seismic zone maps that are included in building codes such as the International Building Code. Building codes that include seismic provisions specify the horizontal force due to lateral acceleration that a building should be able to withstand during an earthquake. PGA values are directly related to these lateral forces that could damage “short period structures” (e.g. single-family dwellings). Longer period response components determine the lateral forces that damage larger structures with longer natural periods (apartment buildings, factories, high-rises, bridges). Table 9-1 lists damage potential and perceived shaking by PGA factors, compared to the Mercalli scale.
9.1.6 USGS Earthquake Mapping Programs
ShakeMaps
The USGS Earthquake Hazards Program produces maps called ShakeMaps that map ground motion and shaking intensity following significant earthquakes. ShakeMaps focus on the ground shaking caused by the earthquake, rather than on characteristics of the earthquake source, such as magnitude and epicenter. An earthquake has only one magnitude and one epicenter, but it produces a range of ground shaking at sites throughout the region, depending on the distance from the earthquake, the rock and soil conditions at sites, and variations in the propagation of seismic waves from the earthquake due to complexities in the structure of the earth’s crust.
A ShakeMap shows the extent and variation of ground shaking immediately across the surrounding region following significant earthquakes. Such mapping is derived from peak ground motion amplitudes recorded on seismic sensors, with interpolation where data are lacking based on estimated amplitudes. Color-coded instrumental intensity maps are derived from empirical relations between peak ground motions and Modified Mercalli intensity. In addition to the maps of recorded events, the USGS creates the following:
• Scenario ShakeMaps of hypothetical earthquakes of an assumed magnitude on known faults • Probabilistic ShakeMaps, based on predicted shaking from all possible earthquakes over a 10,000-year period. In a probabilistic map, information from millions of scenario maps are combined to make a forecast for the future. The maps indicate the ground motion at any given point that has a given probability of being exceeded in a given timeframe, such as a 100-year (1-percent-annual chance) event.
National Seismic Hazard Map
National maps of earthquake shaking hazards provide information for creating and updating seismic design requirements for building codes, insurance rate structures, earthquake loss studies, retrofit priorities and land use 9-4
9. Earthquake
planning. After thorough review of the studies, professional organizations of engineers update the seismic-risk maps and seismic design requirements contained in building codes (Brown et al., 2001). The USGS updated the National Seismic Hazard Maps in 2018. New seismic, geologic, and geodetic information on earthquake rates and associated ground shaking were incorporated into these revised maps. The 2018 map, shown in Figure 9-1, represents the best available data as determined by the USGS.
Source: (U.S. Geological Survey n.d.)
Figure 9-1. Peak Acceleration (%g) with 2% Probability of Exceedance in 50 Years
9.1.7 Liquefaction and Soil Types
Soil liquefaction occurs when water-saturated sands, silts or gravelly soils are shaken so violently that the individual grains lose contact with one another and float freely in the water, turning the ground into a pudding- like liquid. Building and road foundations lose load-bearing strength and may sink into what was previously solid ground. Unless properly secured, hazardous materials can be released, causing significant damage to the environment and people.
A program called the National Earthquake Hazard Reduction Program (NEHRP) creates maps based on soil characteristics to help identify locations subject to liquefaction. NEHRP soil types define the locations that will be significantly impacted by an earthquake. Table 9-2 summarizes NEHRP soil classifications. NEHRP Soils B and C typically can sustain ground shaking without much effect, dependent on the earthquake magnitude. The areas that are commonly most affected by ground shaking have NEHRP Soils D, E and F (Southern California Earthquake Center, 2018). In general, these areas are also most susceptible to liquefaction. The areas that are most affected by ground shaking have NEHRP Soils D, E and F. 9-5
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Table 9-2. NEHRP Soil Classification System
NEHRP Soil
Type Description
Mean Shear Velocity to 30
m (m/s)
A Hard Rock 1,500
B Firm to Hard Rock 760-1,500
C Dense Soil/Soft Rock 360-760
D Stiff Soil 180-360
E Soft Clays < 180
F Special Study Soils (liquefiable soils, sensitive clays, organic soils, soft clays >36 m thick)
9.2 HAZARD PROFILE
9.2.1 Past Events
The Bay Area has experienced significant, well-documented earthquakes. Since 1855, more than 140 earthquakes have been felt in the Santa Rosa area (County of Sonoma 2017). According to the Northern California Earthquake Data Center, two earthquakes of magnitude 5.0 or greater have been felt in Sonoma County since 2000:
• December 14, 2016—A 5.01 magnitude event near The Geysers • August 24, 2014—A 6.02 magnitude event near South Napa The sections below describe major recorded historical earthquakes that have affected Sonoma County.
Pre-1900 Earthquakes
Seven earthquakes are believed to have caused damage to structures in Sonoma County in the 19th century.
Reported damage from these earthquakes indicates a Modified Mercalli rating of VI to VIII. Notable events were the 1868 magnitude 7.2 earthquake on the Hayward Fault and the 1898 magnitude 6.7 earthquake on the Rodgers Creek Fault. Although damage from these two events was limited due to the area’s sparse population at the time, a recurrence of either of these events would result in significant damage today (County of Sonoma 2017). 1906 San Francisco Earthquake
The April 18, 1906, magnitude 8.3 earthquake on the northern segment of the San Andreas Fault caused major damage in Santa Rosa, Sebastopol, Healdsburg, Petaluma and other communities. Santa Rosa is said to have suffered more damage proportionally to its size than any other Bay Area city. The only reported casualties in Sonoma County were in Santa Rosa, where 65 died. The shaking lasted for about 50 seconds. The Santa Rosa Courthouse was totally destroyed by the shaking and ensuing fire, as were approximately eight blocks of commercial buildings. It was reported that almost all non-wood buildings were destroyed by the shaking alone (County of Sonoma 2017).
1969 Rodgers Creek / Healdsburg Fault Earthquake
The last major earthquakes with an epicenter in Sonoma County occurred on October 1, 1969. Two earthquakes of magnitudes 5.6 and 5.7 originated 2 miles north of Santa Rosa. Damage was concentrated in Santa Rosa and principally confined to the partial collapse of unreinforced masonry buildings and wood frame buildings. In all, 99 structures were significantly damaged, approximately half in the business district and half in residential areas. 9-6
9. Earthquake
Total building damage was estimated at $6 million, with dwelling contents losses at $1.25 million. Several County buildings suffered damage, including the library, post office, and veterans memorial building. There was no loss of life from these earthquakes. The mayor of Santa Rosa sought state and federal disaster assistance, but there was not enough damage to public facilities to warrant a declaration. Small Business Administration loans were made available to commercial and residential property owners (County of Sonoma 2017). 1989 Loma Prieta Earthquake
This magnitude 6.9 earthquake was caused by slip along the San Andreas Fault. Damage in Sonoma County was minor (only five dwellings were yellow tagged), but the quake killed 63 people and injured 3,757 throughout Northern California and caused an estimated $6 billion in property damage. It was the largest earthquake to occur on the San Andreas Fault since the 1906 San Francisco earthquake (County of Sonoma 2017). 2014 South Napa Earthquake
On August 24, 2014, a magnitude 6.0 earthquake shook Napa, Solano, and Sonoma County. The epicenter was 9 miles southeast of the City of Sonoma. The earthquake occurred on the West Napa Fault, a fault that was not mapped under the Alquist-Priolo earthquake fault hazard zone. It was the largest event in the Bay Area since the 1989 Loma Prieta earthquake. At least 12 aftershocks followed. The quake injured 257 people and killed one.
Several structures in eastern Sonoma County were severely damaged. The governor issued an emergency proclamation for this event and a federal disaster was declared on September 11, 2014. The total economic loss was estimated at $400 million (County of Sonoma 2017).
2016 The Geysers Earthquake
A 5.0 magnitude earthquake occurred 4 miles west of The Geysers and 14 miles southwest of Clearlake on December 14, 2016, following a series of medium size earthquakes in Mammoth Lakes and the Central Coast.
This event was primarily felt in the Clearlake and Santa Rosa areas but was also felt throughout the Bay Area.
9.2.2 Location
The Mendocino Triple Junction, in the Pacific Ocean near Cape Mendocino, is the point where the Gorda plate, the North American plate, and the Pacific plate meet. This is the location of a change in the broad plate motions that dominate the west coast of North America, linking the convergence boundary of the Cascadia subduction zone, the transform boundary of the San Andreas Fault system, and the Gorda plate’s subduction under the North American plate and simultaneous converging against the Pacific plate.
Fault Locations
Several major faults traverse Sonoma County. The Alquist-Priolo Earthquake Fault Maps identify the following earthquake faults running through or near the county (see Figure 9-2):
• San Andreas Fault—The San Andreas Fault intersects land in Sonoma County at Doran Beach and Bodega Bay as well as south of Fort Ross, traveling north to the county line just east of the coastline.
Studies of the North Coast section of the San Andreas Fault suggest an average recurrence interval of 200 to 300 years, although studies indicate a long interval between the 1906 earthquake and the previous earthquake, which occurred around 1300. Prior to 1300, the intervals were about 200 years (USGS n.d.). 9-7
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Source: (U.S. Geological Survey 2016)
Figure 9-2. Mapped Faults in Sonoma County
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9. Earthquake
• Rodgers Creek Fault—The Rodgers Creek Fault, which lies east of the San Andreas Fault, is the main strand of the North American-Pacific Plate boundary north of San Francisco Bay. The two sides of the fault slip past each other at a rate of 6 to 10 millimeters per year. It is estimated that there is a 33 percent chance of an earthquake of magnitude 6.7 or greater on the combined Rodgers Creek-Hayward fault system over the 30-year period from 2014 through 2043. In 2018, USGS released a more detailed and higher resolution map of the Rodgers Creek Fault. The new map shows the Rodgers Creek Fault extending about 11 miles farther north than previously thought and flanking the east side of the town of Healdsburg. It also showed an overall increase in the known width and complexity of the fault zone.
These new findings indicate a greater hazard than previously thought (U.S. Geological Survey 2018). • Hayward Fault—The Hayward Fault runs along the foot of the East Bay Hills. Its last major earthquake occurred on October 21, 1868, destroying downtown Hayward, killing 5 people, and injuring 30. It had an estimated magnitude of 6.8 and was considered the “Great Earthquake” until 1906. Scientists have found that the most recent five major earthquakes on this fault happened on average every 140 years. It is very likely that the Hayward fault will rupture and produce a significant earthquake within the next 30 years (Berkeley Seismology Lab 2018).
• Maacama Fault—The Maacama Fault passes east of the City of Cloverdale. The Maacama fault zone is one of three major fault zones that make up the San Andreas fault system in northern California. The fault is creeping near the town of Willits. Preliminary studies indicate that over the last 700 years, only fault creep has occurred at the site; however, over the last 3,500 years, slip has been accommodate by both creep and earthquakes with surface rupture. The preliminary minimum long-term slip rate at Haehl Creek on the Maacama fault of greater than or equal to 8 millimeters per year is consistent with rates found on the Hayward and Rodgers Creek segments of this fault system and is consistent with the notion that the fault zone is capable of producing large earthquakes (Larsen 2005).
Faults outside the planning area also can impact its people, property, and economy. A rupture in the Cascadia subduction zone, for example, would have considerable impacts on the planning area (Pacific Northwest Seismic Network, 2018). This is the 600-mile-long offshore zone, from northern Vancouver Island to Cape Mendocino, where the Juan de Fuca plate is being subducted below the North American plate.
NEHRP Soil Type and Liquefaction Mapping
Figure 9-3 shows NEHRP soil classifications in Sonoma County. Figure 9-4 shows areas in that have moderate, high, or very high susceptibility to liquefaction.
9.2.3 Frequency
Historic records of earthquake occurrences may give some indication of future probabilities. Seismic activity was more frequent from 1830 to 1930 than it has been since. This leads some scientists to suspect that pressure is building up along the faults in the Bay Area that can result in a large quake. Such a quake could have dramatic and devastating effects throughout the Bay Area. The USGS reports the following earthquake probabilities for the Bay Area over next 30 years (U.S. Geological Survey n.d.):
• 72 percent probability of an earthquake measuring magnitude 6.7 • 51 percent probability of an earthquake measuring magnitude 7 • 20 percent probability of an earthquake measuring magnitude 7.5 9-9
Cotati
Sonoma
Windsor
Healdsburg
Sebastopol
Rohnert Park
Santa
Rosa
Cloverdale
Petaluma ¬«12
¬«116
¬«
116
¬«
37 ¬«
121
¬«
12 ¬«
128
¬«1
¬«1
£¤101
£¤
101
Figure 9-3. NEHRP Soil Class
±
0 10 5
Miles
Data Sources: Sonoma Co,
CGS
C (Dense soil/soft rock)
D (Stiff soil)
E (Soft clay)
Cities
County Boundary
Highways
Cloverdale
¬128«
£101¤
Healdsburg
¬1«
Windsor
¬116«
Santa ¬12«Rosa
Sebastopol
¬1 Rohnert Park «
£
Cotati
101 Sonoma ¤
Petaluma ¬«12
¬116 ¬«121 «
¬37«
Figure 9-4. Liquefaction Susceptibility
Water Very High
±Very Low Cities 0 5 10
Miles
Low County Boundary
Data Sources: Sonoma Co.,
Moderate Highways USGS
High
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements
9.2.4 Severity
The severity of an earthquake can be expressed in terms of intensity or magnitude (see Section 0). The State of California Department of Conservation probabilistic ground shaking maps, based on current information about fault zones, show the PGA that has a certain probability of being exceeded in a 50-year period. Sonoma County is in a high-risk area, with a 10-percent probability in a 50-year period of ground shaking from a seismic event exceeding 40 to 60 percent of gravity in most parts of the county. Figure 9-5 shows the expected peak horizontal ground accelerations for this probability.
Source: (U.S. Geological Survey n.d.)
Figure 9-5. Peak Horizontal Acceleration (%g) with 10% Probability of Exceedance in 50 Years
9.2.5 Warning Time
There is no current reliable way to predict the day or month that an earthquake will occur at any given location.
Research is being done with warning systems that detect the lower energy compressional waves (P waves) that precede the secondary waves (S waves) experienced as an earthquake. Earthquake early warning systems may provide a few seconds’ or a few minutes’ notice that a major earthquake is about to occur. The warning time is very short, but it could allow for someone to get under a desk, pause hazardous or high-risk work, or initiate protective automated systems in structures or critical infrastructure.
9.2.6 Secondary Hazards
Earthquakes can cause landslides, often as a result of loss of cohesion in clay-rich soils. Earthen dams and levees are highly susceptible to seismic events, and the impacts of their eventual failures can be considered secondary risk exposure to earthquakes. Depending on the location, earthquakes can also trigger tsunamis. Additionally, fires can result from gas lines or power lines that are broken or downed during the earthquake. It may be difficult to control a fire, particularly if the water lines feeding fire hydrants are also broken. 9-12
9. Earthquake
9.3 EXPOSURE
9.3.1 Population
The entire population of the planning area (490,000) is potentially exposed to direct damage from earthquakes or indirect impacts such as business interruption, road closures, and loss of function of utilities.
Socially vulnerable populations living on NEHRP D or E soils were estimated based on data for the Census- defined blocks that lie at least partially within the mapped soil zones. Because many of those Census blocks extend outside the define soils zones, the estimates are greater than the actual exposed populations, but they provide reasonable relative data for use in mitigation planning. Figure 9-6 summarizes the estimated exposure of socially vulnerable populations.
Under Age
16 87,827
(22.1% of
Total)
Over Age 65
54,976
(13.8% of
Total)
Total
398,261
0 50,000
100,000
150,000
200,000
250,000
300,000
350,000
400,000 Number of People in Hazard Area
Low Income
89,645
(58.9% of
Total)
Total
152,245
0 20,000
40,000
60,000
80,000
100,000
120,000
140,000
160,000 Number of Households in Hazard Area
See Section 4.8.1 for the definition of “low income” used in this analysis Figure 9-6. Socially Vulnerable Populations Living on NEHRP D or E Soils Census Blocks
9.3.2 Property
According to County Assessor records, there are 173,000 buildings in the planning area, most of them residential.
All buildings are considered to be exposed to the earthquake hazard.
9.3.3 Critical Facilities
Since the entire planning area has exposure to the earthquake hazard, all critical facilities components are considered to be exposed. The breakdown of the numbers and types of facilities is presented in Table 4-4. Critical 9-13
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements facilities constructed on NEHRP Type D and E soils are particularly at risk from seismic events. Figure 9-7 shows the number of critical facilities built on these soils in the planning area, by type of facility.
Facility Category
253 On NEHRP Type D & E Soils Transportation
385
Planning Area Total
407
Safety & Security
650
143
Health & Medical
219
1,697
Hazardous Material
3,109
68 Food, Water, Shelter
131
28 Energy
74 99
Communications
191
0 500 1,000 1,500 2,000 2,500 3,000
Number of Facilities in Identified Area
3,500
Figure 9-7. Critical Facilities Constructed on NEHRP Type D and E Soils, and Countywide Significant facilities included in the mapped areas of NEHRP Type D and E soils include the following:
• 2 water treatment facilities • 43 fire stations • 230 road bridges • 20 wastewater treatment facilities • 8 police stations • 10 port facilities • 1,697 hazardous material sites • 206 school buildings • 3 airports • 4 hospitals
9-14
-
9. Earthquake
9.3.4 Environment
The entire planning area is exposed to the earthquake hazard, including all natural resources, habitat, and wildlife.
9.4 VULNERABILITY
Earthquake vulnerability data for the risk assessment was generated using a Hazus Level 2 (user-defined) analysis for the for the events listed in Table 9-3. The countywide analysis results are summarized in the sections below.
Detailed information, broken down by municipality, can be found in Appendix D.
Table 9-3. Earthquakes Modeled for Risk Assessment
Event Magnitude Epicenter Location PGA
100-Year Probabilistic Earthquake N/A N/A Figure 9-8
Hayward Fault Scenario 7.57 16 miles southeast of Petaluma Figure 9-9 Maacama Fault Scenario 7.55 26 miles north-northwest of Cloverdale Figure 9-10 Rodgers Creek – Healdsburg Fault Scenario 7.19 3 miles north-northeast of Santa Rosa Figure 9-11 San Andreas Fault Scenario 8.04 16 miles west of Sebastopol Figure 9-12
9.4.1 Population
Hazus estimated impacts on persons and households in the planning area for the selected earthquake scenarios as summarized in Table 9-4.
Table 9-4. Estimated Earthquake Impact on Persons
Scenario Displaced Households Persons Requiring Short Term Shelter 100-Year Probabilistic Earthquake 874 558
Hayward Fault Scenario 5,020 3,250
Maacama Fault Scenario 1,663 1,072
Rodgers Creek – Healdsburg Fault Scenario 3,792 2,466
San Andreas Fault Scenario 584 370
9.4.2 Property
Building Age
Table 9-5 identifies significant milestones in building and seismic code requirements that directly affect the structural integrity of development. Using U.S. Census estimates of housing stock age, estimates were developed of the number of housing units constructed before each of these dates. About a quarter of the planning area’s housing units were constructed after the Uniform Building Code was amended in 1994 to include seismic safety provisions. Housing units built before 1933 when there were no building permits, inspections, or seismic standards, account for only about 3 percent.
Loss Potential
Table 9-6 summarizes Hazus estimates of earthquake damage in the planning area for the evaluated scenarios.
The debris estimate includes only structural debris; it does not include additional debris that may accumulate, such as from trees. In addition, these estimates do not include losses that would occur from any local tsunamis or fires stemming from an earthquake.
9-15
Cloverdale
128 ¬«
101 £¤
1¬«
Healdsburg
Windsor
116 ¬«
1¬«
Sebastopol
Sant 12 ¬«Rosa
a
Rohnert Park
Cotati
101 £¤
Petaluma
116 ¬«
Sonoma
12 ¬«
121 ¬«
37 ¬«
Figure 9-8. 100-Year Probabilistic Earthquake
Mercalli Intensity Scale Cities
±VI (Strong/Light) County Boundary 0 5 10
Miles
VII (Very Strong/Moderate) Highways Data Sources: Sonoma Co., Intensity scale described as: USGS
(perceived shaking / potential damage)
Cloverdale
128 ¬«
101 £¤
1¬«
Healdsburg
Windsor
116 ¬«
1¬«
Sebastopol
Sant 12 ¬«Rosa
a
Rohnert Park
Cotati
101 £¤
Petaluma
116 ¬«
Sonoma
12 ¬«
121 ¬«
37 ¬«
Figure 9-9. Hayward M7.57 Earthquake Scenario
Mercalli Intensity Scale Cities
VI (Strong/Light) County Boundary ±
0 5 10
VII (Very Strong/Moderate) Highways Miles
Data Sources: Sonoma Co., VIII (Severe/Moderate-Heavy)
Intensity scale described as:
USGS
(perceived shaking / potential damage)
Cloverdale
128 ¬«
101 £¤
1¬«
Healdsburg
Windsor
116 ¬«
1¬«
Sebastopol
Sant 12 ¬«Rosa
a
Rohnert Park
Cotati
101 £¤
Petaluma
116 ¬«
Sonoma
12 ¬«
121 ¬«
37 ¬«
Figure 9-10. Maacama M7.55 Earthquake Scenario
Mercalli Intensity Scale Cities
V (Moderate/Very Light) County Boundary ±
0 5 10
VI (Strong/Light) Highways Miles
Data Sources: Sonoma Co., VII (Very Strong/Moderate) USGS VIII (Severe/Moderate-Heavy)
Intensity scale described as: (perceived shaking / potential damage)
1
I
I I
C l o v e r d a l e
1 2 8¬«
1 0 1£¤
1¬«
H e a l d s b u r g
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1¬«
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S a n t a 1 2¬« R o s a
R o h n e r t P a r k
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1 0 1£¤
P e t a l u m a
1 1 6¬«
S o n o m a
1 2¬«
1 2 1¬«
3 7¬«
F i g u r e 9 - 1 . R o d g e r s C r e e k H e a l d s b u r g M 7 . 1 9 M e r c a l l i I n t e n s i t y S c a l e C i t i e s V ( M o d e r a t e / V e r y L i g h t ) C o u n t y B o u n d a r y ± 0 5 10
V I ( S t r o n g / L i g h t ) H i g h w a y s Miles D a t a S o u r c e s : S o n o m a C o . , V I ( V e r y S t r o n g / M o d e r a t e ) U S G S V I ( S e v e r e / M o d e r a t e - H e a v y ) I n t e n s i t y s c a l e d e s c r i b e d a s : ( p e r c e i v e d s h a k i n g / p o t e n t i a l d a m a g e )
Cloverdale
128 ¬«
101 £¤
1¬«
Healdsburg
Windsor
116 ¬«
1¬«
Sebastopol
Sant 12 ¬«Rosa
a
Rohnert Park
Cotati
101 £¤
Petaluma
116 ¬«
Sonoma
12 ¬«
121 ¬«
37 ¬«
Figure 9-12. San Andreas M8.04 Earthquake Scenario
Mercalli Intensity Scale Cities
±VI (Strong/Light) County Boundary 0 5 10
Miles
VII (Very Strong/Moderate) Highways Data Sources: Sonoma Co., VIII (Severe/Moderate-Heavy) USGS
Intensity scale described as:
(perceived shaking / potential damage)
9. Earthquake
Table 9-5. Age of Housing Units in Planning Area
Time Period
Number of Current
Planning Area Housing
Units Built in Period
% of Total
Housing Units Significance of Time Frame
Pre-1933 5,640 3.3% Before 1933, there were no explicit earthquake requirements in building codes. State law did not require local governments to have building officials or issue building permits.
1933-1940 2,980 1.7% In 1940, the first strong motion recording was made. 1941-1960 21,917 12.6% In 1960, the Structural Engineers Association of California published guidelines on recommended earthquake provisions.
1961-1975 35,516 20.5% In 1975, significant improvements were made to lateral force requirements.
1976-1994 66,594 38.4% In 1994, the Uniform Building Code was amended to include provisions for seismic safety.
1995 – present 40,837 23.5% Seismic code is currently enforced.
Total 173,484 100.0%
Note: Number and percent estimates are approximation as housing unit age information does not correspond directly with the time periods indicated. In addition, there are significant margins of error associated with the Census estimates.
Table 9-6. Estimated Impact of Earthquake Scenario Events in the Planning Area Structure Debris Structure + Contents Damage
Earthquake Scenario Event Tons Truckloads Value % of Total Value 100-Year Probabilistic Earthquake 836,820 33,473 $14,392,110,613 6.6% Hayward Fault Scenario 4,117,150 164,686 $31,275,029,312 14.3% Maacama Fault Scenario 1,862,920 74,517 $18,535,479,687 8.5% Rodgers Creek – Healdsburg Fault Scenario 2,592,600 103,704 $24,448,538,642 11.2% San Andreas Fault Scenario 995,710 39,828 $14,504,660,400 6.6%
9.4.3 Critical Facilities
Level of Damage
Hazus classifies the vulnerability of critical facilities to earthquake as no damage, slight damage, moderate damage, extensive damage, or complete damage. Hazus was used to assign a category to each critical facility in the planning area for the assessed earthquake scenarios. Summary results are shown in Figure 9-13 through Figure 9-17.
