Morgan Hill City Council
September 4, 2024

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 9: Inclement Weather 166
Table 54: 2001–2022 Extreme Temperature Events
Dates of Event Event Type Location
7/20/2006 Heat Santa Clara Valley Including
San Jose
7/21/2006 Heat East Bay Hills and the Diablo
Range (Zone)
7/22/2006 Heat San Francisco Bay Shoreline
(Zone)
7/22/2006 Heat Santa Cruz Mountains (Zone)
5/17/2009 Heat Santa Clara Valley Including
San Jose
5/17/2009 Heat San Francisco Bay Shoreline
(Zone)
5/17/2009 Heat East Bay Hills and the Diablo
Range (Zone)
5/17/2009 Heat Santa Cruz Mountains (Zone)
6/19/2017 Heat Santa Clara Valley Including
San Jose
12/25/2018 Cold/Wind Chill Santa Clara Valley including San Jose
1/1/2019 Cold/Wind Chill Santa Clara Valley including
San Jose
11/27/2019 Cold/Wind Chill Santa Clara Valley including San Jose
11/28/2019 Cold/Wind Chill Santa Clara Valley including San Jose
11/29/2019 Cold/Wind Chill Santa Clara Valley including San Jose
6/10/2019 Heat Santa Clara Valley Including
San Jose
6/10/2019 Excessive Heat San Francisco Bay Shoreline
(Zone)
8/14/2020 Excessive Heat San Francisco Bay Shoreline
(Zone)
8/16/2020 Excessive Heat Santa Clara Valley Including
San Jose
9/6/2020 Excessive Heat Santa Clara Valley Including
San Jose
9/7/2020 Excessive Heat San Francisco Bay Shoreline
(Zone)
6/17/2021 Heat Santa Clara Valley Including
San Jose
6/17/2021 Heat Santa Clara Valley Including
San Jose

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 9: Inclement Weather 167
Dates of Event Event Type Location
7/11/2021 Excessive Heat East Bay Hills and the Diablo
Range (Zone)
6/10/2022 Heat Santa Clara Valley Including
San Jose
6/21/2022 Heat San Francisco Bay Shoreline
(Zone)
6/21/2022 Heat San Francisco Bay Shoreline
(Zone)
6/21/2022 Heat Santa Clara Valley Including
San Jose
9/4/2022 Heat Santa Clara Valley Including
San Jose
9/4/2022 Heat San Francisco Bay Shoreline
(Zone)
9/4/2022 Heat Santa Clara Valley Including
San Jose
9/4/2022 Heat Santa Clara Valley Including
San Jose
9/5/2022 Heat San Francisco Bay Shoreline
(Zone)
9/5/2022 Heat Santa Clara Valley Including
San Jose
9/6/2022 Heat San Francisco Bay Shoreline
(Zone)
9/6/2022 Heat Santa Clara Valley Including
San Jose
9/8/2022 Heat Santa Clara Valley Including
San Jose
High Wind
• In addition, the NOAA/NCEI database reports 494 high wind events that have occurred in the OA since 2005. This is the most common inclement weather hazard type to affect the OA, and these events have caused an estimated $7,406,100 in property damage since the first recorded incident in 2005.
• On 1/19/2010-1/22/2000, the NCEI records strong winds that affected the Bay Area. Winds knocked two trees down along East Santa Clara Street in San Jose and around 10,000 customers lost power in the San Jose area. Additional incidents of trees and limbs falling and causing damage were reported. These reports include damage to utilities such as telephone poles and power transformers, damage caused to vehicles, partially blocked roadways, or injured people. Specific impacts were recorded in San Jose, Los Gatos, and Milpitas.

Table 55:2002-2022 High Wind Events

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 9: Inclement Weather 168
Years Number of Wind Events
2005 6
2006 11
2008 16
2009 18
2010 21
2011 15
2012 14
2013 23
2014 32
2015 15
2016 3
2017 51
2018 26
2019 92
2020 52
2021 61
2022 9
Although rare, Santa Clara County has also experienced tornados, which have caused a total of $6,721,000 in damage.

Table 56: 1951–2022 Tornado Events
Date Event Type Location
1/11/1951 F2 Tornado Santa Clara County
12/8/1997 F0 Tornado San Jose
2/6/1998 F0 Tornado Sunnyvale
5/5/1998 F1 Tornado Los Altos
5/5/1998 F2 Tornado Sunnyvale
4/14/2007 EF0 Tornado Gilroy

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 9: Inclement Weather 169
Space Weather
Space weather monitoring is relatively new, and there is limited data available regarding historical occurrences. However, we know that space weather has impacted the OA and is likely to impact the OA in the future.

9.2.2 Location
Heavy precipitation/Rain, extreme heat or extreme cold, high winds, and even space weather can occur anywhere within the OA. The extent is highly variable. Given their widespread nature, these events have the potential to impact large areas simultaneously. Historical records show that these events can happen in one isolated part of the OA, or in the entirety of the planning area. Therefore, inclement weather events are not confined to any geographical barrier and may impact all parts of the OA.

9.2.3 Frequency
Several types of inclement weather have shown an increasing number of events in recent years. This represents a trend that due to factors such as climate change it is likely that inclement weather events will become more frequent over time. Based on the frequency of historic events, inclement weather events can be expected to occur multiple times annually.

Heavy Precipitation: According to the historical events reported by the NCEI database, 44 heavy precipitation events have occurred in the last twenty-one years. Of these 44 events, 33 incidents have occurred in the last five years alone.

Probability in OA: Highly Likely
Extreme Temperature: 33 extreme temperature events were recorded in the NCEI database since 2002. 24 of those have occurred since 2017.

Probability in OA: Highly Likely
High Wind: 470 high wind events have occurred in the OA since 2005. Wind speeds in these events have ranged from 9 to 96 knots.

Probability in OA: Highly Likely
Space Weather: Data was not available to estimate the frequency of space weather events.

Probability in OA: Likely

9.2.4 Severity
Severity will depend on the extent of each event and can range from mild to severe. Inclement weather can cause disruption to critical facilities and extensive damage to property and infrastructure. The NCEI database reports a total of $24,152,120 in property damage from these events in some of the most severe cases, not including space weather.

The levels of severity related to space weather are best described in Table 50, Table 51, and Table 52.

The level of severity can range from minor to extreme and will mostly impact the OA’s technological, communication, and power grid systems.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 9: Inclement Weather 170

9.2.5 Warning Time
The National Weather Service provides forecasting and prediction services for a variety of inclement weather events and maintains an established process for issuing advisories, watches and warnings for each. The amount of warning time varies on the type of weather event. 126F 127
Heavy Precipitation: warning times for significant weather events may vary widely. Initial storm advisories can have a lead time of 36-48 hours. More extreme events like thunderstorms, hail, or other severe weather may have warning of an hour or less.

Extreme Temperature: Advisories regarding extremes in heat or cold can be issued with 0-36 hours of lead time.

High Wind: Wind advisories can have a lead time of 12 to 36 hours.

Space Weather: Space weather prediction services are provided by NOAA’s Space Weather Prediction Center. The SWPC draws on various data sources to provide forecasts, watches, warning and alerts with a possible lead time of hours to days of possible space weather activity. 127F 128 Lead time varies from hours to
days.

9.3 Cascading Impacts
While inclement weather is the most common hazard to occur in the OA, it is important to note the potential cascading impacts of these events.

Heavy precipitation can lead to significant flooding, also contributing to landslides and mudslides, posing serious threats to human lives, property, infrastructure and ecosystems. Flooding and other cascading impacts of heavy rain can interrupt transportation, utility services, or other emergency services.

Extreme temperatures, whether excessively hot or cold, will have impacts on human health, potentially leading to heatstroke, dehydration, and cold-related illnesses. Additionally, extreme temperatures can strain energy resources and infrastructure, increasing the risk of blackouts and disruption of critical services and infrastructure.

High winds have historically caused damage across the OA. They can damage or disrupt essential functions and interrupt service from energy and communication systems.

Space weather events, such as solar flares and geomagnetic storms can be significant and pose substantial risks to modern technological systems and critical infrastructure. Space weather can disrupt satellite communications, GPS, and aviation systems, affecting various sectors, including telecommunications, transportation, and emergency services.

9.4 Exposure

9.4.1 Population
Because of the unconfined and variable nature of inclement weather, the entire population is considered to be exposed. Heavy rain, hail, lightning, high winds, and extreme temperatures all have the potential to harm the health of or injure people within the OA.
127 https://www.weather.gov/safety/
128 Space Weather Watc https://www.weather.gov/safety/space-wwhes, Warnings and Alerts

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 9: Inclement Weather 171
Table 57: Population by Jurisdiction
Jurisdiction 2022
City of Campbell 42,833
City of Cupertino 59,610
City of Gilroy 59,269
City of Los Altos 31,526
Town of Los Altos Hills 8,400
Town of Los Gatos 33,062
City of Milpitas 80,839
City of Monte Sereno 3,488
City of Morgan Hill 46,451
City of Mountain View 83,864
City of Palo Alto 67,473
City of San José 976,482
City of Santa Clara 130,127
City of Saratoga 30,667
City of Sunnyvale 156,234
Unincorporated County 84,458
Total 1,894,783

9.4.2 Property
There are 535,391 buildings in the OA, with a total replacement value of $381 billion. All of these buildings exist in an area susceptible to inclement weather; however, dollar loss estimates for specific building occupancy types or geographic areas are not available. Although inclement weather events are the most frequent hazard in the OA, historic dollar losses have been comparatively moderate.

9.4.3 Critical Facilities and Infrastructure
All critical facilities in the OA are potential exposed to inclement weather. Facilities may be damaged directly by high winds, hail, or by flooding that results from heavy rain. Water, electric, fuel, transportation, and communication services could be disrupted by damage caused by heavy precipitation, high winds, or space weather. Extreme temperature may also disrupt services or strain energy systems.

9.5 Vulnerability
Changes in conditions: Increase in vulnerability
 A review of past events indicates that an increasing number of inclement weather events per year for heavy rain, extreme heat, and high winds.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 9: Inclement Weather 172
 CMRA indicates a trend in increasing number of extreme heat days each year and the number of consecutive hot days. Climate change will continue to increase the amount of very hot days and impacts to residents will increase as a result.
 Climate change affects the frequency and intensity of storms, which contributes to the potential of damaging inclement weather events such as heavy rain, hail, and high wind across the OA.  Overall population and building growth are slow in the OA, and are not expected to be a significant factor in increased vulnerability. Currently the increase in vulnerability appears to be driven more by an increased number of events than changes in development. However, continued monitoring of demographic shifts, new insights regarding the effects of climate change, economic changes that may impact building and other development, and aging of building stock and infrastructure should continue to receive consideration on potential weather impacts.

Individual jurisdictional annexes may note localized growth at a smaller scale.

9.5.1 Population
Because of the unpredictable nature of when and where significant weather events can occur, and variable intensity of these events, all of the OAs population of 1,894,783 residents are potentially vulnerable to impacts of inclement weather. The OA has a population of nearly 10,000 people that are homeless, according to the 2023 Santa Clara Point-In-Time report. This population is especially vulnerable to the impacts of inclement weather, especially with extreme heat and cold events, which can be detrimental to human health. Only 25% of individuals experiencing homelessness are sheltered, and the other 75% may be exposed to the elements and inclement weather events. NCEI has documented cases of illness or death in Santa Clara County related to extreme heat or cold.

All other vulnerable populations such as those in poverty, the elderly, and children can be negatively impacted by inclement weather events. Hazus 6.0 demographic data indicates that 260,705 persons or

13.5 percent of the population in the OA are over 64 years old. Older persons are particularly at risk to health problems caused by extreme heat. 422,719 persons or 21.8 percent of the population within the OA under 18 years of age. It is estimated that 2.45 percent of the people within the OA are economically disadvantaged, defined as having household incomes of $20,000 or less.

People with disabilities, communities of color, persons with limited English proficiency, housing condition, health care access and education are additional factors that affect vulnerability to severe weather hazards identified in the Silicon Valley 2.0 hazard fact sheet for extreme heat. Outdoor workers and unhoused residents are among the population most at risk. According to the Silicon Valley 2.0 report, 62% of the total County population is currently exposed to extreme heat conditions and 42% of those exposed have moderate to high social vulnerability. Approximately 60,000 people within moderate socially vulnerable communities in South County jurisdictions, such as Morgan Hill and Gilroy, could be exposed to frequent extreme heat events.

All business, schools, and government systems reliant on technology are vulnerable to the impacts of space weather. As space weather has the potential to disrupt the OA’s power and communication systems, any entity reliant on technology may experience more severe impacts.

9.5.2 Property, Critical Facilities, Infrastructure
Hazus modeling is not available to quantify potential damage to property, critical facilities, or infrastructure in the OA. Physical damage and service disruption are variable depending on the size and severity of the weather event.

Although the exact location and intensity of weather events makes it difficult to project exactly what physical assets may be at risk, possible impacts might include:

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 9: Inclement Weather 173
Heavy Rain
• Localized pooling of water on roadways has occurred during or following past heavy rain events.

Flooding in early 2023 lead to closures of major transportation routes, including Hwy 101 in Gilroy. Vehicles may lose traction or may be unable to safely traverse some roadways. Impacts to roadways may cause delays to emergency responders.
• Heavy rain has contributed to landslides and other slope failures that have blocked or damaged roadways.
• Pedestrian travel is particularly difficult during a severe weather event. Those who rely on public transportation may have difficulty reaching transit points or difficulty finding shelter if service is interrupted
• Heavy rain has caused localized flooding and subsequent water damage to nearby structures.

Flooding from heavy rain can damage residences, commercial, industrial, or other building types.

High Wind
• Above ground utilities such as communication towers, telephone lines and power lines are vulnerable to damage from high winds. High winds have caused trees to fall which have damaged power lines and transformers, disrupting power service to large areas. • Critical facilities may also lose power if high winds damage power distribution systems and may need to rely on back-up power systems.
• High winds can damage roofs or windows to homes and businesses or other critical facilities.

Winds can also lead to fallen trees which may damage nearby buildings or sidewalks. • Outdoor events such as sporting activities, farmers markets, and community festivals can be disrupted, and participants may have difficulty seeking shelter from a fast-moving storm.

Extreme Heat
• Extreme heat may lead to increased demand on energy systems which may strain their ability provide adequate cooling to facilities and increases potential for power outages. Rolling brownouts have occurred in the Bay Area during past extreme heat events. • High temperatures can cause pavement to soften and expand which can cause rutting, potholes, or other damage to streets.
• Extreme heat has had serious health consequences for people who are unhoused or for outdoor workers.

Extreme Cold
• Extreme cold is less common in the OA, but results in difficult travel if combined with precipitation that may lead to icy roadways, airport runways, and other transportation systems. • NCEI records have documented impacts to highways, airports, and mass transit systems due to icy conditions during period of extreme cold.
• Dangerous travel conditions can lead to damage from collisions, personal injury, or economic disruption.

9.5.3 Environment
Cascading effects of heavy rain could be damaging to the environment, such as erosion caused by increased runoff or flooding, or to systems damage from landslides. Hail can cause extensive damage to plants. High winds can break tree branches or cause other damage to plant life or animal habitat.

9.5.4 Economy
Inclement weather events can disrupt the economy of the OA. Economic impact will largely be associated with the disruption of transportation, power, gas, and telecommunication services caused by high winds, or associated with flooding caused by heavy precipitation. Events severe enough to cause structural damage can also cause business interruptions. The economic sectors affected may vary significantly depending on the extent and severity of the event.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 9: Inclement Weather 174

9.6 Future Trends in Development, Scenario, Issues
Impacts from inclement weather pose a real threat to physical, social and economic well-being in the OA.

Multiple events can be expected to affect the OA every year, with variable severity. OA residents can use weather advisories to prepare themselves for inclement weather events, but sometimes severe events have limited warning time. Inclement weather has the potential to cause cascading impacts and can damage multiple systems in both limited and widespread areas. Although there have been small decreases in population in recent years, Santa Clara County has experienced significant growth in the past. New development may affect drainage patterns and change impacts from heavy rain. An increase in population coincides with an increased number of people at risk to weather events. As discussed further in Section 11: Climate Change, impacts of severe weather are projected to intensify due to climate change.

9.7 Issues
 Inclement weather is inherently uncertain. Conditions can change rapidly, and the severity and exact locations of an event are highly variable.
 The variable nature of inclement weather makes it difficult to identify specific locations, structures, or systems that might be more likely to be damaged in an event. This makes it difficult to identify priorities or areas for risk reduction projects.
 People may not heed weather warnings.
 Heavy rain events may result in ponding or localized flooding in low-lying areas.  Heavy rain may result in inland stormwater flooding after stormwater management systems are overwhelmed.
 Inclement weather can cause secondary hazards such as flooding, landslides, mudslides, and disruption of systems or infrastructure.
 Inclement weather events are directly related to climate change; however, climate change impacts still are poorly understood. Average temperatures are expected to increase and changes in precipitation patterns are anticipated, but it is not yet known how that will impact local weather patterns. Impacts of inclement weather events are anticipated to become more severe.

9.8 Consequence Analysis
Inclement weather events occur frequently, and these events are likely to increase in frequency over time.

They may cause a wide range of impacts, which vary greatly depending on the level of severity and extent for each event. The information in Table 58 provides the consequence analysis of the potential for detrimental impacts of inclement weather done for accreditation with the Emergency Management Accreditation Program (EMAP).

Table 58: EMAP Consequence Analysis Inclement Weather
Subject Ranking Impacts/Inclement Weather
Health and Safety of
Public in the Area of the
Incident
Severe Impact of the immediate area could be severe for affected areas and moderate to light for other less
affected areas.

Responders Moderate Impact to responders could be severe for unprotected personnel and moderate to light for prepared personnel.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 9: Inclement Weather 175
Subject Ranking Impacts/Inclement Weather
Continuity of Operations
(including continued
delivery of services)
Moderate There is a moderate expectation to execute the COOP, though a severe level of impact can be expected for the full extent of an inclement weather event.

Property, Facilities, and
Infrastructure
Minimal to
Severe
There is potential for localized impact to facilities and infrastructure in the incident area, which can range from minimal to severe depending on the extent of the
incident.

Delivery of Services Minimal to
Severe
Delivery of services could be affected if there is any
disruption to the roads and/or utilities due to damages sustained.

Environment Moderate to
Severe
Greatest impact will be to trees, bushes, foliage, crops, and wildlife, which could be severe.

Economic Conditions Minimal to
Severe
Impacts to the economy will greatly depend on the
severity of the inclement weather event, longevity of the storm, and any damages sustained such as utilities and
roads. Public Confidence in the
Government
Minimal to
Severe
Response and recovery will be in question if not timely and effective.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 10: Drought 176
10 Drought
Definitions
 Meteorological Drought: Occurs when rainfall has been deficient for an extended period.  Hydrological Drought: Occurs when rainfall deficits impact the water supply available from streams, reservoirs, lakes, and groundwater.
 Agricultural Drought: Occurs when factors such as rainfall deficits, soil water deficits, reduced groundwater, or low reservoir levels for irrigation result in impacts on agriculture.  Socioeconomic Drought: Occurs when diminished water supply reduces the supply of economic goods such as fruits, vegetables, grains, or meat.
 Ecological Drought: Occurs when a prolonged and widespread deficit in naturally available water supplies—including changes in natural and managed hydrology—creates multiple stresses across ecosystems.

10.1 General Background
Drought is a significant decrease in water supply relative to what is “normal” in a given location. A normal phase in the climate cycle of most geographical regions, drought originates 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.

Determination of when drought begins is based on impacts on water users and assessments of the available water supply, including water stored in surface reservoirs or groundwater basins. Different water agencies have different criteria for defining drought. Some issue drought watch or drought warning announcements. The California Water Code does not include a statutory definition of drought; however, analysis of the code indicates that legal matters most frequently focus on drought conditions during times of water shortages.

10.1.1 Monitoring and Categorizing Drought
Drought monitoring at the national, regional, and local levels is an integral part of drought early warning, planning, and mitigation. 128F
129 Nationally, agencies involved in this effort include the National Oceanic and Atmospheric Administration, National Integrated Drought Information System which produces the U.S.

Drought Monitor, and the National Drought Mitigation Center at the University of Nebraska-Lincoln.

In California, drought is addressed by the California Department of Water Resources (DWR).

The National Oceanic and Atmospheric Administration
The National Oceanic and Atmospheric Administration (NOAA) has developed several indices to measure drought impacts and severity and to map their extent and locations:
 The Palmer Crop Moisture Index measures short-term drought weekly to quantify drought’s impacts on agriculture during the growing season.
129 National Oceanic and Atmospheric Administration. (n.d.). Monitoring Drought. https://www.drought.gov/what-isdrought/monitoring-drought

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 10: Drought 177
 The Palmer Drought Severity Index is based on long-term weather patterns. Long-term drought is cumulative, so the intensity of drought during a given month depends on current weather plus the cumulative weather of previous months. The Palmer Drought Index responds rapidly as weather patterns change quickly.
 The Palmer Z Index measures short-term drought on a monthly scale.  The hydrological impacts of drought (e.g., reservoir levels, groundwater levels, etc.) take longer to develop and it takes longer to recover from them. The Palmer Hydrological Drought Index is a long-term index to quantify hydrology effects. The Palmer Hydrological Drought Index responds more slowly to changing conditions than the Palmer Drought Index.  The Standardized Precipitation Index considers only precipitation. In the Standardized Precipitation Index, an index 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.

The following graphics show the Palmer Drought Index and Palmer Hydrological Drought Index for California Climate Division 4, of which Santa Clara County is a part, from January 1, 2005, to May 20, 2023. Near normal conditions are indicated by -1.9 to +1.9. Drought conditions are indicated by -2.0 to -

4.0 or less and wet conditions are indicated by +2.0 to +4.0 or above.

Figure 45: Palmer Drought Index from January 1, 2005, to May 20, 202312 9F 130
Figure 46: Palmer Hydrological Drought Index from January 1, 2005, to May 20, 202313 0F 131
130 National Centers for Environmental Information. (2023, May). Weekly Palmer Drought Indices Divisional Time Series. https://www.ncei.noaa.gov/access/monitoring/weekly-palmers/time-series/0404 131 National Centers for Environmental Information. (2023, February). North American Drought Monitor. https://www.ncei.noaa.gov/access/monitoring/nadm/indices/spi/div

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 10: Drought 178
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.131F 132
 D0: Abnormally Dry
▪ Short-term dryness slowing planting and growth of crops ▪ Some lingering water deficits
▪ Pastures or crops not fully recovered
 D1: Moderate Drought
▪ Some damage to crops and pastures
▪ Some water shortages are developing
▪ Voluntary water-use restrictions requested
 D2: Severe Drought
▪ Crop or pasture loss likely
▪ Water shortages are 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

10.1.2 Normal Precipitation in California
Most of California’s precipitation comes from storms moving across the Pacific Ocean. Extremely dry and extremely wet years have become more common in California. 13 2F 133 On average, 75 percent of California’s
annual precipitation occurs between November and March, with 50 percent occurring between December and February. A persistent Pacific high-pressure zone over California in mid-winter signals a tendency for a dry water year. A typical water year produces about 100 inches of rainfall over the North Coast, 50 inches of precipitation (combination of rain and snow) over the Northern Sierra, and 15 inches in Santa Clara County. In extremely dry years, these annual totals can fall to as little as one half, or even one third of these amounts.
132 National Integrated Drought Information System. (n.d.). U.S. Drought Monitor (USDM). https://www.drought.gov/data-maps-tools/us-droughtmonitor#:~:text=Thepercent20U.S.percent20Droughtpercent20Monitorpercent20percent28USDMpercent29percent20 ispercent20apercent20map,Severepercent20percent28D2percent29percent2Cpercent20Extremepercent20percent28 D3percent29percent20andpercent20Exceptionalpercent20percent28D4percent29percent20Drought. 133 Office of Environmental Health Hazard Assessment. (2018). Precipitation. https://oehha.ca.gov/media/epic/downloads/09precipitation_19dec2018.pdf#:~:text=Extremelypercent20drypercent20 andpercent20extremelypercent20wetpercent20yearspercent20havepercent20become,thatpercent20providepercent2 0mostpercent20ofpercent20thepercent20statepercentE2percent80percent99spercent20waterpercent20supplies.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 10: Drought 179
As the winter months have become warmer in recent years, more precipitation has been falling as rain instead of snow over the watersheds that provide most of the state’s water supplies. With climate change, more intense dry periods under warmer conditions are anticipated, leading to extended, more frequent drought in California. A higher proportion of precipitation falling as rain instead of snow and an increase in the duration, frequency, and intensity of warm, wet storms are also projected. This can result in greater flooding, and force reservoirs to release more water early in the spring, which means less water will be available for agriculture and municipal uses in the summer and fall. 13 3F 134
The Sierra Nevada snowpack serves as the primary agent for replenishing water in the San Francisco Bay area, including Santa Clara County, and for much of the State of California. A reduction in spring snowpack runoff, whether due to drier winters or to increasing temperatures leading to more rain than snow, can increase the risk of summer or fall water shortages throughout the region.

Increases in temperature are already causing decreases in snowpack. The mountain snowpack provides as much as a third of California’s water supply by accumulating snow during our wet winters and releasing it slowly during our dry springs and summers. Warmer temperatures will melt the snow faster and earlier, making it more difficult to store and use throughout the dry season. The DWR Climate Change Team expects that by the end of this century, California’s Sierra Nevada snowpack is projected to experience a 48–65 percent loss from the historical April 1 average of 66.5 inches. 134F 135

10.1.3 Water Supply Strategy
The Bay Area Water Supply Conservation Agency (BAWSCA) is the main water provider for much of the Bay Area, allowing Santa Clara Valley Water District (SCVWD) to manage the continual water supply necessary to maintain health, safety, and economic wellbeing of residents, businesses, and community organizations. BAWSCA developed a two-phase, long-term water supply strategy for customers throughout the Bay Area, as outlined in the 2015 Long-Term Reliable Water Supply Strategy Phase II Final Report.135F
136 Purposes of its strategy are as follows:
 Quantifying water supply reliability needs of BAWSCA member agencies through 2040.  Identifying water supply management programs or programs that can be developed to meet those regional water reliability needs.
 Developing an implementation plan for the water supply strategy.

This strategy recognized that drought-year shortfalls could be significant but determined that normalyear water supply would be adequate through at least 2040. Dry years could result in system-wide cutbacks of up to 20 percent, but 10 to 15 percent is the more consistent standard. BAWSCA noted that the impacts of water shortages would be regional and could lead to secondary detrimental economic effects. To address this concern, the strategy focused on identifying options for filling all or part of the drought-year supply shortfall and investigating and potentially implementing actions that seem most beneficial.1 36F
137
134 Office of Environmental Health Hazard Assessment. (2018). Precipitation. https://oehha.ca.gov/media/epic/downloads/09precipitation_19dec2018.pdf#:~:text=Extremelypercent20drypercent20 andpercent20extremelypercent20wetpercent20yearspercent20havepercent20become,thatpercent20providepercent2 0mostpercent20ofpercent20thepercent20statepercentE2percent80percent99spercent20waterpercent20supplies. 135 California Department of Water Resources. (n.d.). Climate Change and Water. https://water.ca.gov/Programs/AllPrograms/Climate-Change-Program/Climate-Change-and-Water 136 Bay Area Water Supply Conservation Agency. (2015 February). Long-Term Reliable Water Supply Strategy. https://bawsca.org/water/reliability/strategy
137 Bay Area Water Supply & Conservation Agency. (n.d.). Water Supply & System. https://bawsca.org/water/supply

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 10: Drought 180
BAWSCA also developed a Water Conservation Implementation Plan,137F 138 focusing on the following
objectives:
 Help BAWSCA member agencies evaluate potential water savings and cost-effectiveness associated with implementing additional water conservation measures beyond their commitments of 2004.
 Determine potential water savings in 2018 and 2030 based on a selected range of new conservation measures and the 2004 water conservation commitments.  Determine BAWSCA’s role in helping member agencies achieve individual water conservation goals.
 Develop a coordinated regional plan for water conservation implementation measures to serve as a guideline for member agencies.

In the Santa Clara County OA, the following districts and cities are members of BAWSCA: SCVWD, Milpitas, Mountain View, Palo Alto, San José, City of Santa Clara, Sunnyvale, Purissma Hills Water District, and Stanford University. 138F
139
The SCVWD is the wholesale water and groundwater management agency throughout Santa Clara County, relying on local retailers (municipalities and private companies) to deliver water throughout the County.1 39F
140 The following are the retailer water providers for each municipal planning partner:
 Campbell: San José Water Company
 Cupertino: San José Water Company and California Water Service Company  Gilroy: Gilroy Public Works Department
 Los Altos: California Water Service Company
 Los Altos Hills: Purissima Hills Water District and California Water Service Company  Los Gatos: San José Water Company
 Milpitas: City of Milpitas Community Services
 Monte Sereno: San José Water Company
 Morgan Hill: City of Morgan Hill
 Mountain View: City of Mountain View Public Works
 Palo Alto: City of Palo Alto Utilities Department
 San José: San José Water Company, Great Oaks Water Company, and San José Municipal Water System
 Santa Clara City: City of Santa Clara Water Department  Saratoga: San José Water Company
138 Bay Area Water Supply & Conservation Agency. (n.d.). Conservation Implementation Plan. https://bawsca.org/conserve/reports/plan#:~:text=Thepercent20specificpercent20objectivespercent20ofpercent20thep ercent20WCIPpercent20arepercent20as,beyondpercent20whatpercent20theypercent20hadpercent20committedperc ent20topercent20inpercent202004.
139 Bay Area Water Supply & Conservation Agency. (2020). Member Agency Map. https://bawsca.org/members/map 140 Santa Clara Valley Water District. (2021). Groundwater Management Plan. https://s3.us-west2.amazonaws.com/assets.valleywater.org/2021_GWMP_web_version.pdf

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 Sunnyvale: City of Sunnyvale Public Works Department and California Water Service Company
The SCVWD has its own water supply strategy outlined in the SCVWD Water Supply Master Plan 2040.1 40F 141
The Water Master Plan 2040 outlines a water supply strategy with three key elements:
 Secure existing supplies and facilities.
 Optimize the use of existing supplies and facilities.  Expand water use efficiency efforts.

Some County residents have domestic wells on their property. The North Central Regional Office of California DWR monitors wells for Santa Clara County to help protect groundwater quality.1 41F 142 Under
Ordinance 90-1, as of July 1, 2013, a person must obtain a permit from SCVWD to perform any well activities. In response to extreme and expanding drought conditions, as those existing on March 28, 2022, the state can prohibit Valley Water and other well permitting agencies from issuing a construction permit for a new groundwater well.14 2F
143

10.1.4 Water Supply Infrastructure
Figure 46 shows the SCVWD water supply system. Santa Clara County receives 50 percent of its water supply from the San Francisco Bay-Delta watershed. Of this water, 40 percent comes directly through the Delta watershed or water conveyance systems and 10 percent is from the Hetch-Hetchy System. Another 30 percent of the County’s supply is local, from natural groundwater, reservoirs to groundwater, and reservoirs to drinking water treatment plans. Five percent is recycled water, primarily used for irrigation, industry, and agriculture. A 15 percent water-use reduction by the community is required to reach the needed water supply total.14 3F
144
141 Santa Clara Valley Water District. (2019, November). Water Supply Master Plan 2040. https://www.valleywater.org/sites/default/files/Waterpercent20Supplypercent20Masterpercent20Planpercent202040_

11.01.2019_v2.pdf
142 California Department of Water Resources. (n.d.). Technical Assistance. https://water.ca.gov/Programs/Integrated-Regional-Water-Management/Technical-Assistance 143 Santa Clara Valley Water District. (n.d.). Well Permits and Inspections. https://www.valleywater.org/contractors/doing-businesses-with-the-district/wells-well-owners/well-permits-andinspections 144 Santa Clara Valley Water District. (n.d.). Where Your Water Comes From. https://www.valleywater.org/yourwater/where-your-water-comes

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Figure 47: Santa Clara Valley Water District System Water Supply144F 145
The Hetch-Hetchy Water System was approved in 1913 under the Raker Act, which allowed use of federal lands to build that water system. The water system was constructed by San Francisco over the next 20 years, with first delivery of water in 1934. Although the system is owned by San Francisco, it was designed from the beginning to serve as a regional water supply system. 145F 146 Figure 47 shows the HetchHetchy Water System.
145 Santa Clara Valley Water District. (n.d.). Where Your Water Comes From. https://www.valleywater.org/yourwater/where-your-water-comes 146 Bay Area Water Supply and Conservation Agency. (n.d.). Hetch Hetchy System. https://bawsca.org/water/supply/hetchhetchy

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Figure 48: Hetch Hetchy Water System1 46F
147

10.2 Hazard Profile
Droughts originate from a deficiency of precipitation resulting from an unusual weather pattern. Such patterns can be short-term, lasting for a few weeks or months, or long-term, lasting for many months or for years. It is possible for a region to experience a long-term circulation pattern that produces drought, and to have short-term changes in this long-term pattern that result in short-term wet spells. Likewise, it is possible for a long-term wet circulation pattern to be interrupted by short-term weather spells that result in short-term drought. Droughts typically occur after 2 or 3 years of below-average rainfall during the period from November to March, when about 75 percent of California’s average annual precipitation falls.

10.2.1 Past Events
California is no stranger to drought; it is a recurring feature of our climate. 1 47F 148 The DWR has state
hydrologic data from as far back as the early 1900s which indicate occurrences of multi-year droughts from 1912 to 1913, 1918 to 1920, and 1922 to 1924. Between 1954 and 2016, California experienced one FEMA-declared emergency (EM) classified as a drought: FEMA Declaration EM-3023 in 1977, which applied to 58 California counties, including Santa Clara County. 148F 149 Santa Clara’s drought history includes
years of drought followed by years with little drought.  1976 to 1977: California had one of its most severe droughts during the winters of 1976 and 1977. 1977 was the driest period on record in California, with the previous winter recorded as the fourth driest in California’s hydrological history. The cumulative impact led to widespread water shortages and severe water conservation measures across the state. Only 37 percent of normal Sacramento Valley runoff was received.
147 Bay Area Water Supply and Conservation Agency. (n.d.). Hetch Hetchy System. https://bawsca.org/water/supply/hetchhetchy
148 California Department of Water Resources. (n.d.). Drought. https://water.ca.gov/drought/ 149 Federal Emergency Management Agency. (n.d.). California Drought EM-3023-CA. https://www.fema.gov/disaster/3023

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 Over $2.6 billion in crop damage was recorded in 31 counties. Santa Clara County was included in FEMA-3023-EM-CA declaration on January 20, 1977.
 1987 to 1992: California’s received precipitation was well below average levels for four consecutive years. While the Central Coast was most affected, the Sierra Nevada mountains in Northern California and the San Joaquin Valley (Central Valley) were also affected. Water suppliers did not experience shortages until the third or fourth year of the drought. Reservoir storage provided a buffer against drought impacts during the initial years of the drought. In 1991, the State Water Project sharply decreased deliveries to water suppliers, including the San Francisco Bay Area. The SCVWD implemented drought contingency measures such as rationing and mandatory conservation to reach its 25 percent reduction goal. By February 1991, all 58 counties in California were suffering under drought conditions that affected urban, rural, and agricultural areas. Some counties had declared a local drought emergency, but Santa Clara County was not included.
 1993 to 2006: Rainfall in this period reached previous highs, but severe drought conditions returned in 2007.
 2007 to 2009: A governor’s executive order 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, after the largest court-ordered water restriction in state history up to that time, a state of emergency was proclaimed for the entire state as the severe drought conditions continued. Santa Clara County received about half of its water through the Sacramento-San Joaquin River Delta, which was already significantly limited that year because of pumping restrictions mandated under the Endangered Species Act. 149F 150 Water deliveries through the Delta
were cut by about 20 to 30 percent. The SCVWD had mandatory water conservation and rationing measures in effect to reduce usage by 15 percent.
 2012 to 2017: California’s drought has set several records. From 2012 to 2014, it ranked as the driest three consecutive years for statewide precipitation. New climate records were set in 2014 for statewide average temperatures and for record-low water allocations from State Water Project and Central Valley Project contractors. A statewide drought emergency was declared in January 2014. Minimum annual precipitation records were set for many communities in 2013. Executive orders and regulations called for water conservation and management. A new law requires retail urban water suppliers with more than 3,000 customers to establish rules defining “excessive water use” and impose those rules during drought emergencies. On its website, DWR refers to “the five-year drought that ended in 2016. 150F
151 On April 2, 2017, Governor Brown lifted the drought emergency imposed in 2014 but declared that California must continue water conservation efforts USGS).1 51F
152
 2017 to Present: The National Drought Mitigation Center developed the Drought Impact Reporter1 52F
153 as 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 members of relevant government agencies. The database is being populated beginning with the most recent impacts and working backward in time. 150 U.S. Fish and Wildlife Service. (n.d.). Endangered Species Act. https://www.fws.gov/media/endangered-speciesact 151 California Department of Water Resources. (n.d.). Countywide Drought and Water Shortage Contingency Plans. https://water.ca.gov/Programs/Water-Use-And-Efficiency/2018-Water-Conservation-Legislation/County-DroughtPlanning 152 USGS. (n.d.). 2012-2016 California Drought: Historical Perspective. (https://ca.water.usgs.gov/californiadrought/california-drought-comparisons.html 153 National Drought Mitigation Center. (n.d.). Drought Impact Reporter. https://www.drought.gov/data-mapstools/drought-impact-reporter-dir

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Between January 2017 and January 2023, the Drought Impact Reporter 153F 154 described more than
fifty incidents of drought-related events in the Santa Clara OA. Highlights of that list are presented below.
▪ Feb. 8, 2017: The State Water Resources Control Board opted to keep the water restrictions until spring to see how the rest of winter plays out, in terms of precipitation, before making any changes to the restrictions.
▪ March 22, 2017: Farmers south of the Sacramento-San Joaquin Delta learned that they would receive 65 percent of full allocations from the U.S. Bureau of Reclamation, despite the heavy rainfall the state has received, leading to flooding, full reservoirs, and deep snowpack. ▪ April 11, 2017: The U.S. Bureau of Reclamation announced that South of Delta water contractors would receive a 100 percent allocation from the Central Valley Project. ▪ Jan. 30, 2018: The California Department of Water Resources announced that customers of the State Water Project would receive 20 percent of their requests. ▪ Feb. 22, 2018: Eastside water service contractors will receive 100 percent of their contracts. ▪ 2019: No relevant reports on drought.
▪ Feb. 25, 2020: North-of-Delta agriculture contractors will receive 50 percent of their contracted supply, while South-of-Delta agriculture contractors will receive 15 percent. ▪ Feb. 27, 2020: Northern California ranchers with unirrigated pastures were already giving their cattle supplemental feed as grasses have dried out months earlier than usual. ▪ May 26, 2020: After a dry winter, the State Water Project increased its allocation to 20 percent following above normal precipitation in May.
▪ Feb. 23, 2021: The upcoming March snowpack survey and a planned airborne snow survey will provide more information on the amount of water available for growers. The Tuolumne snowpack measured just 55.6 percent of the historical average for the date. ▪ March 17, 2021: Customers of the Santa Clara Valley Water District were urged to increase their conservation efforts, but no water restrictions were mandated just yet. ▪ March 23, 2021: The California Department of Water Resources updated its initial water allocation for the 2021 water year to 5 percent of requested supplies, down from 10 percent as announced in December 2020.
▪ March 23, 2021: The State Water Board sent notices to California’s 40,000 water users, including small farms and big cities, to alert them to prepare for cuts in water deliveries. ▪ April 28, 2021: The board also voted unanimously to double the price it pays homeowners to use drought-tolerant landscaping from $1 per square foot to $2. The district serves nearly two million people in Santa Clara County.
▪ May 5, 2021: Farmers north of the Sacramento-San Joaquin Delta learned that they will not receive even 5 percent of contracted water from the Central Valley Project. Water deliveries were suspended, due to limited supply, according to the Bureau of Reclamation. ▪ May 26, 2021: The U.S. Bureau of Reclamation announced that municipal water agencies that get water from the Central Valley Project would receive just 25 percent of their allocation. 154 National Integrated Drought Information System. (n.d.). Drought Impact Reporter Dashboard. https://unldroughtcenter.maps.arcgis.com/apps/dashboards/46afe627bb60422f85944d70069c09cf

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▪ May 27, 2021: The federal government announced water cuts to urban areas of more than half, which will lead the Santa Clara Valley Water District. ▪ June 10, 2021: Mandatory water restrictions were issued for Santa Clara County, due to the low water supply.
▪ June 25, 2021: A local emergency was declared in Santa Clara County, due to extreme drought.
▪ July 4, 2021: Farmers were not getting any water from the state or federal projects. ▪ July 8, 2021: California Gov. Gavin Newsom requested that people and businesses curb their water use by 15 percent as intense drought persisted.
▪ Aug 2, 2021: Santa Clara Valley Water District does not have enough water to release to support the fish.
▪ Nov 23, 2021: The San Francisco Public Utilities Commission urged almost 3 million water customers in the Bay Area to curb water use by 10 percent and declared a water shortage emergency.
▪ Dec 2, 2021: The State Water Resources Control Board proposed new emergency drought regulations to discourage water waste as water supplies were low after continued drought. ▪ March 15, 2022: Despite the record dry January and February, Californians are falling short on voluntary water conservation.
▪ May 23, 2022: Urban water use increased 18.9 percent in March. ▪ May 25, 2022: Water users in Santa Clara County could be fined $500, or even up to $10,000, as the Santa Clara Valley Water District enacted new drought rules. ▪ June 7, 2022: Public systems, like the City of San Francisco, will have to have to turn to groundwater or other sources.
▪ Oct 19, 2022: Many trees in California’s forests were turning rust colored as another year of drought and bark beetles or other insects.
▪ Nov 28, 2022: Water storage was near historic lows, with Shasta Reservoir, the largest reservoir in the Central Valley Project, at 31 percent of capacity. ▪ Nov 30, 2022: A survey of California urban water agencies representing about 90 percent of the state’s population showed that about 18 percent, or 73 of the 414 water suppliers, indicated that they will soon face potential shortages. ▪ Dec 1, 2022: The Department of Water Resources announced an initial State Water Project allocation of 5 percent of requested supplies for 2023. ▪ Jan 26, 2023: The series of storms that brought heavy rain and snow to California have partially filled reservoirs, allowing the State Water Project to offer 30 percent of requested water supplies to 29 public water agencies that serve 27 million Californians. The initial allocation in December was just 5 percent.

U.S. Department of Agriculture Disaster Declarations
The U.S. Department of Agriculture (USDA) Farm Service Agency provides assistance for natural disaster losses resulting from drought, flood, fire, freeze, tornadoes, pest infestation, and other natural disasters.

The USDA Secretary of Agriculture is authorized to designate counties as disaster areas to make emergency loans to producers suffering losses in those counties and in contiguous counties.

For drought-related data between 2017 and 2022, the period for which data was available, California has been included in 55 State and County Level Records of Disaster Designation Information made by the US

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Secretary of Agriculture. Santa Clara County was included in seven of these declarations in relation to drought. 15 4F
155
Table 59: U.S. Department of Agriculture Disaster Declarations Including Santa Clara County, 2017–2022
Year Declaration Number(s)
2017 S4144 and S4163
2018 None
2019 None
2020 S4697
2021 S4916, S4969, and S4958
2022 S5146

10.2.2 Location
Drought is a regional phenomenon. A drought that affects the Santa Clara County OA would affect all aspects of the environment and the community simultaneously and has the potential to impact every person directly or indirectly in the county as well as adversely affect the local economy. “The norm for California’s climate is to move back and forth frequently between wet and dry conditions, and water conservation must be a way of life for all who enjoy living in or visiting our state.”
Karla Nemeth, Director of California Department of Water Resources March 12, 2018

10.2.3 Frequency
Historical drought data regarding Santa Clara County indicate four significant droughts over the last 40 years, with drought occurring in 12 of those 40 years. 155F 156 Based on risk factors and this history, droughts
likely will continue to occur cyclically in the Santa Clara County OA.

Further information on the impact of climate change on the probability of drought is included in Section

11. Probability in OA: Likely
155 U.S. Department of Agriculture. (n.d.). Disaster Designation Information. https://www.fsa.usda.gov/programs-andservices/disaster-assistance-program/disaster-designation-information/index 156 National Integrated Drought Information System. (n.d.). Drought Conditions for Santa Clara County. https://www.drought.gov/states/california/county/santapercent20clara

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10.2.4 Warning Time
Empirical studies conducted over the past century have shown that meteorological drought is never the result of a single cause. It is the result of many causes, often synergistic in nature; these include global weather patterns that produce persistent, upper-level high-pressure systems along the West Coast with warm, dry air resulting in less precipitation.

Scientists at this time do not know how to predict drought more than a month in advance for most locations. Predicting drought depends on the ability to forecast precipitation and temperature. Anomalies of precipitation and temperature may last from several months to several decades; California is currently finishing a several-year-long drought, while other areas in the United States may undergo droughts as short as 1 or 2 months. How long droughts last depends on interactions between the atmosphere and the oceans, soil moisture and land surface processes, topography, internal dynamics, and the accumulated influence of weather systems on the global scale.

10.2.5 Extent
Figure 48 displays data from the U.S. Drought Monitor for the State of California from 2000 through 2023.

Several periods of moderate to extreme drought have occurred during this time. D0 Abnormally Dry represents the least severe drought conditions and D4 Exceptional Drought represents the most severe.

As noted in 10.2.1 Past Events, significant droughts have occurred from 2007-2009, 2012-2017 and 2000-2023. During these events, significant portions of the entire state were impacted. The OA has experienced conditions from D0 to D4 at different times within this time frame. Figure 49 displays a snapshot of conditions during the most recent drought. During the fall of 2022, the entire OA was experiencing Severe Drought conditions, with the easternmost part of the county experiencing Extreme Drought. As noted in 10.1.4, a significant portion of the OA water supply originates outside of the area which may experience different drought severity.

Figure 49: U.S. Drought Monitor Time Series California 2000-2023

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Figure 50: U.S. Drought Monitory Map of California

10.2.6 Severity and Impacts
The severity of a drought depends on the degree of moisture deficiency, the duration, 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. 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 property, as do other natural disasters. Drought affects agriculture, business and industry, energy, fire, plants, tourism and recreation, and water supply and quality.