Time to Restore Critical Facilities to Functionality
Hazus estimates the time to restore critical facilities to fully functional use. Results are presented as probability of being functional at specified time increments: 1, 3, 7, 14, 30 and 90 days after the event. For example, Hazus may estimate that a facility has 5 percent chance of being fully functional at Day 3, and a 95 percent chance of being fully functional at Day 90. The analysis of critical facilities in the planning area was performed for the assessed earthquake scenarios. The results are summarized in Figure 9-18 through Figure 9-22. These figures show the average functionality for all critical facilities in each category. 9-21
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Probability of Experiencing Damage Level
Shown, Average of All Facilities in Category (%)
100%
Safety and Security 90%
Food, Water and Shelter
80% Medical and Health
Energy 70% Communications
60% Transportation
Hazardous Materials
50%
40%
30%
20%
10%
0%
Slight Damage or Greater Moderate Damage or Greater Extensive or Complete Damage Probability of Experiencing Damage Level
Shown, Average of All Facilities in Category (%)
Figure 9-13. Critical Facility Damage Potential, 100-Year Probabilistic Earthquake 100%
Safety and Security
Food, Water and Shelter
Medical and Health
90%
80% Energy
Communications 70%
Transportation
60% Hazardous Materials
50%
40%
30%
20%
10%
0%
Slight Damage or Greater Moderate Damage or Greater Extensive or Complete Damage Figure 9-14. Critical Facility Damage Potential, Hayward Fault Scenario 9-22
9. Earthquake
Probability of Experiencing Damage Level
Shown, Average of All Facilities in Category (%)
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
Safety and Security
Food, Water and Shelter
Medical and Health
Energy
Communications
Transportation
Hazardous Materials
Slight Damage or Greater Moderate Damage or Greater Extensive or Complete Damage Figure 9-15. Critical Facility Damage Potential, Maacama Fault Scenario Probability of Experiencing Damage Level
Shown, Average of All Facilities in Category (%)
100%
Safety and Security 90%
Food, Water and Shelter
80% Medical and Health
Energy 70% Communications
60% Transportation
Hazardous Materials
50%
40%
30%
20%
10%
0%
Slight Damage or Greater Moderate Damage or Greater Extensive or Complete Damage Figure 9-16. Critical Facility Damage Potential, Rodgers Creek – Healdsburg Fault Scenario 9-23
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Probability of Experiencing Damage Level
Shown, Average of All Facilities in Category (%)
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
Safety and Security
Food, Water and Shelter
Medical and Health
Energy
Communications
Transportation
Hazardous Materials
Slight Damage or Greater Moderate Damage or Greater Extensive or Complete Damage Figure 9-17. Critical Facility Damage Potential, San Andreas Scenario Probability of Being Fully functional on Day
Shown, Average of All Facilities in Category (%)
100
90 80
70 60
50 40
30 20
10 0
Safety and Security Food, Water and Shelter
Medical and Health Energy
Communications Transportation
Hazardous Materials
at Day 1 at Day 3 at Day 7 at Day 14 at Day 30 at Day 90 Figure 9-18. Critical Facility Functionality, 100-Year Probabilistic Earthquake 9-24
9. Earthquake
Probability of Being Fully functional on Day
Shown, Average of All Facilities in Category (%)
100
90 80
70 60
50 40
30 20
10 0
Safety and Security Food, Water and Shelter
Medical and Health Energy
Communications Transportation
Hazardous Materials
at Day 1 at Day 3 at Day 7 at Day 14 at Day 30 at Day 90 Figure 9-19. Critical Facility Functionality, Hayward Fault Scenario Probability of Being Fully functional on Day
Shown, Average of All Facilities in Category (%)
100
90 80
70 60
50 40
30 20
10 0
Safety and Security Food, Water and Shelter
Medical and Health Energy
Communications Transportation
Hazardous Materials
at Day 1 at Day 3 at Day 7 at Day 14 at Day 30 at Day 90 Figure 9-20. Critical Facility Functionality, Maacama Fault Scenario 9-25
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Probability of Being Fully functional on Day
Shown, Average of All Facilities in Category (%)
100
90 80
70 60
50 40
30 20
10 0
Safety and Security Food, Water and Shelter
Medical and Health Energy
Communications Transportation
Hazardous Materials
at Day 1 at Day 3 at Day 7 at Day 14 at Day 30 at Day 90 Figure 9-21. Critical Facility Functionality, Rodgers Creek – Healdsburg Fault Scenario Probability of Being Fully functional on Day
Shown, Average of All Facilities in Category (%)
100
90 80
70 60
50 40
30 20
10 0
Safety and Security Food, Water and Shelter
Medical and Health Energy
Communications Transportation
Hazardous Materials
at Day 1 at Day 3 at Day 7 at Day 14 at Day 30 at Day 90 Figure 9-22. Critical Facility Functionality, San Andreas Fault Scenario 9-26
9. Earthquake
9.4.4 Environment
Environmental problems as a result of an earthquake can be numerous. Secondary hazards will likely have some of the most damaging effects on the environment. Earthquake-induced landslides can significantly damage surrounding habitat. It is also possible for streams to be rerouted after an earthquake. Rerouting can change the water quality, possibly damaging habitat and feeding areas. Streams fed by groundwater wells can dry up because of changes in underlying geology.
9.5 FUTURE TRENDS IN DEVELOPMENT
As populations grow, it is critical that the services supporting these communities—such as water, sewer, power, roads, hospitals, and public safety agencies—are able to maintain or quickly resume functionality after a disaster.
Land use in the planning area will be directed by general plans adopted under California’s General Planning Law.
The safety elements of the general plans establish standards and plans for the protection of the community from hazards, including seismic hazards. The information in this plan provides a tool to ensure that there is no increase in exposure in areas of high seismic risk. Development in the planning area will be regulated through building standards and performance measures so that the degree of risk will be reduced. Geologic hazard areas are heavily regulated under California’s General Planning Law. The International Building Code establishes provisions to address seismic risk.
9.6 SCENARIO
Based on history and geology, the planning area will be frequently impacted by earthquakes. The worst-case scenario is a higher-magnitude event (7.5 or higher) with an epicenter within 50 miles of the county. Earthquakes of this magnitude or higher could lead to massive structural failure of property on soils prone to liquefaction.
Building and road foundations would lose load-bearing strength. Injuries could occur from debris, such as parapets and chimneys that could topple or be shaken loose and fall on those walking or driving below. Levees and revetments built on these poor soils would likely fail, representing a loss of critical infrastructure. An earthquake event of this magnitude located off the coast could cause a significant local tsunami that would further damage structures and jeopardize lives. An earthquake may also cause minor landslides along unstable slopes, which put at risk major roads and highways that act as sole evacuation routes. This would be even more likely if the earthquake occurred during the winter or early spring.
9.7 ISSUES
Important issues associated with an earthquake include the following:
• A large percentage of the planning area is located on NEHRP D soils, which are prone to liquefaction.
Structures on these soils may experience significant structural damage. • It is estimated more than a third of the planning area’s building stock was built prior to 1975, when seismic provisions became uniformly applied through building code applications. Many structures may need seismic retrofits in order to withstand a moderate earthquake. Residential retrofit programs, such as Earthquake Brace+Bolt, may be able to assist in the costs of these efforts. • Due to limitations in current modeling abilities, the risk to critical facilities in the planning area from the earthquake hazard is likely understated. A more thorough review of the age of critical facilities, codes they were built to, and location on liquefiable soils should be conducted. 9-27
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements • Damage to transportation systems in the planning area after an earthquake has the potential to significantly disrupt response and recovery efforts and lead to isolation of populations. • Earthquakes can cause fires in wooden homes and collapse of essential buildings such as fire stations. • Landslides and tsunamis are major secondary hazards that could have a widespread effect on the county. • Citizens are expected to be self-sufficient up to two weeks after a major earthquake without government response agencies, utilities, private-sector services, and infrastructure components. Education programs are currently in place to facilitate development of individual, family, neighborhood, and business earthquake preparedness. It takes individuals, families, and communities working in concert with one another to be prepared for disaster.
• After a major seismic event, the planning area is likely to experience disruptions in the flow of goods and services resulting from the destruction of major transportation infrastructure across the broader region. • A seismic event can damage communication systems, complicating efforts to coordinate response to the event.
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10. FLOODING
10.1 GENERAL BACKGROUND
10.1.1 Types of Floodplains in the Planning Area
A floodplain is the area adjacent to a river, creek, lake, or the ocean that becomes inundated during a flood. In general, there are two types of floodplains in the planning area: riverine and coastal.
Riverine Floodplains
Riverine floodplains may be broad, as when a river crosses an extensive flat landscape, or narrow, as when a river is confined in a canyon.
When floodwaters recede after a flood event, they leave behind layers of rock and mud. These gradually build up to create a new floor of the floodplain. Floodplains generally contain unconsolidated sediments (accumulations of sand, gravel, loam, silt, and/or clay), often extending below the bed of the stream. These sediments provide a natural filtering system, with water percolating back into the ground and replenishing groundwater. These are often important aquifers, the water drawn from them being filtered compared to the water in the stream. Fertile, flat reclaimed floodplain lands are commonly used for agriculture, commerce, and residential development.
Connections between a river and its floodplain are most apparent during and after major flood events. These areas form a complex physical and biological system that not only supports a variety of natural resources but also provides natural flood and erosion control. When a river is separated from its floodplain with levees and other flood control facilities, natural, built-in benefits can be lost, altered, or significantly reduced.
The frequency and severity of flooding for river systems are based on discharge probability. The discharge probability is the probability that a certain river discharge (flow) level will be equaled or exceeded in a given year.
Flood studies use historical records to determine the probability of occurrence for different discharge levels and storm surge levels. These measurements reflect statistical averages only; it is possible for multiple floods with a low probability of occurrence (such as a 1-percent-annual-chance flood) to occur in a short time period. For riverine flooding, the same flood event can have flows at different points on a river that correspond to different probabilities of occurrence.
Coastal Floodplains
Coastal floodplains are adjacent to the ocean and other tidally influenced areas. Like riverine floodplains, coastal floodplains may be broad or narrow, depending on local topography and natural flood defenses such as dune systems or tidal wetlands. Coastal floods are usually caused by coastal storms that, when combined with normal 10-1
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements tides, push water toward the shore. This is commonly referred to as storm surge. The result can be waves that extend further inland, causing damage to development that would not normally be subject to wave action.
10.1.2 FEMA Regulatory Flood Zones
The extent of flooding associated with a 1-percent annual probability of occurrence (also called the base flood) is used as the regulatory boundary by many agencies. Also referred to as the special flood hazard area (SFHA), this boundary is a convenient tool for assessing vulnerability and risk in flood-prone communities. Many communities have maps that show the extent and likely depth of flooding for the base flood. Corresponding water-surface elevations describe the elevation of water that will result from a given discharge level, which is one of the most important factors used in estimating flood damage.
FEMA defines flood hazard areas as areas expected to be inundated by a flood of a given magnitude. These areas are determined via statistical analyses of records of river flow, storm tides, and rainfall; information obtained through consultation with the community; floodplain topographic surveys; and hydrologic and hydraulic analyses.
Flood hazard areas are delineated on DFIRMs (Digital Flood Insurance Rate Maps), which provide the following information:
• Locations of specific properties in relation to special flood hazard areas • Base flood elevations (1-percent-annual-chance) at specific sites • Magnitudes of flood in specific areas
• Undeveloped coastal barriers where flood insurance is not available • Regulatory floodways and floodplain boundaries (1-percent and 0.2-percent-annual-chance floodplains).
Land covered by floodwaters of the base flood is the special flood hazard area on a DFIRM—an area where NFIP floodplain management regulations must be enforced, and where mandatory purchase of flood insurance applies.
This regulatory boundary is a convenient tool for assessing vulnerability and risk in flood-prone communities, because many communities have maps showing the extent of the base flood and likely depths that will occur.
The base flood elevation (the water elevation of a flood that has a 1-percent chance of occurring in any given year) is one of the most important factors in estimating potential damage from flooding. A structure within a 1-percent-annual-chance floodplain has a 26-percent chance of undergoing flood damage during the term of a 30-year mortgage. The 1-percent-annual-chance flood is used by the NFIP as the basis for insurance requirements nationwide. DFIRMs also depict 0.2-percent-annual-chance flood designations.
DFIRMs and other flood hazard information can be used to identify the expected spatial extent of flooding from a 1-percent and 0.2-percent annual chance event. They depict the following SFHAs and other areas:
• Zone A (Also known as Unnumbered A-zones)—SFHAs where no base flood elevations or depths are shown because detailed hydraulic analyses have not been performed. • Zones A1-30 and AE—SFHAs that are subject to inundation by the base flood, determined using detailed hydraulic analysis. Base flood elevations are shown within these zones. • Zone AH—SFHAs that are subject to shallow flooding, usually in the form of a pond, with an average depth ranging from 1 to 3 feet. Base flood elevations derived from detailed analyses are shown at selected intervals within these zones.
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10. Flooding
• Zone AO—SFHAs subject to inundation by types of shallow flooding where average depths are between 1 and 3 feet. These are normally areas prone to shallow sheet flow flooding on sloping terrain. • Zone AR—Areas with a temporarily increased flood risk due to the building or restoration of flood control system (such as a levee or a dam). Mandatory flood insurance purchase requirements apply, but rates do not exceed the rates for unnumbered A zones if the structure is built or restored in compliance with Zone AR floodplain management regulations.
• Zone A99—Areas with a 1 percent annual chance of flooding that will be protected by a federal flood control system where construction has reached specified legal requirements. No depths or base flood elevations are shown within these zones.
• Zone VE, V1-30—SFHAs along coasts that are subject to inundation by the base flood with additional hazards due to waves with heights of 3 feet or greater. Base flood elevations derived from detailed hydraulic analysis are shown within these zones.
• Zone B and X (shaded)—Zones where the land elevation as been determined to be above the base flood elevation, but below the 500-year flood elevation. These zones are not SFHAs. • Zones C and X (unshaded)—Zones where the land elevation has been determined to be above both the base flood elevation and the 500-year flood elevation. These zones are not SFHAs.
The FEMA designated floodway is the channel of a water course and portion of the adjacent floodplain that is needed to convey the base flood without increasing flood levels by more than a specified amount (typically, 1 foot). A floodway may be designated within the SFHA where the deepest, highest velocity flow is expected and any infrastructure will be at risk. Floodways should be kept free of obstructions and development to allow floodwaters to move downstream unobstructed. Any development in a floodway is subject to severe damage and high risks for occupants and emergency responders.
Flood damage may occur outside of SFHAs. FEMA typically does not designate SFHAs for areas subject to flooding from local drainage problems, particularly in urban areas; drainage basins of less than 1 square mile in area; or hillside areas subject to runoff, erosion, and mudflow. FEMA does not map flooding along the length of all streams or in areas that are undeveloped.
According to FEMA, the coastal high hazard area (or “V zone,” where V stands for velocity wave action) is the most hazardous part of the coastal floodplain, due to its exposure to wave effects. The V zone has an increased degree of flood risk compared to coastal flood areas not within the coastal high hazard area (A zones), and is subject to more stringent regulatory requirements. Figure 10-1 is a typical transect illustrating the coastal V and A zones and the effects of energy dissipation and regeneration of a wave as it moves inland. Wave elevations are decreased by obstructions such as buildings, vegetation, and rising ground surface.
10.1.3 Floodplain Ecosystems and Beneficial Functions
Floodplains can support ecosystems that are rich in plant and animal species. Wetting of the floodplain soil releases a surge of nutrients left over from the last flood or caused by the rapid decomposition of organic matter accumulated since then. Microscopic organisms thrive, and larger species enter a rapid breeding cycle.
Opportunistic feeders (particularly birds) move in to take advantage. The production of nutrients falls away quickly, but the surge of new growth endures. Species growing in floodplains are markedly different from those that grow outside floodplains. For instance, trees that grow in floodplains tend to be very tolerant of root disturbance and very quick-growing compared to non-riparian trees. 10-3
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Figure 10-1. Typical Transect Schematic
Floodplains have many natural beneficial functions, and disruption of them can have long-term consequences for entire regions. Some well-known, water-related functions of floodplains (noted by FEMA) include:
• Natural flood and erosion control • Filter nutrients and impurities from runoff • Provide flood storage and conveyance • Process organic wastes • Reduce flood velocities • Moderate temperatures of water • Reduce flood peaks • Provide groundwater recharge
• Reduce sedimentation • Promote infiltration and aquifer recharge • Surface water quality maintenance • Reduce frequency and duration of low surface flows Areas in the floodplain that typically provide these natural functions are wetlands, riparian areas, sensitive areas, and habitats for rare and endangered species.
10.1.4 Effects of Human Activities
Because they border water bodies, floodplains have historically been popular sites to establish settlements.
Human activities tend to concentrate in floodplains for a number of reasons: water is readily available; riverine floodplain land is fertile and suitable for farming; transportation by water is easily accessible; land is flatter and easier to develop; and there is value placed in ocean views. But human activity in floodplains frequently interferes with the natural function of floodplains. It can affect the distribution and timing of drainage, thereby increasing flood problems. Human development can create local flooding problems by altering or confining drainage channels or causing erosion of natural flood protection systems such as dunes. Flood potential can be increased in several ways: reducing a stream’s capacity to contain flows; increasing flow rates or velocities downstream; and allowing waves to extend further inland. Human activities can interface effectively with a floodplain as long as steps are taken to mitigate the activities’ adverse impacts on floodplain functions. 10-4
10. Flooding
10.2 HAZARD PROFILE
10.2.1 Federal Flood Program Participation
National Flood Insurance Program
The most vulnerable structures in the planning area are those that are not constructed to standards to withstand the impacts of a flood. Such structures may have been built before flood damage prevention regulations were in effect or may not be subject to flood-related building codes because they are outside mapped flood hazard areas.
Table 10-1 summarizes planning area participation in the National Flood Insurance Program (NFIP). The average flood insurance claim paid out in the planning area since participation in NFIP began is $38,200.
Initial FIRM
Table 10-1.
# of Flood
Insurance Policies
Flood Insurance Statistics
Insurance In
Total Written
Premium + Federal Claims through 03/31/2021
Jurisdiction Effective Date as of 03/31/2021 Force Policy Fee Number Value Cloverdale 09/27/1985 35 $9,048,600 $8,766 9 $327,240
Cotati 04/15/1980 72 $22,529,500 $7,412 0 $2,275
Healdsburg 03/04/1980 115 $39,433,000 $11,673 42 $759,335 Petaluma 02/15/1980 362 $128,484,100 $35,705 280 $8,643,392 Rohnert Park 06/01/1981 46 $15,456,500 $9,095 5 $3,639
Santa Rosa 08/03/1981 145 $47,368,200 $12,478 21 $465,602 Sebastopol 06/18/1980 37 $15,563,600 $420 21 $1,602,797 City of Sonoma 01/17/1979 79 $21,724,100 $8,679 28 $683,642 Sonoma County 01/20/1982 2,183 $557,908,100 $804,699 2,986 $116,859,702 Town of Windsor 12/02/2008 62 $19,496,400 $10,270 0 $45,252 Total 3,136 $877,012,100 $909,197 3,392 $129,392,876
Levee Accreditation
For the NFIP, FEMA only recognizes levee systems that meet minimum design, operation, and maintenance standards. CFR 44 (Section 65.10) describes the information needed for FEMA to determine if a levee system provides protection from the 1 percent annual chance flood. This information must be supplied to FEMA by the community or other party when a flood risk study or restudy is conducted, when FIRMs are revised, or upon FEMA request. FEMA reviews the information for the purpose of establishing the appropriate FIRM flood zone.
FEMA coordinates its programs with the U.S. Army Corps of Engineers, who may inspect, maintain, and repair levee systems. The Corps has authority under Public Law 84-99 to supplement local efforts to repair flood control projects that are damaged by floods. Like FEMA, the Corps provides a program to allow public sponsors or operators to address levee system maintenance deficiencies. Failure to do so within the required timeframe results in the levee system being placed in an inactive status in the Corps’ Rehabilitation and Inspection Program. Levee systems in an inactive status are ineligible for rehabilitation assistance under Public Law 84-99.
There are about 60 levees in Sonoma County. Table 10-2 lists the 29 levees shown on the FEMA FIRM, all of which protect population, structures, and/or valuable property from riverine flooding. 10-5
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Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Table 10-2. Levees Shown on FIRM for Sonoma County
Levee Name Population Structures Property Value Length (miles) FIRM Status All Coast Lumber Company and Cloverdale Water
Treatment Plan
51 3 $5.29 million 1.24 Non-Accredited
J. Black #3 6 4 $2.27 million 0.99 Non-Accredited
Petaluma River Left Bank 508 213 $140 million 0.83 Non-Accredited Petaluma River Right Bank 567 176 $104 million 0.89 A99 Sonoma County Levee 106 2 1 $327,000 0.26 Non-Accredited Sonoma County Levee 111 1 1 $595,000 0.09 Non-Accredited Sonoma County Levee 12 121 44 $23 million 2.58 Non-Accredited Sonoma County Levee 13 124 13 $106 million 0.59 Non-Accredited Sonoma County Levee 16 14 7 $9.28 million 1.08 Non-Accredited Sonoma County Levee 17 25 14 $8.33 million 0.7 Non-Accredited Sonoma County Levee 18 18 2 $16.2 million 0.07 Non-Accredited Sonoma County Levee 21 0 2 $4.11 million 1.31 Non-Accredited Sonoma County Levee 25 307 24 $88.8 million 0.24 Non-Accredited Sonoma County Levee 29 102 16 $29.9 million 0.9 Non-Accredited Sonoma County Levee 30 5 2 $844,000 1.93 Non-Accredited Sonoma County Levee 32 63 17 $4.78 million 0.23 Non-Accredited Sonoma County Levee 33 62 21 $13.3 million 0.85 Non-Accredited Sonoma County Levee 34 477 1 $4.47 million 1.39 Non-Accredited Sonoma County Levee 37 4 1 $358,000 0.21 Non-Accredited Sonoma County Levee 38 154 3 $11.8 million 1.47 Non-Accredited Sonoma County Levee 39 4 1 $391,000 0.45 Non-Accredited Sonoma County Levee 4 4 3 $2.04 million 0.39 Non-Accredited Sonoma County Levee 46 12 3 $1.85 million 1.76 Non-Accredited Sonoma County Levee 48 2 2 $2.6 million 0.62 Non-Accredited Sonoma County Levee 50 2 1 $1.03 million 4.82 Non-Accredited Sonoma County Levee 62 0 3 $3.41 million 7.91 Non-Accredited Sonoma County Levee 65 7 4 $5.24 million 5.62 Non-Accredited Sonoma County Levee 67 0 4 $2.16 million 9.56 Non-Accredited Sonoma County Levee 80 13 4 $2.38 million 1.11 Non-Accredited The Community Rating System
Sonoma County and the City of Petaluma currently participate in the CRS program. Table 10-3 summarizes the CRS status of each. Many of the mitigation actions identified in this plan are creditable activities under the CRS program. Therefore, successful implementation of this plan offers the potential to enhance the CRS classification.
Table 10-3. CRS Status of Participating Jurisdictions
Current CRS Premium Discount
Jurisdiction NFIP Community # CRS Entry Date Classification SFHA Non SFHA City of Petaluma 060379 10/01/1991 6 20% 10%
Sonoma County 060375 10/01/1991 10 0% 0%
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10. Flooding
Repetitive Loss
A repetitive loss property is defined by FEMA as an NFIP-insured property that has experienced any of the following since 1978, regardless of any changes in ownership:
• Four or more paid losses more than $1,000
• Two paid losses more than $1,000 within any rolling 10-year period • Three or more paid losses that equal or exceed the current value of the insured property.
The government has instituted programs encouraging communities to identify and mitigate the causes of repetitive losses. Studies have found that many of these properties are outside any mapped 1 percent annual chance (100-year) floodplain. The key identifiers for repetitive loss properties are the existence of NFIP insurance policies and claims paid by the policies.
FEMA further designates as severe repetitive loss (SRL) any NFIP-insured single-family or multi-family residential building for which either of the following is true:
• The building has incurred flood-related damage for which four or more separate claims payments have been made, with the amount of each claim (including building and contents payments) exceeding $5,000, and with the cumulative amount of such claims payments exceeding $20,000 • At least two separate claims payments (building payments only) have been made under NFIP coverage, with the cumulative amount of claims exceeding the market value of the building.
To qualify as an SRL property, at least two of the claims must be within 10 years of each other, and claims made within 10 days of each other are counted as one claim. In determining SRL status, FEMA considers the loss history since 1978, or from the building’s construction if it was built after 1978, regardless of any changes in the ownership of the building.
FEMA-sponsored programs, such as the CRS, require participating communities to identify repetitive loss areas.
A repetitive loss area is the portion of a floodplain holding structures that FEMA has identified as meeting the definition of repetitive loss. Identifying repetitive loss areas helps to identify structures that are at risk but are not on FEMA’s list of repetitive loss structures because no flood insurance policy was in force at the time of loss.
FEMA’s list of repetitive loss properties identifies 969 such properties in the Sonoma County planning area, as of March 28, 2021, as summarized in Table 10-4. These properties likely were flooded by flood events typical for the floodplain reflected in the current mapping. Note that special purpose district planning partners to this plan have no identified repetitive loss properties because districts are not eligible participants in the NFIP.
FEMA recently changed its policies on providing repetitive loss properties information due to implications of the Privacy Act. The “routine use” provision for acquiring the data, which requires certifications on how the data will be used, was not well-defined at the time of this plan update. Repetitive loss data for all planning partners could not be acquired in time for analysis and assessment for this plan. Therefore, the resolution of the repetitive loss data available to support this plan update is limited to property counts only. No location or dates of loss data was available. As the State of California’s largest repetitive loss community, Sonoma County and its planning partners understand the importance of a thorough understanding of the repetitive flood loss problem. The County and its planning partners will seek to meet FEMA requirements for access to this data through plan implementation.
Future updates to this plan will seek to have enhanced resolution for more detailed analysis. 10-7
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Table 10-4. Repetitive Loss Properties in Sonoma County Repetitive Loss Total Number of Payment Made for Losses
Properties Losses Building Payments Contents Payments Total Payments Jurisdiction Healdsburg 8 25 $305,938.77 $52,298.70 $358,237.47
Morro Bay 3 6 $107,144.24 $.00 $107,144.24
Petaluma 39 108 $2,750,657.29 $668,253.68 $3,418,910.97 Santa Rosa 2 4 $27,134.97 $.00 $27,134.97
Sebastopol 9 23 $654,611.00 $665,023.72 $1,319,634.72
Sonoma (City) 4 10 $167,180.17 $40,134.26 $207,314.43
Sonoma County 904 3,217 $69,494,790.53 $15,992,640.94 $85,487,431.47 Total 969 3,393 $73,507,456.97 $17,418,351.30 $90,925,808.27 Source: March 28, 2021 FEMA Repetitive Loss Summary
10.2.2 Typical Flood-Causing Events
There are six types of flood events that can impact the planning area: riverine flooding, urban flooding, coastal flooding, tsunami flooding, flooding from sea level rise, and flooding from a dam failure. This hazard profile focuses on the coastal, riverine, and urban flood hazards. Floods resulting from a dam failure are discussed in Chapter 7. Tsunami flooding is discussed in Chapter 14. Floods from sea level rise are discussed in Chapter 12
10.2.3 Flooding Sources
River Systems
Riverine floods in Sonoma County occur during winter. Typically, they develop within 24 to 48 hours after a storm event and recede within three days after the end of the storm. Damaging floods in the county occur most frequently along the Russian River, Petaluma River, Sonoma Creek, Laguna De Santa Rosa, and their tributaries.
Russian River
The Russian River watershed is the largest in Sonoma County, draining a total of 1,485 square miles. It originates in Mendocino County, flows southward through eastern Sonoma County to Healdsburg, where it turns west; then flows into the Pacific Ocean at Jenner. Nearly 90 percent of the drainage basis lies upstream of the flood-prone areas of the Russian River which includes the unincorporated communities of Monte Rio, Guerneville, Rio Nido and Forestville. The watershed comprises much of the county’s prime agricultural land and has been greatly influenced by urbanization in the vicinity of Windsor and Santa Rosa. West of Forestville, the river’s floodplain narrows significantly as it flows through the Coast Range to the Pacific.
Along the lower Russian River, floods are characterized by high velocity and significant depth of flow due to the relatively narrow floodplain. The frequency of flooding in this portion of the river causes repetitive flood losses in the residential and commercial districts of Mirabel Park, Duncans Mills, Monte Rio, Rio Nido and Guerneville.
The National Weather Service considers the Russian River at flood stage when it reaches a height of 32 feet at the Guerneville Bridge. Floods reaching a gauge height of less than 34 feet at the Guerneville Bridge are common during a typical winter but do not usually present significant problems for the community.
The USGS has maintained stream-gaging stations to record flow levels on the Russian River since 1911. The gages are located in the communities of Cloverdale (USGS gauge #11463000), Healdsburg (USGS gauge 10-8
10. Flooding
#11464000), and Guerneville (USGS gauge # 11467000). The drainage area monitored by these gaging station is about 1,340 square miles, of which 235 square miles are in the drainage areas behind either the Warm Springs Dam or the Coyote Valley Dam. The Guerneville gauge monitors peak discharge 4.3 miles upstream from the Guerneville Bridge and 20.8 miles upstream from the mouth of the Russian River. For County emergency response purposes, a staff gauge on the Guerneville Bridge is used to monitor flood elevation levels and risk to the community of Guerneville. This gauge does not measure or record flow levels and is not an official USGS gaging station (County of Sonoma 2017).
Sonoma Creek
Sonoma Creek frequently floods during relatively small winter storm events that cause flows to overtop the banks.
The flooding is of short duration, but may last several days. The bordering low lands are most impacted, as a result of storm water runoff from the upper watershed (County of Sonoma 2017).
Petaluma River
Petaluma River floods after multi-day storm events due to inadequate storm water infrastructure. Flooding along this river mainly occurs in the Payran area, between Denman Flat and the confluence of Lynch Creek; and the Penngrove area. Between 1997 and 2008, the City of Petaluma significantly reduced its flood exposure in the Payran area by completing $40 million in improvements to flood control infrastructure, including 3,600 feet of channel widening and floodwalls, pump stations, two vehicular bridge replacements, and two railroad bridge replacements (County of Sonoma 2017).
Laguna De Santa Rosa
The Laguna De Santa Rosa is a freshwater estuary that receives stormwater from southern Santa Rosa, Rohnert Park, Cotati, Sebastopol, and large unincorporated areas. Many portions of these cities were developed in or near the Laguna’s historical flood inundation area. The most significant flood impacts along the Laguna and its tributaries occur in western Rohnert Park, Cotati, and eastern Sebastopol. The City of Santa Rosa’s Laguna Treatment Plant has experienced significant flooding on numerous occasions.
Urban Flooding
Urban flooding occurs when available stormwater conveyance systems lack the capacity to convey rainfall runoff to nearby creeks, streams, and rivers. As drainage facilities are overwhelmed, roads and transportation corridors become conveyance facilities. Urban floods can be a great disturbance of daily life in urban areas. Roads can be blocked, and people may be unable to go to work or school. Economic damage can be high, but the number of casualties is usually limited, because of the nature of the flood.
The two key factors that contribute to urban flooding are rainfall intensity and duration. Topography, soil conditions, urbanization and groundcover also play an important role. On flat terrain, the flow speed is low and people can still drive through flooded areas. The water rises relatively slowly and usually does not reach life endangering depths. Risks are much greater where conditions cause floodwaters to flow at higher velocity.
Coastal Flooding
Coastal flooding occurs when intense, offshore low-pressure systems drive ocean water inland. The water pushed ashore is called storm surge. Flooding along the Pacific coast is often associated with the simultaneous occurrence 10-9
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements of very high tides, large waves, and storm swells during the winter. Though at lower risk, the San Pablo Bay shoreline also can be affected by storm surges and tidal actions.
Most flood damage along the California coast is due to the confluence of large waves, storm surges, and high tides during strong El Niño events. These storm events can result in coastal bluff erosion. Much of the Sonoma County coastline is elevated above sea level with dramatic coastal bluffs. The bluffs are subject to erosion from winter storms, wave action, wind, and stormwater runoff and can become unstable. Bluff erosion or retreat may occur suddenly and catastrophically through slope failure due to heavy rain, high wave action, and high tides.
According to the National Academy of Sciences, storms and sea level rise are causing California coastal bluffs, beaches, and dunes to retreat at rates from a few inches to several feet per year. The Academy projects that California coastal bluffs could retreat more than 100 feet by 2100. The ability of coastal bluffs to withstand continuous erosive forces over time depends on the relative resistance of the shoreline rocks. Factors that determine rock resistance are the type of rock, extent of shearing and fracturing, and inclination of the rock layers.
Coastal bluffs consisting of native materials from the Franciscan or Merced geologic formations are the most affected by erosion (County of Sonoma 2017).
Coastal bluff erosion has threatened development in some areas west of State Highway 1, such as Gleason Beach.
Structures and septic systems in these areas were built on or near the edge of coastal bluffs and on steep slopes, which are eroding. Landslides, in conjunction with wave action, failure of shoreline protection measures, and changes in drainage have resulted in severe erosion, bluff failure, and loss of bluff top area. Some houses have been demolished and removed because they posed a public safety risk, and several other houses have been damaged to the extent that they are no longer habitable. Caltrans investigations in 1998 and 2003 determined that coastal erosion rates near Gleason Beach were about 1 foot per year. Coastal erosion threatens the stability of State Highway 1, and Caltrans developed a project to relocate State Highway 1 at Gleason Beach. The project is scheduled to begin in mid-2021.
10.2.4 Past Events
Significant floods occurred on the Russian River in 1955, 1964, 1986, 1995, 1997, 2006, and 2017. The earliest major flood recorded on the Russian River occurred in 1862. This flood predated gauge measurements of river flow, but is estimated to have had a discharge of about 100,000 cubic feet per second. The Petaluma River has also had a history of flooding. According to the Corps of Engineers, floods in 1982, 1986, and 1998 caused over $34 million in damage within the City of Petaluma, particularly in the Payran area. These flood hazards have been significantly reduced by construction of flood control channels and flood walls, completed in 2008 (County of Sonoma 2017). Table 10-5 summarizes the 15 federally declared flood disasters in Sonoma County related to flooding between 1960 and 2020.
The largest flood in recent history occurred between February 14 and 18, 1986, when a peak discharge of 102,000 cubic feet per second was recorded and the flood reached a gage height of 48.6 feet at Guerneville. Heavy rains from December 26, 2005, to January 3, 2006. The Russian River rose above flood stage at all USGS gaging stations in Sonoma County. Significant flooding also occurred on the Petaluma River and Sonoma Creek. At Guerneville, the river crested at 41.6 feet. The rainfall measured in the City of Santa Rosa during this storm was near record-setting at 17.6 inches. A federal disaster declaration was issued for this event, and more than 100 roadways were blocked due to flooding or landslides. Some 2,100 business and residential properties were inundated, and 50,000 residents were without power. Sonoma County business and residential damages were estimated at $104 million (County of Sonoma 2017).
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10. Flooding
Table 10-5. Declared Sonoma County Flood Disaster Events Date
Declaration
# Type of event
Assistance
Typea
Estimated
Damage
February 24 – March 1, 2019 4434 Severe winter storms, flooding, landslides, mudslides PA $155 million February 1 – 23, 2017 4308 Severe winter storms, flooding, mudslides PA $537.1 million January 3 – 12, 2017 4301 Severe winter storms, flooding, and mudslides PA $162.3 million March 29 – April 16, 2006 1646 Severe storms, flooding, landslides, and mudslides PA December 17 – January 3, 2006 1628 Severe storms, flooding, mudslides, and landslides IA & PA $163.2 million February 2 – April 30, 1998 1203 Severe winter storms and flooding IA & PA December 28, 1996 – April 1, 1997 1155 Severe storms/flooding IA & PA February 13 – April 19, 1995 1046 Severe winter storms, flooding, landslides, mud flows IA & PA January 3 – February 10, 1995 1044 Severe winter storms, flooding, landslides, mud flows IA & PA January 5 – March 20, 1993 979 Severe storm, winter storm, mud & landslides, flooding
IA & PA
February 12 – March 10, 1986 758 Severe storms, flooding IA & PA January 21 – March 30, 1983 677 Coastal storms, floods, slides, tornadoes IA & PA December 19, 1981 – January 8, 1983 651 Severe storms, flood, mudslides, high tide IA & PA January 16, 1969 253 Severe storms, flooding IA & PA
December 24, 1964 183 Heavy rains & flooding IA & PA
a. IA = Individual Assistance; PA = Public Assistance; HMGP = Hazard Mitigation Grant Program; N/A = Information not available or applicable
10.2.5 Location
FEMA-generated FIRMs (Flood Insurance Rate Maps) are the principle tool used to identify the extent and location of the flood hazard. FEMA last updated a FIRM for the County of Sonoma on March 7, 2017. Flood hazard zones from that mapping are shown on Figure 10-2. Preliminary data were released in May 2020 for additional FIRM updates, but those changes have not yet become effective.
Additional areas called “flood awareness areas” were delineated using the County’s “functional riparian channels and floodplains” data (see Figure 10-3). The mapped areas overlap with the FEMA and Russian River data in some areas but also include streams not mapped by FEMA.
10.2.6 Frequency
Riverine Flooding
Annual peak gauge heights and discharges for the Russian River at the USGS Guerneville gage indicate that peak flows exceeded flood stage at Guerneville in 34 of 59 years. The number of floods experienced may be greater as some years had more than one high flow event. The future potential for flood frequency and intensity in the near term is expected to be similar to the observed historic probabilities. Based on this, the planning area can expect at least one episode of minor river flooding most winters. In the longer term, the effects of development and of climate change storm intensity, frequency and flooding are unknown (County of Sonoma 2017). 10-11
Cloverdale
128 ¬«
101 £¤
1¬«
Healdsburg
Windsor
116 ¬«
Sebastopol
Santa
Rosa
12¬«
1¬« Rohnert Park
Cotati
101 £¤
Petaluma
116 ¬«
Sonoma
12¬«
121 ¬«
37¬«
Figure 10-2. FEMA FIRM Flood Hazard Areas
1% Annual Chance Flood (100-Year)
0.2% Annual Chance Flood (500-Year)
±
01 05
Miles
Cities
County Boundary
Data Sources: Sonoma Co.,
FEMA
Highways
Cotati
Sonoma
Windsor
Healdsburg
Sebastopol
Rohnert Park
Santa
Rosa
Cloverdale
Petaluma ¬«12
¬«116
¬«
116
¬«
37 ¬«
121
¬«
12 ¬«
128
¬«1
¬«1
£¤101
£¤
101
Figure 10-3. Sonoma County Flood Awareness Areas
±
0 10 5
Miles
Data Sources: Sonoma Co.
Flood Awareness Areas
Cities
County Boundary
Highways
- - - -
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Flood frequency for riverine flooding is often evaluated by examining peak discharges; Table 10-6 lists peak flows on the Petaluma and Russian Rivers used in FEMA’s flood insurance study to define flows with a 10 percent, 2 percent, 1 percent, and 0.2 percent probability of occurring in any given years. These values are often referred to a flows likely to occur once every 10, 50, 100, or 500 years, respectively. FEMA’s peak flows for all surface waters in the county are included in Appendix E.
Table 10-6. Peak Riverine Discharges on the Petaluma and Russian Rivers Discharge (cubic feet/second)
Source/Location
10 Percent Annual
Chance
2 Percent Annual
Chance
1 Percent Annual
Chance
0.2 Percent Annual
Chance
Petaluma River
Downstream of confluence with Adobe Creek 8,672 11,034 11,910 15,044 At Highway 101 bridge 6,675 9,149 10,494 13,694
Downstream of confluence of Washington
Creek
5,758 8,459 9,757 13,056
Downstream of confluence of Lynch Creek 5,246 7,492 8,671 11,563 Downstream of confluence of Capri Creek 4,653 6,583 7,728 10,523 Downstream of confluence of Willow Brook 3,587 4,825 5,360 6,733 Upstream of confluence of Willow Brook 1,701 2,947 3,529 4,801 Russian River
At Pacific Ocean 76,000 102,000 114,000 135,000
Upstream of Duncan Mills 75,000 100,000 112,000 133,000 Upstream of confluence of Austin Creek 74,000 98,000 107,000 131,000 Upstream of Summerhome Gage 73,000 97,000 106,000 130,000 Downstream of confluence of Mark West Creek 67,000 92,000 97,000 126,000 Upstream of confluence of Mark West Creek 60,000 88,000 103,000 140,000 Upstream of confluence of Dry Creek 56,000 79,000 90,000 129,000 Upstream of confluence of Brooks Creek 55,000 78,000 88,000 127,000 Upstream of confluence of Maacama Canal 51,000 73,000 82,000 115,000 Upstream of confluence of Sausal Creek 50,000 71,000 81,000 111,000 Upstream of confluence of Lytton Creek 50,000 70,000 80,000 110,000 Upstream of confluence of Miller Creek 48,000 68,000 79,000 106,000 Upstream of confluence of Gill Creek 47,000 67,000 76,000 105,000 Upstream of confluence of Big Sulphur Creek 46,000 58,000 73,000 100,000 Upstream of confluence of Oat Valley Creek 40,000 56,000 64,000 85,000 Russian River Split Flow
At Healdsburg Avenue * 215 640 9,140
* Data not available
Source: FEMA Flood Insurance Study Number 06097CV001E, Sonoma County, California and Incorporated Areas, March 7, 2017 Coastal Flooding
The frequency and severity of coastal flooding are based on storm surge height, which is the height of water accounting for waves. Table 10-7 summarizes the still-water elevations along the San Pablo Bay coastline, representing the steady state water depth not accounting for breaking waves. These are the projected elevations of 10-14
10. Flooding
floodwaters in the absence of waves resulting from wind or seismic effects. Table 10-8 shows the storm surge water levels used for mapping the coastal floodplains in the planning area.
Table 10-7. Summary of Still-Water Elevations Along San Pablo Bay Low 8.4 9.8 10.6 10.9
Median 9.9 11.4 12.3 12.1
High 15.7 15.1 17.2 16.5
a. Elevation in 1988 North American Vertical Datum
Source: FEMA Flood Insurance Study Number 06097CV001E, Sonoma County, California and Incorporated Areas, March 7, 2017 Table 10-8. Regional Storm Surge Water Elevations
Regional Storm Surge Water Elevations (feet, North American Vertical Datum) Point Reyes Bodega Harbor Arena Cove
Still Water Elevationa (feet)
10 percent 2 percent 1 percent 0.2 percent
-
- - - -
50-percent 7.6
20-percent 7.9 7.9 8.2
10-percent 8.2
4-percent 8.5 8.5 8.8
2-percent 8.8
1-percent 9.1 9.1 9.3
0.2 percent 9.8
7.6 8.2
8.8 9.8
7.8 8.4
9.0 10.1
Source: FEMA Flood Insurance Study Number 06097CV001E, Sonoma County, California and Incorporated Areas, March 7, 2017
10.2.7 Severity
The principal factors affecting flood damage are flood depth and velocity. The deeper and faster flood flows become, the more damage they can cause. Shallow flooding with high velocities can cause as much damage as deep flooding with slow velocity. Wave action has significant velocity, and waves as small as 1.5 feet can cause substantial damage to structures and other development. Table 10-9 summarizes impacts and estimated costs of recent federally declared flood disasters in Sonoma County.
10.2.8 Warning Time
Due to the sequential pattern of meteorological conditions needed to cause serious flooding, it is unusual for a flood to occur without warning. Warning times for floods can be between 24 and 48 hours. Flash flooding can be less predictable, but potential hazard areas can be warned in advanced of potential flash flooding danger. Flash flooding is infrequent in the planning area.
As major storm systems approach, the National Weather Service, in coordination with the California Department of Water Resources, monitors weather conditions and real-time precipitation and river stage data; forecasts the amount and timing of expected precipitation; and issues official river forecasts and hydrologic statements.
Updated a minimum of twice daily, these river forecasts are available as both text products and as graphical river guidance plots, which provide river stage information for each official forecast point for the next five days following the forecast issuance. As storm events continue with streams and rivers rising to threatening levels, these forecasts may be updated more frequently if needed. 10-15
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Table 10-9. Damage and Estimated Losses from Recent Floods in Sonoma County Date
Loss
Estimatesa Damage
January 8-31, 1995 $21 million • Over 50 roads closed
• 15,000 residents without power
• Total displaced persons exceeded 2,000, of which 456 flood victims were evacuated by air • 13 medical cases were treated and 2 flood-related fatalities occurred March 7-15, 1995 $13.3 million • Over 100 roads closed
• 45,000 residents without power At least 3,000 residents displaced • Up to 30 containers of possible toxic materials identified in the flood zone December 30, 1996
– January 4, 1997
$31 million • Up to 200 roads were closed or damaged temporarily 463 homes damaged • 12,000 residents without power
• Over 1,200 victims evacuated their residences and 2 storm-related deaths occurred • Sewage and treatment plants overflowed
February 2, 1998 $28 million • 200 roads were listed as flooded or closed 6,400 residents without power • 250+ homes were inundated
• 1,200 residents voluntarily evacuated 4 storm-related deaths December 30, 2005
– January 3, 2006
$104 million • Over 100 roads closed due to flooding and landslides Approximately 50,000 county residents without power 2106 properties inundated, 67 declared uninhabitable Unknown number of self-evacuations
• Laguna Wastewater Treatment Plant flooded with partially treated sewage spill December 2014 $1.1 million • 48 businesses and single-family dwellings damaged along Foss Creek January – February
2017
$155 million • 1,900 homes (1,760 with major damage) and 578 businesses damaged by flood waters • Emergency operations center activated
February 2019 $155 million • 2,000 buildings damaged and 1 fatality • 3,500 evacuated
• Emergency operations center activated
a. Dollar amounts in the year of occurrence
Graphical river guidance plots can be accessed at these websites:
• http://www.cnrfc.noaa.gov
• http://cdec.water.ca.gov/guidance_plots/
10.2.9 Secondary Hazards
The most problematic secondary hazard for flooding is bank or coastal erosion. In many cases the threat and effects of erosion are worse than actual flooding. This is especially true on the upper courses of rivers where there are steep gradients. Floodwaters in these reaches may pass quickly and without much damage, but scour the banks, edging properties closer to the floodplain or causing them to fall in. Flooding is also responsible for hazards such as landslides when high flows over-saturate soils on steep slopes, causing them to fail. Hazardous materials spills are also a secondary hazard of flooding if storage tanks rupture and spill into streams, rivers or drainage sewers.
10-16
10. Flooding
10.3 EXPOSURE
A quantitative assessment of exposure to the dam failure hazard was conducted using the asset inventory developed for this plan, flood mapping by FEMA for the most recent Sonoma County FIRM (see Figure 10-2), and Sonoma County’s mapped flood awareness areas (see Figure 10-3). Detailed results by jurisdiction are included in Appendix D; countywide summaries are provided below.
10.3.1 Population
Table 10-10 summarizes the estimated population living in the evaluated flood hazard areas.
Table 10-10. Exposed Population in Evaluated Flood Hazard Zones 1% Annual Chance Flood
Zone
0.2% Annual Chance Flood
Zone Flood Awareness Areas
Population Exposed 7,768 17,861 7,524
% of Total Planning Area Population 1.6% 3.7% 1.55%
Socially vulnerable populations living in the mapped flood zones were estimated based on data for the Census- defined blocks that lie at least partially within the mapped flood zone. Because many of those Census blocks extend outside the flood zone, the estimates are greater than the actual exposed populations, but they provide reasonable relative data for use in mitigation planning. Figure 10-4 summarizes the estimated exposure of socially vulnerable populations.
10.3.2 Property
Table 10-11 summarizes the estimated property exposure in the evaluated flood hazard areas. Figure 10-5 shows the occupancy class defined by Hazus for all buildings in the mapped floodplains. These occupancy classes provide an indication of land use within the mapped hazard area. Some land uses are more vulnerable to flood risks, such as single-family homes, while others are less vulnerable, such as agricultural land or parks.
10.3.3 Critical Facilities
The breakdown of critical facility exposure by facility type is shown in Figure 10-6. Critical facilities exposed to the flood hazard represent the following percentages of all critical facilities in the planning area:
• 7.9 percent (377 facilities) of all critical facilities are in the 1-percent-annual-chance flood hazard area. • 11.6 percent (550 facilities) of all critical facilities are in the 0.2-percent-annual-chance flood hazard area. • 8.8 percent (418 facilities) of all critical facilities are in the flood awareness area. 10-17
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Under Age 16
20,667
(20.9% of Total)
Under Age 16
25,283
(21.5% of Total) Under Age 16
14,845
(17.5% of Total)
Over Age 65
14,916
(15.1% of Total)
Over Age 65
17,038
(14.5% of Total)
Over Age 65
15,981
(18.9% of Total) Total
99,044
Total
117,440
Total
84,757
0 20,000
40,000
60,000
80,000
100,000
120,000
1% Annual Chance 0.2% Annual Chance Flood Awareness Area Number of People in Hazard Area
Low-Income
23,536
(60.2% of Total)
Low-Income
27,141
(59.3% of Total)
Low-Income
19,436
(54.0% of Total) Total
39,076
Total
45,776
Total
35,986
0 5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
45,000
50,000
1% Annual Chance 0.2% Annual Chance Flood Awareness Area Number of Households in Hazard Area
See Section 4.8.1 for the definition of “low income” used in this analysis Figure 10-4. Socially Vulnerable Populations in Flood Zone Census Blocks Table 10-11. Exposed Property in Evaluated Flood Hazard Zones 1% Annual Chance Flood
Zone
0.2% Annual Chance Flood
Zone Flood Awareness Areas
Acres of inundation area 58,495 64,542 —
Number of Buildings Exposed 4,570 8,416 4,654
Value of Exposed Structures $6,282,146,827 $9,094,421,934 $5,241,949,336 Value of Exposed Contents $6,062,349,168 $8,493,127,383 $4,978,042,951 Total Exposed Property Value $12,344,495,994 $17,587,549,317 $10,219,992,287 Total Exposed Value as % of Planning
Area Total
5.6% 8.0% 4.68%
10-18
10. Flooding
1% ANNUAL CHANCE FLOOD
Residential
3,155
69%
Education Commercial 11 4360% 10%
Government
240 Religion Agriculture Industrial
6375% 12 79
0% 14% 2%
0.2% ANNUAL CHANCE FLOOD
Residential
6,353
76%
Commercial Education 83415 10%0%
Government Religion Agriculture Industrial
283 20 763 148
3% 0% 9% 2%
Residential
3,357
72%
Commercial
313
7%
Industrial
46 Agriculture
759Religion
13 Government
157
4%
Education
9 0%
FLOOD AWARENESS AREAS
16%0% 1%
Figure 10-5. Building Occupancy Classes in the Mapped Flood Zones 10-19
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements 191
74 131
3,109
219
650
385
2 7
8 199
11 20
130
16 8
12 322
20 35
137
2 3
10 142
8 15
238
0 500 1,000 1,500 2,000 2,500 3,000 3,500
Communications
Energy
Food, Water, Shelter
Hazardous Material
Health & Medical
Safety & Security
Transportation
Number of Facilities in Identified Area
Facility Category
Flood Awareness Area
0.2% Annual Chance Flood
1% Annual Chance Flood
Planning Area Total
Figure 10-6. Critical Facilities in Mapped Flood Hazard Areas and Countywide Significant facilities included in the 1 percent annual chance flood zone include the following:
• 4 wastewater treatment facilities • 5 fire stations
• 199 hazardous material sites • 115 road bridges
• 5 schools • 14 port facilities
• 1 airport
10-20
-
- - - -
10. Flooding
All levees line flood-prone rivers and are therefore exposed to the flood hazard. This includes the 29 levees, totaling 50 miles in length, listed in Table 10-2, which have been identified as protecting people and valuable assets from floodwaters.