The Drought Impact Reporter contains information on 154 impacts from droughts that affected Santa Clara County from January 2017 through January 18, 2023. The following are the categories and reported number of impacts. Note that some impacts have been assigned to more than one category.

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Figure 51: Reported Number of Drought Impacts by Category156F 157
The National Drought Mitigation Center uses three categories to describe drought impacts:
 Economic Impacts: These impacts of drought cost people or businesses money. They include farmers’ loss of crops, costs for irrigation or drilling new wells to address low water supply, lost business for companies that sell boats or fishing equipment, and water companies’ costs for additional water supplies.
 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.

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. Much of the flow in streams comes from groundwater, especially during the summer when there is less precipitation and after snowmelt ends. Reduced groundwater levels mean that even less water will enter streams when steam flows are lowest.
157 National Drought Mitigation Center. (n.d.). Drought Impact Reporter. https://www.drought.gov/data-mapstools/drought-impact-reporter-dir

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10.2.7 Responses to Recent Drought
Table 60: Recent Federal and State Drought Responses
Date Federal and State Drought Response
April 2021 Drought emergency proclamation for parts of the state.

May 2021 Drought emergency proclamation issued in April 2021 expanded to include Santa Clara County and $5.1 billion package to:
 Address immediate emergency needs.
 Build regional capacity to endure drought.
 Safeguard water supplies for communities, the economy, and the environment.

July 2021 Voluntary 15 percent water use reduction.

October 2021 State Water Board empowered to prohibit wasteful uses of potable water such as washing sidewalks or driveways.

January 2022 State Water Board prohibited using drinking water for activities such as filling decorative fountains/ponds, washing sidewalks and driveways, watering lawns during and right after rain, and using hoses without automatic shutoff nozzles.

March 2022 Local water suppliers called to move to Level 2 of their Water Shortage Contingency Plans. U.S Department of Agriculture Livestock Forage Disaster Program activated.

August 2022 California’s Water Supply Strategy, which includes creating additional water storage space, recycling and reusing water, increasing efficient water use and conservation, and diversifying water supplies, was released.

September 2022 California Legislature provides additional funds to state residents to replace their lawns with drought-resistant plants and landscaping.

November 2022 The California Department of Water Resources Control Board adopted new performance standards that require urban retail water suppliers to monitor and reduce leakage in their distribution systems. The California Legislature authorized over a billion dollars in funding to the California Department of Water Resources for drought relief in 2021 and 2022.

December 2022 The California Department of Water Resources Control Board extends its emergency regulation to January 2024.

Urban Water Suppliers’ Responses to Defined Drought Stages The California Water Code, Sec. 10632 was amended in 2016 to provide guidance on stages of action to be undertaken by urban water suppliers. It requires them to develop a plan that incorporates an urban water shortage contingency analysis that includes each of the following elements that are within the authority of the urban water supplies. Stages of action to be undertaken by the urban water supplier in response to water supply shortages, including up to a 50 percent reduction in water supply.

The extreme drought and water shortage emergency condition in summer 2022 called for water use restrictions. In responding to the drought condition, the SCVWD declared a water shortage emergency condition in June 2021 that called for 15 percent water use reduction to minimize water shortage risk. The call for water conservation has been instrumental in reducing county-wide water use and helps alleviate the negative consequence of the ongoing drought.

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Since the call, county-wide water use has been reduced by 6 percent cumulatively against a 2019 baseline.157F
158 In winter 2023 when this plan is being written, conditions continue to improve; but the county is still in a “D-1 Moderate Drought”. Participating municipality retail water providers’ drought contingency measures are described in the annexes in Volume 2 of this hazard mitigation plan, as applicable.

10.3 Cascading Hazards
When natural hazard events overlap or occur in quick succession, the events can compound and cause detrimental effects. Drought is particularly likely to be part of a cascading hazard because it can cover a large area and go on for a long time.158F
159

10.3.1 Drought and Wildfire
The hazard most associated with drought is wildfire (see Section 8). A prolonged lack of precipitation dries out vegetation, which becomes increasingly susceptible to ignition as the duration of the drought extends. Reduced ponds, streams, and reservoir levels can also limit withdrawal sources for fighting wildfires. The extreme conditions can also increase the likelihood of shrub and tree mortality by wildfire in previously fire-adapted ecosystems, Millions of board feet of timber have been lost, and in many cases, erosion occurred, which caused considerable damage to aquatic life, irrigation, and power production by heavy silting of streams, reservoirs, and rivers in addition to habitat and infrastructure losses and threats to animal and human life.

10.3.2 Drought and Extreme Heat
Drought is often accompanied by extreme heat, exposing people to the risk of sunstroke, heat cramps and heat exhaustion. Periods of extreme heat increase evaporation, leading to reduced water availability in soils and surface water supplies. Periods of drought can cause extreme heat due to lack of water in the atmosphere, soils, and rivers, where decreased water availability in the system reduces the amount of evaporation happening at the surface, quickly increasing temperatures. Extreme heat can also increase water demands, in which human activities can reduce water supplies, leading to human-caused drought.

These hazards occurring together can compound health impacts, reduce energy production, cause loss of aquatic life due to reduced stream and reservoir levels and increased water temperatures, kill vegetation, and create dangerous air quality issues.

10.3.3 Drought and Flooding
Drought, along with wildfires that can stem from drought, increases flood risk. Extended drought and wildfire can stress and reduce the amount of vegetation. When it does rain, the reduction of vegetation can increase flooding due to faster runoff rates, compared to normal conditions when abundant vegetation slows runoff and increases water absorption into the ground. Drought or wildfire conditions prior to flooding can also cause water quality deterioration from the increased soil and ash particles in the runoff. On farmlands, drought conditions prior to flooding may also cause a surge of farm chemicals applied to crops to enter streams through runoff. These factors can affect the water quality for aquatic life, animals, and humans, who are all dependent on the water source. Increased instances of flash flooding may also occur.
158 Santa Clara Valley Water District. (2022, November 22). Board of Directors Meeting Agenda. https://scvwd.legistar.com/Calendar.aspx
159 American Planning Association. (2019). Falling Dominoes: A Planner’s Guide to Drought and Cascading Impacts. https://abag.ca.gov/sites/default/files/falling-dominoes-planners-guide-to-drought-and-cascading-impacts.pdf

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10.3.4 Drought and Landslides
Droughts can indirectly cause landslides through a cascade of natural hazards. For example, drought can cause dry conditions and increased fuel loads for wildfires that, in turn, can increase the likelihood of flooding. The ash-infused topsoil, which is water repellent, and loss of vegetation can increase runoff and take large amounts of earthen material with them, causing devastating impacts to populations in the path of the landslide event. Such events could cause the loss of infrastructure and life. From an environmental standpoint, they may also affect the water quality of downstream rivers and streams and the habitat for animals, flora, and fauna. Landslides can also alter the topography of the landscape, which can modify surface and groundwater flow patterns.

10.4 Vulnerability
Changes in conditions: increase in vulnerability
 The CMRA tool in Figure 51 shows an increasing number of days without precipitation and an increase in the consecutive number of days without precipitation.  Extended periods of drought have the potential to impact the water supply in the OA.  Drought results in dryer vegetation and increased tree mortality, which compounds conditions that contribute to wildfire risk.
 Loss of vegetation due to drought may increase vulnerability to landslides.  Ongoing drought may contribute to land subsidence if groundwater is not recharged.  SCVWD is implementing recycled and purified water programs to provide access to droughtresistant water supply.  The National Risk Index reports and Expected Annual Loss of $1,771,981 for drought based on crop impacts.
 Santa Clara has adopted the Water Efficient Landscape Ordinance to promote water use efficiency and sustainable landscape design practices.

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Figure 52: CMRA Drought Hazard: Days without Precipitation Indicator for Santa Clara County

10.4.1 Population
The entire population of the Santa Clara County OA is vulnerable to drought events. Drought can affect people’s health and safety, including health problems related to low water flows, insect infestation, poor water quality, or dust. Droughts can also lead to loss of human life. 15 9F 160 The Centers for Disease Control
and Prevention (CDC) has identified the indirect role of drought in people’s deaths through disruptions of agriculture and water systems, poor air quality, and increased heat-related and respiratory illnesses. In a recent study, they have also addressed the increased occupational psychosocial stress among U.S. farmers.160F
161 Other possible impacts include recreational risks; effects on air quality; diminished living conditions related to energy, air quality, and hygiene; compromised food and nutrition.

These negative health effects are more likely to impact populations that already have higher health risks, such as the old or young. Hazus 6.0 demographic data indicates that 260,705 persons or 13.5 percent of the population in the OA are over 64 years old and 422,719 persons or 21.8 percent of the population within the OA under 18 years of age. These people may experience worse effects from poor air quality or reduced nutrition resulting from agricultural production loss. It is also estimated that 2.45 percent of the people within the OA are economically disadvantaged, defined as having household incomes of $20,000 or less. Those with fewer financial resources are often less resilient to hazard impacts.

The SCVWD, BAWSCA, regional water purveyors, and other regional stakeholders have devoted considerable time and effort to protect life, safety, and health during times of consecutive dry years, such as the current drought. Provisions and measures have been taken to analyze and account for anticipated 160 National Oceanic and Atmospheric Administration. (2016, June 15). Drought in America: Slow Moving, Far Reaching. https://www.noaa.gov/explainers/drought-in-america-slow-moving-far-reaching 161 National Integrated Drought Information Systems. (2021, July 24). The Association Between Drought Conditions and Increased Occupational Psychosocial Stress Among U.S. Farmers. https://www.drought.gov/documents/association-between-drought-conditions-and-increased-occupationalpsychosocial-stress

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water shortages. With coordination from its cities, the SCVWD has the ability to minimize and reduce impacts on residents and water consumers in the Santa Clara County OA.

10.4.2 Property
Significant depletion of groundwater supplies—from drought, excessive groundwater pumping or both— can lead to subsidence, which is the downward collapse of the land surface when groundwater aquifers lack the water to support the weight of the ground. Compaction of aquifer systems is the greatest cause of subsidence in California. Although this is typically due to groundwater pumping rather than drought, drought creates a need for greater groundwater pumping as freshwater sources disappear. Droughtinduced subsidence is not as common as wildfire or extreme heat, but it can significantly impact the local environment, floodplain/wetlands, and water supply, and it typically is irreversible. It may cause wetlands to change size and shape, migrate to lower elevations, or disappear entirely. Rivers may change course, and patterns of erosion and deposition may change.161F
162
No structures will be directly affected by drought conditions, though droughts often lead to reduced local fire suppression capabilities which could threaten structures. Some structures may become vulnerable to wildfires, which are more likely following years of drought.

Drought conditions may also be the cause of serious foundation problems and have significant impacts on landscapes, which could cause a financial burden to property owners. However, these impacts are not considered critical in planning for impacts from the drought hazard.

Drought is detrimental to agricultural assets and other natural resources. Although the agricultural sector is relatively small in the OA, a significant area in the southeast area of the county near Gilroy is involved in agricultural production. Extended drought may diminish the productivity of agricultural lands and impact their value. The National Risk Index reports and Expected Annual Loss of $1,771,981 to crops due to drought in Santa Clara County. This area would likely feel more significant impact of drought due to crop losses caused by drought or impacts to water supplies.

10.4.3 Critical Facilities
Critical facilities as defined for this plan will continue to be operational during a drought. Critical facility elements such as landscaping may not be maintained due to limited resources, but the risk to the Santa Clara County OA’s critical facilities functions is low.

10.4.4 Environmental Impact
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.

Wildlife habitat, for example, may be degraded through the loss of wetlands, lakes, and vegetation.

However, many species will eventually recover from this temporary aberration. The degradation of landscape quality, including increased soil erosion, may lead to a more permanent loss of biological productivity. Although environmental losses are difficult to quantify, growing public awareness and 162 United States Geological Survey. (2000, December). Land Subsidence in the United States. https://water.usgs.gov/ogw/pubs/fs00165/#:~:text=Thepercent20sitepercent20ispercent20inpercent20thepercent20Sa npercent20Joaquinpercent20Valley,resultedpercent20inpercent20permanentpercent20subsidencepercent20andperc ent20relatedpercent20groundpercent20failures.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 10: Drought 196
concern for environmental quality has forced public officials to focus greater attention and resources on these effects.

Santa Clara County Tree Mortality Exposure
Large numbers of trees have died in California since the beginning of the 2012-2016 drought. Most of these trees were stressed from higher temperatures and a shortage of water, making them more vulnerable to insects and diseases. California’s pattern of tree mortality corresponds with global trends that are linked to increasingly dry and hot climatic conditions. Prolonged periods of drought, combined with the increased infestation of native bark beetles, have contributed to the death of millions of trees on federal, state, and private lands across the state. Removal of these dead trees can be costly and challenging, which can add to the financial impacts of drought. “Many trees in California’s forests were turning rust colored as another year of drought and bark beetles or other insects led to higher tree mortality. Trees were stressed from inadequate water and could not produce enough sap or pitch to defend themselves against insects.” Drought Impact Reporter Dashboard
October 19, 2022
The following figures show the areas where the state is focused for removal of dead trees. The Tier 1 Tree Mortality High Hazard Zones (see Figure 52) represent areas of tree mortality in direct proximity to assets determined to be important to life and property (including communications, transportation, recreation, communities, and utilities). These areas were designated by state and local governments as being in greatest need of dead tree removal, pursuant to the California Governor’s Emergency proclamation on October 30, 2015.

The Tier 2 Tree Mortality High Hazard Zones, shown in Figure 53, are defined by watersheds that have elevated tree mortality as well as significant community and natural resource assets.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 10: Drought 197
Figure 53: Tier 1 Tree Mortality High Hazard Zones162F
163
Figure 54: Tier 2 Tree Mortality High Hazard Zones163F
164
163 California State Geoportal. (2022 June 2). California High Hazard Zones (Tier 1). https://gis.data.ca.gov/maps/a71a85136b0b414ea734fdfbe3d7674a/explore?layer=0&location=37.231739%2C121.694821%2C9.99 164 California State Geoportal. (2022 June 2). California High Hazard Zones (Tier 2). https://hub.arcgis.com/maps/e50b7577426c4367a518b80b38e9b5d8/explore?location=37.371146%2C121.549960%2C10.88

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10.4.5 Economic Impact
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). In addition to losses in yields in crop and livestock production, drought is associated with increased insect infestations, plant diseases, and wind erosion. Water- and lake-related recreational activities including, but not limited to, fishing, swimming, rafting, and canoeing are valuable for the local and regional economy, particularly when the lake/reservoir is located in a rural area with limited other recreational/tourism opportunities. 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. When a drought occurs, the agricultural industry faces risk of economic impact and damage. During droughts, crops do not mature, resulting in smaller crop yields, undernourishment of wildlife and livestock, decreases in land values, and ultimately financial losses to farmers. 164F 165 Agriculture production has been a
significant and growing factor in Santa Clara County, especially as agricultural effects on the economy start to normalize (after a period of decline).

Evaluation of direct effects (i.e., excluding indirect and induced spending benefits) can occur based on information conveyed in USDA reports. According to the 2017 Census of Agriculture, 890 farms were present in Santa Clara County, encompassing 288,084 acres of total farmland. The average farm size was 324 acres.

Santa Clara County farms had a total market value of products sold of $310.2 million ($293.7 million in vegetable crops including nursery and greenhouse; and $18.4 million in cattle, layers, and horses, and related products), averaging $348,524 per farm.

A prolonged drought can affect a community’s economy significantly. Increased demand for water and electricity may result in shortages and higher costs of these resources. Industries that rely on water for business may be impacted the most (e.g., landscaping businesses). Although most businesses will still be operational, they may be affected aesthetically—especially the recreation and tourism industry. Moreover, droughts within another area could affect food supply/price of food for residents within the Santa Clara County OA.

10.5 Future Trends in Development
Land use planning is also directed by general plans adopted under California’s General Planning Law.

Municipal planning partners are encouraged to establish General Plans with policies directing land use and dealing with issues of water supply and protection of water resources. These plans increase capability at the local municipal level to protect future development from impacts of drought. All planning partners reviewed their general plans under the capability assessments undertaken for this effort.

Deficiencies revealed by these reviews can be identified as mitigation actions to increase capability to deal with future trends in development.

To confront the challenges of increasing drought vulnerability, several efforts are underway to address water needs in the OA. Santa Clara Valley Water District is investing in locally reliable, sustainable and efficient water supplies such as recycled and purified water. Santa Clara County has adopted a modified Water Efficient Landscape Ordinance to guide future landscape and water management practices to promote long-term water efficiency.
165 National Integrated Drought Information System. (n.d.). Agriculture. https://www.drought.gov/sectors/agriculture

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 10: Drought 199

10.6 Scenario
Drought is a cyclic part of the climate of California. Continuation or exacerbation of the current situation across the State (i.e., an extreme, multiyear drought associated with record-breaking rates of low precipitation and high temperatures) is the worst-case scenario for Santa Clara County. Low precipitation and high temperatures increase possibility of wildfires throughout the County, increasing need for water when water is already in limited supply. Surrounding counties, also under drought conditions, could increase their demand for the water supplies on which Santa Clara County also relies, triggering social and political conflicts. The higher density population of the Bay Area increases the likelihood of such conflicts. Additionally, the longer drought conditions last in or near the Santa Clara OA, the greater the effect on the local economy; water-dependent industries especially will undergo setbacks. According to the USGS, “Climate change exacerbates droughts by making them more frequent, longer, and more severe.”165F
166

10.7 Issues
Important issues associated with drought in the OA include the following:
 Identification and development of alternative water supplies.  Utilization of groundwater recharge techniques to stabilize the groundwater supply.  The probability of increased drought frequencies and durations due to climate change.  The promotion of active water conservation even during non-drought periods.  Monitoring of implementation and benefits of the Long-Term Reliable Water Supply Strategy projects, Water Conservation Implementation Plan projects, and water system capital improvement upgrades.
 Application of alternative techniques (groundwater recharge, water recycle, local capture and reuse, desalination, and transfer) to stabilize and offset Sierra Nevada snowpack water supply shortfalls.
 Regular occurrence of drought or multiyear droughts that may limit the Operational Area’s ability to successfully recover from or prepare for more occurrences-particularly noteworthy due to longevity of the current ongoing drought.

Table 61: EMAP Consequence Analysis: Drought
Subject Ranking Impacts/Drought
Health and Safety of
Persons in the Area of the
Incident
Minimal to
moderate
Drought impact tends to be agricultural; however,
because of the lack of precipitation that leads to drought, water supply disruptions can occur, which can affect
people. The impact is expected to be minimal.

Responders Minimal With proper preparedness and protection, the impact on the responders is expected to be minimal.

Continuity of Operations Minimal There is minimal expectation for utilization of the COOP. 166 U.S. Geological Survey. (n.d.). Droughts and Climate Change. https://www.usgs.gov/science/scienceexplorer/climate/droughts-and-climate-change

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 10: Drought 200
Subject Ranking Impacts/Drought
Property, Facilities, and
Infrastructure
Minimal to
severe
Impact on property, facilities, and infrastructure could be minimal to severe, depending on the length and intensity of the drought. The structural integrity of buildings and buckling of roads could be affected.

Delivery of Services Minimal The impact on the delivery of services should be nonexistent or minimal.

Environment Minimal to
severe
The impact on the environment could be severe. Drought
can severely affect farming, ranching, wildlife, and plants due to the lack of precipitation.

Economic Conditions Minimal to
moderate
Impacts on the economy will be dependent on how
extreme the drought is and how long it lasts.

Communities that depend on water recreation could be
tested, as well as agricultural.

Public Confidence in
Jurisdiction’s Governance
Minimal Confidence could be an issue during periods of extreme drought if planning is not in place to address intake needs and the loss of agricultural crops.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 11: Climate Change 201
11 Climate Change

11.1 General Background
Definitions
 Climate Change: changes in average weather conditions that persist over multiple decades or longer1.
 Climate Mitigation: action taken to curb climate change by reducing or preventing the emission of greenhouse gases.
 Climate Adaptation: action taken to protect the community from the impacts of a changing climate.  Adaptative Capacity: an estimate of the community’s current ability to deal with the projected impacts of climate change.

Climate, consisting of patterns of temperature, precipitation, humidity, wind and seasons, plays a fundamental role in shaping natural ecosystems and the human economies and cultures that depend on them. Climate change is defined as “changes in average weather conditions that persist over multiple decades or longer. Climate change encompasses both increases and decreases in temperature, as well as shifts in precipitation, changing risk of certain types of inclement weather events, and changes to other features in the climate system.” 166F
167 A key indicator of climate change is the increase of global temperatures.

Multiple temperature records from all over the world have shown a warming trend. The Intergovernmental Panel on Climate Change (IPCC) has stated that the warming of the climate system is unequivocal. 16 7F 168
The 2022 global average surface temperature was 1.55 °F warmer than the 20th-century average of 57.0 °F and about 1.90 ˚F warmer than the pre-industrial period (1880-1900). In fact, the ten warmest years on record have all occurred since 2010, with the last nine years of 2014-2022 among the ten warmest years.168F
169 2022 ranked as the sixth-warmest year on record since 1880. 169F170 Although this temperature change may seem small, it means a significant increase in accumulated heat worldwide which is driving regional and seasonal temperature extremes, reducing snow cover and sea ice, intensifying heavy rainfall, and changing habitat ranges for plants and animals—expanding some and shrinking others. 17 0F 171
167 Globalchange.gov. (No Date). Glossary. https://www.globalchange.gov/climate-change/glossary 168 The Intergovernmental Panel on Climate Change. (n.d.). Summary for Policymakers. https://www.ipcc.ch/report/ar6/wg2/downloads/report/IPCC_AR6_WGII_SummaryForPolicymakers.pdf 169 https://www.noaa.gov/news/2022-was-worlds-6th-warmest-year-on-record 170 National Oceanic and Atmospheric Administration. (2023, January 12). 2022 Was World’s 6th -Warmest Year on
Record. https://www.noaa.gov/news/2022-was-worlds-6th-warmest-year-on-record 171 National Oceanic and Atmospheric Administration. (2023, January 18). Climate Change: Global Temperature. https://www.climate.gov/news-features/understanding-climate/climate-change-global-temperature#SnippetTab

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Figure 55: Global Temperature 1880-2022171F
172
The global warming temperature trend and its related impacts are caused by an exponential increase of greenhouse gas emissions including carbon dioxide, methane, nitrous oxide, and fluorinated gases.

Emissions of these gases come from a variety of sources, such as the combustion of fossil fuels for energy and transportation, agricultural production, changes in land use and volcanic eruptions. According to the U.S. Environmental Protection Agency (EPA), carbon dioxide concentrations measured about 280 parts per million before the industrial era began in the late 1700s and reached 414 parts per million in 2021, a 48% increase.1 72F
173 See Figure 54. In addition, the concentration of methane has more than doubled since pre-industrial times, and nitrous oxide is being measured at a record high of 334 parts per billion. 17 3F 174
172 NASA. (n.d.). World of Change: Global Temperatures. https://earthobservatory.nasa.gov/world-of-change/globaltemperatures 173 Environmental Protection Agency. (n.d.) Climate Change Indicators: Atmospheric Concentrations of Greenhouse Gases. https://www.epa.gov/climate-indicators/climate-change-indicators-atmospheric-concentrations-greenhousegases#:~:text=Carbon%20dioxide%20concentrations%20have%20increased,is%20due%20to%20human%20activitie s
174 Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock, eds. (2017). Climate science special report: Fourth National Climate Assessment, volume I. U.S. Global Change Research Program. https://science2017.globalchange.gov/.