10.3.4 Environment
Because floodplain management measures place restrictions on development in areas affected by flooding, floodplains often have a higher portion of area that is undeveloped open space or natural area. These undeveloped areas represent environment exposed to the flood hazard.
10.4 VULNERABILITY
The results of the vulnerability assessment indicate estimated damage for the 1-percent and 0.2-percent-annual- chance flood hazards. Detailed results by jurisdiction are included in Appendix D; countywide summaries are provided below.
10.4.1 Population
Impacts on persons and households for evaluated flood events were estimated for each event through the Level 2 Hazus analysis. Table 10-12 summarizes the results.
Table 10-12. Estimated Flood Impacts on Residents
Displaced Population
Number of Residents Requiring Short Term
Shelter
1% Annual Chance Flood Zone 1,684 85
0.2% Annual Chance Flood Zone 4,802 273
10.4.2 Property
Table 10-13 summarizes Hazus estimates of flood damage in the planning area. The debris estimate includes only structural debris and building finishes; it does not include additional debris that may result from a flood event, such as from trees, sediment, building contents, bridges or utility lines.
Table 10-13. Estimated Impact of a Flood Event in the Planning Area Damage Type 1% Annual Chance Event 0.2% Annual Chance Event Structure Debris (Tons) 657,111 784,257
Buildings Impacted 3,594 5,264
Total Value (Structure + Contents) Damaged $3,890,916,991 $4,480,381,334 Damage as % of Total Value 1.8% 2.0%
10.4.3 Critical Facilities
Estimated Damage
Hazus was used to estimate the percent of damage to the building and contents of critical facilities, using depth/damage function curves. The results are summarized in Figure 10-7 and Figure 10-8. 10-21
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements % Damage to Structure % Damage to Contents Number of Facilities Affected 100 180
158
160
Average Damage as % of Total Value Average Damage as % of Total Value 80 74.7 140
120
9760
20.2 29.2
17.6 7 8.0 5 31.8 1.4 2 2.4
13.0 100
44.5 8038.6 40 34.0
20 9.2 15
60 21.4
40 20
0 0
Safety and Food, Water and Health and Energy Communications Transportation Hazardous Security Sheltering Medical Material
Figure 10-7. Estimated Damage to Critical Facilities from 1% Annual Chance Flood % Damage to Structure % Damage to Contents Number of Facilities Affected 100 226
47.1 42.9
47.3 113
17.7 30.8 31.3
18.0 9.4 22
10 8.7 10
1.8 3 1.4
11.7 10
3.4 11.1
250
80 200
15060
40 100
20 50
0 0
Safety and Food, Water and Health and Energy Communications Transportation Hazardous Security Sheltering Medical Material
Number of Facilities Affected Number of Facilities Affected Figure 10-8. Estimated Damage to Critical Facilities from 0.2% Annual Chance Flood 10-22
10. Flooding
Impacts on Hazardous Materials
During a flood event, containers holding hazardous materials can rupture and leak into the surrounding area.
These facilities could release chemicals that cause cancer or other human health effects, significant adverse acute human health effects, or significant adverse environmental effects.
Impacts on Utilities and Infrastructure
Roads that are blocked or damaged can isolate residents and can prevent access throughout the planning area, including for emergency service providers needing to get to vulnerable populations or to make repairs. Bridges washed out or blocked by floods or debris also can cause isolation. Underground utilities can be damaged. Levees can fail or be overtopped, inundating the land that they protect. Floodwaters can back up drainage systems, causing localized flooding. Culverts can be blocked by debris from flood events, also causing localized urban flooding. Floodwaters can get into drinking water supplies, causing contamination. Sewer systems can be backed up, causing wastewater to spill into homes, neighborhoods, rivers and streams.
10.4.4 Environment
Flooding is a natural event, and floodplains provide many natural and beneficial functions. Nonetheless, flooding can impact the environment in negative ways. Migrating fish can wash into roads or over dikes into flooded fields, with no possibility of escape. Pollution from roads, such as oil, and hazardous materials can wash into rivers and streams. During floods, these can settle onto normally dry soils, polluting them for agricultural uses.
Human development such as bridge abutments and levees, and logjams from timber harvesting can increase stream bank erosion, causing rivers and streams to migrate into non-natural courses.
Loss estimation platforms such as Hazus are not currently equipped to measure environmental impacts of flood hazards. The best gauge of vulnerability of the environment would be a review of damage from past flood events.
Capturing loss data from future events that segregates damage to the environment could be beneficial in measuring the vulnerability of the environment for future updates.
10.5 FUTURE TRENDS IN DEVELOPMENT
The County and its planning partners are equipped to handle future growth within flood hazard areas. All municipal planning partners have general plans that address frequently flooded areas in their safety elements. All partners have committed to linking their general plans to this hazard mitigation plan update. This will create an opportunity for wise land use decisions as future growth impacts flood hazard areas. In addition, partners who are participating in good standing in the NFIP have agreed to regulate new development in the mapped floodplain according to standards that equal or exceed those specified under 44 CFR Section 60.3. This will ensure that any development allowed in the floodplain will be constructed such that the flood risk exposure is eliminated or significantly reduced.
Development in floodplain areas outside of an urban service area is generally constrained by septic requirements.
New standards of onsite wastewater treatment (septic) systems became effective in May 2016 pursuant to new regulations adopted by the State Water Resources Control Board. These standards establish a risk-based, tiered approach for the management of septic system installations and replacements. They mandate that these systems meet increasing levels of performance and protection if they are adversely affecting water quality in nearby water bodies. These higher standards may limit development on some parcels in the floodplain. The County Riparian 10-23
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Corridor Zoning combining district further restricts development in many portions of the floodplain (County of Sonoma 2017).
10.6 SCENARIO
The major river systems in Sonoma County flood at irregular intervals, but generally in response to a succession of intense winter rainstorms. Storm patterns of warm, moist air usually occur between early November and late March. A series of such storms can cause severe flooding in Sonoma County. The worst-case scenario is a series of storms that flood numerous drainage basins in a short time. This would overwhelm city and County response and floodplain management departments. Major roads would be blocked, preventing access for many residents and critical functions. High river flows could cause rivers to scour, possibly washing out roads and creating more isolation problems. In the case of multi-basin flooding, the County would not be able to make repairs quickly enough to restore critical facilities.
10.7 ISSUES
The planning team has identified the following flood-related issues relevant to the planning area:
• Structures in the planning area built before any regulations existed on floodplain development may be particularly vulnerable to the flood hazard.
• The accuracy of the existing flood hazard mapping produced by FEMA in reflecting the true flood risk within the planning area is questionable, especially along the Russian River. • The extent of the flood-protection currently provided by flood control facilities (dams, dikes and levees) is not known due to the lack of an established national policy on flood protection standards. • Older levees are subject to failure or do not meet current building practices for flood protection. • The risk associated with the flood hazard overlaps the risk associated with other hazards such as earthquake, landslide, and severe weather. This provides an opportunity to seek mitigation alternatives with multiple objectives that can reduce risk for multiple hazards. • There is no area-wide degree of consistency in land-use and floodplain management practices. • Climate change may cause more extensive flood problems due to possible sea level rise and more severe weather patterns. The 0.2 percent-annual-chance floodplain inundation area may become a higher probability risk. Coastal flood hazard ratings may also need to be reviewed. • More information is needed on flood risk to support the concept of risk-based analysis of capital projects. • There needs to be a sustained effort to gather historical damage data, such as high-water marks on structures and damage reports, to measure the cost-effectiveness of future mitigation projects. • Ongoing flood hazard mitigation will require funding from multiple sources. • Coordinated hazard mitigation efforts among jurisdictions affected by flood hazards in the county are recommended.
• Floodplain residents should continue to be educated about flood preparedness and the resources available during and after floods.
• The concept of residual risk should be considered in the design of future capital flood control projects and should be communicated with residents living in the floodplain. 10-24
10. Flooding
• The promotion of flood insurance as a means of protecting private property owners from the economic impacts of frequent flood events should continue.
• The economy affects a jurisdiction’s ability to manage its floodplains. Budget cuts and personnel losses can strain resources needed to support floodplain management. • Sonoma County is the State of California largest “repetitive loss” community. Challenges in the acquisition of repetitive loss data from FEMA have made it difficult to acquire data necessary to study the repetitive flood loss problem in depth.
10-25
11. LANDSLIDE/MASS MOVEMENT
11.1 GENERAL BACKGROUND
11.1.1 Mass Movement Types
Mass movement is the movement of rock and soil down slope under the influence of gravity. Landslides are the most commonly recognized type of mass movement. Other common mass movement types include the following:
• Block slides—Blocks of rock that slide along a slip plane as a unit down a slope. • Coastal bluff erosion—The collapse of coastal bluffs due to undercutting erosive forces of wave action. • Creep—A slow-moving landslide often only noticed through crooked trees and disturbed structures. • Debris avalanche—A debris flow that travels faster than about 10 miles per hour (mph). Speeds in excess of 20 mph are not uncommon, and speeds in excess of 100 mph, although rare, can occur. The slurry can travel miles from its source, growing as it descends, picking up trees, boulders, cars, and anything else in its path.
• Earth flows—Fine-grained sediments that flow downhill and typically form a fan structure. • Mudslides or Debris Flows—Rivers of rock, earth, organic matter and other soil materials saturated with water. They develop in the soil overlying bedrock on sloping surfaces when water rapidly accumulates in the ground, such as during heavy rainfall or rapid snowmelt. • Rock falls—Blocks of rock that fall away from a bedrock unit without a rotational component. • Rock topples—Blocks of rock that fall away from a bedrock unit with a rotational component. • Rotational slumps—Blocks of fine-grained sediment that rotate and move down slope. • Transitional slides—Sediments that move along a flat surface without a rotational component.
11.1.2 Factors Causing Mass Movements
Mass movements are caused by a combination of geological and climate conditions, as well as encroaching urbanization. Vulnerable areas are affected by residential, agricultural, commercial, and industrial development and the infrastructure that supports it.
Factors causing mass movements fall into two categories:
• Factors that increase driving forces:
Steepening the slope
Adding weight to (loading) the slope, especially the upper parts 11-1
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Increasing the height of a slope (either by human or natural downcutting) Seismic shaking
• Factors that reduce resisting forces:
Adding water to the slope, which causes increased pore pressure, which reduces frictional strength Steepening the slope, which reduces normal stress, and thus reduces internal friction Bedding, jointing, or foliation parallel to slope or dipping out of slope—these discontinuities are low- strength zones along which the rock can fail and slide out of the slope Intrinsically weak materials (e.g., deeply weathered, sheared, unconsolidated, or clay-rich materials) Undercutting the slope, which reduces support
Removing vegetation, especially trees, which reduces root strength and leads to increased water in soil due to reduced evaporation losses
Seismic shaking
Coastal bluff erosion caused by wave action
11.2 HAZARD PROFILE
11.2.1 Past Events
Table 11-1 lists the known damage-causing mass movements that have occurred in the county. The following sections describe specific past significant events.
Table 11-1. Landslide/Mass Movement Events in Sonoma County Dates of Event Primary Event Type
FEMA
Disaster # Losses/Impacts
February 24 – March 1, 2019 Severe Winter Storms, Flooding, Landslides, and Mudslides
4434
February 1 – 23, 2017 Severe Winter Storms, Flooding,
Mudslides
4308
January 3 – 12, 2017 Severe Winter Storms, Flooding,
and Mudslides
4301
December 23, 2012 Debris Flow Mudslides closed Annapolis Road for several hours, east of Highway 1. $1,000 in property
damage
December 5, 2012 Debris Flow Highway 1 was closed north of Jenner due to a significant mud and rockslide blocking the lanes
of the road. $15,000 in property damage.
March 29 – April 16, 2006 Severe Storms, Flooding,
Landslides, and Mudslides
1646
December 17, 2005 – January 3, 2006 Severe Storms, Flooding, Mudslides, and Landslides
1628
Sources: FEMA 2020, National Centers for Environmental Information Storm Events Database, 2020 April 2006
One of the most recent and most destructive mass movements in Sonoma County occurred in April 2006.
Persistent heavy rainfall caused a massive number of landslides across the Sonoma and Marin County valleys area during the first half of April. About $20 million of damage was done to agriculture, with over $9 million spent in road repair damage in Sonoma County alone. Over $5 million worth of damage was done to single family 11-2
11. Landslide/Mass Movement
dwellings in Sonoma - with lesser (but still substantial) amounts of damage experienced to businesses and public buildings. In Marin County the hardest hit areas were Mill Valley, Fairfax, and San Rafael. In Mill Valley, a man was killed after he was buried in a mudslide in his backyard.
January – March 1998
Another example of Sonoma County’s landslide risk occurred during the El Niño winter storms of January 6 – 7, 1998, in the community of Rio Nido. The upper portion of the slide consisted of a large rotational block failure that occurred near the top of the ridge, approximately 600 feet above the elevation of the canyon floor. Two debris flow failures, which are characterized by fluid and high speed downhill flows, were initiated from the face of the block. The southern debris flow traveled 1,500 feet down a narrow ravine, causing the destruction of three homes and damaging four others in Upper Canyon Three. The northern debris flow traveled down an adjacent drainage ravine north of the homes and came to rest within a long-jam 15 to 20 feet high, located about 800 feet from the canyon floor. Additional debris flows occurred in the same area on February 21 and March 12, 1998 as a result of additional moderate rainfall.
Residents were evacuated until the stability of the slides could be determined. Geologic studies were performed and movement of the slides monitored for years. Evacuation zones maps were periodically revised and residents gradually permitted to return to some areas. Other damaging slides occurred in the communities of Monte Rio, Gold Ridge, Hidden Acres, Blucher Valley, Fitch Mountain, and the coastal community of Gleason’s Beach.
The widespread damage caused across the state by these storms prompted FEMA and Cal OES to initiate the first federally funded landslide acquisition program. The program was designed to permanently remove the properties destroyed, damaged, or still at risk from the landslides. Sonoma County received funds for the acquisition of 45 properties in the four communities that suffered the greatest damage.
11.2.2 Location
Dormant Sites of Previous Mass Movements
One of the best predictors of where mass movements might occur is the location of past landslides, which can be recognized by distinctive topographic shapes that can remain in place for thousands of years. Such sites range from a few acres to several square miles. Many show no evidence of recent movement and are not currently active. A few may become active in any given year. The recognition of ancient dormant landslide sites is important in the identification of areas susceptible to landslides because they can be reactivated by earthquakes or by exceptionally wet weather. These dormant sites are also vulnerable to construction-triggered sliding. The shoreline contains many large, deep-seated dormant landslides.
Landslide Susceptibility Mapping
In 2011, the California Geological Survey conducted a statewide analysis using a combination of regional rock strength and slope data to create classes of susceptibility to deep-seated landslides. The analysis assumed, in general, that susceptibility to deep-seated landslides is low on very low slopes in all rock materials and increases with slope and in weak rocks. The analysis also factored in locations of past landslides. Figure 11-1 shows deep- seated landslide susceptibility classes (none, low, moderate, high, and very high). 11-3
Cotati
Sonoma
Windsor
Healdsburg
Sebastopol
Rohnert Park
Santa
Rosa
Cloverdale
Petaluma ¬«12
¬«116
¬«
116
¬«
37 ¬«
121
¬«
12 ¬«
128
¬«1
¬«1
£¤101
£¤
101
Figure 11-1. Susceptibility to Deep-Seated Landslides
±
0 10 5
Miles
Data Sources: Sonoma Co.,
CGS
low
moderate
high
very high
Cities
County Boundary
Highways
11. Landslide/Mass Movement
11.2.3 Frequency
Mass movements are often triggered by other natural hazards such as earthquakes, heavy rain, floods or wildfires, so their frequency is often related to the frequency of the precipitating hazards. In Sonoma County, landslides typically occur during and after severe storms, so the potential for landslides largely coincides with the potential for sequential severe storms that saturate steep, vulnerable soils. Most weather-induced landslides in the county occur in the winter after the water table has risen. Landslides that result from earthquakes can occur at any time.
The probability of a landslide event occurring in the county in any given year is high. Approximately 50 landslides occur each winter that block county roads (County of Sonoma 2017). Table 11-1 lists five landslide events in the county for which federal disaster declarations were issued between 2006 and 2019, an average of one such major event every three years.
11.2.4 Severity
Mass movements destroy property and infrastructure and can claim human lives. They have the potential of destabilizing the foundation of structures, which may result in monetary loss for residents. Slope failures in the United States result in an average of 25 to 50 lives lost per year (USGS, 2020). Movements can pose a serious hazard to properties on or below hillsides. They can cause block access to roads, which can isolate residents and businesses and delay commercial, public and private transportation. This can result in economic losses for businesses. Vegetation or poles on slopes can be knocked over, resulting in possible losses to power and communication lines. Mass movements also can damage rivers or streams, potentially harming water quality, fisheries, and spawning habitat.
Landslides along the county’s coastline, in conjunction with wave action, have resulted in seawall failure, severe erosion, cliff failure, and loss of bluff top area that threatens development. Lots have been significantly reduced in size in the last 25 years, and several houses have been damaged to the extent that they are no longer habitable.
Poor road design and construction can contribute to landslide hazards through side-casting on sloping lands, over steepened cut slopes and inadequate drainage facilities (County of Sonoma 2017).
11.2.5 Warning Time
The velocity of landslides ranges from a slow creep of inches per year to many feet per second, depending on slope angle, material and water content. Some methods used to monitor landslides can provide an idea of the type of movement and the amount of time prior to failure. It is also possible to determine what areas are at risk during general time periods. Assessing the geology, vegetation and amount of predicted precipitation for an area can help in these predictions. However, there is no practical warning system for individual landslides. The current standard operating procedure is to monitor situations on a case-by-case basis, and respond after the event has occurred.
Generally accepted warning signs for landslide activity include the following:
• Springs, seeps, or saturated ground in areas that have not typically been wet before • New cracks or unusual bulges in the ground, street pavements or sidewalks • Soil moving away from foundations
• Ancillary structures such as decks and patios tilting and/or moving relative to the main house • Tilting or cracking of concrete floors and foundations • Broken water lines and other underground utilities
11-5
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements • Leaning telephone poles, trees, retaining walls or fences • Offset fence lines
• Sunken or down-dropped road beds
• Rapid increase in creek water levels, possibly accompanied by increased turbidity (soil content) • Sudden decrease in creek water levels though rain is still falling or just recently stopped • Sticking doors and windows, and visible open spaces indicating jambs and frames out of plumb • A faint rumbling sound that increases in volume as the landslide nears • Unusual sounds, such as trees cracking or boulders knocking together.
11.2.6 Secondary Hazards
Mass movements are not generally known to result in secondary hazards. A landslide that blocks a river or stream does have the potential to cause flooding.
11.3 EXPOSURE
A quantitative assessment of exposure to the mass movement/landslide hazard was conducted using the landslide hazard mapping and the asset inventory developed for this plan, with an emphasis on the zones with the highest degree of susceptibility (Zones V through X). Detailed results by jurisdiction are provided in Appendix D. Results for the whole planning area are presented in the sections below.
11.3.1 Population
Population exposure was estimated by calculating the number of buildings in each hazard area as a percent of total planning area buildings, and then applying this percentage to the estimated planning area population. Table 11-2 summarizes the estimated countywide population living in the mapped landslide risk areas.
Table 11-2. Exposed Population in Mapped Landslide Hazard Zones Moderate Landslide
Risk (Susceptibility
Categories V and VI)
High Landslide Risk
(Susceptibility
Categories VII, VIII, IX)
Very High Landslide Risk
(Susceptibility Category X;
Includes existing landslides)
Population Exposed 54,240 51,796 6,919
% of Total Planning Area Population 11.2% 10.7% 1.4%
Socially vulnerable populations exposed to the landslide hazard were estimated based on data for the Census- defined blocks that lie at least partially within the mapped high and very high landslide hazard zones. Because many of those Census blocks extend outside the mapped hazard zones, the estimates are greater than the actual exposed populations, but they provide reasonable relative data for use in mitigation planning. Figure 11-2 summarizes the estimated exposure of socially vulnerable populations. 11-6
11. Landslide/Mass Movement
Under Age
16 45,358
(19.7% of
Total)
Over Age 65
39,864
(17.3% of
Total)
Total
230,019
0 40,000
80,000
120,000
160,000
200,000
240,000 Number of People in Hazard Area
Low Income
50,734
(54.2% of
Total)
Total
93,574
0 10,000
20,000
30,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000 Number of Households in Hazard Area
See Section 4.8.1 for the definition of “low income” used in this analysis Figure 11-2. Socially Vulnerable Populations in High and Very High Landslide Risk Zones Census Blocks
11.3.2 Property
Table 11-3 summarizes the estimated property exposure in the evaluated landslide hazard areas. Figure 11-3 shows the occupancy class defined by Hazus for all buildings in the mapped landslide hazard areas. These occupancy classes provide an indication of land use within the mapped hazard area. Some land uses are more vulnerable to landslides, such as single-family homes, while others are less vulnerable, such as agricultural land or parks.
Table 11-3. Exposed Property in Mapped
Moderate Landslide
Risk (Susceptibility
Categories V and VI)
Landslide Hazard Zon
High Landslide Risk
(Susceptibility
Categories VII, VIII, IX)
es
Very High Landslide Risk
(Susceptibility Category X;
Includes existing landslides)
Number of Buildings Exposed 21,473 24,283 3,173
Value of Exposed Structures $16,099,740,155 $27,506,743,592 $3,361,526,190 Value of Exposed Contents $12,591,969,732 $24,546,419,412 $2,936,809,711 Total Exposed Property Value $28,691,709,887 $52,053,163,004 $6,298,335,902 Total Exposed Value as % of Planning Area Total 13.1% 23.8% 2.9% 11-7
85%
Commercial
417
2%Industrial Agriculture Religion
Government
116
Education
31 0%
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Residential
18,289
22 0%
2,548
12%
50 0%
1%
MODERATE LANDSLIDE RISK
Residential
18,720
0%4,530
19%
47 0%
267
1%
HIGH LANDSLIDE RISK
77%
Commercial
649
3%
Industrial
40Agriculture Religion Government
Education
30 0%
Residential
2,478
78%
Commercial
63 2%
Industrial
3 0%
Agriculture
582
19%
Religion
4 0%
Government
40 1%
Education
3 0%
VERY HIGH LANDSLIDE RISK
Figure 11-3. Building Occupancy Classes in the Mapped Landslide Hazard Zones
11.3.3 Critical Facilities
The breakdown of exposure of critical facilities by susceptibility class and facility type is shown in Figure 11-4.
11.3.4 Environment
All natural areas within the high susceptibility zones for landslide are considered to be exposed to the hazard. 11-8
11. Landslide/Mass Movement
Facility Category
Transportation
Safety & Security
Health & Medical
Hazardous Material
Food, Water, Shelter
Energy
Communications
191
74 131
3,109
219
650
385
32 17
26 386
15 71
173
High or Very High Landslide Risk
Planning Area Total
0 500 1,000 1,500 2,000 2,500 3,000 3,500
Number of Facilities in Identified Area
Figure 11-4. Critical Facilities in Mapped Landslide Susceptibility Classes and Countywide
11.4 VULNERABILITY
Vulnerability estimates for the landslide hazard are described qualitatively. No loss estimation of these facilities was performed because damage functions have not been established for the landslide hazard.
11.4.1 Population
All people exposed the landslide hazard are potentially vulnerable to landslide impacts. Populations with access and functional needs as well as elderly populations and the very young are more vulnerable to the landslide hazards as they may not be able to evacuate quickly enough to avoid the impacts of a landslide. 11-9
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements
11.4.2 Property
Loss estimations for the landslide hazard are not based on modeling utilizing damage functions, because no such damage functions have been generated. Instead, loss estimates were developed representing 1, 10, 30, and 50 percent of the replacement value of exposed structures. This allows emergency managers to select a range of economic impact based on an estimate of the percent of damage to the general building stock. Damage in excess of 50 percent is considered to be substantial by most building codes and typically requires total reconstruction of the structure. Table 11-4 shows potential losses in the areas with the highest degree of landslide susceptibility.
Table 11-4. Loss Estimation for Mass Movement
Exposed Value Loss Value Loss as % of Total Planning Area Replacement Value Moderate Landslide Susceptibility Zone
Loss = 1% of Exposed Value $287 million less than 1%
Loss = 10% of Exposed Value $2.9 billion 1.3%
Loss = 30% of Exposed Value $28.7 billion $8.7 billion 3.9% Loss = 50% of Exposed Value $14.4 billion 6.6%
High Landslide Susceptibility Zone
Loss = 1% of Exposed Value $520 million less than 1%
Loss = 10% of Exposed Value $5.2 billion 2.4%
Loss = 30% of Exposed Value $52.0 billion $15.6 billion 7.1% Loss = 50% of Exposed Value $26.0 billion 11.9%
Very High Landslide Susceptibility Zone
Loss = 1% of Exposed Value $63 million less than 1%
Loss = 10% of Exposed Value $630 million less than 1%
Loss = 30% of Exposed Value $6.3 billion $1.9 billion less than 1% Loss = 50% of Exposed Value $3.2 billion 1.5%
11.4.3 Critical Facilities
Highly susceptible areas of the county include mountain and coastal roads and transportation infrastructure. At this time all infrastructure and transportation corridors identified as exposed to the landslide hazard are considered vulnerable until more information becomes available. A more in-depth analysis of the mitigation measures taken by landslide-exposed critical facilities to prevent damage from landslides should be done to determine if they could withstand impacts of a mass movement.