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Figure 56: Global Carbon Dioxide Concentrations Over Time174F 175
Scientists can place this rise in carbon dioxide in a longer historical context through the measurement of carbon dioxide in ice cores. According to these records and illustrated in Figure 55, carbon dioxide concentrations in the atmosphere are the highest that they have been in 650,000 years.175F 176
The major scientific agencies of the United States and the world—including NASA, NOAA, and the Intergovernmental Panel on Climate Change (IPCC)—agree that long-term climate change is occurring.

There is broad scientific consensus (97 percent of scientists) that the current, unprecedented climatewarming trends are very likely due to human activities. 1 76F 177 Unless emissions of greenhouse gases are
substantially reduced, this warming trend is expected to continue.

11.1.1 Climate Change Indicators
Climate change will affect the people, property, economy, and ecosystems of the Santa Clara County Operational Area (OA) in a variety of ways. Some of these impacts are already being felt. Widespread, pervasive impacts to ecosystems, people, settlements, and infrastructure have resulted from observed increases in the frequency and intensity of climate and weather extremes, including temperature extremes on land and in the ocean, heavy precipitation, drought, and fire weather. 177F 178 Rising global
temperatures have also been accompanied by other more localized changes in weather and climate. 2022 serves as a good example of this.
175 Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock, eds. (2017). Climate science special report: Fourth National Climate Assessment, volume I. U.S. Global Change Research Program. https://science2017.globalchange.gov/.
176 NASA. (2023). Carbon Dioxide. https://climate.nasa.gov/vital-signs/carbon-dioxide/ 177 NASA. (n.d.). Scientific Consensus: Earth’s Climate is Warming. https://climate.nasa.gov/scientific-consensus/ 178 The Intergovernmental Panel on Climate Change. (n.d.). Summary for Policymakers. https://www.ipcc.ch/report/ar6/wg2/downloads/report/IPCC_AR6_WGII_SummaryForPolicymakers.pdf

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The OA started the year in the drought, experienced record high temperatures in the summer that reached as high as 109 degrees Fahrenheit in San José and ended with the start of multiple atmospheric rivers which brought record rainfall and flooding.

California’s number of extreme heat events (including both days and nights) has increased. In particular, there has been an increase in nighttime temperatures. Scientists have demonstrated that nighttime temperatures are more sensitive to the greenhouse gases which cause climate change178F 179. This example
helps demonstrate the consequence of climate change that are already being felt throughout the region.

The planet’s oceans and glaciers have also experienced changes: oceans are warming and becoming more acidic, ice caps are melting, and sea levels are rising. 17 9F 180,
180F
181 Sea level along the U.S. coastline is
projected to rise, on average, 10 to 12 inches in the next 30 years (2020–2050), which will be as much as the rise measured over the last 100 years (1920–2020). This has already put some coastal homes, beaches, roads, bridges, and wildlife at risk. As a coastal county, sea-level rise is one of the most concerning impacts of climate change facing the OA. At the time of the development of this plan, NASA reports the following trends1 81F
182:
 Carbon Dioxide: Increasing trend, currently at 419 parts per million as of February 2023.  Global Temperature: Increasing trend, increase of 1.6 ºF since 1880.  Arctic Ice Minimum: Decreasing trend, 12.6% per decade.  Land Ice: Decreasing trend, Antarctica is losing approximately 150 billion tons a year and Greenland is losing about 270 billion tons per a year.
 Sea Level: Increasing trend, 3.8 inches per year.

Climate change impacts are most frequently associated with negative consequences, for example, warmer average temperatures could increase air conditioning costs and affect the spread of diseases like Lyme disease but could also improve conditions for growing some crops. More extreme variations in weather are also a threat to society. More frequent and intense extreme heat events can increase illnesses and deaths, especially among vulnerable populations, and damage some crops. While increased precipitation can replenish water supplies and support agriculture, intense storms can damage property, cause loss of life and population displacement, and temporarily disrupt essential services such as transportation, telecommunications, energy, and water supplies. 1 82F
183 The most important effect for the development of this plan is that climate change will have a measurable impact on the occurrence and severity of natural hazards.

11.1.2 Projected Future Impacts
The Fourth National Climate Assessment Report for the United States 183F 184 indicates that impacts resulting
from climate change will continue through the 21st century and beyond. Evidence of human-caused climate change is overwhelming and continues to strengthen. 179 California Office of Environmental Health Hazard Assessment. (2019, February 11). Extreme Heat Events. https://oehha.ca.gov/epic/changes-climate/extreme-heat-events 180 National Oceanic and Atmospheric Administration. (2023, January 12). 2022 Was World’s 6th -Warmest Year on
Record. https://www.noaa.gov/news/2022-was-worlds-6th-warmest-year-on-record 181 U.S. Geological Survey. (2022, March 1). Global and Regional Sea Level Rise Scenarios for the United States. https://www.usgs.gov/publications/global-and-regional-sea-level-rise-scenarios-united-states 182 NASA. (n.d.). Ice Sheets. https://climate.nasa.gov/vital-signs/ice-sheets/ 183 Environmental Protection Agency. (n.d.). Climate Change Indicators: Weather and Climate. https://www.epa.gov/climate-indicators/weather-climate
184 U.S. Global Change Research Program. (n.d.). Fourth National Climate Assessment. https://nca2018.globalchange.gov/

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 11: Climate Change 205
The impacts of climate change are intensifying across the country and represent a real threat to Americans’ physical, social, and economic well-being are rising. These impacts are projected to intensify, however how much they intensify will depend on actions taken to reduce global greenhouse gas emissions and to adapt to the risks from climate change now and in the coming decades. 184F 185
The California Climate Adaptation Planning Guide (APG) outlines the following climate change impact concerns for the Bay Area Region communities 18 5F
186:
 Increased temperature
 Reduced precipitation
 Sea level rise – coastal inundation and erosion
 Public health – heat and air pollution
 Reduced agricultural productivity
 Inland flooding
 Reduced tourism
According to the Fourth U.S. National Climate Change Assessment, after the third climate assessment where there were more than twice as many high temperature records as low temperature records broken between 2001 and 2012, global high temperature records continued to be broken in 2014, 2015, and 2016. Heavy rainfall events and large forest fire incidents are becoming more frequent and more severe.

Long-term impacts, like a continued decline in arctic ice and increase chronic drought, are expected.

Cal-Adapt,186F
187 a resource for public information on how climate change might impact local communities, based on the most current data available, has projected increases in temperature within the OA in particular. Table 62 shows the expected increases in average maximum temperatures and Table 63 addresses the number of extreme heat days per year. The increase in average surface temperatures can also lead to more intense heat waves that can be exacerbated in urbanized areas by what is known as the urban heat island effect.

Table 62: Average Maximum Temperature – Santa Clara County187F 188
Baseline (1961–1990)
Medium Emissions (RCP 4.5) High Emissions (RCP 8.5)
Mid-Century End-Century Mid-Century Mid-Century
30-year
average

68.6 oF 72.0 oF 73.1 oF 72.8 oF 76.0 oF
30-year range 68.4–68.8 oF 70.5–73.5 oF 71.0–75.7 oF 70.9–75.0 oF 73.2–80.5 oF 185 U.S. Global Change Research Program. (n.d.). Fourth National Climate Assessment, Chapter 1: Overview. https://nca2018.globalchange.gov/chapter/1/
186 Cal OES. (June 2020). Climate Adaptation Planning Guide. https://www.caloes.ca.gov/wpcontent/uploads/Hazard-Mitigation/Documents/CA-Adaptation-Planning-Guide-FINAL-June-2020-Accessible.pdf 187 Cal-Adapt. (n.d.). Explore and analyze climate data from California’s Climate Change Assessment. https://caladapt.org/ 188 Cal-Adapt. (n.d.). Annual Averages. https://cal-adapt.org/tools/annual-averages

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 11: Climate Change 206
Table 63: Number of Extreme Heat Days Per Year
(Heat Is above 92.7 oF) Santa Clara County188F
189
Baseline (1961-1990)
Medium Emissions (RCP 4.5) High Emissions (RCP 8.5)
Mid-Century End-Century Mid-Century End-Century
30-year
average
4 days/yr. 12 days/yr. 17 days/yr. 17 days/yr. 31 days/yr. 30-year range 0–16 days/yr. 0–32 days/yr. 2–44 days/yr. 1–61 days/yr. 0–101 days/yr.

Although the California Climate Adaptation Planning Guide projected reduced precipitation in the region, recent Cal-Adapt projections show significant increases in Santa Clara County’s average annual precipitation levels (see Table 64).

Table 64: Average Annual Precipitation – Santa Clara County18 9F 190
Baseline (1961-1990)
Medium Emissions
(RCP 4.5)
High Emissions
(RCP 8.5)
Mid-Century End-Century Mid-Century End-Century
30-year
average

23.8 inches 26.0 inches 26.0 inches 26.3 inches 9.1–68.6 inches
30-year range

8.2–51.5
inches

9.2–54.5
inches

9.0–51.3
inches 29.2 inches 8.6–63.6 inches
Climate change projections contain inherent uncertainty, largely derived from the fact that they depend on future greenhouse gas emission scenarios. Generally, the uncertainty in greenhouse gas emissions is addressed by the presentation of differing scenarios: low-emissions to high-emissions scenarios. In lowemissions scenarios, there is an effort to limit greenhouse gas emissions leading to emissions starting to decline close to mid-century. In high-emissions scenarios, greenhouse gas emissions continue to increase at current rates through the end of the century. Different climate scenarios can also be described in terms of likelihood and confidence. Likelihood refers to the statistical probability of the effect described occurring. Confidence refers to how valid that result is likely to be, based on available data and its consistency with current literature. There will always be some level of uncertainty when estimating future conditions. Uncertainty in outcomes is addressed by averaging a variety of climate change model outcomes or providing a range of outcomes. Despite this general uncertainty, climate change projections present valuable information to help guide decision-making. 189 Cal-Adapt. (n.d.). Extreme Heat Days & Warm Nights. https://cal-adapt.org/tools/extreme-heat 190 Cal-Adapt. (n.d.). Annual Averages. https://cal-adapt.org/tools/annual-averages

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 11: Climate Change 207
Figure 57: Impacts of Climate Change on Human Health190F 191

11.1.3 How Climate Change Impacts Hazard Mitigation
An essential aspect of hazard mitigation is estimating the likelihood of hazard events. Traditionally, the probability of a hazard event occurring has been expressed as a statistical projection based on records of past events. This approach assumes that the likelihood of hazard events occurring remains essentially unchanged over time. Thus, averages based on the past frequencies of floods, for example, are used to estimate future flooding frequencies: if a river has flooded an average of once every five years for the past 100 years, then it can be expected to continue to flood an average of once every five years.

For hazards that are affected by climate conditions, the assumption that future behavior will be equivalent to past behavior may not be valid given that climate conditions are changing. As flooding is generally associated with precipitation frequency and quantity, for example, the frequency of flooding will not remain constant if broad precipitation patterns change over time. 191 California Department of Public Health. (February 2017). Climate Change and Health Profile Report – Santa Clara County.
https://www.cdph.ca.gov/Programs/OHE/CDPH%20Document%20Library/CHPRs/CHPR085SantaClara_County2- 23-17.pdf

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Specifically, as hydrology changes, storms currently considered to be a one percent annual chance flood event (100-year flood) might strike more often, leaving communities at greater risk of flooding. The risks of landslide, severe storms, extreme heat events and wildfire are all affected by climate patterns as well.

For this reason, an understanding of climate change is pertinent to efforts to mitigate natural hazards.

Information about how climate patterns are changing provides insight on the reliability of future hazard projections used in mitigation analysis. This section summarizes current understandings about climate change in order to provide a context for the recommendation and implementation of hazard mitigation measures.

Table 65: Potential Direct and Related Climate Change Impacts in the Operational Area Direct Impacts Related Impacts
Rising temperatures  Heat wave
 Changes in wind patterns 191F
192
 Drought
 Reduced snowpack
 Increased extreme events, including severe storms and wildfires  Shifting human health and disease patterns.
 Sea Level Rise
 Permanent inundation of previously dry land
 Larger area impacted by extreme high tide
 Increased coastal erosion
 Saltwater intrusion into freshwater systems
Changes in precipitation1 92F
193  Changed seasonal patterns
 Flooding
 Saturated earth
 Reduced snowpack
 Drought
The links between these climate change indicators and most of the natural and other hazards of concern profiled in this MJHMP are direct but less clear for other hazards as illustrated in Table 66 and discussed later in this section.
192 Columbia Climate School. (2021, January 6). Will Global Warming Bring a Change in the Winds? https://news.climate.columbia.edu/2021/01/06/westerly-winds-climate-change/ 193 Columbia Climate School. (2021, January 6). Will Global Warming Bring a Change in the Winds? https://news.climate.columbia.edu/2021/01/06/westerly-winds-climate-change/

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 11: Climate Change 209
Table 66: Climate Change Impacts on Natural and Other Hazards Negative Impact on Natural and Other Hazards
Climate Change
Indicator
Natural Hazards Other
Dam and Levee
Failure
Drought
Earthquake
Flood
Landslide
Inclement
Weather
Tsunami
Wildfire
Sea Level Rise
Terrorism
Technological
Incidents
Power Outages
Epidemic/Pandemic
Fog
Rising temperatures X X X X X X X X X
Heat wave X X X X X X X
Changes in wind
patterns1 93F
194

X X X X X X X
Drought N/A X X X
Reduced snowpack X X X
Increased extreme events,
including severe storms
and wildfires

X X X N/A X
Shifting human health and
disease patterns
X
Sea Level Rise X X X X X N/A X X
Changes in precipitation1 94F
195 X X X X X X X X X
Changes in seasonal
patterns

X X X X X X X X X
Flooding X N/A X X X X
Saturated earth X X X X X
194 Columbia Climate School. (2021, January 6). Will Global Warming Bring a Change in the Winds? https://news.climate.columbia.edu/2021/01/06/westerly-windsclimate-change/

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11.2 Responses to Climate Change
Communities and governments worldwide are working to address, evaluate and prepare for climate changes that are likely to impact communities in coming decades. Climate change discussions encompass two separate but inter-related considerations: mitigation and adaptation. The term “mitigation” can be confusing because its meaning changes across disciplines:
 Mitigation in restoration ecology and related fields generally refers to policies, programs or actions that are intended to reduce or to offset the negative impacts of human activities on natural systems. Mitigation can be understood as avoiding, minimizing, rectifying, reducing, or eliminating, or compensating for known impacts. 195F
196
 Mitigation in climate change discussions is defined as “reducing emissions of and stabilizing the levels of heat-trapping greenhouse gases in the atmosphere.” 1 96F 197 The goal is to stabilize the
climate and avoid significant human interference.
 Mitigation in emergency management is typically defined as the effort to reduce loss of life and property by lessening the impact of disasters. 197F
198
In this section, mitigation is used as defined by the climate change community. In the other sections of this plan, mitigation is primarily used in an emergency management context. The IPCC defines adaptation as “the process of adjustment to actual or expected climate and its effects.” Mitigation and adaptation are related, as the world’s ability to reduce greenhouse gas emissions will affect the degree of adaptation that will be necessary. Moreover, some initiatives and actions can both reduce greenhouse gas emissions and support adaptation to likely future conditions. The ability to adapt to changing conditions is often referred to as adaptive capacity, which is “the ability of systems, institutions, humans and other organisms to adjust to potential damage, to take advantage of opportunities, or to respond to consequences.” 198F 199
Societies across the world are facing the need to adapt to changing conditions and to identify ways to increase their adaptive capacity. Some efforts are already underway. Farmers are altering crops and agricultural methods to deal with changing rainfall and rising temperature; architects and engineers are redesigning buildings; planners are looking at managing water supplies to deal with droughts or flooding.

Adaptive capacity goes beyond human systems, as some ecosystems show a remarkable ability to adapt to change and to buffer surrounding areas from the impacts of change. Forests can bind soils and hold large volumes of water during times of plenty, releasing it through the year; floodplains can absorb vast volumes of water during peak flows; coastal ecosystems can hold out against storms, attenuating waves and reducing erosion. Other ecosystem services—such as food provision, timber, materials, medicines, and recreation—can provide a buffer to societies in the face of changing conditions. Ecosystem-based adaptation is the use of biodiversity and ecosystem services as part of an overall strategy to help people adapt to the adverse effects of climate change. This includes the sustainable management, conservation and restoration of specific ecosystems that provide key services. 196 Environmental Protection Agency. (n.d.). Types of Mitigation Under CWA Section 404. https://www.epa.gov/cwa404/types-mitigation-under-cwa-section-404-avoidance-minimization-and-compensatorymitigation#:~:text=The%20White%20House%20Council%20on%20Environmental%20Quality%20%28CEQ%29,rectif ying%2C%20reducing%20over%20time%2C%20and%20compensating%20for%20impacts. 197 NASA. (N.D.). Responding to Climate Change. https://climate.nasa.gov/solutions/adaptation-mitigation/ 198 Federal Emergency Management Agency. (2017, November). Fact Sheet, Planning for a Resilient Community. https://www.fema.gov/sites/default/files/documents/fema_planning-resilient-communities_fact-sheet.pdf 199 Intergovernmental Panel on Climate Change. (n.d.). Annex II, Glossary. https://www.ipcc.ch/site/assets/uploads/2018/02/WGIIAR5-AnnexII_FINAL.pdf

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One type of mitigation measure that is also important to acknowledge is nature-based solutions (NBS).

FEMA defines NBS as sustainable planning, design, environmental management, and engineering practices that weave natural features or processes into the built environment to promote adaptation and resilience19 9F
200. These actions usually have multiple benefits such as reducing flood risk, reducing urban heat, adding recreation space, protecting nature spaces like shorelines and wetlands, and improving water quality in addition to fighting climate change. They may be more cost-effective than traditional grey infrastructure projects and are often more palatable by the public due to their hard to quantify benefits like additional recreation space, community beautification, increased property values, and better public health.

The immediacy of some of these benefits can also increase public support for NBS when compared to less clear, long-term benefits of other climate adaptation measures. Many communities within the OA have taken an interest in NBS, also known as green infrastructure, and actively engage in and encourage implementation of NBS separately or in conjunction with traditional infrastructure projects. Further information is included in Volume 2.

A coordinated response to climate change is necessary to implement effective risk reduction measures across the OA. Initiatives including the Silicon Valley 2.0 Climate Change Preparedness Tool, the Santa Clara County Climate Collaborative and Climate Resilience Tool, and local climate action and adaptation plans reflect the OA’s commitment to assessing and implementing climate mitigation and adaptation actions. Assessment of the current efforts and adaptive capacity of the planning partners participating in this hazard mitigation plan are included in the jurisdiction-specific Annexes in Volume 2.

11.3 Vulnerability Assessment – Hazards of Concern
The following sections provide information on how each identified hazard of concern for this planning process may be impacted by climate change and how these impacts may alter current exposure and vulnerability to these hazards for the people, property, critical facilities, and the environment in the OA.

11.3.1 Earthquake
Climate Change Impacts on Earthquake
Currently, the impact, if any, of climate change on earthquakes is not well understood. “Climate-related stress changes might or might not promote an earthquake to occur, but we have no way of knowing by how much.”200F
201 Some scientists say that melting glaciers could induce tectonic activity. As ice melts and water runs off, tremendous amounts of weight are shifted on the earth’s crust. As newly freed crust returns to its original, pre-glacier shape, it could cause seismic plates to slip and stimulate volcanic activity, according to research into prehistoric earthquakes and volcanic activity. NASA and USGS scientists found that retreating glaciers in southern Alaska may be opening the way for future earthquakes.20 1F
202 Other researchers are studying whether the stress of alternating periods of drought and heavy precipitation in the Sierra Nevada could potentially be felt on faults in or near the range and whether the increased pumping of groundwater in the Central Valley during times of drought could have an effect on the seismicity on the adjacent San Andreas Fault. 202F 203
200 FEMA. (May 2023). Nature-Based Solutions. https://www.fema.gov/emergency-managers/riskmanagement/nature-based-solutions 201 NASA. (2019, October 29). Can Climate Affect Earthquakes, Or Are the Connections Shaky? https://climate.nasa.gov/news/2926/can-climate-affect-earthquakes-or-are-the-connections-shaky/ 202 NASA. (2004, August 2). Retreating Glaciers Spur Alaskan Earthquakes. https://www.nasa.gov/home/hqnews/2004/jul/HQ_04252_glaciers.html 203 NASA. (2019, October 29). Can Climate Affect Earthquakes, Or Are the Connections Shaky? https://climate.nasa.gov/news/2926/can-climate-affect-earthquakes-or-are-the-connections-shaky/

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Secondary impacts of earthquakes could be magnified by climate change. Soils saturated by repetitive storms or heavy precipitation could experience liquefaction or an increased propensity for slides during seismic activity due to the increased saturation. Dams storing increased volumes of water due to changes in the hydrograph could fail during seismic events.

Exposure, Sensitivity, and Vulnerability
Because impacts on the earthquake hazard are not well understood, increases in exposure and vulnerability of the local resources are not able to be determined. In general, everywhere that is susceptible to earthquake-induced liquefaction may be more susceptible if saturated with water due to climate change conditions when an earthquake occurs.

11.3.2 Wildfire
Climate Change Impacts on Wildfire
Wildfire risk is determined by climate variability, local topography, and human intervention. Climate change has the potential to affect multiple elements of the wildfire system: fire behavior, ignitions, fire management, and vegetation fuels. The frequency of extreme wildfires which burn over 25,000 acres is also expected to increase by nearly 50 percent across the state by 2100 according to the state’s Fourth Climate Change Assessment. Under both high- and medium-emissions scenarios, the change in acres burned in Santa Clara County is likely to increase until 2050 and then decrease by the end of the century.

Fire season for the OA is also expected to begin earlier in the year and last longer 20 3 F 204. The Cal-Adopt
projections20 4F
205 demonstrating how wildfire risk in the areas surrounding the OA is expected to increase over the next century is shown in Table 67.

Hot, dry spells create the highest fire risk. Increased temperatures may intensify wildfire risk by warming and drying out vegetation. Changes in climate patterns may impact the distribution and perseverance of insect outbreaks that create dead trees and vegetation, increasing the amount of available fire fuel. Wetter periods followed by a drought can result in increased dry vegetative ready to be burned. When climate alters fuel loads and fuel moisture, forest susceptibility to wildfires changes. Climate change also may increase winds that spread fires.