11.4.4 Environment
Natural Resources
Landslides can destroy natural assets that are highly valued by the community:
• Landslides that fall into streams may significantly impact fish and wildlife habitat, as well as affecting water quality.
• Hillsides that provide wildlife habitat can be lost due to landslides. • Endangered species and their critical habitat in the planning area may be located in landslide hazard areas. 11-10
11. Landslide/Mass Movement
Agricultural and Timber Resources
Agricultural resources include rangelands, timberlands, cultivated farmlands and dairy lands. Landslides can have major consequences to such resources, primarily timberland, due to the large percentage of such land in remote locations on steep slopes. Roads accessing timberlands are often susceptible to slides and frequently are contributing factors to landslides. Mass movement activity on these roads can remove them from production.
Cultural Resources
Many cultural sites are at risk from landslides, which can destroy artifacts and structures.
Scenic Resources
Sonoma County features a broad range of scenic resources, including the coastline and Pacific Ocean, mountains, hills, ridgelines, inland water features, forests, agricultural features, and distinctive rural communities. Many of these resources or access routes to them are vulnerable to landslides.
11.5 FUTURE TRENDS IN DEVELOPMENT
Land use controls (such as prohibiting development on unstable soils or steep slopes) are the most cost effective way to prevent loss of life and property. The County and its planning partners are equipped to handle future growth within landslide hazard areas. All municipal planning partners have general plans that address landslide risk areas in their safety elements. All partners have committed to linking their general plans to this hazard mitigation plan update. This will create an opportunity for wise land use decisions as future growth impacts landslide hazard areas.
The California Building Standards Code has adopted the International Building Code (IBC) by reference. The IBC includes provisions for geotechnical analyses in steep slope areas that have soil types considered susceptible to landslide hazards. These provisions assure that new construction is built to standards that reduce the vulnerability to landslide risk. Building construction and grading activities are subject to County code that require a geotechnical report or slope stability analysis under specific slope conditions. The County requires a site evaluation prior to building plan check. Geologic maps are reviewed during the site evaluation and where building or grading is proposed in areas mapped with landslides, expansive soils, liquefaction potential, or fault rupture hazards, a geotechnical report is required and design mitigations identified.
11.6 SCENARIO
Major landslides in Sonoma County occur as a result of soil conditions that have been affected by severe storms, groundwater or human development. Landslides are most likely during late winter when the water table is high.
After heavy rains, soils become saturated with water. As water seeps downward through upper soils that may consist of permeable sands and gravels and accumulates on impermeable silt, it will cause weakness and destabilization in the slope. The worst-case scenario for landslide hazards in the planning area would generally correspond to a severe storm with heavy rain and flooding and/or high ocean waves, followed by a damaging earthquake. An earthquake that occurs when water tables are high and soils are saturated has the potential to trigger a significant number of landslides in the planning area. 11-11
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements
11.7 ISSUES
Important issues associated with landslides in the planning area include the following:
• An accurate picture of where landslides occurred during previous storms is vital in making intelligent land use planning and mitigation decisions.
• Landslides may result in isolation of neighborhoods and communities, due to the fact that large portions of the transportation infrastructure are in areas of high and moderate slope instability. Isolation may result in food shortages, loss of power, and severely reduced economic productivity. • There are critical facilities in areas of unstable slopes that could result in interruption to utility services, particularly water and power. This creates a need for mitigation and for continuity of operations planning to develop procedures for providing services without access to essential facilities. • Landslides may result in loss of water quality to the environment and for drinking purposes, due to increased sediment delivery into surface waterways.
• There are existing homes in landslide hazard areas throughout the planning area. The degree of vulnerability of these structures depends on the codes and standards the structures were constructed to.
Information to this level of detail is not currently available. • The impact of climate change on landslides is uncertain. If climate change impacts the timing and intensity of rain event, then the frequency of landslide events may increase. • The risk associated with the landslide hazard overlaps the risk associated with other hazards such as earthquake, flood, and wildfire. This provides an opportunity to seek mitigation alternatives with multiple objectives that can reduce risk for multiple hazards.
• California’s Disclosures in Real Property Transactions law requires disclosure if a property is in a landslide hazard area. Such disclosure is dependent upon knowledge by the seller or the seller’s real estate agent or the posting of a landslide hazard map at the offices of the County recorder, County assessor, and County planning agency and a notice identifying the location of the map and any changes to it. • Future development could lead to more homes in landslide risk areas. • Mapping and assessment of landslide hazards are constantly evolving. As new data and science become available, assessments of landslide risk should be reevaluated. • Coastal bluff erosion is particularly susceptible to ocean wave height and the direction of wave approach.
El Niño conditions often result in substantial increases in the of coastal bluff retreat. Roads and residential developments are most exposed to these hazards.
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12. SEA LEVEL RISE
12.1 GENERAL BACKGROUND
In the past century, global mean sea level has increased by 7 to 8 inches. The two major causes of global sea level rise are thermal expansion of warming oceans and the melting of land-based glaciers and polar ice caps. Given current trends in greenhouse gas emissions and increasing global temperatures, sea level rise is expected to accelerate in the coming decades, with scientists projecting as much as a 66-inch increase in sea level along segments of California’s coast by 2100. While over the next few decades, the most damaging events are likely to be dominated by large El Niño - driven storm events in combination with high tides and large waves, impacts will generally become more frequent and more severe in the latter half of this century (California Coastal Commission 2019).
Approximately 85 percent of California’s population live and work in coastal counties. The sea level along California’s coasts has risen nearly 8 inches in the past century and is projected to rise by as much as 20 to 55 inches by the end of the century. A 55-inch sea level rise could put nearly half a million people at risk of flooding by 2100, and threaten $100 billion in property and infrastructure, including roadways, buildings, hazardous waste sites, power plants, and parks and tourist destinations. Coastal erosion could have a significant impact on California’s ocean-dependent economy, which is estimated to be $46 billion per year.
As sea levels rise, saltwater contamination of the State’s delta and levee systems will increase. Saltwater contamination of the Sacramento/San Joaquin Delta will threaten wildlife and the source of drinking water for 20 million Californians. Farmland in low areas may also be harmed by salt-contaminated water (California Office of the Attorney General 2021). The third National Climate Assessment cites strong evidence showing that the cost of doing nothing exceeds the costs associated with adapting to sea level rise by 4 to 10 times.
12.2 HAZARD PROFILE
Sonoma County’s ocean coast regularly experiences erosion, flooding, and significant storm events, and sea level rise would exacerbate these processes. Sea level at the San Francisco tide gauge has risen 8 inches over the past century, and the National Research Council projects that by 2100 sea level in California south of Cape Mendocino may rise 66 inches. It is critically important that Sonoma County plan and prepare to adapt to sea level rise to ensure public resources and coastal communities are resilient for present and future generations. Future development considerations should include future vulnerabilities to sea level rise and corresponding habitat migration (County of Sonoma, 2019).
The 2018 report Conserving California’s Coastal Habitats: A Legacy and a Future with Sea Level Rise identifies numerous potential impacts of sea level rise on coastal and bay land natural resources and natural protections in Sonoma County (Heady et al. 2018):
12-1
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements • As sea levels rise, coastal habitats may be squeezed into an ever-shrinking area between rising seas and human development and infrastructure. Coastal habitats are at risk of being submerged, and their associated species will be lost without immediate conservation and management actions to give them the space and ability to move inland.
• Natural land and open space may become inundated, including important undeveloped areas that serve as protection of developed areas from flooding.
• Higher seas will mean a higher reach for storm surge, which will increase coastal flooding. • Saltwater intrusion into surface and groundwater aquifers will push further and further inland, potentially altering natural habitat communities and impacting agricultural practices. • Currently protected and conserved coastal areas in California and their habitat value, which represents a real and substantial investment for the future, may be impacted or lost.
The report highlights recommendations to conserve California’s coastal habitats in the face of rising sea levels by maintaining resilient conservation lands, conserving resilient landscapes, and managing in place for resilience.
12.2.1 Previous Assessments
Sonoma County Local Coastal Plan
First mandated in 1979 by the California Coastal Act, the Sonoma County Local Coastal Plan is an important planning document in managing the conservation and development of the county’s coastal regions. The current Local Coastal Plan was written in 1981 and amended in 2001. The County is currently in the process of updating the plan. General findings of the plan are as follows (County of Sonoma 2019):
• Rising seas increase the risk of coastal flooding, storm surge inundation, bluff and coastal erosion, shoreline retreat, saltwater intrusion, and wetland loss or migration. • The net result of coastal storms and sea level rise is coastline retreat, ranging from a few centimeters per year for bluffs made of resistant bedrock to several meters for beaches and dunes. These rates of coastline retreat will increase with rising sea levels and are likely to further increase if waves become higher. • The impacts of sea level rise will vary according to local factors such as shoreline characteristics and topography, the location and extent of development, and local drainage and wind patterns. • Sea level rise will result in more frequent flooding and gradual inundation, as well as increased bluff, dune, and shoreline erosion. This flooding and erosion will affect transportation facilities, utility systems, storm water systems, ports and harbors, large wetland areas, and coastal development (i.e., homes and businesses).
Bodega Bay Vulnerability Assessment, Sonoma County Local Coastal Plan Sonoma County’s Bodega Bay Focused Sea Level Rise Vulnerability Assessment and Adaptation Strategies is an appendix to that Local Coastal Plan that focuses on Bodega Bay, the coastal community most at risk from the impacts of sea level rise. The Bodega Bay vulnerability assessment identifies coastal areas and assets in Bodega Bay exposed to sea level rise and storm events. It analyzes the location and extent of assets projected to be inundated by sea level rise and flooded by storm events and assesses the impacts of inundation and flooding. The assessment also identifies potential adaptation measures to minimize the risks and impacts of inundation and flooding. The sea level rise and storm scenarios used in the analysis are based California sea level rise projections adopted by the National Research Council in 2012 and the Our Coast Our Future website. The assessment 12-2
12. Sea Level Rise
selected five sea level rise and storm scenarios that cover a full range of impact to affected coastal communities by the end of the century. Its findings by area are as follows (Permit Sonoma 2017):
• The northern section of Bodega Bay—the Bodega Harbor area—contains all the marinas, the only rural residential development, and the largest area of urban residential development in the Bodega Bay study area. By 2100 under the worst-case scenario, permanent inundation from sea level rise would affect 59 to 99 percent of marinas; 28 to 76 percent of County roads; 53 percent of a coastal wetland, and up to 14 percent of residential areas.
• The eastern section of Bodega Bay—the Highway 1 area—contains all the commercial development and the only public utility (Bodega Bay PUD Wastewater Treatment Plant) in the Bodega Bay study area. By 2100 under the worst case scenario, permanent inundation from sea level rise would affect 9 to 70 percent of commercial areas, 51 percent of the Bodega Harbour Yacht Club, 13 to 22 percent of residential areas, and 2 percent of a public access and recreation area.
• The southern section of Bodega Bay—the County Regional Parks Area—contains the only County parks (Westside and Doran Beach Regional Parks) and institutional development (U.C. Davis Bodega Marine Laboratory) in the Bodega Bay study area. By 2100 under the worst case scenario, permanent inundation from sea level rise would affect 20 to 73 percent of coastal wetlands, almost 100 percent of Westside Regional Park and 36 percent of Doran Beach Regional Park, 26 to 39 percent of County roads, and 23 percent of the Links at Bodega Harbor Golf Course.
12.2.2 Location
Sea level rise is likely to affect all coastal areas of Sonoma County. The habitats fringing a coastline attenuate waves and thus reduce storm-related damage to shorelines from erosion and inundation. North of the Russian River mouth to the northern extent of Sonoma County, kelp forest habitat backed by rocky cliffs dominate the landscape and are generally low exposure. In contrast, south of the Russian River mouth, a greater diversity of habitats (e.g., wetlands, beaches, dune systems) are present and are habitats that are highly exposed to erosion and inundation during storms compared to north of the river mouth. As coastal development and rising sea levels alter or damage these habitats, coastlines and nearby infrastructure become increasingly vulnerable to storms (County of Sonoma 2019).
The USGS’s Coastal Storm Modeling System sea level rise data were used in the risk assessment for this hazard mitigation plan. The data indicate sea-level rise inundation areas for a sea-level rise of 200 centimeters with and without a 100-year storm event. The mapped inundation areas for these two scenarios (200-cm SLR and 200-cm SLR + 100-Yr) are shown on Figure 12-1 and Figure 12-2.
12.2.3 Frequency
Sea level rise is an ongoing phenomenon that will likely impact the frequency and severity of coastal storms.
Storms and flooding in California typically occur during the winter from November to April and are influenced by several climate patterns, most prominently the El Niño Southern Oscillation. Every two to seven years, the Southern Oscillation alternates between two phases—La Niña and El Niño. El Niño years generally result in persistently low air pressure, greater rainfall, and high winds. The water levels reached during these large, short- term events have exceeded mean sea levels projected for 2100, so understanding their additive effects is crucial for coastal planning (County of Sonoma 2019).
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Cotati
Sonoma
Windsor
Healdsburg
Sebastopol
Rohnert Park
Santa
Rosa
Cloverdale
Petaluma ¬«12
¬«116
¬«
116
¬«
37 ¬«
121
¬«
12 ¬«
128
¬«1
¬«1
£¤101
£¤
101
Figure 12-1. Estimated Inundation Area for 200-Centimeter Sea Level Rise with No Storm ±
0 10 5
Miles
Data Sources: Sonoma Co.,
OCOF
Inundation Area
Cities
County Boundary
Highways
Cotati
Sonoma
Windsor
Healdsburg
Sebastopol
Rohnert Park
Santa
Rosa
Cloverdale
Petaluma ¬«12
¬«116
¬«
116
¬«
37 ¬«
121
¬«
12 ¬«
128
¬«1
¬«1
£¤101
£¤
101
Figure 12-2. Estimated Inundation Area for 200-Centimeter Sea Level Rise with a 100-Year Storm Event
±
0 10 5
Miles
Data Sources: Sonoma Co.,
OCOF
Inundation Area
Cities
County Boundary
Highways
7 Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Low air pressure during a storm causes an immediate rise in sea level above predicted tides, referred to as storm surge. It also increases wind activity, generating erosive waves on top of the already high sea level. This combination of factors during an El Niño event can cause widespread damage in coastal areas. As sea level rises, flooding from storms will become more frequent and potentially more hazardous (County of Sonoma 2019).
Severity
Models projecting sea level rise provide a range of severity scenarios based on assumptions about the rate of climate change. One medium-rate sea level rise scenario for Sonoma County predicts increases from the 1992 baseline sea level as shown in Figure 12-3. While Sonoma County’s ocean coast regularly experiences erosion, flooding, and significant storm events, sea level rise would exacerbate these natural processes and lead to significant social, environmental, and economic impacts. The third National Climate Assessment cites strong evidence showing that the cost of doing nothing exceeds the costs associated with adapting to sea level rise by 4 to 10 times (County of Sonoma 2019).
Source: Climate Central, 2021
0.5 0.7
1 1.3
1.7 2.1
2.6 3
3.6 3.9
4.2 4.7
5.3 5.9
0 1
2 3
4 5
6 2020 2030 2040 2050 2060 2070 2080 2090 2100 2110 2120 2130 2140 2150 Sea Level Rise Above 1992 Baseline (feet)
Figure 12-3. Medium Projection for Sonoma County Sea Level Rise Above 1992 Baseline
12.3 EXPOSURE
12.3.1 Population
The planning team overlaid the sea-level-rise projection data on the population and asset data developed for the hazard risk assessment for this plan. Detailed results by district are provided in Appendix D; results for the total planning area are presented in Table 12-1.
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- - -
12. Sea Level Rise
Table 12-1. Estimated Population Exposure for Sea Level Rise 200 cm SLR 200 cm SLR + 100 Yr
Population Exposed 356 1,106
% of Total Planning Area Population 0.07% 0.23%
Socially vulnerable populations exposed to the sea level rise hazard were estimated based on data for the Census- defined blocks that lie at least partially within the mapped 200-cm SLR + 100-Yr inundation zone. Because many of those Census blocks extend outside the mapped inundation zone, the estimates are greater than the actual exposed populations, but they provide reasonable relative data for use in mitigation planning. Figure 12-4 summarizes the estimated exposure of socially vulnerable populations.
Number of People in Hazard Area
12,000
10,000
8,000
6,000
4,000
2,000
0 Over Age 65
1,209
(17.5% of Total)
Under Age 16
1,138
(16.5% of Total)
200-cm SLR
Total
6,907
Over Age 65
1,609
(15.9% of Total) Total
10,099
Under Age 16
1,896
(18.8% of Total)
200-cm SLR + 100-Yr Storm
Number of Households in Hazard Area
5,000
4,500
4,000
3,500
3,000
2,500
2,000
1,500
1,000
500
0 Low-Income
1,824
(56.9% of Total)
200-cm SLR
Total
3,204
Low-Income
2,564
(57.5% of Total)
Total
4,457
200-cm SLR + 100-Yr Storm
See Section 4.8.1 for the definition of “low income” used in this analysis Figure 12-4. Socially Vulnerable Populations in 200-cm SLR + 100-Yr Inundation Zone Census Blocks 12-7
- - -
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements
12.3.2 Property
Detailed results for property exposed to the sea-level rise hazard by district are provided in Appendix D; results for the total planning area are presented in Table 12-2. Current land use distribution, as represented by building occupancy class, in the areas affected by sea level rise is shown in Figure 12-5 for the 200-centimeter sea level rise scenarios with and without a 100-year storm event.
Table 12-2. Estimated Property Exposure for Sea Level Rise 200 cm SLR 200 cm SLR + 100 Yr
Number of Buildings Exposed 328 737
Value of Exposed Structures $871,633,025 $1,487,128,142 Value of Exposed Contents
Total Exposed Property Value
$832,486,802
$1,704,119,828
$1,434,604,620
$2,921,732,763
Total Exposed Value as % of Planning Area Total 0.78% 1.34% 200-CM SLR
Commercial
53 16%
Residential
126
39%
Education
0 0%
Government Religion
14 0
4% 0%
Industrial
13 4%
Agriculture
122
37%
200-CM SLR + 100-YR
Commercial Residential 169363 23%49%
Education Industrial 5 321% 4%
Agriculture Government Religion
13929 0
19%4% 0%
Figure 12-5. Building Occupancy Classes in the Mapped Flood Zones
12.3.3 Critical Facilities
The exposure of critical facilities by sea level rise inundation zone and facility type is shown in Figure 12-6.
There are over 50 hazardous waste facilities in the 200-Centimeter SLR + 100-Year Storm inundation zone.
Inundation of hazardous waste facilities poses a number of important risks to the immediate community, including public health concerns.
12.3.4 Environment
All sea level rise inundation areas are exposed and vulnerable to impacts. Many of the SLR inundation areas include important environmental and natural resources, which are often important elements in nature-based SLR and flooding strategies.
12-8
12. Sea Level Rise
Facilty Type
25 Transportation 22 200-Centimeter SLR + 100-Year Storm 385
200-Centimeter SLR
11 Safety & Security 4 Planning Area Total
650
6 Health & Medical 1
219
73 Hazardous Material 24
3,109
2 Food, Water, Shelter 1
131
2 Energy 1
74 6
Communications 3
191
0 500 1,000 1,500 2,000 2,500 3,000
Number of Facilities in Identified Area
Figure 12-6. Critical Facilities in Sea-Level Rise Inundation Areas 3,500
12.4 VULNERABILITY
No quantitative vulnerability analysis was performed for the sea level rise hazard, as no hazard mitigation models for quantitative analysis have yet been developed or validated. The vulnerability analysis for this hazard is based on qualitative projections. The following potential and critical impacts were identified in a qualitative review of recent scientific research on the topic of sea level rise in California:
• Storm drainage systems may experience backups as a result of higher level of daily tidal flooding, especially if outfalls are located within sea level rise inundation areas. • Important coastal habitat may be lost as sea level rise permanently inundates areas, or it may be damaged due to extreme tide and storm surge events.
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Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements • Saltwater intrusion into freshwater resources may occur, further altering habitat and ecosystems.
Protective ecosystem services may be lost as land area and wetlands are permanently inundated.
Residents, businesses, and industries that currently thrive on the shoreline will be at risk of flooding by the middle of the century if nothing is done to protect, elevate or relocate them. A 16-inch sea level rise (relative to sea level in 2000) could expose 281 square miles of Bay shoreline to flooding, and a 55-inch rise could expose 333 square miles. If no adaptation measures were taken, a 55-inch rise would place an estimated 270,000 people in the Bay Area at risk from flooding, 98 percent more than are currently at risk. The estimated value of Bay Area shoreline development at risk from a 55-inch rise in sea level is $62 billion—two-thirds of all the estimated value of development vulnerable to sea level rise along California’s entire coastline. Any increased storm activity resulting from climate change, in combination with higher sea level, could cause even greater flooding (San Francisco Bay Conversation and Development Commission, 2011).
Populations seeking areas to relocate as they retreat from rising sea levels could significantly affect Sonoma County. This could impact housing needs, thus increasing exposure within Sonoma County to the other hazards profiled in this plan (San Francisco Bay Conversation and Development Commission, 2011).
12.5 FUTURE TRENDS IN DEVELOPMENT
The overall land area of Sonoma County will decrease as sea level rise permanently inundates the County’s lowest areas. This will have significant impacts on land use and planning in local communities. Local general plans as well as climate action/adaptation plans in the planning area will guide this future development. State mandates have sought to strengthen land use application in areas impacted by sea level rise. Local general plans should be referenced and cross-referenced with the results of this Plan to mitigate future development in areas most vulnerable to sea level rise.
12.6 SCENARIO
Sea levels along the Sonoma County coast will rise over the next 80 years and beyond, and the county and coastal cities will be adversely impacted by that rise. The impacts are already happening and will progress over time. The planning partners are already preparing for these impacts using programs such as the Local Coastal Plan and other current projections customized for the immediate region. Mitigating the impacts from sea-level rise will take resources and tough land use decisions over the next 30 years, starting immediately.
12.7 ISSUES
The planning team has identified the following sea-level-rise-related issues:
• Available funding is not adequate to mitigate the impacts from sea level rise. • The County should consider the adoption of higher regulatory standards to mitigate impacts of sea-level rise on redevelopment.
• The data and science that measure sea-level rise impacts progress rapidly. The County should commit to staying in line with the best available data and science on sea-level rise as it evolves. • The costs to mitigate impacts from sea-level rise will be extensive and potentially beyond the County’s means.
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12. Sea Level Rise
• There needs to be a determination of where people can go when the only option to mitigate the impacts from sea-level rise is to retreat.
• The County will need to find ways to equitably mitigate impacts from sea-level rise. • Risk communication will be crucial to the successful mitigation of this hazard. 12-11
13. SEVERE WEATHER
13.1 GENERAL BACKGROUND
Severe weather refers to any dangerous meteorological phenomena with the potential to cause damage, serious social disruption, or loss of human life. The most common severe weather events to impact the planning area are thunderstorms, damaging winds and extreme heat. For this risk assessment, the term “severe weather” refers to these three event types in aggregate. They are assessed as a single hazard for the following reasons:
• Records indicate that each of these weather event types has impacted the planning area to some degree, and all have similar frequencies of occurrence.
• None of these weather event types have a clearly defined extent or location. Therefore, no quantitative, geospatial analysis is available to support exposure or vulnerability analysis; the analyses for this hazard are qualitative.
13.1.1 Thunderstorms, Lightning and Hail
NOAA classifies a thunderstorm as a storm with lightning and thunder produced by cumulonimbus clouds, usually producing gusty winds, heavy rain, and sometimes hail. Thunderstorms are usually short in duration (seldom more than two hours), but they may deliver enough rainfall to cause urban or flash flooding.
Lightning is an electrical discharge that results from the buildup of positive and negative charges within a thunderstorm. When the buildup becomes strong enough, lightning appears as a “bolt.” This flash of light usually occurs within the clouds or between the clouds and the ground. A bolt of lightning instantaneously reaches temperatures approaching 50,000 ºF. The rapid heating and cooling of air near the lightning causes thunder.
Hail occurs when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere where they freeze into ice. Eventually, the hailstones encounter downdraft air and fall to the ground. Hailstones can begin to melt and then re-freeze together, forming large and very irregularly shaped hail.
13.1.2 Damaging Winds
Straight-Line Winds
Straight-line wind is a general term used to describe damaging winds that are not tornadoes. They are many different types of straight-line winds. Most damaging straight-line winds are generated by thunderstorm systems, although some result from other types of weather phenomena. Damaging winds are those that exceed 50 to 60 mph. The Beaufort Wind Chart (Table 13-1) provides terminology and a description of potential impacts at different levels (National Severe Storms Laboratory, 2018). 13-1
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Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Table 13-1. Beaufort Wind Chart
Beaufort
Number
Range
(mph) Terminology Description
0 0 Calm Calm. Smoke rises vertically.
1 1-3 Light air Wind motion visible in smoke.
2 4-7 Light breeze Wind felt on exposed skin. Leaves rustle. 3 8-12 Gentle breeze Leaves and smaller twigs in constant motion. 4 13-18 Moderate breeze Dust and loose paper is raised. Small branches begin to move. 5 19-24 Fresh breeze Smaller trees sway
6 25-31 Strong breeze Large branches in motion. Whistling heard in overhead wires. Umbrella use is difficult. 7 32-38 Near gale Whole trees in motion. Some difficulty when walking into the wind. 8 39-46 Gale Twigs broken from trees. Cars veer on road. 9 47-54 Sever gale Light structure damage.
10 55-63 Storm Trees uprooted. Considerable structural damage. 11 64-73 Violent storm Widespread structural damage.
12 74-95 Hurricane Considerable and widespread damage to structures.