Table 67: Wildfire – Projected Santa Clara County Acres Burned 204 County of Santa Clara Office of Sustainability. (n.d.). Climate Change Projections in Santa Clara County. https://siliconvalleytwopointzero.org/climateprojections/wildfire 205 https://cal-adapt.org/tools/wildfire
Baseline (1961–1990)
Medium Emissions (RCP 4.5) High Emissions (RCP 8.5)
Mid-Century End-Century Mid-Century End-Century
30-year
average
6212.0 acres 6848.3 acres 6897.2 acres 6957.8 acres 6613.5 acres 30-year range
6143.0–
6366.3 acres
6512–7295.2
acres
6459.2–7388.5
acres
6570.1–7599.1
acres
6158.9–7378.2
acres

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Exposure, Sensitivity, and Vulnerability
Population and Property
While previous Cal-Adapt projections showed wildfire risk in the areas surrounding the OA decreasing over the next century, current projections show increased risk to wildfire, with increases in annual acres burned. Table 68 indicates the population and buildings at risk in the moderate, high, and very high FHSZs. Should the risk increase as Cal-Adapt projects, these totals are likely to increase also. WUI fires are an increased concern of the OA. While not all OA residents may experience increased risk to wildfire directly, secondary impacts, such as smoke and poor air quality will impact many more. These impacts are likely to be felt disproportionally. Vulnerable populations, including the elderly, low-income populations, and people with disabilities, will face some of the greatest negative impacts. Long-term impacts to public health, including mental health, will also be important to consider as the frequency and severity of wildfires increase.

Table 68: Population and Buildings at Risk – Santa Clara County Moderate FHSZ High FHSZ Very High FHSZ
Buildings Population Buildings Population Buildings Population 995 3,714 2,599 9,622 9,547 33,167
Critical Facilities
The risk to critical facilities would increase with the overall increased risk of wildfires. Key infrastructure including utilities, water, gas, electric, and communications infrastructure can be damaged during a wildfire. Secondary impacts, like increased power outages, transportation delays or disruptions, and smoke, are also likely to impact the operations of critical facilities.

Environment
It is possible that the exposure and vulnerability of the environment will be impacted by changes in wildfire risk due to climate change. Natural fire regimes may change, resulting in more frequent or higher intensity burns. While the California ecosystem is adapted to some fire, these types of events and fuel conditions represent risk beyond the norm for the local environment. These changes may alter the composition of the ecosystems in areas in and surrounding the OA including destroying vulnerable fish and wildlife habitats.

Economy
Recent fires destroyed residences, burned thousands of acres, forced people to evacuate. Costs involved included personal and business losses, lost economic activity, and the cost of containing a wildfire.

Seasonal agricultural workers and workers outside on poor air quality days are most likely to experience increased negative impacts or disruptions to their work due to increase wildfire events. Transportation infrastructure could also be impacted.

11.3.3 Inclement Weather
Climate Change Impacts on Inclement Weather
Climate change presents a challenge for risk management associated with inclement weather. While only slight changes in annual rainfall is expected in the OA, there is an increased risk that this rainfall will occur during an extreme precipitation event. These severe storms will be an essential to replenish fresh water supplies particularly during times of drought however, they may result in increased flooding. The number of weather-related disasters during the 1990s was four times that of the 1950s and led to 14 times as much in economic losses.

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Historically, the County has experienced just two extreme precipitation events per year. Depending on the emissions scenario, the OA is predicted to experience 3-5 events per year by the end of the century.205F 206
The type of inclement weather event may also change. Fewer snow events and additional rain events are predicted.

Climate projections such as found in the CMRA tool show an increasing number of days over 95 and over 100 degrees through mid to late century, in both lower and high emission scenarios. Projections also anticipate higher maximum temperatures averaged over a 5 day period, which indicates there may also be extended periods of extreme heat.

Extreme cold events are less common in the OA. The Temperature Minimum explorer as part of the ClimRR climate tool, shows the minimum temperature increasing through mid- to end-century. Current projections suggest that climate change may reduce vulnerability to extreme cold in the OA.

It is not fully understood how climate change may impact high wind events. Climate change is expected to contribute to more severe weather in the future. However, the Wind Explorer in the ClimRR tool shows little difference in wind miles per hour between mid-and end-century. Further investigation may be needed to fully understand how climate change may impact high winds in the OA.

Exposure, Sensitivity, and Vulnerability
Population and Property
Population and property exposure and vulnerability would be unlikely to increase as a direct result of climate change impacts on the inclement weather hazard. Inclement weather events may occur more frequently, but exposure and vulnerability will remain the same. Secondary impacts, such as the extent of localized flooding, may increase, impacting greater numbers of people and structures.

Critical Facilities
Critical facility exposure and vulnerability would be unlikely to increase as a result of climate change impacts on the inclement weather hazard; however, critical facility owners and operators may experience more frequent disruption to service provision. For example, more frequent and intense storms may cause more frequent disruptions in power service.

Environment
Exposure and vulnerability of the environment would be unlikely to increase; however, more frequent storms and heat events and more intense rainfall may place additional stressors on already stressed systems.

Economy
Climate change impacts on the inclement weather hazard may impact the local economy through more frequent disruption to services, such as power outages.

11.3.4 Drought
Climate Change Impacts on Drought
Due to a warmer climate, droughts could become more frequent, more severe, and longer lasting. As stated in the National Climate Resilience Toolkit, “higher surface temperatures brought about by global warming increase the potential for drought. Evaporation and the higher rate at which plants lose moisture 206 Santa Clara County. (February 2023) Draft Unincorporated Santa Clara County Climate Vulnerability Assessment.

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through their leaves both increase with temperature. Unless higher evapotranspiration rates are matched by increases in precipitation, environments will tend to dry, promoting drought conditions.20 6F 207
Drought is one of the most expensive hazards due to its impacts across sectors, particularly the agricultural industry. Valley Water indicates that it poses a severe threat to their normal operations due to the region’s dependency on imported water and increased demand for water across the region. The OA could likely experience what stresses other regions globally have started to experience:
 Growing populations
 Increased competition for available water
 Poor water quality
 Environmental claims
 Uncertain reserved water rights
 Groundwater overdraft
 Aging urban water infrastructure
California is particularly well-aware of the potential negative impacts of prolonged drought. Until storms in early 2023, the entire state, including the entire OA, was in severe to extreme drought. DWR has noted impacts of climate change on statewide water resources by charting changes in snowpack, sea level, and river flow. As temperatures rise and more precipitation comes in the form of rain instead of snow, these changes will likely continue or grow even more significant.

Cal-Adapt indicated in early 2023 that Santa Clara County should expect future April snowpack levels to be reduced by up to 25 inches from the baseline (1961-1990) by 2099.207F 208 These future projections may or
may not be adjusted following the area’s recent record-breaking snowpack levels. In addition to snowpack resources, the OA’s water supply is derived from groundwater and surface water resources. Increased incidence of drought may cause a drawdown in groundwater resources without allowing for the opportunity for aquifer recharge. Under the HadGEM-ES simulation, a high-emissions scenario, an extended drought is predicted for California from 2025–2075.20 8F 209
Exposure, Sensitivity and Vulnerability
Population and Property
Population exposure and vulnerability to drought are unlikely to change as a result of climate change given that the entire OA is already exposed to this hazard. Greater numbers of people may need to engage in behavior change, such as water saving efforts, significant life or health impacts are unlikely so long as water supplies can be managed to account for the additional strain. Property exposure and vulnerability may increase as a result of increased drought resulting from climate change, although this would most likely occur in non-structural property such as crops and landscaping. It is unlikely that structure exposure and vulnerability would increase as a direct result of drought, although secondary impacts of drought, such as wildfire, may increase and threatenstructures. 207 U.S. Climate Resilience Toolkit. (n.d.). Drought.
https://toolkit.climate.gov/topics/water/drought#:~:text=Higher%20surface%20temperatures%20brought%20about%2 0by%20global%20warming,environments%20will%20tend%20to%20dry%2C%20promoting%20drought%20condition s.
208 Cal-Adapt. (n.d.). Snowpack. https://cal-adapt.org/tools/snowpack/ 209 Santa Clara County. (February 2023) Draft Unincorporated Santa Clara County Climate Vulnerability Assessment.

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Critical Facilities
Critical facility exposure and vulnerability are unlikely to increase as a result of increased drought resulting from climate change; however, critical facility operators may be sensitive to changes and need to alter standard management practices and actively manage resources, particularly in water-related service sectors. Water-related infrastructure may experience disruptions.

Environment
The vulnerability of the environment may increase as a result of increased drought resulting from climate change. Ecosystems and biodiversity in the Bay Area are already under stress from development and water diversion activities. Prolonged or more frequent drought resulting from climate change may further stress the ecosystems in the region, which include many special status species.

Economy
Increased incidence of drought could increase the potential for impacts on the local economy including the agricultural and recreational sectors, the wine industry, and related tourism activities. Crop-related losses would be expected to be particularly high given a severe drought.

11.3.5 Dam and Levee Failure
Climate Change Impacts on Dam and Levee Failure
Dams and levees are engineered barriers designed to retain surface water based on assumptions including information on a river’s flow behavior (depicted on a hydrograph). Safeguards are built into these structures but there is increased risk associated with hazard events that surpass what the impacted dam or levee was designed to withstand. Substantial increases in rainfall or/or snowmelt in an area can have significant effects on the hydrograph used for the design of a dam or levee. If the hydrograph changes, it is conceivable that the dam or levee can lose some or all of its designed margin of safety, also known as freeboard.

In the case of dams, if freeboard is reduced, dam operators may be forced to release increased volumes early in a storm cycle in order to maintain the required margins of safety. Such early releases of increased volumes can increase flood potential downstream. According to the California Department of Water Resources (DWR), flood flows on many California rivers have been record-setting since the 1950s. This means that water infrastructure, such as dams and levees, have been forced to manage flows for which they were not designed. The California Division of Dam Safety (DSOD) has indicated that climate change may result in the need for increased safety precautions to address higher winter runoff, frequent fluctuations of water levels, and increased potential for sedimentation and debris accumulation from changing erosion patterns and increases in wildfire events. According to the DSOD, climate change also will impact the ability of dam operators to estimate extreme flood events.209F 210
Dams are constructed with safety features known as “spillways.” Spillways are put in place on dams as a safety measure in the event of the reservoir filling too quickly. Spillway overflow events, often referred to as “design failures,” result in increased discharges downstream and increased flooding potential.

Although climate change will not increase the probability of catastrophic dam failure, it may increase the probability of design failure.

In the case of levees, a reduction in freeboard caused by a changing hydrograph means that a levee may no longer protect an area against the design-storm standard for which it was originally built. This means that risk to the area that a levee is protecting from inundation will increase. 210 California Department of Water Resources. (n.d.). Climate Change Basics. https://water.ca.gov/WaterBasics/Climate-Change-Basics

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Exposure, Sensitivity and Vulnerability
Population and Property
While the exposure and vulnerability of population and property are unlikely to change significantly as result of climate change alone, the likelihood of failure of water infrastructure (dams and levees) is generally expected to increase because of more frequent exposure to extreme events.210F 211 Dam failures
may experience increase overtopping or breaches due to extreme precipitation events, even if the overall precipitation in the OA is expected to decrease.

This problem is exacerbated because of the age of many of the dams in the OA and the increasing population living in potential inundation areas. It should be noted that dams, and those levees in the OA that are accredited, are mapped in a FEMA special flood hazard area where flood insurance applies. If a levee loses its accreditation, additional people and property can be considered exposed to increased flood risk. There are a number of ongoing projects in the OA which would impact the existing FEMA flood layers including the Lower Berryessa Flood Protection Project, Permanente Creek Flood Protection Project, Upper Berryessa Creek Flood Protection Project, and Sunnyvale East and West Channels.211F 212
These projects may reduce the local risk to riverine flooding.

Critical Facilities
Dam owners and operators are sensitive to the risk and may need to alter maintenance and operations to account for changes in the hydrograph and increased sedimentation. Critical facility owners and operators in levee failure inundation areas should always be aware of residual risk from flood events that may overtop the levee system.

Environment
The exposure and vulnerability of the environment to dam and levee failure may change as a result of climate change. Cascading hazards, as discussed throughout this plan, could have a notable impact on the environment. For example, if there is a long period of drought caused by climate change prior to a dam failure, it could reduce the land’s ability to hold water resulting in increased runoff and damage.

Such a scenario could result in additional disaster events, such as landslides and mudslides.

Ecosystem services may be used to mitigate some factors that could increase the risk of design failures, such as increasing the natural water storage capacity in watersheds above dams. The use of nature-based solutions or green stormwater infrastructure systems could also help with stormwater collection around the dam.

Economy
Changes in the dam failure hazard related to climate change may affect the local economy. More frequent flooding events due to dam failure would negatively impact the local economy. Economic impacts may also result from changes to the levee failure hazard if accreditation is lost.

11.3.6 Flood
Climate Change Impacts on Flood
Climate change is expected to impact both precipitation-driven riverine and surface flooding as well as coastal flooding in the OA. High frequency flood events (e.g., 10-year floods) in particular will likely 211 Mallakpour, I., AghaKouchak, A., & Sadegh, M. (2019). Climate‐induced changes in the risk of hydrological failure of major dams in California. Geophysical Research Letters. https://agupubs.onlinelibrary.wiley.com/doi/pdfdirect/10.1029/2018GL081888 212 Silicon Valley 2.0. Memorandum SV2.0 Tool Update – Flood Hazar Layer Map Updates. (June 2021). https://siliconvalleytwopointzero.org/downloads/SiliconValley2.0_Flood-Hazard-Layer-Map-Update-Memo.pdf

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increase with a changing climate. What is currently considered a 1-percent-annual-chance (100-year flood) also may strike more often, leaving many communities at greater risk.

Climate change is already impacting water resources, and resource managers have observed the following:
 Historical hydrologic patterns can no longer be solely relied upon to forecast the water future.  Precipitation and runoff patterns are changing, increasing the uncertainty for water supply and quality, flood management and ecosystem functions.
 Extreme climatic events will become more frequent, necessitating improvement in flood protection, drought preparedness and emergency response.

The amount of snow is critical for water supply and environmental needs, but so too is the timing of snowmelt runoff into rivers and streams. Rising snowlines caused by climate change will allow more mountain areas, such as the Sierra Nevada watersheds, to contribute to peak storm runoff. Changes in watershed vegetation and soil moisture conditions will likewise change runoff and recharge patterns. As stream flows and velocities change, erosion patterns will also change, altering channel shapes and depths, possibly increasing sedimentation behind dams, and affecting habitat and water quality. Intense dry periods followed by wet periods will result in additional flooding. With potential increases in the frequency and intensity of wildfires due to climate change, there is potential for more floods following fire, which increase sediment loads and water quality impacts.

Use of historical hydrologic data has long been the standard of practice for designing and operating water supply and flood protection projects. For example, historical data are used for flood forecasting models and to forecast snowmelt runoff for water supply. This method of forecasting assumes that the climate of the future will be similar to that of the period of historical record. However, the hydrologic record cannot be used to predict changes in frequency and severity of extreme climate events such as floods. Scientists project greater storm intensity with climate change, resulting in more direct runoff and flooding. Going forward, model calibration must happen more frequently, new forecast-based tools must be developed, and a standard of practice that explicitly considers climate change must be adopted.

Exposure, Sensitivity, and Vulnerability
Population and Property
Population and property exposure and vulnerability may increase as a result of climate change impacts on the flood hazard. Runoff patterns may change, resulting in flooding in areas where it has not previously occurred. People experiencing homelessness are particularly susceptible to the impacts of climate change, including flooding.

Critical Facilities
Critical facility exposure and vulnerability may increase as a result of climate change impacts on the flood hazard. Runoff patterns may change, resulting in risk to facilities that have not historically been at risk from flooding. Additionally, changes in the management and design of flood protection for critical facilities may be needed as additional stress is placed on these systems. Planners will need to factor a new level of safety into the design, operation, and regulation of flood protection facilities such as dams, bypass channels and levees, as well as the design of local sewers and storm drains.

Environment
The exposure and vulnerability of the environment may increase as a result of climate change impacts on the flood hazard. Changes in the timing and frequency of flood events may have broader ecosystem impacts that alter the ability of already stressed species to survive. The destruction due to fire or sea level rise of habitats with important flood protection ability may impact the vulnerability of the OA.

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Economy
If flooding becomes more frequent, there may be impacts on the local economy. More resources may need to be directed to response and recovery efforts, and businesses may need to close more frequently due to loss of service or access during flood events. Flood damage to essential utilities will also present a major concern for the local economy.

11.3.7 Landslide
Climate Change Impacts on Landslide
Climate change may impact storm patterns, increasing the probability of more frequent, intense storms with varying duration. Increase in global temperature is likely to affect the snowpack and its ability to hold and store water. Warming temperatures also could increase the occurrence and duration of droughts, which would increase the probability of wildfire, reducing the vegetation that helps to support steep slopes.

All of these factors would increase the probability for landslide and/or mudslide occurrences.

Exposure, Sensitivity, and Vulnerability
Population and Property
Population and property exposure and vulnerability would be unlikely to increase as a result of climate change impacts on the landslide hazard. Landslide events may occur more frequently, but the extent and location should be contained within mapped hazard areas or recently burned areas.

Critical Facilities
Critical facility exposure and vulnerability would be unlikely to increase as a result of climate change impacts on the landslide hazard; however, critical facility owners and operators may experience more frequent disruption to service provision as a result of landslide hazards. For example, transportation systems may experience more frequent delays if slides blocking these systems occur more frequently.

Towers supporting power lines and bridges could also collapse during a landslide event. In addition, increased sedimentation resulting from landslides may negatively impact flood control facilities, such as dams and levees.

Environment
Exposure and vulnerability of the environment would be unlikely to increase as a result of climate change, but more frequent slides in river systems may impact water quality and have negative impacts on stressed species.

Economy
Changes to the landslide hazard resulting from climate change are unlikely to result in significant impacts on the local economy. The economy of the OA is considered to be highly adaptive to landslide risk.

11.3.8 Tsunami
Climate Change Impacts on Tsunami
The impacts of global climate change on tsunami probability are unknown. Some scientists say that melting glaciers could induce tectonic activity, inducing earthquakes, which could result in tsunamis. Other scientists have indicated that underwater avalanches (also caused by melting glaciers), may also result in additional tsunamis. Even if climate change does not increase the frequency with which tsunamis occur, it may result in more destructive waves. As sea levels continue to rise, tsunami inundation areas would likely reach further into communities than current mapping indicates.

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Exposure, Sensitivity, and Vulnerability
As land area likely to be inundated by tsunami waves increases, exposure and vulnerability to the tsunami hazard may increase for population, property, critical facilities, and the environment. Changes to the tsunami hazard from climate change may result in more direct economic impacts on a greater number of businesses and economic centers, as well as the infrastructure systems that support those businesses.

Population and Property
The population and property identified in the tsunami inundation areas are mostly likely to experience the most detrimental impacts of a tsunami event. This area is described in Figure 61. While most of the OA is unlikely to experience these impacts, vulnerable populations are at the highest risk of these impacts, including the homeless, elderly, children, and even the historically underserved communities.

Based on the National Structure Inventory, 14 commercial buildings and 2 industrial buildings are potentially vulnerable to tsunami. A tsunami has the potential to cause over $6 million in damages to these buildings.

If sea level rise increases the tsunami inundation area, then the population and property at risk will increase.

Critical Facilities
The risk to critical facilities would be similar to that of the flood risk. Key infrastructure including utilities, water, gas, electric, and communications infrastructure can be damaged during a tsunami. Destructive waves can damage buildings, homes, and businesses. Severe flooding, and other impacts, like power outages, transportation disruptions, are also likely to impact the operations of critical facilities.

Environment
A tsunami has the potential to cause detrimental impacts on the OA’s environment. This may include uprooted trees and plants and destroyed animal habitats such as nesting sites for birds. Tsunami’s may result in the disappearance or abrupt erosion of beaches. During a tsunami event hazardous material infrastructure and pipelines may become damaged, causing the contamination of the environment with oil, raw sewage, and poisonous pollutants.

Economy
The cost of repair after a tsunami can impacts the economy. The tourism industry may be impacted as historic coastal properties and popular coastal businesses are damaged. Critical facilities and other important assets may be damaged by temporary inundation, resulting in loss of services such as power or wastewater treatment. Local tax revenue may decline as areas that were previously occupied by houses and businesses that may permanently close or have difficulty repairing.

11.3.9 Extreme Temperatures
Climate Change Impacts on Extreme Temperatures
Extreme temperatures are a serious consequence of climate change. Extreme heat is of particular concern to the OA. Climate change is expected to bring longer, more frequent, and more severe extreme heat events to the region.212F
213 By 2100, the area can expect 6 to 10 heat waves a year. Also, by the end of the century according to a medium emissions model, there will also be an estimated 2-44 days per year 213 County of Santa Clara Office of Sustainability. (N.D.). Climate Change Projections in Santa Clara County – Extreme Heat. https://siliconvalleytwopointzero.org/climateprojections/extremeheat

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defined as extreme heat days – or days where the maximum temperature is above 92.7 °F.21 3F 214 Extreme
heat days followed by warm nights are dangerous because they don’t allow time for the population, landscapes, and the built environment to cool off. This can result in increased mortality, health issues, and wildfire risk.

Exposure, Sensitivity and Vulnerability
Population and Property
Extreme temperature days can directly harm human health. While flooding is the most common natural disaster, extreme temperatures are the deadliest. Extreme heat can result in a variety of heat-related illness from mild heat stress to fatal heat stroke. Mental health stress and other illnesses can also increase. Vulnerable populations, including the elderly, children, and people with pre-existing conditions will be disproportionally impacted by extreme heat. Societally inequities, such as the distribution of minority populations in areas more exposed to the urban heat island effect, can be exasperated by extreme heat conditions.

Critical Facilities
The impacts to critical facilities from extreme heat can range from barely noticeable to profound. More and more the value of using building materials that can reduce or withstand the impacts of extreme temperatures is being acknowledged. When building materials are pushed beyond their temperature thresholds, they are at risk of failure. Transportation infrastructure may also be damaged or destroyed, including asphalt, rail tracks, and cars. There is increased demand on energy utilities as the need for air conditioning rises. Back-up generators or alternative cooling methods such as community resilience hubs may be necessary if power supplies are insufficient to cool the community.

Environment
Extreme temperatures, especially hotter temperatures, can wreak havoc on unprepared ecosystems.

They are responsible for reducing biodiversity, resulting in numerous species death. Water supplies are also strained during extreme temperature events. Climate change also alters the range, biogeography, and growth of microbes which could result in additional foodborne and waterborne illnesses.21 4F 215
Economy
The economy will be impacted by the increase in extreme heat days. Extreme heat is associated with loss of productivity, increased chances of mortality, and increased of a workplace accident. A study from UCLA found that workers comp claims occurred more frequently on hotter days for both indoor and outdoor workers. An estimated 15,000 injuries occur per year in California and the financial cost may be between $750 million to $1.25 billion per year.21 5F
216 It also costs power more to maintain normal operations, such as keep the air conditioning running in an office. Some industries, such as construction, simply may not be able to function in too hot an environment. Research indicates that overall, economic growth declines and crop yields drop during periods of extreme heat. 214 Cal-Adapt. (N.D.) Extreme Heat Days & Warm Nights. https://cal-adapt.org/tools/extreme-heat/ 215 California Department of Public Health. (February 2017). Climate Change and Health Profile Report Santa Clara County.
https://www.cdph.ca.gov/Programs/OHE/CDPH%20Document%20Library/CHPRs/CHPR085SantaClara_County2- 23-17.pdf
216 UCLA Luskin. (June 2021). High Temperatures Increase Workers’ Injury Risk Whether They’re Outdoors or Inside. https://luskin.ucla.edu/high-temperatures-increase-workers-injury-risk-whether-theyre-outdoors-orinside#:~:text=A%20UCLA%20study%20published%20today%20shows%20that%20hot,from%20California%E2%80 %99s%20workers%E2%80%99%20compensation%20system%2C%20the%20nation%E2%80%99s%20largest.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 11: Climate Change 222

11.3.10 Sea Level Rise
Climate Change Impacts on Sea Level Rise
In addition to impacts on the identified hazards of concern, climate change presents risks related to sea level rise. Sea level rise will cause currently dry areas to be permanently inundated. The scope of temporary inundation from extreme tide events and storm surge also will also expand. Within the OA, the cities of Palo Alto, Mountain View, Sunnyvale, Milpitas, and San José are expected to experience the most impacts from sea level rise. 216F
217 Multiple possible sea level rise scenarios are presented in Table 69. Although the exact extent and timing of sea level rise is still uncertain, assessing potential areas at risk provides important information appropriate for planning purposes.