Source: Lewis, n.d.
Tornado
A tornado is a violently rotating column of air with circulation reaching the ground. It almost always starts as a funnel cloud and may be accompanied by a loud roaring noise. Tornadoes are extremely destructive on a local scale (NOAA, NWS, 2009). A tornado is the smallest and potentially most dangerous of local storms. It is formed by the turbulent mixing of layers of air with contrasting temperature, moisture, density, and wind flow. The mixing layers of air account for most of the tornadoes occurring in April, May, and June, when cold, dry air meets warm, moister air moving up from the south. Tornado severity classified on the Fujita Tornado Damage Scale is shown in Table 13-2.
Table 13-2. Operational Enhanced Fujita Scale
Enhanced Fujita Number 3 Second Gust (mph)
0 65-85
1 86-110
2 111-135
3 136-165
4 166-200
5 Over 200
Source: NOAA, 2018a
13.1.3 Extreme Heat
Extreme heat is defined as temperatures that hover 10 ºF or more above the average high temperatures for a region for several days or weeks. Extreme heat events can lead to an increase in heat-related illnesses and deaths, cause drought, and impact water supplies. Such events do not typically impact buildings; however, losses may be associated with the urban heat island effect and overheating of heating, ventilation, and air conditioning systems.. 13-2
13. Severe Weather
Extreme heat is the primary weather-related cause of death in the United States. In a 10-year record of weather fatalities across the nation (2006 – 2015), excessive heat claimed more lives each year than floods, lightning, tornadoes, and hurricanes. According to the California Climate Adaptation Strategy, heat waves have claimed more lives in California than all other declared disaster events combined. Despite this history, not a single heat emergency was proclaimed in California at the state or federal level between 1960 and 2016. Heat waves do not strike victims immediately, but their cumulative effects slowly cause harm to vulnerable populations. Older adults, children, and sick or overweight individuals are at greater risk from extreme heat.
13.1.4 Public Safety Power Shutoff Events
Some combinations of weather conditions—particularly high winds, extreme heat, and low humidity—pose increased risks of wildfire. In 2012, the California Public Utilities Commission ruled that California Public Utilities Code gives electric utilities authority to shut off electric power to protect public safety, since power supply systems have the potential ignite wildfires (California Public Utilities Commission 2021). Such shutoffs are referred to as public safety power shutoff events. Given the long, connected nature of power supply systems, a shutoff event targeted to a small at-risk area can affect a larger area outside the risk zone. The duration of a shutoff is tied directly to the severe weather that triggers it; the shutoff typically ends within 24 hours after the severe weather has passed (Pacific Gas & Electric n.d.).
13.2 HAZARD PROFILE
13.2.1 Past Events
Table 13-3 summarizes severe storm, wind, and heat events in the planning area since 2020 with recorded deaths, injuries, or property damage.
Table 13-3. Recorded Past Severe Weather Events in the Planning Area Date Event Type Deaths (Direct or Indirect) Injuries (Direct or Indirect) Property or Crop Damage 10/27/2019 Strong Wind 0 0 $3,000
6/11/2019 Heat 1 0 $0
5/16/2019 Strong Wind 0 0 $1,000
4/16/2019 Strong Wind 0 4 $0
1/16/2019 Heavy Rain 1 0 $0
1/16/2019 Strong Wind 1 0 $0
12/31/2018 Strong Wind 1 0 $0
11/29/2018 Strong Wind 0 0 $10,000
2/8/2015 Heavy Rain 0 0 $25,000
2/6/2015 High Wind 0 0 $23,500
12/30/2014 Strong Wind 0 0 $70,600
2/28/2014 Strong Wind 0 0 $5,000
11/21/2013 Strong Wind 0 1 $35,000
10/4/2013 Strong Wind 0 0 $2,000
5/1/2013 Strong Wind 0 0 $80,000
4/8/2013 Strong Wind 0 0 $9,500
12/23/2012 Strong Wind 0 0 $13,000
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Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Date Event Type Deaths (Direct or Indirect) Injuries (Direct or Indirect) Property or Crop Damage 12/22/2012 Heavy Rain 1 2 $30,000
12/5/2012 Strong Wind 0 0 $5,000
12/2/2012 Strong Wind 0 0 $134,000
11/30/2012 Strong Wind 1 0 $50,100
11/28/2012 Strong Wind 0 0 $1,000
3/27/2012 Strong Wind 0 0 $150,000
3/16/2012 Strong Wind 0 0 $412,500
3/14/2012 Strong Wind 0 0 $24,000
3/13/2012 Strong Wind 0 0 $65,000
1/23/2012 Strong Wind 0 0 $22,500
1/21/2012 Strong Wind 0 0 $60,000
1/20/2012 Strong Wind 0 0 $4,500
12/3/2011 Strong Wind 0 0 $200,000
11/30/2011 Strong Wind 0 0 $62,000
11/1/2011 Strong Wind 0 0 $17,000
10/5/2011 Strong Wind 0 0 $500
6/4/2011 Heavy Rain 0 0 $20,015,000
3/24/2011 Heavy Rain 0 0 $43,500
3/19/2011 High Wind 1 0 $35,000
3/18/2011 Tornado 0 0 $50,000
2/17/2011 Strong Wind 0 0 $35,000
2/16/2011 Heavy Rain 0 0 $25,000
2/15/2011 High Wind 0 0 $250,000
12/28/2010 Strong Wind 1 0 $85,000
12/20/2010 Strong Wind 0 0 $25,000
12/6/2010 Strong Wind 0 0 $45,000
10/24/2010 Strong Wind 0 0 $85,000
2/23/2010 Strong Wind 0 0 $15,000
1/25/2010 Strong Wind 0 0 $50,000
1/20/2010 Strong Wind 0 0 $435,000
1/19/2010 High Wind 0 0 $225,000
1/18/2010 Strong Wind 0 0 $110,000
11/28/2009 Strong Wind 0 0 $85,000
10/13/2009 High Wind 0 0 $160,000
5/5/2009 Heavy Rain 1 1 $50,000
4/14/2009 Strong Wind 0 0 $140,000
3/22/2009 Strong Wind 0 0 $2,000
2/26/2009 Strong Wind 0 0 $15,000
2/22/2009 Strong Wind 1 0 $5,000
2/15/2009 High Wind 0 0 $50,000
12/16/2008 Heavy Rain 1 0 $25,000
11/1/2008 Strong Wind 0 0 $16,000
12/27/2006 Strong Wind 1 0 $500,000
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13. Severe Weather
Date Event Type Deaths (Direct or Indirect) Injuries (Direct or Indirect) Property or Crop Damage 7/21/2006 Heat 1 0 $0
1/27/2005 Tornado 0 0 $150,000
12/29/2004 Tornado 0 0 $3,000
1/1/2002 Heavy Rain 0 0 $200,000
1/25/2001 Lightning 0 1 $1,000,000
Total 13 9 $25,445,200
Source: NOAA Storm Events Database, 2021
13.2.2 Location
Severe weather events have the potential to happen anywhere in the planning area. Mountainous regions experience heavier snowfall and a greater risk of road closures. Wind events are most damaging to areas that are heavily wooded. Under most conditions, the planning area’s highest winds come from the southwest.
13.2.3 Frequency
Table 13-3 lists 65 storm, wind, or heat severe weather events in the planning area since 2000 that caused death, injury or property damage. This amounts to a little more than three damaging severe weather events every year on average. The probability of a severe weather event impacting the planning area is high.
13.2.4 Severity
Of the 65 damaging weather events listed in Table 13-3:
• 53 were associated with high winds, including three tornadoes. These events caused seven deaths and five injuries, and resulted in $4 million in property or crop damage. • Two deaths were caused by extreme heat.
• One injury was caused by lightning. Lightning also caused $1 million in property or crop damage. • Nine heavy rain events caused four deaths, three injuries, and $20.4 million in property or crop damage.
13.2.5 Warning Time
Meteorologists can often predict the likelihood of a severe weather event. This can give several days of warning time. However, meteorologists cannot predict the exact time of onset or severity of a storm. Some storms may come on quickly, with only a few hours of warning time.
13.2.6 Secondary Hazards
Major riverine or urban flooding can result from heavy rain. Rain falling on saturated soils on slopes or on areas recently burned by wildfire may lead to landslides. Lightning during thunderstorms presents a risk of starting a wildfire. Extreme heat can contribute to fire-prone dry vegetation. 13-5
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements
13.3 EXPOSURE
All people and property and the entire environment of the planning area is exposed to some degree to the severe weather hazard.
13.4 VULNERABILITY
13.4.1 Population
The most common problems associated with severe weather events are immobility and loss of utilities. Although all populations in the planning area are exposed to severe weather events, some populations are more vulnerable.
Vulnerable populations are the elderly, low income or linguistically isolated populations, people with life- threatening illnesses, and residents living in areas that are isolated from major roads. Power outages can be life threatening to those dependent on electricity for life support. Populations living at higher elevations with large stands of trees or power lines may be more susceptible to wind damage and black out, while populations in low- lying areas are at risk for possible flooding. In general, populations who lack adequate shelter during severe weather events, those who are reliant on sustained sources of power in order to survive, and those who live in isolated areas with limited ingress and egress options are the most vulnerable. The most common impacts of specific weather event types on people are as follows:
• Thunderstorms, Lightning and Hail—California and the planning area are not particularly prone to thunderstorm events and there are no recorded fatalities from lightning within the planning area.
Thunderstorm-related deaths and injuries in the planning area are most likely to result from accompanying wind and heavy rain.
• Damaging Winds—Damaging winds can cause injuries and fatalities in a number of ways. Downed trees may fall on homes or cars, killing or injuring those inside. Objects that are not secured can be picked up in wind events and become projectiles. Structures that collapse or blow over during damaging wind events, especially tornadoes, may kill or injure those inside. • Extreme Heat—Individuals with physical or mobility constraints, cognitive impairments, economic constraints, or social isolation are typically at greater risk from the adverse effects of excessive heat events. The average summertime mortality for excessive heat events is dependent upon the methodology used to derive such estimates. Certain medical conditions, such as heat stroke, can be directly attributable to excessive heat, while others may be exacerbated by excessive heat, resulting in medical emergencies.
Individuals who lack shelter and heating are particularly vulnerable to extreme cold and wind chill.
13.4.2 Property
All property is vulnerable during severe weather events, but properties in poor condition or in particularly vulnerable locations may risk the most damage. The most common impacts of specific weather event types on property are as follows:
• Thunderstorms, Lightning and Hail—Damage from thunderstorms in the planning area is most likely to be related to secondary hazards accompanying the event, such as flooding, landslides or damaging winds. If lightning directly strikes a building, it may cause substantial damage and may even set the structure on fire.
• Damaging Winds—Mobile homes can be seriously damaged by wind gusts over 80 mph, even if they are anchored (National Severe Storms Laboratory, 2018). Properties at higher elevations or on ridges may be 13-6
13. Severe Weather
more prone to wind damage. Falling trees can result in significant damage to structures. A major tornado could cause widespread damage to property in the planning area, but such an event is unlikely. • Extreme Heat—Extreme heat is generally not a threat to damage property.
No modeling is available for quantitative loss estimations for the severe weather hazard. Instead, loss estimates were developed representing 1 percent, 3 percent and 5 percent of the replacement value of exposed structures:
• Loss of 1 percent of planning area replacement value—$2.2 billion • Loss of 3 percent of planning area replacement value—$6.6 billion • Loss of 5 percent of planning area replacement value—$11 billion
13.4.3 Critical Facilities
All critical facilities are vulnerable during severe weather events, especially those that lack backup power generation capabilities. When facilities supplying power to planning area land line telephone systems are frequently disrupted, significant issues arise with communication in the planning area. In addition, some facilities are particularly vulnerable to specific types of severe weather events:
• Thunderstorms—Facilities located in areas prone to localized or major flooding are vulnerable.
Transportation systems are vulnerable to disruption from flooding or secondary hazard such as landslides. • Damaging Winds—Critical facilities in the direct path of a tornado would be particularly vulnerable.
Facilities located near trees or power lines that are likely to fall are also vulnerable. Roads and other transportation infrastructure could be blocked by downed trees or other debris. • Extreme Heat— Extreme heat is generally not a threat to damage facilities or infrastructure.
13.4.4 Environment
The environment is highly vulnerable to severe weather events. Natural habitats such as streams and trees exposed to the elements during a severe storm risk major damage. Prolonged rains can saturate soils and lead to slope failure. Flood events caused by severe weather or snowmelt can produce river channel migration or damage riparian habitat. Storm surges can erode beachfront bluffs and redistribute sediment loads.
13.5 FUTURE TRENDS IN DEVELOPMENT
All future development will be affected by severe weather events. The ability to withstand impacts lies in sound land use practices and consistent enforcement of codes and regulations for new construction. The planning partners have adopted the International Building Code in response to California mandates. This code is equipped to deal with the impacts of severe weather events. Land use policies identified in general plans within the planning area also address many of the secondary impacts (flood and landslide) of the severe weather hazard.
With these tools, the planning partners are well equipped to deal with future growth and the associated impacts of severe weather.
13.6 SCENARIO
A worst-case severe-weather event would involve prolonged high winds during a thunderstorm with large amounts of precipitation after soils are already saturated. Such an event would have both short-term and long-term 13-7
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements effects. Initially, schools and roads would be closed due to power outages caused by high winds and downed tree obstructions. Some areas of the county could experience limited ingress and egress. Prolonged rain could produce flooding, overtopped culverts with ponded water on roads, mud over roadways, and landslides on steep slopes.
Floods and landslides could further obstruct roads and bridges, further isolating residents. If major landslides impact major highways in the planning area, significant transportation disruption could result.
13.7 ISSUES
Important issues associated with severe weather in the planning area include the following:
• The most common direct impact from severe weather events is loss of power. Power outages that disrupt land line service could cause significant communication disruption. • Older building stock in the planning area is built to low code standards or none at all. These structures could be highly vulnerable to severe weather events such as damaging winds. • Redundancy of power supply must be evaluated, especially for critical facilities. • Those residing in higher elevations with limited transportation routes may have the greatest vulnerability to isolation from storms. Another group at risk is the portion of the county population that is over the age of 65.
• Climate change may cause more severe weather patterns that could impact vulnerable populations within the planning area. Increased frequency and intensity of storms may result in greater damage. • Detailed spatial analysis is needed to locate the most vulnerable populations, followed by focused public education and outreach mitigation activities for these populations. • The risk associated with the severe weather hazard overlaps the risk associated with other hazards such as earthquake, landslide, and flood. This provides an opportunity to seek mitigation alternatives with multiple objectives that can reduce risk for multiple hazards. • Isolated population centers are most vulnerable to the severe weather hazard. Rural areas frequently experience extended power outages, loss of communications, and damage to roads due to severe weather. 13-8
14. TSUNAMI
14.1 GENERAL BACKGROUND
A tsunami is a series of high-energy waves that radiate outward like pond ripples from an area where a generating event occurs, arriving at shorelines over an extended period. Tsunamis can be induced by earthquakes, landslides and submarine volcanic explosions (see Figure 14-1). Tsunamis are typically classified as local or distant, depending on the location of their source in comparison to where waves occur:
• The waves nearest to the generating source represent a local tsunami. Such events have minimal warning time, leaving few options except to run to high ground after a strong, prolonged local earthquake. Damage from the tsunami adds to damage from the triggering earthquake due to ground shaking, surface faulting, liquefaction, and landslides.
• The waves far from the generating source represent a distant tsunami. Distant tsunamis may travel for hours before striking a coastline, giving a community a chance to implement evacuation plans if a warning is received.
Figure 14-1. Common Sources of Tsunamis
In the open ocean, a tsunami may be only a few inches or feet high, but it can travel with speeds approaching 600 miles per hour. As a tsunami enters the shoaling waters near a coastline, its speed diminishes, its wavelength decreases, and its height increases greatly. At the shoreline, tsunamis may take the form of a fast-rising tide, a cresting wave, or a bore (a large, turbulent wall-like wave). The bore phenomenon resembles a step-like change in the water level that advances rapidly (from 10 to 60 miles per hour). The first wave is usually followed by several larger and more destructive waves.
The configuration of the coastline, the shape of the ocean floor, and the characteristics of advancing waves play important roles in the destructiveness of the waves. Bays, sounds, inlets, rivers, streams, offshore canyons, islands, and flood control channels may cause various effects that alter the level of damage. Offshore canyons can focus tsunami wave energy, and islands can filter the energy. It has been estimated that a tsunami wave entering a 14-1
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements flood control channel could reach a mile or more inland, especially if it enters at high tide. The orientation of the coastline determines whether the waves strike head-on or are refracted from other parts of the coastline. A wave may be small at one point on a coast and much larger at other points. The inundation area for a tsunami event is often described as runup as illustrated in Figure 14-2.
Source: UNESCO, Retrieved from Different Directions: Tsunami, n.d.
Figure 14-2. Runup Distance and Height in Relation to the Datum and Shoreline
14.2 HAZARD PROFILE
14.2.1 Past Events
California is at risk from both local and distant tsunamis. About 80 possible or confirmed tsunamis in California have been observed or recorded, including the following:
• The most recent recorded tsunamis affecting California were the March 11, 2011 tsunami caused by an earthquake near Japan, which resulted in nearly $100 million in damage to the California maritime community, and the February 27, 2010 minor recorded tsunami inundation in California caused by an earthquake near Chile.
• A 1960 Chilean earthquake produced a tsunami that impacted the entire Pacific basin. Damage was reported in California ports and harbors from San Diego to Crescent City and losses exceeded $1 million. • A 1964 tsunami generated by a Magnitude-9.2 Alaska earthquake (see Figure 14-3) killed 12 in Northern California and caused over $15 million in damage. Wave oscillations in San Francisco Bay lasted more than 12 hours, causing nearly $200,000 in damage to boats and harbor structures. Sonoma County experienced slight tsunami impacts from this event.
14-2
14. Tsunami
Source: National Centers for Environmental Information, 2018 Figure 14-3. 1964 Alaska Earthquake Tsunami Event
14.2.2 Location
Sonoma County’s rugged cliffs and generally elevated coastline reduces its exposure and vulnerability to tsunamis. The areas in Sonoma County that have the greatest exposure to potential damages by a tsunami are those coastal communities along the southern Sonoma County coast from Jenner to Bodega Bay. Tsunami inundation maps for the Sonoma Coast area near Jenner, Bodega Bay, and the San Pablo Bay were released in 2009 and form the basis for the County’s Tsunami Response Plan.
Spud Point Marina is on the coast in Bodega Bay near the San Andreas Fault. Port Sonoma Marina is at the mouth of the Petaluma River in an area of potentially high liquefaction 3 miles west of the Rodgers Creek Fault. Both of these facilities face a potential risk from earthquake-induced tsunamis.
Figure 14-4 shows the mapped extent of the tsunami inundation areas for the Sonoma County planning area used for this risk assessment, as developed by the California Department of Conservation.
14.2.3 Frequency
The National Tsunami Hazard Mitigation Program rates the risk from the tsunami hazard to the U.S. west coast as a whole as high to very high (Dunbar and Weaver 2015). However, the historical record of tsunami events in Sonoma County includes only one minor event. The frequency of tsunami events in the planning area can be assumed to be low.
14-3
Cotati
Sonoma
Windsor
Healdsburg
Sebastopol
Rohnert Park
Santa
Rosa
Cloverdale
Petaluma ¬«12
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¬«116
¬«37
¬«
121
¬«
12 ¬«
128
¬«1
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£¤101
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Figure 14-4. Tsunami Inundation Zones
±
0 10 5
Miles
Data Sources: Sonoma Co.,
CGS
Tsunami Inundation Zones
Highways
Cities
14. Tsunami
14.2.4 Severity
In 2009 Cal OES, the California Geological Survey, and the University of Southern California mapped the tsunami run-up zone for the maximum credible earthquake along the Sonoma Coast, using NOAA’s National Tsunami Hazard Mitigation Program. The modeling projected that a tsunami of 25 feet could occur in the coastal areas of Sonoma County, and that areas off San Pablo Bay could experience a 5 foot tsunami. According to the National Tsunami Hazard Mitigation Program, tsunami events with runups of more than 3 feet are the most likely to be dangerous to people and property.
A tsunami’s size and speed, as well as the coastal area’s form and depth, affect the impact of the tsunami. At some locations, the advancing turbulent wave front will be the most destructive part of the tsunami wave. In other situations, the greatest damage will be caused by the outflow of water back to the sea between crests, sweeping away items on the surface and undermining roads, buildings, bulkheads, and other structures. This outflow action can carry enormous amounts of highly damaging debris, resulting in further destruction. Ships and boats, unless moved away from shore, may be forced against breakwaters, wharves, and other craft, or be washed ashore and left grounded after the withdrawal of the seawater (National Tsunami Hazard Mitigation Program, 2001).
14.2.5 Warning Time
Visible Indications
Tsunamis are difficult to detect in the open ocean; with waves generally less than 3 feet high. The first visible indication of an approaching tsunami may be either a rise or drop in water surface levels (National Tsunami Hazard Mitigation Program, 2001):
• A drop in water level (draw down) can be caused by the trough preceding the advancing, large inbound wave crest. Rapid draw down can create strong currents in harbor inlets and channels that can severely damage coastal structures due to erosive scour around piers and pilings. As the water’s surface drops, piers can be damaged by boats or ships straining at or breaking their mooring lines. The vessels can overturn or sink due to strong currents, collisions with other objects, or impact with the harbor bottom. • The advancing tsunami may initially arrive as a strong surge increasing the sea level. This can be similar to the rising tide, but the tsunami surge rises faster and does not stop at the shoreline. Even if the wave height appears to be small, 3 to 6 feet for example, the strength of the accompanying surge can be deadly.
Waist-high surges can cause strong currents that float cars, small structures, other debris, and hazardous materials. Boats and debris are often carried inland by the surge and left stranded when the water recedes.
Warning System
Tsunami Warning System for the Pacific Ocean
The tsunami warning system for the Pacific Ocean evolved from a program initiated in 1946. It is a cooperative effort involving 26 countries along with numerous seismic stations, water level stations and information distribution centers. The National Weather Service operates two regional information distribution centers: The Pacific Tsunami Warning Center in Ewa Beach, Hawaii; and the National Tsunami Warning Center covering the California coast in Palmer, Alaska. The warning centers issue tsunami watches, warnings, and advisories. A watch is issued when a large earthquake has occurred far away from the region and the threat is still being determined. A warning is issued when damaging tsunami waves inundating dry land are expected. An advisory is issued when tsunami waves less than 1 meter high and dangerous strong currents will occur in harbors. The 14-5
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements warning system is activated when a Pacific basin earthquake of magnitude 6.5 occurs or an earthquake is widely felt along the North American coast. When this occurs, the following sequence of actions occurs:
• Data is interpolated to determine epicenter and magnitude of the event. • If the earthquake is of the right type, depth, magnitude, and is far away from California coast, a TSUNAMI WATCH is typically issued for the California coastline. • A TSUNAMI WATCH is upgraded to a TSUNAMI WARNING if tsunami wave heights are forecast to be 1 meter or larger. A TSUNAMI ADVISORY is issued if tsunami wave heights are forecast to be
0.3 meters to less than 1 meter.
• Tsunami travel times are calculated, and the warning is transmitted to disseminating agencies who relay it to the public.
• The National Tsunami Warning Center will cancel/expire watches, warnings, or advisories if tide gauges and buoys indicate no significant tsunami was generated or if tsunami waves no longer meet the criteria for at least 3 hours.
This system is not considered to be effective for communities close to the tsunami source, because the first wave would arrive before the data can be processed and analyzed, and communications systems may be impacted by the precipitating event. In this case, strong ground shaking would provide the first warning of a potential tsunami and evacuations should begin immediately.
2010 Sonoma County Operational Area Tsunami Response Plan Sonoma County’s 2010 Tsunami Response Plan incorporates the 2009 mapping of the tsunami run-up zone for the maximum credible earthquake along the Sonoma Coast. The plan establishes notification and evacuation response procedures to help minimize casualties from tsunamis.
NOAA, Cal OES, and local emergency managers coordinate tsunami warning communications for the planning area. This emergency notification system is routinely tested and includes broadcasts on NOAA Weather Radio All Hazards, social media, local television and radio stations, sirens, and aircraft public address system. The Wireless Emergency Alert System will also be activated during a real event.
Estimated Travel Times
The NOAA National Centers for Environmental Information website provides maps that show estimated travel times to coastal locations for various tsunami-generating events. Figure 14-5 shows one example of the travel time for a tsunami generated in Aburatsu, Japan to reach the planning area—approximately 11 hours.
14.2.6 Secondary Hazards
Wherever water transport is a vital means of supply, disruption of coastal systems caused by tsunamis can have far-reaching economic effects.
14-6
14. Tsunami
Source: National Centers for Environmental Information, 2018c Figure 14-5. Potential Tsunami Travel Times in the Pacific Ocean, in Hours
14.3 EXPOSURE
Exposure and vulnerability estimates are based on tsunami inundation maps. The value of exposed buildings in the tsunami inundation zone was generated by overlaying the inundation areas on the general building stock. The population living in tsunami hazard zones was estimated using the percent of buildings within the tsunami inundation areas and applying this percent to the estimated planning area population. Detailed results by jurisdiction are provided in Appendix D; results for the total planning area are presented below.
14.3.1 Population
The estimated total population living in the evaluated dam failure inundation zone is 102 (0.02 percent of the total planning area population). People recreating along beaches, low-lying coastal areas, tidal flats, and stream deltas that empty into ocean-going waters also would be exposed.
Socially vulnerable populations exposed to the tsunami hazard were estimated based on data for the Census- defined blocks that lie at least partially within the mapped inundation zone. Because many of those Census blocks extend outside the mapped inundation zone, the estimates are greater than the actual exposed populations, but they provide reasonable relative data for use in mitigation planning. Figure 14-6 summarizes the estimated exposure of socially vulnerable populations.
14-7
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements 700
600
Over Age 65
Total
665
184
(27.7% of Total)
Under Age 16
71 (10.7% of Total)
Low Income
177
(53.3% of
Total)
Total
332
0 50
100
150
200
250
300
350 Number of Households in Hazard Area
Number of People in Hazard Area
500
400
300
200
100
0 See Section 4.8.1 for the definition of “low income” used in this analysis Figure 14-6. Socially Vulnerable Populations in the Mapped Tsunami Inundation Zone Census Blocks
14.3.2 Property
Table 14-1 summarizes the estimated property exposure in the evaluated tsunami inundation areas. Figure 14-7 shows the Hazus-defined occupancy class of all buildings in the tsunami inundation areas. These occupancy classes provide an indication of land use within the mapped hazard area. Some land uses are more vulnerable to inundation, such as single-family homes, while others are less vulnerable, such as agricultural land or parks.
Table 14-1. Exposed Property in the Tsunami Inundation Zone Number of Buildings Exposed 77
Value of Exposed Structures $41,894,144
Value of Exposed Contents $44,710,276
Total Exposed Property Value $86,604,420
Total Exposed Value as % of Planning Area Total 0.04%
14.3.3 Critical Facilities
The breakdown of critical facility exposure by facility type is shown in Figure 14-8. The total exposed facilities (22) is a very small percentage of total critical facilities in the planning area. They include two wastewater treatment facilities, one fire station, one school, one bridge and 14 port facilities. 14-8
14. Tsunami
Facility Category
Residential
51 66%
Commercial
9 12%
Industrial
2 3%
Agriculture
2 3%Religion
0Government Education
1 12 0%
1% 15%
Figure 14-7. Structures in the Tsunami Inundation Zone, by Building Occupancy Class Transportation
Safety & Security
Health & Medical
Hazardous Material
Food, Water, Shelter
Energy
Communications
191
74 131
3,109
219
650
385
0 0
2 2
0 3
15 Tsunami Inundation Area
Planning Area Total
0 500 1,000 1,500 2,000 2,500 3,000 3,500
Number of Facilities in Identified Area
Figure 14-8. Critical Facilities in Mapped Tsunami Inundation Zone and Countywide 14-9
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements
14.3.4 Environment
All waterways and beaches would be exposed to the effects of a tsunami; inundation of water and introduction of foreign debris could be hazardous to the environment. All wildlife inhabiting the area also is exposed.
14.4 VULNERABILITY
No quantitative vulnerability analysis was performed for the tsunami hazard. The following potential impacts were identified:
• The populations most vulnerable to the tsunami hazard are the elderly, disabled and very young who reside near beaches, low-lying coastal areas, tidal flats, and river deltas that empty into ocean going waters.
• In the event of a local tsunami generated in or near the planning area, there would be little warning time, so more of the population would be vulnerable.
• The impact of tsunami waves and the scouring associated with debris that may be carried in the water could be damaging to all structures along beaches, low-lying coastal areas, tidal flats and river deltas. The most vulnerable are those in the front line of tsunami impact and those that are structurally unsound. • Structures that were built to current floodplain regulations in the tsunami inundation area may have some level of protection, particularly if they were built to withstand wave action. In addition to structure damage, ships moored at piers and in harbors often are swamped and sunk or are left battered and stranded high on the shore.
• The following infrastructure is vulnerable to damage:
• Water Proximate Infrastructure—Breakwaters and piers collapse, sometimes because of scouring actions that sweep away their foundation material and sometimes because of the sheer impact of the tsunami waves.
• Flood Control Systems—Floodwaters can back up drainage systems, causing localized flooding.
Culverts can be blocked by debris from tsunami events, also causing localized urban flooding. • Utility Systems—Floodwaters can get into drinking water supplies, causing contamination. Sewer systems can be backed up, causing waste to spill into homes, neighborhoods, rivers and streams.
Tsunami waves can knock down power lines and radio/cellular communication towers. Power generation facilities can be severely impacted by wave action and by inundation from floodwater. • Tsunami waves can carry destructive debris and pollutants that can have devastating impacts on all facets of the environment. Environmental impacts on local waterways and wildlife would be most significant in areas closest to the point of impact. The vulnerability of aquatic habit and associated ecosystems in low- lying areas close to the coastline is high.
14.5 FUTURE TRENDS IN DEVELOPMENT
According to population projections by the California Department of Finance, Sonoma County’s population should decrease to 485,017 by 2040. This represents a 3.8 percent decrease from the 2018 population. Though new development will continue, the rate of development to accommodate future county growth will not be high.
The County is subject to state general planning laws and the California Coastal Act. The County and its cities have adopted critical areas and resources lands regulations pursuant to these laws. The information in this plan 14-10
14. Tsunami
provides the planning partners a tool to ensure that there is no increase in exposure within the mapped tsunami inundation area of the planning area.
The County of Sonoma was officially recognized as a TsunamiReady community in March 2016 by National Weather Service representatives. This designation recognizes voluntary community programs that promote collaborative tsunami hazard preparedness efforts. In order to become a TsunamiReady community, the County developed a local Tsunami Response Plan, mapped inundation areas along the coast, identified evacuation routes, established refuge areas, installed over 160 tsunami signs in the hazard zones, provided education to the public, deployed and maintained redundant and reliable means to disseminate tsunami warnings and participated in readiness exercises (County of Sonoma 2017).
14.6 SCENARIO
The tsunami scenario with the greatest potential impact on the planning area is a tsunami triggered by a major seismic event along the Cascadia subduction zone. Historical records suggest that tsunami wave heights on the order of 15 to 60 feet could be generated by a Cascadia subduction event (see Figure 14-9). The most destructive tsunami will be associated with a local source Cascadia event and will be preceded by strong ground shaking.
Significant damage will result from the ground shaking, tsunami wave forces, and impacts associated with debris.
A major tsunami event in the region would have devastating impacts on the people, property, and economy of the planning area.
Source: National Centers for Environmental Information, 2018 Figure 14-9. 1700 Cascadia Subduction zone Earthquake Tsunami Event 14-11
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements A tsunami from a more local earthquake, such along the San Andreas fault, might be less severe than a Cascadia subduction event. Tsunamis are less commonly associated with strike-slip faults such as the San Andreas system (County of Sonoma 2017). However, a local source tsunami presents a high risk to people, as there would not be time to initiate evacuation; the first surge could arrive in as few as 10 minutes. Strong ground shaking preceding the tsunami could damage buildings, communications and electric utility infrastructure, roads, and bridges, further impairing the community’s ability to evacuate safely.
14.7 ISSUES
Important issues associated with a tsunami in the planning area include the following:
• Risk from tsunami inundation is not subject to the State of California real estate disclosure law at this time.
• Structures in the planning area built before the cities and County entered the NFIP may not be designed to resist tsunami forces.
• Present building codes and guidelines do not adequately address the impacts of tsunamis on structures. It is anticipated that future updates to the California Building Code will include amendments that address these issues.
• As tsunami warning technologies evolve, the tsunami warning capability within the planning area will need to be enhanced to provide the highest degree of warning to planning partners with tsunami risk exposure.
• With the future impacts from climate change, the issue of sea level rise may become an important consideration as probable tsunami inundation areas are identified through future studies. • Special attention will be focused on vulnerable communities and tourists in the tsunami zone and on hazard mitigation through public education and outreach. 14-12
15. WILDFIRE
15.1 GENERAL BACKGROUND
A wildfire is an unplanned, unwanted, uncontrolled fire in an area of combustible vegetation. Wildfires typically start in rural areas but can burn into urban areas. A wildfire requires fire suppression to prevent damage to the natural or human environment. Though most wildfires are started by humans, they can occur naturally, and are important to many ecosystem processes.
15.1.1 Factors Affecting Fire Behavior
Fire behavior is based on factors such as the following:
• Fuel—Fuel may include living and dead vegetation on the ground, along the surface as brush and small trees, and above the ground in tree canopies. Lighter fuels such as grasses, leaves and needles quickly expel moisture and burn rapidly, while heavier fuels such as tree branches, logs and trunks take longer to warm and ignite. Trees killed or defoliated by forest insects and diseases can be more susceptible to wildfire. Structures in the human-built environment also represent a fuel component. • Weather—Relevant weather conditions include temperature, relative humidity, wind speed and direction, cloud cover, precipitation amount and duration, and the stability of the atmosphere. When the temperature is high, relative humidity is low, wind speed is increasing and coming from the east (offshore flow), and there has been little or no precipitation so vegetation is dry, conditions are very favorable for extensive and severe wildfires. These conditions occur more frequently inland where temperatures are higher and fog is less prevalent.
• Topography—Topography includes slope and elevation. The topography of a region influences the amount of moisture retained in fuels; the impact of weather conditions such as temperature and wind; potential barriers to fire spread, such as roads, vineyards, and lakes; and elevation and slope of landforms (fire spreads more easily uphill than downhill). In steep terrain, common geographic features such as drainages, gulches and canyons can funnel air to act as chimneys, pulling hot air, gases, and embers ahead or outside of the main fire. The direction that a slope faces also has a major influence on fire behavior.
South-facing slopes receive heating and drying solar radiation from early in the morning until sunset, whereas north-facing slopes only receive solar radiation during a short period of the day when the sun is high in the sky.
15.1.2 Secondary Hazards
Wildfires can generate a range of secondary effects, which in some cases may cause more widespread and prolonged damage than the fire itself. Fires can cause direct economic losses in the reduction of harvestable timber and indirect economic losses in reduced tourism. Wildfires cause the contamination of reservoirs, destroy transmission lines and contribute to flooding. They strip slopes of vegetation, exposing them to greater amounts 15-1
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements of runoff. This in turn can weaken soils and cause failures on slopes. Major landslides can occur several years after a wildfire. Most wildfires burn hot and for long durations that can bake soils, especially those high in clay content, thus increasing the imperviousness of the ground. This increases the runoff generated by storm events, thus increasing the chance of flooding. These secondary impacts of wildfire can also affect the quantity and quality of water, which can pose a significant challenge to drinking water utilities.
15.2 HAZARD PROFILE
Virtually all of Sonoma County is at risk to wildfire. Risks include, but are not limited to, the following:
• Extensive building in wildland urban interface/intermix (WUI) areas • Lack of vegetation management near homes and in wildland areas • Structures not built or retrofitted with ignition-resistant building materials that can increase resistance to a wildfire’s heat and embers
• A significant likelihood of high wind events during the dry fall months Common fire causes in Sonoma County include electrical transmission line failures, equipment use, vehicle fires spreading into wildlands, and accidental starts from warming or debris fires. Due to heavy fuel loading, when fires start during high wind conditions, rapid rates of wildfire spread can result.
15.2.1 Wildfire Factors for the Planning area
Topography
Two steep ranges dominate the western and eastern lengths of Sonoma County, and most of the county’s wildland-urban interface is in the hills and valleys of these two ranges. The hills of the Coastal Range rise abruptly from the Pacific shoreline to over 2,000 feet. The slopes of the Mayacamas Mountains on the county’s eastern boundary rise from sea level valleys, including the Santa Rosa Plain, up to 4,500 feet on the slopes of Mount St. Helena. Sonoma County’s valleys and foothills are predominantly devoted to agriculture but also contain most of the urbanized areas and population.
Weather
Sonoma County’s primary wildland fire season typically spans the months May through November, with possible extension on both ends. In the fall, strong and dry northeast “Foehn” or “Diablo” winds significantly increase the likelihood and severity of wildland fires across California. With the exception of areas immediately along the coast, the weather during fire season is generally warm and dry during the day. Gradient winds, generally out of the south/southwest, typically strengthen in the late afternoon and diminish by dark.
Vegetation and Fuels
According to Sonoma County’s 2016 Community Wildfire Protection Plan, there were 513,388 acres of coniferous forests and oak woodlands in Sonoma County at that time—more than 50.5 percent of county land area. Most of the oak woodland, and over 68 percent of the coniferous forestland (132,000 acres), is in private ownerships of 50 acres and less. Much of this forest acreage is not regularly maintained and contains dry/dead material and overcrowded trees and brush. The wide variation of ecosystems and microclimates in Sonoma 15-2
15. Wildfire
County make for a wide variety of vegetative fuels. In cooler climates there are redwood ecosystems as well as coastal prairie grasses. In the hills of the Mayacamas, chaparral, Douglas fir and other conifers, as well as oak grass lands, dominate (Fire Safe Sonoma 2016).
Firefighting Resources
A planning area’s ability to suppress or fight fires when they start is a key factor affecting wildfire impacts.
Federal, state, and local fire protection agencies that share resources and knowledge experience more effective and coordinated outcomes. Initial response to all fire, medical, and similar emergencies is the responsibility of 23 local fire departments in the Sonoma County Operational Area (FireDepartment.net, 2021). The County is divided into two types of responsibility areas:
• Local Responsibility Areas (LRA) and municipalities, where a local agency has primary responsibility for fire and emergency response. In LRA areas, local agencies have primary command, though they may request support from CAL FIRE.
• State Responsibility Areas (SRA), where the state firefighting agency, CAL FIRE, has primary responsibility for wildland fires and fires that pose a threat of spreading into the wildland. CAL FIRE has primary command of SRA fires as soon as their units arrive on the scene. SRAs cover 793,793 acres in Sonoma County—78 percent of the county.
Sonoma County is in CAL FIRE’s Sonoma-Lake-Napa Unit, one of 21 CAL FIRE administrative units statewide.
This unit covers 2.1 million acres in Sonoma, Lake, Napa, Yolo, Colusa, and Solano counties. It is served by three divisions and 10 field battalions. Sonoma County is the West Division, which contains four battalions and covers 793,793 acres.
The 23 local fire agencies in Sonoma County include fire protection districts, community services districts, and municipal fire departments. The City of Santa Rosa has its own fire department. The cities of Sonoma and Cotati and the Town of Windsor are served by local fire districts. Many of the fire districts in unincorporated areas are staffed by paid firefighters and supplemented by volunteers. In the areas most prone to wildland fires, fire suppression services are highly dependent on part-time and volunteer fire-fighting personnel, and the number of volunteer fire fighters has decreased in recent years. Volume 2 of this plan provides more information on the local fire protection districts participating in this multi-jurisdictional hazard mitigation planning effort.
15.2.2 Past Events
Wildland fires, particularly wildland/urban interface fires, have historically occurred in Sonoma County. As development and human activity in Sonoma County increased over the decades, the incidence of human-caused fires increased. Some of the largest or costliest fires since 1964 are listed in Table 15-1.
County residents today are greatly aware of the dangers that wildfire poses. From 2017 through 2020, a series of large, damaging wildfires directly affected Sonoma County. These fires burned over 300,000 acres in Sonoma County, destroyed nearly 7,000 structures, and killed 24 people. Wildfire has become an overwhelming and constant presence throughout the summer and fall, bringing long periods of toxic smoke, multiple red-flag warnings, planned power shutdowns, large-scale evacuations, and fear and trauma on the part of urban and rural residents alike.
15-3
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Table 15-1. Sonoma County Fires Since 1964
Name Acres Burned Structures Burned
1964 Hanley 52,700 108
1964 Nuns Canyon 10,400 27
1965 Knight’s Valley 6,000 0
1965 Pocket Ranch 4,000 0
1965 Austin Creek 7,000 0
1972 Bradford 1,760 4
1978 Creighton Ridge 11,405 64
1988 Cloverdale 1,833 100
1988 Geysers 9,000 7
1996 Porter Creek 300 0
1996 Cavedale 2,100 0
1999 Geyser Road 1,300 0
2000 Berryessa 5,731 15
2004 Geysers 12,000 6
2008 85 322 0
2008 Pine 989 0
2013 McCabe 3,505 —
2015 Valley 76,067 (5,000 in Sonoma County) 1,955
2017 Sonoma Complex Fires (Tubbs, Nuns, Pocket, Presley, Young) 86,039 5,636 2017 Tubbs 36,807 5,643
2019 Kincade 77,753 371
2020 LNU Lightning Complex (Walbridge and Meyers) 57,563 303 2020 Glass 67,484 661
Wildfire Chronology
When humans moved into the landscape some 14,000 years ago, fire became a common feature on the landscape, as native people used fire to increase food production, keep more open landscapes, and promote other ecological values. It is estimated that grasslands and oak woodlands were burned about every 5 years, with forested areas burning about every 12 to 20 years.
Fire frequency decreased during post-statehood years, as European settlers did not perceive the ecological value of burning as it had been practiced by Native Americans. However, wildfires continued to ignite and burn, especially in historical wildfire corridors, These were primarily in the Mayacamas Mountains on the county’s eastern boundary in the Coast Ridges on the west. Large destructive fires have occurred in both areas, most significantly in 1923, 1954, 1964, 1978, and 2017, 2019, and 2020.
In 1964, the Hanley and Nuns Canyon fires simultaneously burned 63,100 acres and 135 structures near the cities of Santa Rosa and Sonoma. Since then, there has been significant growth in rural WUI areas in the county, vastly increasing risks to life and property. The conversion of properties from agriculture, timber production, and grazing uses to residential use has left much of the county’s wildland vegetation to grow unchecked by human activity or fire, significantly increasing potential for destructive wildfire. 15-4
15. Wildfire
In 2017, the Sonoma Complex fires (the Tubbs, Nuns, Adobe, and Pocket and Young fires) killed 24 residents and burned 110,716 acres. The 2017 Tubbs and Nuns Fires burned in nearly identical footprints to 1964’s Hanley and Nuns Canyon fires, but because of development in the area nearly 6,000 structures were lost, compared with 135 in the earlier fire. Impacts like those from the 2017 fires will continue to impact the County and its residents for decades.
Sonoma County’s recent wildfires reflect increasing fire size and intensity across California and the west.
Table 15-2, derived from CAL FIRE data, shows the increasing frequency of destructive fires over the past 80 years. The 20 most destructive fires in state history all occurred in the previous 30 years. Of those 20 most destructive fires, the last 5 years represent:
• 75 percent of the number of events
• 85 percent of acres burned
• 82 percent of structures lost
• 76 percent of lives lost.
Table 15-2. CAL FIRE Destructive Wildfire Statistics, 1939-2020 Years Number of fires Acres Burned Structures Burned Lives Lost 20 Most Destructive Fires
2015-2020 15 255,080 42,418 158
1991-2014
Total
5 20
30,201
285,281
9,327
51,745
49 207
Other Major Fires in Previous 50 Years
1939 -1990 90 209,999 Not Available Not Available
Total 110 495,280 51,745 207
Source: CAL FIRE, Created by Permit Sonoma GIS. Data subject to change as better data and analysis become available Twelve of the 15 destructive wildfires in the past five years took place in Northern California, and seven of them burned within Sonoma County and/or in a bordering county. These data indicate a future characterized by increasing fire frequency, size, and destruction. With climate change, the potential for drought, hotter temperatures year-round, and increasing lightning events as experienced in 2020 is cause for significant concern.
Recent fires also indicate that wildfire does not impact only rural residents or forested areas. More than half (2,575) of the structures lost in the 2017 Tubbs Fire were in urban areas rated as “moderate” Fire Hazard Severity Zone or “urban/unzoned.” Only 1,205 structures were lost in areas ranked as “very high” or “high” Fire Hazard Severity Zones
1964 Nuns Canyon Fire
The Nuns Canyon fire in the Sonoma Valley started on the same day as the Hanley fire and burned for six days.
By the third and fourth days, the fire had burned 9,500 acres and reached Highway 12 and Boyes Hot Springs. By the sixth day, when the fire was brought under control, it had destroyed 27 homes and more than 10,000 acres. 15-5
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements 1964 Hanley Fire
The Hanley fire started on September 19, 1964, on the Hanley property off Highway 29 on the slopes of Mt. St.
Helena in Napa County. By the end of the next day, firefighters had contained the fire, but late in the night, winds drove the flames down the slopes to encircle Calistoga on two sides. Several homes on the perimeter of town were burned. On the third day, an ember ignited a spot fire on the ridge west of Highway 128 between Calistoga and Kellogg, in Sonoma County. The fire then raced into Knights Valley and turned southward into Franz Valley. By nightfall, the fire, driven by 70 mph winds, headed down Mark West Canyon toward Santa Rosa. The Sonoma County Hospital was threatened, with embers falling on the rooftop, and 40-foot high flames in nearby trees. To the east, the fire burned over the hills and down into the Rincon Valley area, where it was again stopped. The fire was not brought under control until the morning of September 26. The fire consumed 105 million board feet of timber valued at $1.5 million and destroyed 84 homes and 24 summer cabins. More than 52,000 acres were blackened. No human lives were lost.
2004 Geysers Fire
A number of fires have ignited in the area known as the Geysers. A fire on Labor Day weekend 2004 burned 12,500 acres in the Mayacamas Mountains in Sonoma and Lake counties over a five-day period, cost over $14 million to suppress, and caused over $10 million in property damage. The fire consumed six cabins and destroyed equipment and vehicles belonging to several companies operating in the area, including Calpine Corp., PG&E and AT&T. Firefighters were able to save pumping stations and geothermal power plants worth hundreds of millions of dollars. The 2004 Sonoma Lake Napa Fire Management Plan indicated that vegetation management was one of the primary reasons the geothermal facilities were not destroyed. 2015 Valley Fire
The Valley Fire was located mainly in southern Lake County but moved into Sonoma County, where it burned 5,000 acres near the Geysers and destroyed four steam cooling towers at the CalPine geothermal facility. Starting on September 12, 2015, the fire burned 76,067 acres and destroyed 1,958 structures including 1,280 homes, 27 multi-family structures, 66 commercial properties, and 585 minor structures such as outbuildings and sheds. An additional 93 structures were damaged, including 41 homes, 7 commercial properties and 45 other minor structures. Four firefighters were injured and there were 4 civilian fatalities. 2017 Sonoma Complex Fires—Tubbs, Nuns, and Pocket
On October 8, 2017, an historic wind event led to the worst firestorms in Sonoma County history, followed by almost three weeks of fire. In total, the Nuns, Tubbs, Pocket and Young fires (together comprising the 2017 Sonoma Complex Fire) claimed 24 lives, burned over 110,700 acres in Sonoma, Napa, and Lake counties, and destroyed 6,997 structures with total direct losses exceeding $7.8 billion. The following sections describe the three main fires in the complex that affected Sonoma County.
Tubbs Fire
The Tubbs Fire was the most destructive wildfire in California history when it occurred, burning parts of Napa, Sonoma, and Lake counties. The greatest losses were in the city of Santa Rosa. The Tubbs Fire was one of more than a dozen large fires that broke out in early October 2017 and simultaneously burned eight northern California counties. By the time of its containment on October 31, the fire was estimated to have burned 36,810 acres. At least 24 people in Sonoma County were believed to have been killed by the fire. The fire destroyed more than 15-6
15. Wildfire
5,643 structures, half of which were homes in Santa Rosa. Santa Rosa’s economic loss from the Tubbs Fire was estimated at $1.2 billion, with 5 percent of the city’s housing stock destroyed. The fire incurred $100 million in fire suppression costs.
Nuns Fire
The Nuns Fire broke out in a field in the community of Glen Ellen when strong winds knocked an alder tree into a powerline conductor. It merged with five other fires that together burned an area larger than the city of Oakland. It burned 56,556 acres and destroyed about 1,527 structures before being contained on October 31, 2017.
Pocket Fire
The Pocket Fire started on October 9, 2017 and was contained on October 31, 2017. The fire burned 17,357 acres within Sonoma County (Wildfire Today, 2018). This was a vegetation fire that started near Pocket Ranch Road east of the community of Geyserville. It began during a red flag warning issued by the National Weather Service.
The fire was reported to have destroyed six structures and damaged two others (CalFire Investigation Report, 107CALNU010057, 10/9/2017).
2019 Kincade Fire
The Kincade Fire started northeast of Geyserville in the Mayacamas Mountains on October 23, 2019, and burned 77,753 acres and 371 structures before it was fully contained on November 6, 2019. The fire threatened over 90,000 structures and caused widespread evacuations (198,785 residents) throughout Sonoma County, including the communities of Geyserville, Healdsburg, and Windsor. The majority of Sonoma County and parts of Lake County were under evacuation warnings. The fire was the largest of the 2019 California wildfire season, and the largest ever in Sonoma County.
2020: LNU Lightning Complex Fire
Early on August 16, 2020, following a series of very hot days, thunderstorms hit California. Within the next 72 to 96 hours, over 12,000 lightning strikes were recorded over Northern California. These lightning strikes sparked up to 585 wildfires, many of which grew to be very large at a rapid pace due to parched brush (Wikipedia, 2021).
The Sonoma Lightning Complex, consisting of the Walbridge Fire (55,209 acres), and Meyers Fire (2,616 acres) burned 61,875 acres and 303 structures within Sonoma County. At the same time, in the Sonoma-Lake-Napa CAL FIRE Unit (LNU), the Hennessey Fire consumed 305,651 acres and caused six fatalities.
Firefighting resources were so stretched by the more than 500 wildfires burning across the state during the 2020 lightning siege that each engine company assigned to the fires in Sonoma County was responsible for more than 700 acres (Nicholls, 2020).
2020 Glass Fire
The 2020 Glass fire burned 67,484 acres and 611 structures in Sonoma County, threatening urban neighborhoods in eastern Santa Rosa, and prompting large-scale evacuations from Santa Rosa to Glen Ellen. The Glass Fire sparked in Napa Valley early on Sunday, September 27, 2020, growing at a rate of around 1 acre every five seconds between Sunday night and Monday morning, according to satellite images from the National Oceanic and 15-7
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Atmospheric Administration. An estimated 70,000 people were under evacuation orders in the region surrounding the Glass Fire.