Table 69: Silicon Valley 2.0 Santa Clara County Sea Level Rise Projections and Inundation Layers SV 2.0 Tool Scenarios OPC 2018 Projections Permanent Sea Level Rise 100-year Storm Surge (+47”) Timeframe Scenario Risk
Tolerance
Projection
(in)
Inundation Layer Used in
Tool (in)
Projection
(in)
Inundation Layer Used in
Tool (in)
Baseline Current n/a 0 none 47 48
Mid-Century
Medium Top of likely
range

13.2 12 60.2 66
High 1 in 200 22.8 24 69.8 66
Late-Century
Medium Top of likely
range

40.8 36 87.8 84
Medium-High 1 in 20 52.8 52 99.8 96
High 1 in 200 82.8 84 129.8 131
217 County of Santa Clara Office of Sustainability. (n.d.). Climate Change Projections in Santa Clara County. https://siliconvalleytwopointzero.org/climateprojections/slr

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 11: Climate Change 223
Exposure, Sensitivity, and Vulnerability
The following assessment was conducted using data provided by the San Francisco Bay Conservation and Development Commission. A sea level rise of 48 inches above current mean higher high water was assumed.

Population
Sea level rise will increase the population exposed to both permanent and temporary inundation.

Currently, approximately 0.94 percent of the OA population is estimated to reside in areas subject to sea level rise impacts. The vast majority of these individuals reside in Palo Alto. Table 70 shows exposed population by jurisdiction.

Table 70: Estimated Population Residing in Sea Level Rise Inundation Areas Jurisdiction Estimated
Population
Estimated Population
Exposed
% of Population
Exposed
Campbell 43,442 0 0.0%
Cupertino 60,646 0 0.0%
Gilroy 59,472 0 0.0%
Los Altos 31,809 0 0.0%
Los Altos Hills 8,452 0 0.0%
Los Gatos 33,526 0 0.0%
Milpitas 80,248 0 0.0%
Monte Sereno 3,450 0 0.0%
Morgan Hill 45,037 0 0.0%
Mountain View 82,097 60 0.07%
Palo Alto 68,523 9,373 13.68%
San José 1,014,125 6,275 0.62%
Santa Clara (city) 127,608 0 0.0%
Saratoga 31,039 0 0.0%
Sunnyvale 154,808 2,421 1.56%
Unincorporated
County 90,253 36 0.04%
Total 1,934,535 18,165 0.94%
Property
The majority of losses from sea level rise are related to residential structures, closely followed by commercial and industrial. The majority of these assets are in Sunnyvale, San José, and Palo Alto.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 11: Climate Change 224
Table 71: Structure Type in Sea Level Rise Inundation Areas Jurisdiction Residential Commercial Industrial Agricultural Religious Government Education Total Campbell $0 $0 $0 $0 $0 $0 $0 $0
Cupertino $0 $0 $0 $0 $0 $0 $0 $0
Gilroy $0 $0 $0 $0 $0 $0 $0 $0
Los Altos $0 $0 $0 $0 $0 $0 $0 $0
Los Altos Hills $0 $0 $0 $0 $0 $0 $0 $0
Los Gatos $0 $0 $0 $0 $0 $0 $0 $0
Milpitas $0 $0 $0 $0 $0 $0 $0 $0
Monte Sereno $0 $0 $0 $0 $0 $0 $0 $0
Morgan Hill $0 $0 $0 $0 $0 $0 $0 $0
Mountain View $0 $128,886,000 $83,412,000 $1,034,000 $836,000 $6,966,000 $18,923,000 $240,057,000 Palo Alto $1,415,492,000 $525,812,000 $209,418,000 $4,075,000 $21,582,000 $15,911,000 $206,524,000 $2,398,814,000 San José $489,324,000 $562,847,000 $634,429,000 $4,243,000 $47,844,000 $13,429,000 $16,031,000 $1,768,147,000 Santa Clara
(city) $0 $0 $0 $0 $0 $0 $0 $0
Saratoga $0 $0 $0 $0 $0 $0 $0 $0
Sunnyvale $314,344,000 $755,253,000 $774,439,000 $377,000 $238,000 $5,129,000 $0 $1,849,780,000 Unincorporated
County $0 $998,000 $0 $0 $0 $0 $0 $998,000.00
Total $2,219,160,000 $1,973,796,000 $1,701,698,000 $9,729,000 $70,500,000 $41,435,000 $241,478,000 $6,257,796,000

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 11: Climate Change 225
Table 72: Structure and Contents Value in Sea Level Rise Inundation Areas Jurisdiction Structures
Exposed
Estimated
Value of
Exposed
Structures
Estimated
Value of
Exposed
Contents
Estimated
Total Value
% of Total
Replacement
Value
Campbell 0 $0 $0 $0 0.0%
Cupertino 0 $0 $0 $0 0.0%
Gilroy 0 $0 $0 $0 0.0%
Los Altos 0 $0 $0 $0 0.0%
Los Altos
Hills 0 $0 $0 $0 0.0%
Los Gatos 0 $0 $0 $0 0.0%
Milpitas 635 $468,554,661 $386,407,648 $854,962,309 4.5% Monte
Sereno 0 $0 $0 $0 0.0%
Morgan Hill 0 $0 $0 $0 0.0%
Mountain
View 102 $1,012,240,021 $1,110,560,396 $2,122,800,417 8.5% Palo Alto 4,014 $2,069,879,805 $1,642,022,511 $3,711,902,316 14.4% San José 912 $2,573,152,965 $2,275,265,284 $4,848,418,248 2.3% Santa Clara
(city) 450 $1,273,778,027 $1,228,024,465 $2,501,802,492 5.8% Saratoga 0 $0 $0 $0 0.0%
Sunnyvale 355 $2,632,745,163 $3,074,816,827 $5,707,561,990 13.3% Table 73: Structure and Contents Value in Sea Level Rise Inundation Areas, Unincorporated Santa Clara County
Jurisdiction Structures
Exposed
Estimated Value of
Exposed Structures
Estimated Value of
Exposed Contents
Estimated Total
Value
Unincorporated
County 1 $262,260 $131,130 $393,390
County + All
Cities 6,469 $10,030,612,900 $9,717,228,260 $19,747,841,162 Critical Facilities
There are 72 critical facilities located in OA areas subject to impacts from sea level rise.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 11: Climate Change 226
Table 74: Critical Facilities in Sea Level Rise Inundation Areas Jurisdiction Essential
Facilities Transportation Utilities Community Assets
Hazardous
Materials Total
Campbell 0 0 0 0 0 0
Cupertino 0 0 0 0 0 0
Gilroy 0 0 0 0 0 0
Los Altos 0 0 0 0 0 0
Los Altos Hills 0 0 0 0 0 0
Los Gatos 0 0 0 0 0 0
Milpitas 0 0 0 0 0 0
Monte Sereno 0 0 0 0 0 0
Morgan Hill 0 0 0 0 0 0
Mountain View 1 0 0 1 2 3
Palo Alto 9 12 3 5 9 29
San José 2 7 1 5 7 20
Santa Clara (city) 0 1 0 0 0 1
Saratoga 0 0 0 0 0 0
Sunnyvale 1 2 0 1 11 14
Unincorporated
County 0 0 0 1 0 1
Total 13 26 4 13 29 72
Environment
All sea level rise inundation areas are exposed and vulnerable to impacts. 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. Saltwater intrusion into freshwater resources may occur, further altering habitat and ecosystems and threatening the water supply. Protective ecosystem services may be lost as land area and wetlands are permanently inundated.

Economy
Sea level rise will impact the local economy. The tourism industry may be impacted as historic coastal properties are inundated. Critical facilities and other important assets may be damaged by temporary inundation, resulting in loss of services such as power or wastewater treatment. Coastal businesses may relocate to other areas rather than face high costs from increased risk of storm surge and costs associated with managed retreat. Local tax revenue may decline as areas that were previously occupied by houses and businesses are permanently inundated.

Future Development
The land area of the OA will be reduced as sea level rise permanently inundates areas. This will have significant impacts on land use and planning in local communities. Local General Plans in the OA will guide this future development.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 11: Climate Change 227

11.4 Issues
This assessment of climate change led to identification of the following issues throughout the Santa Clara County OA:
 Planning for climate change related impacts can be difficult due to inherent uncertainties in projection methodologies.
 Average temperatures are expected to continue to increase in the OA, which may lead to a host of primary and secondary impacts, such as an increased incidence of heat waves.  Expected changes in precipitation patterns are still poorly understood and could have significant impacts on the water supply and flooding in the OA.
 Some impacts of climate change are poorly understood such as potential impacts on the frequency and severity of earthquakes, thunderstorms, and tsunamis.  Heavy rain events may result in inland stormwater flooding after stormwater management systems are overwhelmed.
 Permanent and temporary inundation resulting from sea level rise has the potential to impact significant portions of the population and assets in the OA.

Table 75: EMAP Consequence Analysis: Climate Change
Subject Ranking Impacts/Climate Change
Public Minimal to
Severe
Residents across the OA will be impacted by climate change.

Noticeable impacts will include changes in temperature, increased frequency and severity of natural hazards, and the permanent inundation of part of the community due to sea level rise. Even residents outside of areas immediately impacted by hazard events will experience secondary effects such as smoke, a reduction in air quality, disruption of services, and transportation delays or disruptions.

Responders Minimal to
moderate
Responders will be relied upon when a disaster event occurs. They may need to consider climate change and the changing likelihood of future events when allocating resources towards certain hazards.

However, overall, since climate change is a long-term challenge with occurs over many years, there is unlikely to be significant changes needed to responder’s immediate response and recovery efforts.

Continuity of
Operations
(including
continued
delivery of
services)
Minimal to
Severe
The impacts on continuity of operations depends largely on facility and critical infrastructure location, hazard location, frequency, and severity, and human interference. Certain types of infrastructure, such as wastewater treatment plants which could be inundated by sea level rise or power utilities in high-risk wildfire areas, are more likely to be disrupted or destroyed. Delivery of services may also be slowed or stopped in impacted areas. Since climate change occurs over time, there is the opportunity for at-risk facilities and organizations to develop continuity of operations plans which address the increased risks associated with climate change.

Property,
Facilities, and
Infrastructure
Minimal to
Severe
The localized impact to properties, facilities, and infrastructure could be severe. Sea level rise impact estimates are one of the best examples of how climate change could directly relate to the complete loss of property, facilities, and infrastructure. Increased frequency and severity of other hazard events will likely result in damage, destruction, or disturbances to other structures and infrastructure across the OA.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 11: Climate Change 228
Subject Ranking Impacts/Climate Change
Environment Minimal to
Severe
California’s ecosystem has adapted to some of the impacts of climate change including wildfires. However, the increased frequency and severity of hazard events could strain the adaptative capacity of an ecosystem already stressed by increased human development.

Climate change could result in changes to the watershed, water supply, destroy or damage fish and wildlife habitats, destroy crucial flood protective habitats near the ocean, and increase the spread of invasive spices and the death of trees and vegetation.

Economic
Conditions
Minimal to
Severe
Climate change may impact the local economy in multiple different ways. Increased severe hazards events will test the financial resilience of the OA. Certain sectors, including agriculture and tourism, are likely to experience the worse impacts of climate change. Disturbances to transportation, communication, power, and water infrastructure due to climate change can also have a significant impact on the economy.

Impacts largely depend on how effectively the population and built environment can respond to changing climate conditions.

Public
Confidence in
the Government
Minimal to
Severe
Climate change is at the forefront of the national discussion on natural disasters. The public often has intense opinions on the subject. In general, climate change adaptation measures are widely supported in the OA. Solutions which consider short-term benefits, such as planting trees which increase shade and enhance the community’s curb appeal while at the same time reducing the urban heat island effect, are likely to be the most supported. Consistent messaging, accurate and timely public information, and clear reasoning for climate change adaptation measures will increase public’s confidence in the government.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 229
12 Dam and Levee Failure
Definitions
 Dam: Any artificial barrier, together with appurtenant works, which does or may impound or divert water, and which is or will be either 25 feet or more in height from the natural bed of the stream or watercourse at the downstream toe of the barrier, as determined by the department, or from the lowest elevation of the outside limit of the barrier, as determined by the department, if it is not across a stream channel or watercourse, to the maximum possible water storage elevation or has or will have an impounding capacity of 50 acre-feet or more.40  Levee: A man-made structure, usually an earthen embankment, designed and constructed to prevent inland flooding from major storm events and extreme water levels.  Emergency Action Plan: A formal document that identifies potential emergency conditions at a dam and specifies actions to be followed to minimize property damage and loss of life. It contains procedures and information to help the dam owner issue early warning and notification messages and inundation maps to show emergency management authorities the critical areas for action in case of an emergency.
 High Hazard Dam: Dams where failure or improper operation will probably cause loss of at least one human life.
 Extremely High Hazard Dam: California’s Division of Safety of Dams (DSOD) has classified some High Hazard Dams as Extremely High Dams. These are expected to cause considerable loss of human life or would result in an inundation area with a population of 1,000 or more.

12.1 General Background
Dams and levees are designed to mitigate flood events but sometimes floods larger than the estimated risk occur which can cause a partial or total failure. Other causes of dam and levee failure vary.

12.2 Causes of Dam and Levee Failure
A dam failure occurs when the barrier constructed does not obstruct or restrain water as designed, which can rapidly result in a large area of completely inundated land. Levees, though similar, are embankments built to prevent the overflow of a river or stream.

12.2.1 Causes of Dam Failure
Dam failures can be catastrophic to human life and property downstream. Dam failures in the United States typically occur in one of four ways:
 Overtopping of the primary dam structure, which accounts for 34 percent of all dam failures, can occur due to inadequate spillway design, settlement of the dam crest, blockage of spillways, and other factors.
 Foundation defects due to differential settlement, slides, slope instability, uplift pressures, and foundation seepage can also cause dam failure. These account for 30 percent of all dam failures.  Failure due to piping and seepage accounts for 20 percent of all failures. These are caused by internal erosion due to piping and seepage, erosion along hydraulic structures such as spillways, erosion due to animal burrows, and cracks in the dam structure.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 230
 Failure due to problems with conduits and valves, typically caused by the piping of embankment material into conduits through joints or cracks, constitutes 10 percent of all failures.

The remaining six percent of U.S. dam failures are due to miscellaneous causes. Many dam failures in the United States have been secondary results of other disasters. The prominent causes are earthquakes, landslides, extreme storms, massive snowmelt, equipment malfunction, structural damage, foundation failures, and sabotage.

The most likely disaster-related causes of dam failure in the OA are earthquakes, excessive rainfall, and landslides. 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.

12.2.2 Causes of Levee Failure
A levee breach occurs when part of a levee gives way, creating an opening through which floodwaters may pass. A breach may occur gradually or suddenly. The most dangerous breaches happen quickly during periods of high water. The resulting torrent can quickly swamp a large area behind the failed levee with little or no warning. When a levee system fails or is overtopped, severe flood damage can occur.

Earthen levees can be damaged in several ways. Strong river currents and waves can erode the surface.

Trees growing on a levee can blow over, leaving a hole where the root wad and soil used to be.

Burrowing animals, such as the California ground squirrel, the salt marsh harvest mouse, or the western burrowing owl can create holes that enable water to pass through a levee. If severe enough, any of these situations can lead to a zone of weakness that could cause a levee breach. In seismically active areas, earthquakes and ground shaking can cause a loss of soil strength, weakening a levee and possibly resulting in failure. Seismic activity can also cause levees to slide or slump, both of which can lead to failure.

No levee provides protection from events for which it was not designed, and levees require maintenance to continue to provide the level of protection they were designed and built to offer. Maintenance responsibility belongs to a variety of entities including local, state, and federal government and private landowners.

12.3 Hazard Profiles
Levees and dams serve different purposes. Though both can be made with the same materials, their relationship to water is different. Levees run parallel to water while dams lie across. A levee’s primary focus is to reduce flood risk and protect life and property. Dams also serve risk management functions but deliver other infrastructure benefits for communities and industry. The aging dam infrastructure in the OA is notable. Table 76 shows that the Lake Ranch Dam was built in 1877 and ten dams were built in the 1930s.

12.3.1 Past Dam Failure Events
According to the 2018 State of California Multi-Hazard Mitigation Plan, there have been ten dam failures in the state since 1950. The most recent dam emergency occurred in February 2017 at Oroville Dam in northern California’s Butte County when it was on the verge of overflow. The concrete spillway was damaged by erosion and a massive hole developed. The auxiliary spillway was used to prevent overtopping of the dam and it experiences erosion problems also. Evacuation orders were issued out of concern about a potential large uncontrolled release of water from Lake Oroville. Such a release was ultimately prevented, and evacuees returned to their homes.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 231
Table 76: Dams Classified as a High Hazard or an Extremely High Hazard Risk 21 7F 218,
218F
219
Name National
ID #
Hazard
Class Owner Dam Type
Dam
Height
(feet)
Crest
Length
(feet)
Reservoir
Capacity
(acrefeet)
Year
Built
Inundation
Map
Closest
City
Almaden CA00289 Extremely
High SCVWD Earth 110 500 2,000 1936 Yes San José
Almaden Valley CA00661 High San José
Water Co. Earth 38 1,100 27 1955 Yes San José
Austrian 622.013 Extremely
High
San José
Water Co. Earth 185 700 6,200 1950 Yes Los Gatos
Calero 72.003 Extremely
High SCVWD Earth 90 840 9,850 1935 Yes San José
Cherry Flat CA00158 Extremely
High
City of San
José Earth 60 230 500 1936 Yes San José
Coyote CA00287 Extremely
High SCVWD Earth and Rock 140 980 22,541 1936 Yes Gilroy Coyote Percolation
(Steel flashboard dam
being replaced with
an inflatable rubber
dam)
CA00286 High SCVWD Steel
Flashboard 24 204 72 1934 Yes San José
DeBell CA00686 High City of Gilroy Earth 53 580 120 1952 Yes Gilroy Ed R. Levin CA00890 Extremely
High
County of
Santa Clara Earth 38 470 150 1968 Yes Milpitas
Elmer J. Chesbro CA00806 Extremely
High SCVWD Earth and Rock 95 690 8,086 1955 Yes Morgan Hill 218 National Inventory of Dams. (n.d.). Dams of the Nation. https://nid.sec.usace.army.mil/#/ 219 California Department of Water Resources. (n.d.). California Dam Breach Inundation Maps. https://fmds.water.ca.gov/maps/damim/

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 232
Name National
ID #
Hazard
Class Owner Dam Type
Dam
Height
(feet)
Crest
Length
(feet)
Reservoir
Capacity
(acrefeet)
Year
Built
Inundation
Map
Closest
City
Felt Lake CA00670 Extremely
High
Stanford
University
Board of
Trustees
Earth 67 590 900 1930 Yes Palo Alto
Foothill Park CA00868 High City of Palo
Alto
Earth 86 600 67 1988 Yes Palo Alto
Guadalupe CA00290 Extremely
High
SCVWD Earth 142 695 3,460 1935 Yes San José
Higuera CA00687 Extremely
High
Wells Fargo
Bank
Earth 44 525 65 1953 Yes Milpitas
James J. Lenihan,
“Lexington Reservoir”
CA00293 Extremely
High
SCVWD Earth 208 810 21,430 1953 Yes Los
Gatos
Kuhn CA00683 Extremely
High
Private Entity Earth 67 312 85 1947 Yes San José
Lake Ranch CA00676 High San José
Water Co.

Earth 38 160 215 1877 Yes San José
Leroy Anderson
(inactive)
CA00294 Extremely
High
SCVWD Earth 235 1,430 91,300 1950 Yes Morgan
Hill
North Fork CA00299 Extremely
High
Pacheco
Pass Water
District
Earth 100 600 6,150 1939 No Hollister
Rinconada Reservoir CA00295 High SCVWD Earth 40 240 46 1969 Yes Campbell Stevens Creek CA00292 Extremely
High
SCVWD Earth 132 1,080 3,074 1935 Yes Cupertino
Uvas CA00807 Extremely
High
SCVWD Earth 118 1,100 10,000 1957 Yes Morgan
Hill
Vasona Percolating CA01516 Extremely
High
SCVWD Earth 34 1,00 410 1935 Yes Los
Gatos
San Mateo County Dam with an Inundation Area extending into the OA.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 233
Name National
ID #
Hazard
Class Owner Dam Type
Dam
Height
(feet)
Crest
Length
(feet)
Reservoir
Capacity
(acrefeet)
Year
Built
Inundation
Map
Closest
City
Searsville CA00669 Extremely
High
Stanford
Board of
Trustees
Masonry 68 260 1,840 1890 Yes Palo Alto

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 234
Historically, overtopping caused two of the state’s nine dam failures; the others were caused by seepage or leaks. One failure, the 1963 Baldwin Hills Dam Failure in Los Angeles County, resulted in three deaths because the leak turned into a washout. The historical record indicates that California has had about 45 failures of non-federal dams. The failures occurred for a variety of reasons, the most common being overtopping. Other reasons include shortcomings in the dams or an inadequate assessment of surrounding geomorphologic characteristics.

One dam failure event was recorded in the OA in 2017. After a series of slow-moving storms fronts, despite attempts to release water through at outlet at the bottom of the dam, the man-made Anderson Reservoir in Morgan Hill reached its peak capacity and water overtopped Anderson Dam, resulting in flooding downstream along Coyote Creek. The flooding was estimated to have cause $100 million in damages and displaced 14,00 residents 219F
220
.

12.3.2 Dam Location and Extent
According to the USACE National Inventory of Dams (NID), there are 44 regulated dams in the OA (Figure 57). Extremely high hazard dams are indicated in red, high hazard dams are indicated in orange, and dams with a significant hazard are shown in green. Low hazard dams are not mapped.

The DSOD has certified all these dams and has designated all of them with a Condition Assessment of “Satisfactory.” In addition, these dams all have the emergency action plan (EAP) required for all dams that do not have a low downstream hazard potential designation22 0F 221
. The EAP requirements include an
early warning system and operational procedures that may be used, such as reducing reservoir levels and reducing downstream flows, as well as procedures for notifying affected residents and agencies responsible for emergency management. The Searsville Dam, an Extremely High Hazard Dam located in San Mateo County, has an inundation area that could impact parts of the City of Palo Alto. This dam has been added to Table 76.