15.2.3 Location
CAL FIRE has identified several “historic wildland fire corridors” in Sonoma County, including the steep ridges of the coastal ridges in the northwest county, which experienced fires in 1923, 1951, and 1978; and the Geysers area in the northeast Mayacamas range, which has experienced fires in 2004, 2013 and 2019. The 1964 Hanley and Nunn’s fire footprints were nearly identical to the 2017 Tubbs and Nuns fires. The Mayacamas mountains south of Santa Rosa above the Sonoma Valley also have a history of repetitive fire loss, where the Cavedale fires of 1925 and 1996 and the 1964 Nunns fire, and 2017 Sonoma Complex fires caused significant damage.
Figure 15-1 shows the Sonoma County Wildfire Hazard Index, a model that predicts relative wildfire hazard on the landscape. The hazard index has the following categories: Very Low Relative Hazard, Low Relative Hazard, Moderate Relative Hazard, High Relative Hazard, and Very High Relative Hazard. The index is based on inputs that inform potential fire behavior, inputs that represent fire probability occurrence at any location, and a model of wildfire suppression difficulty. The hazard index reflects landscape conditions through the 2018 fire season. The wildfire hazard data was developed for the 2021 update of the County’s Community Wildfire Protection Plan Update. The data is preliminary, pending further peer review.
In general, the mapping shows higher-hazard areas on the hills and mountains of the Coast and Mayacamas ranges. Lower-hazard areas line the Pacific coast, San Pablo Bay, and the Sonoma valley.
15.2.4 Frequency
The overall probability of some wildfire event impacting the planning area is high. Table 15-1 shows 23 fires of 300 acres or more in Sonoma County between 1964 and 2020, an average of one large fire every 2.6 years.
Wildfire probability varies with time of year and size of fire. Wildland fire season in Sonoma County spans the months after the last spring rains have fallen and until the first significant fall or winter rains occur. August, September, and October have the greatest potential for wildland fires as vegetation dries out, humidity levels fall, and offshore winds blow. Changing climate conditions are beginning to extend the local fire season. Drought conditions are of special concern, as is the potential for changing weather patterns, including the potential for more dry lightning storms during fire season.
15.2.5 Severity
As seen in Figure 15-2, the frequency and severity of wildfires in Sonoma County has changed over the past 60 years, especially over the last five years (2015 to 2020). As the size of fire has increased, so has the number of structures burned. This correlates to an increase in fire severity. The more structures that burn, the higher the probability for fatalities, which is the ultimate measure for severity in a mitigation planning context. Except for the Hanley fire in 1964, no fires prior to 2015 had burned more than 12,000 acres, but the burn areas for fires from 2015 to 2020 range from 37,000 to 78,000 acres. Similarly, no fire before 2015 burned more than 108 structures, but fires from 2015 to 2020 burned from 370 to 5,600 structures. 15-8
¬1«
Cloverdale
128 ¬«
101 £¤
Healdsburg
Windsor
116 ¬«
Santa
Rosa
12¬«
Sebastopol
1¬« Rohnert Park
Cotati
101 £¤ Sonoma
Petaluma 12¬«
116 ¬« 121 ¬«
37¬«
Figure 15-1. Fire Hazard Severity Zones
Very Low Relative Hazard Cities
±
Low Relative Hazard County Boundary 0 5 10
Moderate Relative Hazard
High Relative Hazard
Highways
Miles
Data Sources: Sonoma Co.
Very High Relative Hazard
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Acres Burned
80,000
60,000
40,000
20,000
0 1960 1970 1980 1990 2000 2010
Acres Burned Structures Burned
Figure 15-2. Acres and Structures burned, 1964 - 2020
6,000
4,500
3,000
1,500
0 2020
Structures Burned
Potential losses from wildfire include human life, structures and other improvements, natural resources, and natural systems such as water and watersheds. Damage to local economy can include loss of jobs due to direct fire losses or disruption from power safety shutoffs, or loss of crops from fire or smoke damage.
In 2017, at least 24 Sonoma County residents lost their lives due to wildfires. Reducing potential for loss of life should remain a top focus for planning and project implementation. Likewise, efforts should continue to help county residents create homes and communities that can better withstand exposure to the heat and embers of wildfires such as defensible space, structure hardening, and near-community “wildfire calming zones.” Fire hazards present a significant risk to vegetation and wildlife habitats. Short-term loss caused by a wildfire can include the destruction of watersheds, timber, wildlife habitat, and scenic vistas. Long-term effects include smaller timber harvests, reduced access to affected recreational areas, and destruction of cultural and economic resources and community infrastructure. In addition, wildfire can lead to ancillary impacts such as landslides and flooding due to the impacts of silt in local watersheds.
Economic impacts due to wildfires include costs and losses due to burned or smoke-damaged crops, damaged public infrastructure and private property, interrupted transportation corridors, and disrupted communication lines. They also include diminished real property values and thus tax revenues, loss of retail sales, and relocation expenses of temporarily or permanently displaced residents. Power safety shutoffs also have significant impacts on local businesses and residents. Likewise, large-scale evacuations can have enormous impacts on local businesses.
Smoke and air pollution from wildfires can be a health hazard, especially for sensitive populations. Smoke generated by wildfire contains particulate matter (soot, tar, water vapor, and minerals), gases (carbon monoxide, carbon dioxide, nitrogen oxides) and toxics (formaldehyde, benzene). There has been a significant increase in scientific research regarding the impacts of wildfire smoke on public health. Impacts associated with wildfire include difficulty in breathing and increases in incidence of asthma, heart attacks and strokes. People who are over 65 years of age have a higher chance of heart attacks and strokes after two to three days of bad air quality due to wildfire smoke. Smoke worsens health conditions that are already more prevalent in lower-income locations, including some communities of color. One 2016 study in northern California found that people in 15-10
15. Wildfire
lower-income zip code areas were disproportionately likely on wildfire smoke days to visit emergency rooms for asthma complications (Climate Connections, 2021).
Wildfire may also threaten the health and safety of those fighting the fires. First responders are exposed to the dangers from the initial incident and after-effects from smoke inhalation and heat stroke.
15.2.6 Warning Time
If a fire breaks out and spread rapidly, residents may need to evacuate within hours or minutes. Wildfires are mostly caused by human activities and systems, intentionally or accidentally. There is no way to predict when one might break out. Dry conditions, wind and droughts greatly increase fire likelihood. Dry lightning, whose incidence may increase due to changing climate, triggered devastating wildfires across Sonoma County and the state in 2020. Reliable National Weather Service lightning warnings are available on average 24 to 48 hours prior to a significant electrical storm, so special attention can be paid during weather events that may include lightning.
Typically, fires burn with the greatest severity between 1 p.m. and 6 p.m. Once a fire has started, fire alerting is reasonably rapid in most cases. However, when fires start under red flag conditions late at night, as was the case with the Sonoma Complex fires of 2017, alerting and evacuation can pose greater challenges for agencies and the public. Sonoma County has augmented systems and methodologies for alerting and evacuations since 2017 by developing and publicizing evacuation zones, and increasing the means for delivery of evacuation notification.
Many Sonoma County communities, through programs such as Citizens Organized to Prepare for Emergencies (COPE) have organized to help notify their neighborhoods of emergencies.
Following a series of natural disasters, including Hurricane Katrina, that revealed shortcomings in the nation’s ability to effectively alert populations at risk, Congress passed the Warning, Alert, and Response Network (WARN) Act in 2006. Today, new technologies such as smart phones and social media platforms offer new ways to communicate with the public, and the information ecosystem is much broader, including additional official channels, such as government social media accounts, opt-in short message service (SMS)-based alerting systems, and reverse 911 systems. Less official channels include mainstream media outlets and weather applications on connected devices. Unofficial channels include first-person reports via social media (National Academies of Sciences, Engineering, and Medicine, 2018).
15.3 EXPOSURE
A quantitative assessment of exposure to the wildfire hazard was conducted using the wildfire hazard mapping and the asset inventory developed for this plan, with an emphasis on the zones with the highest degree of susceptibility (moderate, high and very high fire risk). Summary results for the complete planning area are presented below. Detailed results by jurisdiction are provided in Appendix D.
15.3.1 Population
Population exposure was estimated by calculating the number of buildings in each hazard area as a percent of total planning area buildings and applying this percentage to the planning area population. Table 15-3 summarizes the estimated countywide population living in the mapped risk areas. In addition to populations who reside in risk areas where fires may occur, visitors, hikers and campers may be exposed to wildfires. The entire population of the planning area has the potential to be exposed to smoke from nearby wildfires. 15-11
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Table 15-3. Exposed Population in Mapped Relative Fire Hazard Zones Moderate Relative Hazard High Relative Hazard Very High Relative Hazard Population Exposed 68,365 8,368 1,158
% of Total Planning Area Population 14.1% 1.7% 0.2%
Socially vulnerable populations exposed to the wildfire hazard were estimated based on data for the Census- defined blocks that lie at least partially within the mapped high and very high relative fire hazard zones. Because many of those Census blocks extend outside the mapped hazard zones, the estimates are greater than the actual exposed populations, but they provide reasonable relative data for use in mitigation planning. Figure 15-3 summarizes the estimated exposure of socially vulnerable populations.
Under Age 16
44,557
(20.2% of Total)
Over Age 65
37,211
(16.9% of Total)
Total
220,253
0 50,000
100,000
150,000
200,000
250,000 Number of People in Hazard Area
Low Income
47,904
(54.4% of
Total)
Total
88,072
0 10,000
20,000
30,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000 Number of Households in Hazard Area
See Section 4.8.1 for the definition of “low income” used in this analysis Figure 15-3. Socially Vulnerable Populations in High and Very High Fire Hazard Zones Census Blocks
15.3.2 Property
Table 15-4 summarizes the estimated countywide property exposure in the mapped landslide risk areas.
Figure 15-4 shows the occupancy class for all buildings in the mapped fire hazard areas. These occupancy classes provide an indication of land use within the mapped hazard area. Some land uses are more vulnerable to fire, such as single-family homes, while others are less vulnerable, such as agricultural land or parks.
15.3.3 Critical Facilities
The breakdown of critical facilities exposure in the high and very high severity zones by facility type is shown in Figure 15-5.
15-12
15. Wildfire
Table 15-4. Exposed Property in Mapped Relative Fire Hazard Zones Moderate Relative Hazard High Relative Hazard Very High Relative Hazard Number of Buildings Exposed 26,245 4,798 1,175
Value of Exposed Structures $20,143,725,511 $9,058,841,363 $3,802,457,456 Value of Exposed Contents $16,645,566,386 $8,613,600,440 $3,746,872,248 Total Exposed Property Value $36,789,291,897 $17,672,441,803 $7,549,329,704 Total Exposed Value as % of Planning Area Total 16.8% 8.1% 3.5%
MODERATE RELATIVE HAZARD
Residential
22,312
85%
Education Commercial
21 542
0% 2%
Government Religion Agriculture Industrial
283 37 3,008 42
1% 0% 12% 0%
HIGH RELATIVE HAZARD
Residential Commercial 2,954 13062% 3%
Industrial
10 Education 0%
3 0% Agriculture Government Religion 1,617 72 12 34%1% 0%
VERY HIGH RELATIVE HAZARD
Commercial Industrial
51 2
Residential
412
35%
5% 0% Agriculture
695
59%
Religion
3 0%
Government
11 1%
Education
1 0%
Figure 15-4. Structures in the High or Very High Relative Fire Hazard Zones, by Land Use Type 15-13
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements Facility Category
Transportation
Safety & Security
Health & Medical
Hazardous Material
Food, Water, Shelter
Energy
Communications
191
74 131
3,109
219
650
385
32 1
3 92
3 7
19 High or Very High Relative Hazard
Planning Area Total
0 500 1,000 1,500 2,000 2,500 3,000 3,500
Number of Facilities in Identified Area
Figure 15-5. Critical Facilities in Mapped Relative Fire Hazard Zones and Countywide
15.3.4 Environment
All natural resources and habitats in mapped relative fire hazard zones are exposed to the risk of wildfire. 15-14
15. Wildfire
15.4 VULNERABILITY
15.4.1 Population
All people exposed to the wildfire hazard are potentially vulnerable to wildfire impacts. Persons with access and functional needs, the elderly and very young may be especially vulnerable to a wildfire if there is not adequate warning time for them to evacuate. People outside the mapped risk areas are susceptible to health hazards associated with smoke and air pollution from wildfires, especially sensitive populations including children, the elderly, and those with respiratory and cardiovascular diseases. Wildfires also threaten the health and safety of those fighting the fires.
15.4.2 Property
All property exposed to the wildfire hazard is vulnerable. Structures that were not constructed to standards designed to protect a building from a wildfire may be especially vulnerable. As of 2008, California State Building code requires minimum standards be met for new buildings in relative fire hazard zones. Less than 10 percent of housing in the planning area was built since this code requirement (U.S. Census Bureau n.d.).
Estimates were developed to indicate the loss that would occur if wildfire damage were equal to 1, 10, 30 or 50 percent of the exposed property value, as summarized in Table 15-5. Damage in excess of 50 percent is considered to be substantial by most building codes and typically requires total reconstruction of the structure.
Table 15-5. Loss Estimates for Wildfire Hazard
Exposed Value Loss Value Loss as % of Total Planning Area Replacement Value Moderate Relative Hazard
Loss = 1% of Exposed Value $370 million less than 1%
Loss = 10% of Exposed Value $3.68 billion 1.7%
Loss = 30% of Exposed Value $36.79 billion $11.04 billion 5.0% Loss = 50% of Exposed Value $18.40 billion 8.4%
High Relative Hazard
Loss = 1% of Exposed Value $180 million less than 1%
Loss = 10% of Exposed Value $1.77 billion less than 1%
Loss = 30% of Exposed Value $17.67 billion $5.30 billion 2.4% Loss = 50% of Exposed Value $8.84 billion 4.0%
Very Relative Hazard
Loss = 1% of Exposed Value $80 million less than 1%
Loss = 10% of Exposed Value $760 million less than 1%
Loss = 30% of Exposed Value $7.55 billion $2.26 billion 1.0% Loss = 50% of Exposed Value $3.78 billion 1.8%
15.4.3 Critical Facilities
Critical facilities not built to fire protection standards, utility poles and lines, and facilities containing hazardous materials are most vulnerable to the wildfire hazard. Most roads would not be damaged except in the worst scenarios, although roads and bridges can be blocked by debris or other wildfire-related conditions and become impassable. Additionally, heavy vehicle traffic during incidents and in post-fire recovery and rebuild can have 15-15
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements significant impact on road surfaces. The following critical facilities are located in very high and high severity zones and their vulnerability could complicate response and recovery efforts during and following an event:
• Hazardous Materials and Fuel Storage—During a wildfire event, these materials could rupture due to excessive heat and act as fuel for the fire, causing rapid spreading and escalating the fire to unmanageable levels. In addition, they could leak into surrounding areas, saturating soils and seeping into surface waters, and have a disastrous effect on the environment. • Communication Facilities—If these facilities are damaged and become inoperable, it would exacerbate already difficult communication in the planning area.
15.4.4 Environment
Sonoma County’s ecosystems are fire adapted. Native plant species have evolved with fire in the landscape, and occurrence of fire is an integral component of forest health. Over millennia, nature has selected for species that survive the passage of wildfire, and for the healthiest, best-placed trees to survive. Some species require fire in order to propagate or germinate seeds. However, severe wildfire behavior, can also cause severe environmental impacts, such as the following:
• Soil Erosion—The protective covering provided by foliage and dead organic matter is removed, leaving the soil fully exposed to wind and water erosion. Accelerated soil erosion occurs, causing landslides and threatening aquatic habitats.
• Spread of Invasive Plant Species—Non-native woody plant species frequently invade burned areas.
When weeds become established, they can dominate the plant cover over broad landscapes, and become difficult and costly to control.
• Disease and Insect Infestations—Unless diseased or insect-infested trees are swiftly removed, infestations and disease can spread to healthy forests and private lands. Timely active management actions are needed to remove diseased or infested trees. • Damaged Fisheries—Critical fisheries can suffer from increased water temperatures, sedimentation, and changes in water quality.
• Destroyed Endangered Species Habitat—Wildfire can have negative consequences for endangered species by degrading their habitat.
• Soil Sterilization—Some wildfires burn so hot that they can sterilize the soil. Topsoil exposed to extreme heat can become water repellant, and soil nutrients may be lost. • Reduced Timber Harvesting—Timber can be destroyed and lead to smaller available timber harvests. • Reduced Agricultural Resources—Wildfire can have disastrous consequences on agricultural resources, removing them from production and necessitating lengthy restoration programs. In addition to fire directly impacting winery facilities and vineyards, smoke can impact grapes on the vine, tainting the grapes so that they become unusable for wine production. • Damaged Cultural and Historical Resources—The destruction of cultural and historic resources may occur, scenic vistas can be damaged, and access to recreational areas can be reduced.
15.5 FUTURE TRENDS IN DEVELOPMENT
Urbanization tends to alter the natural fire regime and can lead to expansion of urbanized areas into wildland areas. Placement of additional housing in the wildland/urban interface areas located in high or very high relative 15-16
15. Wildfire
fire hazard zones can increase the fire threat, particularly in the historical fire corridors north and east of Santa Rosa and in the Sonoma Valley. Development in these areas can burden existing fire protection services, particularly in areas dependent on volunteer firefighters.
Most of the homes in Sonoma County’s WUI areas were constructed before 2008, when California’s WUI Building Code (California Code Chapter 7A) went into effect. This code requires ignition-resistant building materials in WUI areas. Structures built before it took effect and those without adequate vegetation management are at higher risk to wildland fire ignition. In Sonoma County, there are 27,286 structures in the WUI.
Approximately 12,600 of those (all structures with a footprint greater than 1,500 square feet) are in areas with high and very high risk of wildland fires.
Patterns of land use and development have placed extensive residential infrastructure into places that recent, large fires have placed at severe risk. These fires have resulted in the loss of over 6,000 homes in Sonoma County, as well as thousands of other structures. This risk is expected to increase in coming decades. Fire will likely recur within similar footprints and place the same infrastructure at risk into the foreseeable future, as well as occurring in previously unburned areas.
The expansion of development toward wildfire hazard areas can be managed with strong land use and building codes. State and local policies and regulations require landowners to carry out activities such as maintaining defensible space and reducing vulnerability to damage or loss from wildfire. In Sonoma County, defensible space is regulated in Local Responsibility Areas through Sonoma County Code Chapter 13A. In the State Responsibility areas, the California Building Code includes minimum standards related to the design and construction of buildings in fire hazard zones. Any newly permitted buildings must conform to standards that manage flammable materials from around the building (defensible space laws) and construct buildings from fire-resistant material.
New residential construction in high hazard areas in the State Responsibility Areas must be built according to the standards of the 2007 WUI Building Code. Defensible SPACE in the SRA is regulated through Public Resource Code 4290 and 4291, Title 14 of the California Code of Regulations, and Government Code Sections 51175 through 51189.
The State of California has enacted significant legislation that attempts to manage and mitigate wildfire risk.
Appendix B provides a summary of this legislation, much of which will have an impact on future development that interfaces a wildfire hazard severity zone. In addition, the planning partners’ general plans include policies that address managing development in relative fire hazard zones. The planning area is well equipped with these tools, and this planning process has asked each planning partner to assess its capabilities with regards to the tools.
As the planning area experiences future growth, it is anticipated that the exposure to this hazard will remain as assessed or even decrease over time due to these capabilities.
15.6 SCENARIO
A major wildfire in the planning area might begin with a wet spring, which could encourage growth of light flashy fuels, such as grasses and brush. The summer could see the onset of insect infestation or plant pathogens that increase tree mortality. A dry summer could follow the wet spring, exacerbated by dry hot winds.
Carelessness with combustible materials, equipment use, a vehicle fire, a tossed lit cigarette, or a lightning storm could trigger a multitude of small isolated fires.
The embers from these smaller fires could be carried miles by hot, dry winds. The deposition zone for these embers could be deep in forested areas or in urban areas. Fires that start in flat areas move slower, but wind still 15-17
Sonoma County Multijurisdictional Hazard Mitigation Plan Update 2021; Volume 1—Area-Wide Elements pushes them. It is not unusual for a wildfire pushed by wind to burn the ground fuel and later climb into the crown and reverse its track. This is one of many ways that fires can escape containment, typically during periods when response capabilities are overwhelmed. These new small fires would most likely merge. Suppression resources would be redirected from protecting the natural resources to saving lives, homes, and communities. In 2017, the Tubbs fire became an urban conflagration fire, as embers from wildland areas carried across multiple lane Highway 101 to ignite businesses and the community of Coffey Park, where approximately 1,300 homes were lost along with commercial buildings.
The worst-case scenario would include an active fire season throughout the American west, spreading resources thin. Firefighting teams would be exhausted or unavailable. Many federal assets would be responding to other fires that started earlier in the season.
To further complicate the problem, heavy rains could follow, causing flooding and landslides and releasing tons of sediment into rivers, permanently changing floodplains and damaging sensitive habitat and riparian areas. Such a fire followed by rain could release millions of cubic yards of sediment into streams for years, creating new floodplains and changing existing ones. With the forests removed from the watershed, stream flows could easily double. Floods that could be expected every 50 years may occur every couple of years. With the streambeds unable to carry the increased discharge because of increased sediment, the floodplains and floodplain elevations would increase.
15.7 ISSUES
Wildfire is an inevitable and normal ecological process in the fire-adapted landscape of Sonoma County. Nearly 100 years of aggressive fire suppression has contributed to the high wildfire risk of today. Absent fire for many years, wildland areas became overstocked with highly flammable vegetation. At the same time, expansion of homes into rural WUI areas increased the number of homes in high-risk areas. Typically, residential property owners do not maintain forested lands, exacerbating wildfire potential. On public lands, available budget for large-scale wildland fuels maintenance is an ongoing issue. Overcrowded conditions degrade overall forest health and degrade the environmental values provided by forest ecosystems.
While in a few areas, recent wildfires burned hot enough to damage wildland ecosystems, in general wildland ecosystems have not sustained irrevocable damage. In many cases fires were beneficial. Large, uncontrolled wildfires can cause significant damage to ecosystem services, however life, home and economic losses to residents and communities must be considered along with environmental consequences.
Research shows that home loss in wildland fires is primarily driven by two equally important factors:
• The vulnerabilities of buildings that make them prone to ignition—Embers cause 80 percent of wildland fire home ignitions. The following elements are most vulnerable to embers but can be retrofitted on existing homes to reduce risk of ignition:
Non-Class A roofs
Roof edges and soffits
Combustible plants and materials within 5 feet of house walls Non-WUI approved venting products that allow for ember entry into structures Wooden attachments, such as fences and decks
Non-WUI rated windows
Siding
15-18
15. Wildfire
• The vegetative fuels within 100 feet of structures (the area referred to as defensible space)—Good defensible space, wherein vegetation has been reduced to reduce fire intensity and spread, is critical to reduce ignition.
Most of the homes in Sonoma County’s WUI areas were constructed before 2008, when the WUI Building Code went into effect. This code requires ignition-resistant building materials in WUI areas. Structures built before it took effect and those without adequate vegetation management are at higher risk to wildland fire ignition.
Outside of the home and the 100-foot defensible space zone, surrounding wildland fuels can play a role in home destruction, as fire and embers can spread from nearby wildland areas into communities. It is in this area that vegetation management can come into play. This refers to actions taken to alter natural vegetation or plant communities that abut communities, usually on the scale of 10s to 1,000s of acres. Vegetation management can include prescribed fire, prescribed grazing, timber harvest techniques, invasive plant removal, or mechanical treatment to remove fine fuels, dense stands of fire-prone species, shrubs, and dead and dying vegetation. Fuels are reduced in order to create “community calming zones” or restore ecosystems to less flammable conditions.
Strategically placed calming zones can reduce near-community fire intensity and spread, provide safe anchors that firefighters can use to stop forward progress of the fire, and supplement and support near-home mitigation strategies. Roadside fuels treatment can support emergency ingress and egress, increasing community and firefighter safety.
Although the patterns of land use, natural plant communities, topography, weather, soils, and geology vary across the landscapes of Sonoma County, notable patterns are discernible. An approach is needed for deploying existing techniques at the scale of whole communities. Such an approach would be informed by the principles of landscape ecology. It would view the natural lands where fires tend to originate and the built infrastructure of human communities that abut the natural landscapes as a coupled system. Mitigating large-scale loss of life and property can be achieved using relatively well-established techniques of home hardening, defensible space and vegetation management at the scale of whole communities and the natural landscapes that surround them. 15-19
16. CLIMATE CHANGE
16.1 GENERAL BACKGROUND
16.1.1 What is Climate Change?
Climate is the result of long-term weather patterns—including temperature, precipitation, humidity, wind and seasons—and plays a fundamental role in shaping natural ecosystems and the human economies and cultures that depend on them. “Climate change” refers to changes over a long period of time, with units of climate change measurements often conducted in 30-year increments.
The well-established worldwide warming trend of recent decades and its related impacts are caused by increasing concentrations of carbon dioxide and other greenhouse gases in the earth’s atmosphere. Greenhouse gases are gases that trap heat in the atmosphere, resulting in a warming effect. Carbon dioxide is the most commonly known greenhouse gas; however, methane, nitrous oxide and fluorinated gases also contribute to warming.
Emissions of these gases come from a variety of sources, such as the combustion of fossil fuels, agricultural production and changes in land use. According to the National Aeronautics and Space Administration (NASA), carbon dioxide concentrations measured about 280 parts per million (ppm) before the industrial era began in the late 1700s and have risen dramatically since then, surpassing 400 ppm in 2013 for the first time in recorded history (see Figure 16-1).