The Anderson Dam, located three miles east of U.S. 101 in Morgan Hill, was discussed in the 2019 MJHMP but on February 24, 2020, the Federal Energy Regulatory Commission ordered that Anderson Lake should be drained due to earthquake risk. At the time of writing this update, work on the $576-million Anderson Dam Seismic Retrofit Project continues. The project is expected to be complete by 2031.

Dam Inundation Mapping
The most significant issue associated with dam failure involves the potential impact on properties and populations in the inundation zones. As listed in Table 76, all but two of the dams have an approved inundation map. Flooding because of a dam failure would significantly impact these areas. Figure 58 shows dam inundation depth grids within the County.
220 San José Spotlight. (2022, June 1). Valley Water settles with San José residents over 2017 flood. San José flood victims get multimillion-dollar settlement - San José Spotlight (sanjosespotlight.com) 221 California Legislative Information. (n.d.). Water Code, Division 3, Dams and Reservoirs. Chapter 8. https://leginfo.legislature.ca.gov/faces/codes_displayexpandedbranch.xhtml?tocCode=WAT&division=3.&title=&part= &chapter=&article=&nodetreepath=4

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 235
Figure 58: Location of Dams Impacting Santa Clara County221F 222
222 California Department of Water Resources, Division of Safety of Dams. (n.d.). Dam Breach Inundation Map Web Publisher. https://fmds.water.ca.gov/webgis/?appid=dam_prototype_v2

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 236
Figure 59: Dam Inundation Depth Grids

12.3.3 Levee Location
Currently, there are 89 levees listed by the United States Army Corps of Engineers (USACE) in Santa Clara County,222F
223 a significant increase from the seven levees listed in 2017. As shown in Figure 59, most of these structures are located in the South San Francisco Bay area. 223 National Levee Database. (n.d.). Levees of the Nation. https://levees.sec.usace.army.mil/#/

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 237
Figure 60: Levees in Santa Clara County2 23F
224
The Santa Clara Valley Water District (SCVWD) owns and maintains most of these levees. It manages approximately 100 miles of levees in Santa Clara County. About 50 miles provide 100-year flood protection and nearly 18 miles were constructed in partnership with the Corps. Other levee sponsors include U.S. Fish and Wildlife Service (USFWS) (12), the City of Palo Alto (2) Caltrans District 4, and the Alameda County Flood Control District with Santa Clara County (1). Table 77 provides information about levees in the County.

Most of the levees have not been screened by the USACE for risk. Of the seven that have been screened, three are considered to be low risk, three are considered moderate risk and one review is labeled “No Verdict.” Of the 89 levees, a total of 14 have been accredited by FEMA and recognized on a Flood Insurance Rate Map (FIRM). One levee is designated as a Provisionally Accredited Levee (PAL).

This designation may be used for a levee system that FEMA has previously accredited as providing base flood hazard reduction on an effective FIRM, and for which FEMA is awaiting data and/or documentation that will show the levee system is compliant with 44 CFR 65.10. 224 National Levee Database. (n.d.). Levees of the Nation. https://levees.sec.usace.army.mil/#/

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 238
Table 77: Levees in Santa Clara County 22 4F
225

FEMA - NFIP/FIRM
Information Only
listing active levees County Sponsor Total
Miles
What is Behind the Levee
Risk
Accredited
Levee*
USACE Rehab**
Levee Name Population Buildings Property
Value
Coyote Creek, Santa
Clara – LB
Santa Clara SCVWD 6.72 9,477 1,879 $1.59B Low A A
Coyote Creek, Santa
Clara – RB
Santa Clara SCVWD 4.9 26,188 4,721 $3.47B Moderate A A
Coyote Creek, Santa
Clara – RB Bypass
Alameda,
Santa Clara
SCVWD 1.67 1,247 21 $219M No Verdict N A
Cunningham Flood
Detention Facility
Certification Project
Santa Clara SCVWD 4.32 20,689 3,588 $3.08B Not Screened A N Cunningham Flood
Detention Facility
Certification Project 2
Santa Clara State 4.5 32,882 6,174 $3.64B Not Screened A N Guadalupe River - LB Santa Clara SCVWD 8.48 30,391 3,364 $3.85B Moderate A A Guadalupe River - RB Santa Clara SCVWD 6.9 24,361 2,335 $3.21B Moderate A A King & Lyons Alameda and
Santa Clara
Alameda
County
Flood
Control

3.5 3,497 62 $449M Low PAL A
Los Gatos Creek Santa Clara Caltrans
District 4
.41 92 27 $29.9M Not Screened A N
Santa Clara County
Levee 1
Alameda,
Santa Clara
Local 1.33 0 0 0 Not Screened N N
225 National Levee Database. (n.d.). Levees of the Nation. https://levees.sec.usace.army.mil/#/

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 239

FEMA - NFIP/FIRM
Information Only
listing active levees County Sponsor Total
Miles
What is Behind the Levee
Risk
Accredited
Levee*
USACE Rehab**
Levee Name Population Buildings Property
Value
Santa Clara County
Levee 10
Santa Clara USFWS 0.68 0 1 0 Not Screened N N
Santa Clara County
Levee 11
Santa Clara USFWS 2.28 0 0 0 Not Screened N N
Santa Clara County
Levee 12
Santa Clara SCVWD 0 0 4 $212M Not Screened N N
Santa Clara County
Levee 13
Santa Clara City of Palo
Alto

0.62 0 0 0 Not Screened N N
Santa Clara County
Levee 14
Santa Clara Local 1.06 0 0 0 Not Screened N N
Santa Clara County
Levee 15
Santa Clara SCVWD 1.13 1,632 298 $436M Not Screened N N Santa Clara County
Levee 16
Santa Clara USFWS 8.66 0 0 0 Not Screened N N
Santa Clara County
Levee 17
Santa Clara SCVWD 1.39 140 21 $248M Not Screened N N
Santa Clara County
Levee 18
Santa Clara USFWS 1.67 0 0 0 Not Screened N N
Santa Clara County
Levee 19
Santa Clara SCVWD 1.42 2,181 727 $450M Not Screened N N Santa Clara County
Levee 20
Santa Clara SCVWD 0.19 269 89 $134M Not Screened N N
Santa Clara County
Levee 21
Santa Clara SCVWD 1.97 1,512 420 $364M Not Screened N N Santa Clara County
Levee 22
Santa Clara USFWS 3.24 0 0 0 Not Screened N N
Santa Clara County
Levee 23
Santa Clara SCVWD 1.56 1,722 628 $401M Not Screened N N

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 240

FEMA - NFIP/FIRM
Information Only
listing active levees County Sponsor Total
Miles
What is Behind the Levee
Risk
Accredited
Levee*
USACE Rehab**
Levee Name Population Buildings Property
Value
Santa Clara County
Levee 24
Santa Clara USFWS 1.24 0 0 0 Not Screened N N
Santa Clara County
Levee 25
Santa Clara USFWS 0.43 0 0 0 Not Screened N N
Santa Clara County
Levee 26
Santa Clara SCVWD 2.86 0 0 0 Not Screened N N
Santa Clara County
Levee 27
Santa Clara USFWS 0.66 0 0 0 Not Screened N N
Santa Clara County
Levee 28
Santa Clara SCVWD 4.01 0 0 0 Not Screened N N
Santa Clara County
Levee 29
Santa Clara SCVWD 3.05 5,409 1,719 $1.08B Not Screened N N Santa Clara County
Levee 3
San Mateo,
Santa Clara
SCVWD 7.56 17,748 6,351 $4.45B Not Screened N N
Santa Clara County
Levee 30
Santa Clara SCVWD 1.31 3,389 750 $628M Not Screened A N Santa Clara County
Levee 30.2
Santa Clara - 1.21 4,107 1,315 $777M Not Screened N N
Santa Clara County
Levee 31
Santa Clara USFWS 1.01 0 0 0 Not Screened N N
Santa Clara County
Levee 32
Santa Clara - 0.66 0 0 0 Not Screened N N
Santa Clara County
Levee 33
Santa Clara - 0.11 0 2 $1.09 Not Screened N N
Santa Clara County
Levee 34
Santa Clara USFWS 1.8 0 0 0 Not Screened N N
Santa Clara County
Levee 35
Santa Clara SCVWD 5.7 21,352 715 $6.03B Not Screened A N

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 241

FEMA - NFIP/FIRM
Information Only
listing active levees County Sponsor Total
Miles
What is Behind the Levee
Risk
Accredited
Levee*
USACE Rehab**
Levee Name Population Buildings Property
Value
Santa Clara County
Levee 36
Santa Clara SCVWD 5.19 0 0 0 Not Screened N N
Santa Clara County
Levee 37
Santa Clara SCVWD 2.76 32,113 5,873 $10.6B Not Screened A N Santa Clara County
Levee 37 North
Santa Clara - 8.89 0 0 0 Not Screened N N
Santa Clara County
Levee 38
Santa Clara SCVWD 3.78 0 0 0 Not Screened N N
Santa Clara County
Levee 4
Santa Clara - 0.08 0 0 0 Not Screened N N
Santa Clara County
Levee 40
Santa Clara SCVWD 1.26 8,031 43 $169M Not Screened N N
Santa Clara County
Levee 41
Santa Clara SCVWD 3.7 2,454 208 $951M Not Screened A N
Santa Clara County
Levee 42
Santa Clara SCVWD 3.52 11,654 4,092 $2.99B Not Screened N N Santa Clara County
Levee 43
Santa Clara SCVWD 0.02 0 0 0 Not Screened N N
Santa Clara County
Levee 44
Santa Clara - 0.26 0 0 0 Not Screened N N
Santa Clara County
Levee 45
Santa Clara SCVWD 3.56 0 0 0 Not Screened N N
Santa Clara County
Levee 46
Santa Clara SCVWD 1.11 5,616 53 $641M Not Screened N N
Santa Clara County
Levee 47
Santa Clara - 0.54 0 0 0 Not Screened N N
Santa Clara County
Levee 48
Santa Clara City of Palo
Alto

0.29 865 33 $82M Not Screened N N

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 242

FEMA - NFIP/FIRM
Information Only
listing active levees County Sponsor Total
Miles
What is Behind the Levee
Risk
Accredited
Levee*
USACE Rehab**
Levee Name Population Buildings Property
Value
Santa Clara County
Levee 49
Santa Clara USFWS 0.41 0 0 0 Not Screened N N
Santa Clara County
Levee 5
Santa Clara - 0.21 0 0 0 Not Screened N N
Santa Clara County
Levee 50
Santa Clara - 1.44 0 0 0 Not Screened N N
Santa Clara County
Levee 52
Santa Clara USFWS 2.5 0 0 0 Not Screened N N
Santa Clara County
Levee 53
Santa Clara - 0.03 0 0 0 Not Screened N N
Santa Clara County
Levee 54
Santa Clara SCVWD 1.83 8,645 688 $1.54B Not Screened A N Santa Clara County
Levee 55
Santa Clara City of Palo
Alto

0.33 2 2 $3.93M Not Screened N N
Santa Clara County
Levee 56
Santa Clara - 0.72 0 0 0 Not Screened N N
Santa Clara County
Levee 57
Santa Clara - 0.36 0 0 0 Not Screened N N
Santa Clara County
Levee 58
Santa Clara - 1.78 0 0 0 Not Screened N N
Santa Clara County
Levee 6
Santa Clara SCVWD 1.51 6,548 1,840 $1.2B Not Screened N N Santa Clara County
Levee 61
Santa Clara SCVWD 1.58 2,370 382 $247M Not Screened N N Santa Clara County
Levee 62
San Benito,
Santa Clara
SCVWD 2.41 41 14 $24.4M Not Screened N N
Santa Clara County
Levee 64
Santa Clara SCVWD 1.06 50,774 1,610 $10.2B Not Screened N N

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 243

FEMA - NFIP/FIRM
Information Only
listing active levees County Sponsor Total
Miles
What is Behind the Levee
Risk
Accredited
Levee*
USACE Rehab**
Levee Name Population Buildings Property
Value
Santa Clara County
Levee 65
Santa Clara - 1.39 12,714 2,494 $2.6B Not Screened N N
Santa Clara County
Levee 67
Santa Clara - 0.88 180 48 $82.7M Not Screened N N
Santa Clara County
Levee 7
Santa Clara - 0.2 0 0 0 Not Screened N N
Santa Clara County
Levee 72
Santa Clara SCVWD 0.86 0 5 $2.27M Not Screened N N
Santa Clara County
Levee 73
Santa Clara SCVWD 0.94 475 186 $97.6M Not Screened N N
Santa Clara County
Levee 74
Santa Clara - 0.62 413 128 $63.7M Not Screened N N
Santa Clara County
Levee 75
Santa Clara SCVWD 1.09 181 21 $100M Not Screened N N
Santa Clara County
Levee 79
Santa Clara - 0.87 6,131 1,840 $917M Not Screened N N
Santa Clara County
Levee 8
Santa Clara - 0.56 0 0 0 Not Screened N N
Santa Clara County
Levee 80
Santa Clara - 0.72 47 7 $1.87M Not Screened N N
Santa Clara County
Levee 81
Santa Clara SCVWD 0 0 0 Not Screened N N
Santa Clara County
Levee 82
Santa Clara SCVWD 0 0 0 Not Screened N N
Santa Clara County
Levee 83
San Benito,
Santa Clara
SCVWD 2.65 149 19 $22.5M Not Screened N N
Santa Clara County
Levee 84
Santa Clara SCVWD 0.54 1,647 759 $253M Not Screened N N

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 244

FEMA - NFIP/FIRM
Information Only
listing active levees County Sponsor Total
Miles
What is Behind the Levee
Risk
Accredited
Levee*
USACE Rehab**
Levee Name Population Buildings Property
Value
Santa Clara County
Levee 85
Santa Clara - 0.87 1,767 551 $246M Not Screened N N
Santa Clara County
Levee 86
Santa Clara SCVWD 1.88 0 0 0 Not Screened N N
Santa Clara County
Levee 88
Santa Clara SCVWD 0.75 1,517 244 $633M Not Screened N N Santa Clara County
Levee 89
Santa Clara SCVWD 1.04 9,275 2,261 $1.97B Not Screened A N Santa Clara County
Levee 9
Santa Clara SCVWD 3.41 12,080 1,617 $2.79B Not Screened N N Santa Clara County
Levee 90
Santa Clara SCVWD 0.81 2,052 344 $191M Not Screened N N Santa Clara County
Levee 92
Santa Clara SCVWD 0.21 5,982 295 $4224M Not Screened N N Santa Clara County
Levee 93
Santa Clara - 1.3 13 5 $238M Not Screened N N
Santa Clara County
Levee 96
Santa Clara - 0.5 273 5 $27.7M Not Screened N N
Uvas Creek - LB Santa Clara SCVWD 2.19 14,505 4,377 $1.72B Low A A *Accredited Levee column abbreviations: accredited (A), not accredited (N), and Provisionally Accredited Levee (PAL). **USACE Rehab column abbreviations: active (A) and not active (N).

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 245

12.3.4 Frequency of Dam and Levee Failure
Dam and levee failure events are infrequent and usually coincide with events that cause them, such as earthquakes, landslides and excessive rainfall and snowmelt. There is a “residual risk” associated with dams that remains after safeguards have been implemented. The residual risk is associated with events beyond those that the facility was designed to withstand. However, the probability of occurrence of any type of dam failure event is low in today’s regulatory and safety oversight environment.

Further information on the impact of climate change on the probability of dam failure is included in Section 11.

Probability in OA: Unlikely

12.3.5 Severity
FEMA categorizes the downstream hazard potential of a dam into three categories in increasing severity:

Low, Significant, and High. The state’s Division of Safety of Dams (DSOD) adds a fourth category of "Extremely High." The OA has dams in all four categories; this plan focuses on the Extremely High Hazard and High Hazard dams.

Table 78: Potential Downstream Hazard from Dams225F
226
Hazard
Potential Potential Downstream Impacts to Life and Property Number of Dams in the OA Low No probable loss of human life and low economic and environmental losses. Losses are expected to be principally limited to the owner’s property.
13 Significant No probable loss of human life but can cause economic loss, environmental damage, impacts to critical facilities, or other significant impacts.
4 High Expected to cause loss of at least one human life. 6 Extremely High Expected to cause considerable loss of human life or would result in an inundation area with a population of 1,000 or more. 18 12.3.6 Warning Time for Dam Failure
EAPs are critical in identifying areas downstream from dams requiring warning and evacuation in the event of dam failure. Warning and evacuation time can dramatically influence the number of persons at risk and the number of fatalities per dam failure.
226 California Department of Water Resources. (2021, September). Downstream Hazard. https://water.ca.gov/- /media/DWR-Website/Web-Pages/Programs/All-Programs/Division-of-Safety-of-Dams/Files/Publications/Division-ofSafety-of-Dams-Definitions-for-Downstream-Hazard-and-Condition-Assessment.pdf

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 246
Advance Warning of Failure
Warning time for dam failure varies depending on the cause of the failure. In events of extreme precipitation or massive snowmelt, evacuations can be planned with sufficient time. In the event of a structural failure due to earthquake, there may be no warning time.

Time for the Failure to Occur
A dam’s structural type also 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. 22 6F 227
Time after Failure for Notification
Time available to notify those in the impacted area will depend on the dam’s distance from it and the river flow. The warning and protective action process is divided into the following three time periods:
 Warning delay time is the period between when a threat is first detected, or when an emergency manager is first notified of the threat and when an emergency manager issues a first alert/warning.
 Warning diffusion time is the period after the first alert/warning is issued and the time that people receive that alert/ warning.
 Protective action initiation time is the period after people receive the first alert/ warning and when they initiate protective action. In this time period, most people take a range of actions to prepare to implement a protective action and may receive subsequent warning messages. 22 7F 228
Santa Clara County and its planning partners have established protocols for emergency warning and response through the County’s adopted emergency operations plan2 28F 229
. The SCVWD Dam Safety
Program maintains the operation of its dams and works with Santa Clara County Emergency Management to provide copies of the most recent dam EAPs and inundation maps and uses this information to plan notification needs for downstream areas in the event of a failure.

12.3.7 Warning Time for Levee Failure
As with dam failure, warning time for levee failure depends on the cause of the failure. A levee failure caused by structural failure can be sudden and occur with little to no warning. If heavy rains are impacting a levee system, communities located in the immediate danger zone can be evacuated before a failure occurs. If the levee failure is caused by overtopping, the community may or may not be able to recognize the impending failure and evacuate. If a levee failure occurs suddenly, evacuation may not be possible. 227 Starosolszky, O. and Melder, O.M. (2014, April 23) Hydrology of Disasters: Proceedings of the World Meteorological Organization Technical Conference Held in Geneva, November 1988. https://books.google.com/books?id=DSFpAwAAQBAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=o nepage&q&f=false
228 U.S. Army Corps of Engineers. (2019 April 30). A Guide to Public Alerts and Warnings for Dam and Levee Emergencies. https://www.publications.usace.army.mil/Portals/76/Users/182/86/2486/EP%201110-2-

17.pdf?ver=2019-06-20-152050-550
229 Santa Clara County. (2017, January). County of Santa Clara Emergency Operations Plan. Emergency Operations Plan (EOP) - Office of Emergency Management - County of Santa Clara (sccgov.org)

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 247

12.4 Exposure
Exposure to the dam failure hazard was assessed by use of GIS spatial analysis, overlaying the inundation areas with data from the underlying population, buildings, and critical facilities. The consistency of the data available to support this risk assessment varied greatly within the OA. The level of analyses varied based on available data.

12.4.1 Population
The estimated total population living in the inundation area of an Extremely High Hazard, or a High Hazard dam is 32.17% of the total county population.

12.4.2 Property
Table 79 summarizes the estimated property exposure in the inundation area of an Extremely High Hazard or a High Hazard dam. These estimates were determined using a combination of Hazus and GIS data.

Table 79: Estimated Exposure and Value of Structures in the Dam Failure Inundation Area Exposure County Total
Inundation Area 162.90 square miles
Number of Buildings Exposed 168,271
Percentage of Exposed Buildings in the County 31.43%
Exposed Value in the County $131,358,283,000
Total Percentage of Exposed Value in the County 34.48%

12.4.3 Environment
The environment would be exposed to a number of risks in the event of dam failure. The inundation could introduce many foreign elements into local waterways. This could result in destruction of downstream habitat and could have detrimental effects on many species.

12.5 Vulnerability
The vulnerability of people, property, critical facilities, and the environment was evaluated for the combined dam failure inundation area. Countywide summaries are provided below.

Change in conditions: no change in vulnerability
 Growth in population and building is slow and not concentrated in areas at particular risk to dam failure.
 No new dams have been constructed in the OA.
 Dams with significant or high hazard potential are required to have an EAP.  Dam safety programs include inspection, maintenance, and ongoing monitoring of dams to ensure stability and proper functioning of dams.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 248
 DSOD and dam owners such as SCVWD periodically undertake special engineering studies to ensure that its dams are compliant with the latest design guidelines and regulations to minimize seismic risks to its dams.
• Although vulnerability to dams is not increasing, risk to people and structures in dam inundation remains high. In the unlikely even of a failure, the high volume and velocity of resulting flood waters can have devastating impacts downstream.

12.5.1 Population
All populations downstream from dam failures that are incapable of escaping the area within the allowable time period are vulnerable. Those that particularly vulnerable includes the elderly and children, people with medical conditions, people who are visually impaired and/or have vision loss, people with public transportation needs, people with non-visible disabilities, people with limited English proficiency. The vulnerable population also includes those who would not have adequate warning from a television or radio emergency warning system or cell phone alert or people who may unable to get themselves out of the inundation area. The potential for loss of life is affected by the capacity and number of evacuation routes available to populations living in areas of potential inundation. Those who are located closest to the dam would have the least warning time.

Table 80 shows the number of households and population within potential dam inundation areas in each city in the OA. These values were obtained using Hazus 6.0 dasymetric census blocks that were within a dam inundation boundary.

Table 80: Population in Dam Inundation Area
Jurisdiction Population Households
Campbell 32,482 13,326
Cupertino 840 284
Gilroy 27,185 7,752
Los Altos 3,242 1,097
Los Altos Hills 0 0
Los Gatos 5,721 2,523
Milpitas 14,242 4,315
Morgan Hill 25,469 8,296
Monte Sereno 0 0
Mountain View 17,931 7,215
Palo Alto 8,200 3,318
San Jose 396,703 137,686
Santa Clara 51,519 18,932
Saratoga 350 111
Sunnyvale 30,143 11,832
Unincorporated 8,955 2,629
Total 622,982 219,316

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 249

12.5.2 Property
Vulnerable properties are those closest to the dam inundation area. 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. Dam inundation areas can be extensive depending on the volume of water contained by the dam. Table 81 shows the value of buildings exposed to potential dam inundation within the OA. This does not take into account any damage functions based on depth of flooding, it is only a summary of building values within the potential inundation zone.

12.5.3 Critical Facilities
Typical vulnerabilities of critical facilities impacted by dam failure include road failure with road segments and bridges washed out creating isolation issues., This includes all roads, railroads, and bridges in the path of the dam inundation. 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 could also be damaged from the rushing water. Other critical facilities buildings can become flooded and inoperable. Loss of these utilities could create additional isolation issues for the inundation areas. Emergency response could also be delayed. The analysis in Santa Clara County shows the following facilities could be impacted by dam failure:
 Highway bridges: 352
 Rail bridges: 67
 Light rail bridges: 6
 Light rail facilities: 36
 Wastewater treatment facilities: 1
 Schools: 158
 Fire stations: 24
 Hospitals: 7
 Communication Facilities: 6
 Electric Power Facilities: 7

12.5.4 Environment
The environment would be vulnerable to multiple 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 coho salmon. The extent of the vulnerability of the environment is the same as the exposure of the environment.

Reservoirs held behind dams affect many ecological aspects of a river. River topography and dynamics depend on a wide range of flows, but rivers below dams often experience long periods of very stable flow conditions or saw-tooth flow patterns caused by releases followed by no releases. Water releases from dams usually contain very little suspended sediment; this can lead to scouring of riverbeds and banks.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 250
Table 81: Value of Exposed Structures in the Dam Inundation Area by Jurisdiction Jurisdiction Residential Commercial Industrial Agricultural Religious Government Educational Total Campbell $4,338,205,000 $1,534,627,000 $418,973,000 $13,221,000 $90,790,000 $14,829,000 $216,448,000 $6,627,093,000 Cupertino $160,643,000 $6,496,000 $2,027,000 $0 $675,000 $1,054,000 $5,339,000 $176,234,000 Gilroy $2,768,627,000 $1,566,395,000 $507,771,000 $19,205,000 $24,057,000 $66,266,000 $204,184,000 $5,156,505,000 Los Altos $687,482,000 $6,339,000 $12,817,000 $0 $4,831,000 $0 $19,788,000 $731,257,000 Los Altos Hills $0 $0 $0 $0 $0 $0 $0 $0
Los Gatos $758,943,000 $615,193,000 $67,764,000 $251,000 $32,366,000 $28,170,000 $107,868,000 $1,610,555,000 Milpitas $1,497,684,000 $626,566,000 $811,058,000 $156,000 $23,605,000 $1,175,000 $31,025,000 $2,991,269,000 Monte Sereno $0 $0 $0 $0 $0 $0 $0 $0
Morgan Hill $4,114,421,000 $1,308,828,000 $773,853,000 $17,254,000 $96,415,000 $46,581,000 $161,812,000 $6,519,164,000 Mountain View $2,615,045,000 $1,500,804,000 $562,361,000 $3,282,000 $67,756,000 $19,574,000 $57,915,000 $4,826,737,000 Palo Alto $1,478,921,000 $420,664,000 $27,422,000 $464,000 $5,489,000 $0 $157,677,000 $2,090,637,000 San José $46,073,871,000 $17,490,269,000 $7,035,829,000 $96,030,000 $748,644,000 $925,107,000 $4,665,413,000 $77,035,163,000 Santa Clara
(city) $6,161,892,000 $3,784,586,000 $2,203,267,000 $22,795,000 $175,731,000 $44,789,000 $123,534,000 $12,516,594,000 Saratoga $66,875,000 $202,000 $5,927,000 $0 $0 $0 $0 $73,004,000 Sunnyvale $4,084,357,000 $1,325,027,000 $745,125,000 $303,000 $36,082,000 $15,642,000 $110,192,000 $6,316,728,000 Unincorporated
County $2,183,239,000 $1,458,486,000 $315,766,000 $406,032,000 $40,527,000 $83,070,000 $200,223,000 $4,687,343,000 Total $76,990,205,000 $31,644,482,000 $13,489,960,000 $578,993,000 $1,346,968,000 $1,246,257,000 $6,061,418,000 $131,358,283,000

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 12: Dam and Levee Failure 251

12.6 Cascading Hazards
Dam and levee failures can lead to cascading hazards including landslides, bank erosion, and destruction of downstream habitat. It could also impact future drought events by releasing stored water resources.

12.7 Regulatory Oversight for Dams
Responsibility for dam safety in the OA is distributed among federal agencies, state agencies, the SCVWD and private dam owners.

12.7.1 National Dam Safety Act
Potential for catastrophic flooding due to dam failures led to passage of the National Dam Safety Act (Public Law 92-367). The goal of this FEMA-monitored effort is to identify and mitigate the risk of dam failure so as to protect the lives and property of the public. The National Dam Safety Program is a partnership among the states, federal agencies, and other stakeholders that encourages individual and community responsibility for dam safety. Under FEMA’s leadership, state assistance funds have allowed all participating states to improve their programs through increased inspections, emergency action planning, and purchases of needed equipment. FEMA has also expanded existing and initiated new training programs. Grant assistance from FEMA provides support for improvement of dam safety programs that regulate most of the dams in the United States. 229F 230

12.7.2 U.S. Army Corps of Engineers Dam Safety Program
The USACE is responsible for safety inspections of some federal and non-federal dams in the United States that meet size and storage limitations specified in the National Dam Safety Act. The USACE has inventoried dams; surveyed each state and federal agency’s capabilities, practices, and regulations regarding design, construction, operation, and maintenance of dams; and developed guidelines for inspection and evaluation of dam safety.230F
231

12.7.3 Federal Energy Regulatory Commission Dam Safety Program The Federal Energy Regulatory Commission (FERC) cooperates with a large number of federal and state agencies to ensure and promote dam safety. More than 3,000 dams are part of regulated hydroelectric projects in the FERC program. Two-thirds of these are more than 50 years old. As dams age, concern about their safety and integrity grows, so oversight and regular inspection are important.

FERC inspects hydroelectric projects on an unscheduled basis to investigate the following:
 Potential dam safety problems
 Complaints about constructing and operating a project  Safety concerns related to natural disasters
 Issues concerning compliance with the terms and conditions of a license 230 Federal Emergency Management Agency. (n.d.). Dam Safety. https://www.fema.gov/emergency-managers/riskmanagement/dam-safety 231 U.S. Army Corps of Engineers. (n.d.). Dam Safety Program. https://www.usace.army.mil/Missions/CivilWorks/Dam-Safety-Program/

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Every five years, an independent consulting engineer, approved by the FERC, must inspect and evaluate projects with dams higher than 32.8 feet (10 meters), or with a total storage capacity of more than 2,000 acre-feet.

FERC monitors and evaluates seismic research in geographic areas such as California where there are concerns about possibly seismic activity. This information is applied in investigating and performing structural analyses of hydroelectric projects. FERC also evaluates the effects of potential and actual large floods on the safety of dams. During and following floods, FERC visits dams and licensed projects, determines the extent of damage, if any, and directs any necessary studies or remedial measures the licensee must undertake. The FERC publication Engineering Guidelines for the Evaluation of Hydropower Projects (2021) guides the FERC engineering staff and licensees in evaluating dam safety. The publication is frequently revised to reflect current information and methodologies.

FERC requires dam licensees to prepare EAPs and conducts training sessions on how to develop and test these plans. The EAPs outline an early warning system if there is an actual or potential sudden release of water from a dam due to failure. The plans include operational procedures that may be used, such as reducing reservoir levels and reducing downstream flows, as well as procedures for notifying affected residents and agencies responsible for emergency management. These plans are frequently updated and evaluated to ensure that everyone knows what to do in emergency situations.2 31F 232

12.7.4 State of California
One of the earliest state regulatory programs was enacted in California in the 1920s. California’s Division of Safety of Dams (DSOD) monitors dam maintenance and safety at the state level. When a new dam is proposed, DSOD engineers and geologists inspect the site and the subsurface. Upon submittal of an application, DSOD reviews the plans and specifications prepared by the owner to ensure that the dam is designed to meet minimum requirements and that the design is appropriate for the known geologic conditions. After approval of the application, DSOD inspects all aspects of the construction to ensure that the work accords with the approved plans and specifications. After construction, DSOD inspects each dam annually to ensure performance as intended and to identify developing problems. DSOD periodically reviews stability of dams and their major appurtenances in light of improved design approaches, requirements, and new findings regarding earthquake hazards and hydrologic estimates in California. 232F 233
Finally, on June 27, 2017, SB 92 required an EAP for all dams that do not have a low downstream hazard potential designation.233F
234
An EAP is a dam owner’s formal plan that identifies potential emergency conditions at a dam and specifies actions to be followed to minimize loss of life and property damage. It includes information that dam owners use to notify local emergency management officials and state and federal dam safety regulators. The EAPs must do all of the following:
 Be based upon an inundation map approved by DWR pursuant to Section 6161 of the state’s Water Code.
 Be developed by the dam’s owner in consultation with any local public safety agency that may be impacted by an incident involving the dam, to the extent a local public safety agency wishes to consult.
232 Federal Energy Regulatory Commission. (2015, July). Chapter VI, Emergency Action Plans. https://cmsstage.ferc.gov/sites/default/files/2020-04/chap6.pdf 233 California Department of Water Resources. (2021, June 1). Dam Safety and the Importance of the Division of Safety of Dams with Andy Mangney. https://water.ca.gov/News/Blog/2020/June/DSOD-Andy-MangneyQA#:~:text=DSOD%20also%20conducts%20independent%20reviews%20of%20applications%20for,in%20light%20of %20improved%20design%20approaches%20and%20requirements. 234 California Legislative Information. (2021). Government Code Title 2, Division 1, Chapter 7. https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?sectionNum=8589.5&lawCode=GOV

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 Adhere to FEMA guidelines, and include, at a minimum, all of the following:
▪ Notification flowcharts and contact information
▪ The response process
▪ The roles and responsibilities of the dam owner and impacted jurisdictions following an incident involving the dam
▪ Preparedness activities and exercise schedules
▪ Inundation maps approved by the Department of Water Resources pursuant to Section 6161 of the Water Code
▪ Any additional information that may impact life or property

12.7.5 Santa Clara Valley Water District
The SCVWD Dam Safety Program includes four main components 23 4F 235:
 Periodic Special Engineering Studies
The SCVWD periodically undertakes special engineering studies to ensure that its dams are compliant with the latest design guidelines and regulations to keep pace with the growing body of knowledge surrounding earthquakes. In addition to seismic studies, the SCVWD periodically conducts other special engineering studies to minimize the risks to its dams. Though not required by the regulatory agencies, the water district has proactively expanded this potential failure mode analysis approach to the remainder of our dams.
 Surveillance and Monitoring
Instrumentation placed in and on the dam furnishes data for water district engineers to determine if the structure is functioning as intended. SCVWD continuously monitors the conditions of its dams and uses automated instrumentation at five dams. The equipment can collect, check, record, and archive the collected data and alert staff when parameters exceed set threshold limits.
 Dam Inspections and Maintenance
SCVWD routinely inspects and monitors the condition of each dam and provides an annual surveillance report to the DSOD and each year they also jointly inspect each of its dams with DSOD.
 Emergency Response and Preparedness
At the core of the Dam Safety Program’s emergency response and preparedness is its postearthquake dam evaluation program team. After significant earthquakes, trained personnel inspect the dams for any signs of damage or potential for failure. SCVWD works with various agencies on emergency action planning and training exercises each year. Under the Dam Safety Program, SCVWD is developing updated maps which estimate what areas could flood in the highly unlikely event of a dam failure.
235 Santa Clara Valley Water District. (n.d.). Dam Safety Program. https://www.valleywater.org/flooding-safety/damsafety-program

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12.7.6 Regulatory Oversight for Levees
Regulatory oversight depends on whether the levee is accredited or not. The USACE has recently updated its guidance for evaluating, designing, and constructing levees. Engineer Manual (EM) 1110-2- 1913. The document has been available for review and publication was expected at the end of December 2022.

Federal Emergency Management Agency (FEMA)
FEMA does not evaluate the performance of a levee system—this is the responsibility of the levee owner.

FEMA is responsible for establishing levee system evaluation and mapping standards for an accredited levee, determining flood insurance risk zones, and reflecting these determinations on FIRMs.

State of California
The Department of Water Resources (DWR), Division of Flood Management inspects the levees annually and prepares a report which addresses vegetation, animal control, slope stability, erosion, and vehicle traffic.

12.8 Future Trends in Development
Land use in the OA is directed by general plans adopted under state law and local regulations. The safety elements of the general plans establish standards and plans for the protection of the community from hazards. Dam and levee failure are currently not addressed as stand-alone hazards in the safety elements, but flooding is. Municipalities participating in this MJHMP have established comprehensive policies regarding sound land use in identified flood hazard areas. Most of the areas vulnerable to the more severe impacts from dam and levee failure intersect the mapped flood hazard areas. Flood-related policies in the general plans will help to reduce the risk associated with dam and levee failure hazard for all future development in the OA.

12.9 Scenario
An earthquake in the region could lead to liquefaction of soil around a dam. This could occur without warning during any time of the day. A terrorist or other intentional attack also could cause a catastrophic failure of a dam that impacts the OA. While the probability of dam failure is very low, the probability of flooding associated with changes to dam operational parameters in response to climate change is higher.

Dam designs and operations are developed based on hydrographs with historical record. If these hydrographs experience significant changes over time due to the impacts of climate change, the design and operations may no longer be valid for the changed condition. This could have significant impacts on dams that provide flood control. Specified release rates and impound thresholds may have to be changed.

This would result in increased discharges downstream of these facilities, thus increasing the probability and severity of flooding.

12.10 Issues
There is often limited warning time for dam failure. These events are frequently associated with other natural hazard events such as earthquakes, landslides, or inclement weather, which limits their predictability and compounds the hazard. Important issues associated with dam failure hazards include the following:
 Federally regulated dams have an adequate level of oversight and sophistication in the development of an EAP 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.

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 Mapping for federally regulated dams is required and available; however, mapping for nonfederal-regulated dams that estimates inundation depths is needed to better assess the risk associated with dam failure from these facilities. Moreover, although mapping is required for federally regulated dams, development downstream of dams and upgrades to older dams may have altered inundation areas; however, these inundation maps may not have been updated for significant periods of time. Encouraging property owners of dams to update EAPs and inundation maps will ensure availability of the most accurate data to assist emergency planners and local officials.
 Most dam failure mapping required at federal levels requires determination of the probable maximum flood. While the probable maximum flood represents a worst-case scenario, it is generally the event with the lowest probability of occurrence. For non-federal-regulated dams, mapping of dam failure scenarios that are less extreme than the probable maximum flood but have a higher probability of occurrence can be valuable to emergency managers and community officials downstream of these facilities. This type of mapping can illustrate areas potentially impacted by more frequent events to support emergency response and preparedness.  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.  Addressing security concerns and the need to inform the public of the risk associated with dam failure is a challenge for public officials.
 Limited financial resources for dam maintenance during economic downturns result in decreased attention to dam structure operational integrity because available funding is often directed to more urgent needs. This could increase potential for maintenance failures. Dam failure inundation areas are often not considered special flood hazard areas under the NFIP, so flood insurance coverage in these areas is not common.

12.11 Consequence Analysis
When a dam fails, the stored water can be suddenly released and have catastrophic effects on life and property downstream. The amount of warning time depends largely on the nature of the failure. Homes, bridges, and roads may be demolished in minutes. The impact of the event may be felt for an extended period of time. Residents near a significant or high-hazard dam should become familiar with the dam’s emergency action plans if one is available. EAPs written for dams include procedures for notification and coordination with law enforcement and other governmental agencies; information on the potential inundation area; plans for warning and evacuation; and procedures for making emergency repairs.

The information in Table 82 provides the consequence analysis of the potential for detrimental impacts of dam failure done for accreditation with the Emergency Management Accreditation Program (EMAP).

Table 82: EMAP Consequence Analysis: Dam Failure
Subject Ranking Impacts/Dam Failure
Health and Safety of
Public in the Area of
the Incident
Severe The localized impact is expected to be severe for the inundation area and moderate to minimal for other affected areas.

Responders Minimal The impact on responders is expected to be minimal with proper training. The impact could be severe if there is a lack of training.

Continuity of
Operations (including
continued delivery of
services)
Minimal Temporary relocation may be necessary if inundation affects government facilities.

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Subject Ranking Impacts/Dam Failure
Property, Facilities,
and Infrastructure
Minimal to
severe
The localized impact could be severe for facilities and infrastructure in the inundation area of the incident. The farther away from the incident area, the more likely the damage will lessen, from moderate to minimal.

Delivery of Services Minimal to
severe
Delivery of services could be affected if there is any disruption to the roads and/or utilities due to the inundation. Minimal to severe, depending on area size and location affected.

Environment Severe The impact will be severe for the immediate area. The impact will lessen as distance increases from the immediate incident area.

Economic Conditions Minimal to
severe
Impacts on the economy will greatly depend on the scope of the inundation and the amount of time it takes for the water to recede.

Public Confidence in
the Government
Minimal to
severe
The public’s confidence will vary, depending on the perception of whether the failure could have been prevented, the warning time, and the time it takes for response and recovery.

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13 Flood
Definitions
 Flood: The inundation of normally dry land.
 Floodplain: The land area along the sides of a river that becomes inundated with water during a flood.
 1-Percent-Annual-Chance (100-Year) Floodplain: The area flooded by the flood that has a 1- percent chance of being equaled or exceeded in a given year. The 1-percent-annual-chance flood is the standard used by most federal and state agencies.  0.2-Percent-Annual-Chance (500-Year) Floodplain: The area flooded by the flood that has a 0.2- percent chance of being equaled or exceeded in a given year.  Regulatory Floodway: Channel of a river or other water course and adjacent land areas that must be reserved for discharge of the base flood without cumulatively increasing water surface elevation more than a designated height. Communities must regulate development in these floodways to ensure no increases in upstream flood elevations.
 Return Period: The average number of years between occurrences of a hazard (equal to the inverse of the annual likelihood of occurrence).
 Riparian Zone: The area along the banks of a natural watercourse.

13.1 General Background
Flooding is a temporary condition in which normally dry land is partially or completely inundated. There are number of ways in which flooding can happen. The water levels in bodies such as streams, rivers, lakes, and reservoirs can exceed the water body’s banks, causing water to overflow into nearby areas.

Heavy precipitation can overwhelm the ability of soil to absorb water or local storm drains to carry it away, causing water to build up on the surface. Water from oceans and bays can inundate shoreline areas during exceptionally high tides or be pushed ashore by the winds of an intense storm during coastal floods. Flooding may also occur from infrastructure failure, such as a burst water tank or pipe. Dam inundation, a specific type of infrastructure failure flood that occurs when a dam partially or completely fails, is discussed separately under Dam Failure. Flooding is California’s second most frequent disaster after wildfire.

A floodplain is the area adjacent to a river, creek, or lake that becomes inundated during a flood.

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

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floodplain with levees and other flood control facilities, natural, built-in benefits can be lost, altered, or significantly reduced.

Coastal communities face additional unique flooding hazards. This includes storm surge, waves, and erosion. Coastal flooding can be especially dangerous when high waters combine with the power of waves. Storm surge refers to the abnormal rise in seawater level during a storm. It can cause major coastal and inland flooding. Waves are generated by wind blowing across water. During a storm, wind speeds tend to be higher and last longer, creating larger, more powerful waves. These waves can cause significant damage to anything they impact along the coast. In this case, erosion refers to the wearing away of beaches, dunes, or bluffs by the forces of waves, flowing water, and/or winds. This process can quickly change the appearance of a coastline.

13.1.1 Measuring Floods and Floodplains
Flood frequency and severity are calculated using a number of variables. Flood studies determine the likelihood that a certain river discharge (flow) level will occur in a given year. For example, the 100-year flood has a 1-percent chance of being equaled or exceeded in any given year. For many years, federal agencies referred to the flood frequency statistics in terms of reoccurrence internals (i.e., the “100-year” or “500-year” flood). However, these measurements reflect statistical averages only; it is possible for two or more floods with a 100-year or higher recurrence interval to occur in a short time. The same flood can have different recurrence intervals at different points on a river. In recent years, the terminology has changed to the annual exceedance probability (i.e., 1%, 0.2% chance of occurring) to clear up this confusion.23 5F
236 The “annual flood” is the greatest flood event expected to occur in a typical year.

The extent of flooding associated with a 1-percent annual probability of occurrence (the base flood or 100-year flood) is the standard regulatory boundary for many organizations. 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 important to understand for floodplain management and community development in order to mitigate risk to new and existing structures.

13.1.2 Floodplain Ecosystems
Floodplains can support ecosystems that are rich in plant and animal species. A floodplain can contain 100 or even 1,000 times as many species as a river. Wetting of the floodplain soil releases an immediate surge of nutrients: those left over from the last flood, and those that result from the rapid decomposition of organic matter that has 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 peaks and falls away quickly, but the surge of new growth endures for some time. This makes floodplains valuable for agriculture. Species growing in floodplains are markedly different from those that grow outside floodplains. For instance, riparian trees (trees that grow in floodplains) tend to be very tolerant of root disturbance and very quick growing compared to non-riparian trees.236F 237
236 U.S. Geological Survey. (2018, June 7). The 100-Year Flood. https://www.usgs.gov/special-topics/water-scienceschool/science/100-yearflood#:~:text=The%20USGS%20and%20other%20agencies%20often%20refer%20to,and%20this%20corresponds% 20to%20a%205-year%20recurrence-interval%20flood.
237 International Union of Forest Research Organizations. (n.d.). Riparian and Coastal Ecosystems. https://www.iufro.org/science/divisions/division-8/80000/80100/80105/

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13.1.3 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; land is fertile and suitable for farming; transportation by water is easily accessible; and land is flatter and easier to develop. But human development in floodplains frequently interferes with the natural function of floodplains. It can affect the distribution and timing of drainage, thereby increasing flood problems. Urbanization can create local flooding problems by altering or confining drainage channels.

This increases flood potential in two ways: it reduces the stream’s capacity to contain flows, and it increases flow rates or velocities downstream during all stages of a flood event. River debris, such as the dumping of waste and rubbish, can decrease the conveyance of the river channels. Mining and other industries can change water patterns. Sometimes, humans intentionally make changes, through structural flood control measures such as dams, levees, and embankments.

13.1.4 Federal Flood Programs
FEMA Regulatory Flood Zones
FEMA Flood Zones are geographic areas FEMA has defined according to their levels of flood risk. 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 analysis. These zones are described in terms of high-, moderate-, and low-risk. The “low-risk” area does not mean “no-risk.” Anywhere it rains, it can flood. About 25-percent of NFIP claims come from properties in a low-risk flood zone. Everyone is in a flood zone, but not everyone is in a Special Flood Hazard Area (SFHA). The SFHA is the high-risk area, or the land area covered by floodwaters during the base flood where communities that participate in the NFIP must enforce floodplain management regulations and where mandatory purchase of flood insurance applies for federally backed mortgages. A structure within a 1-percent annual chance floodplain (the SFHA) has a 26-percent chance of undergoing flood damage during the term of a 30-year mortgage.

FEMA Flood Zones are further defined as follows:
 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 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.  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 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): Moderates flood hazard areas between the limits of the base flood and the 0.2-percent-annual-chance (or 500-year) flood.
 Zones C and X (unshaded): Areas of minimal flood hazards outside the SFHA with an elevation higher than the 0.2-percent-annual-chance flood.

Modern visualizations can help residents understand their flood risk. DFIRMS identify locations of specific properties in relation to SFHAs; base flood elevations (1-percent annual chance) at specific sites; magnitudes of flood within specific areas; undeveloped coastal barriers where flood insurance is not available; and regulatory floodways and floodplain boundaries (1-percent and 0.2-percent annual chance floodplain boundaries). FEMA’s National Flood Hazard Layer is a compilation of GIS data including a

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nationwide digital Flood Insurance Rate Map and updated data, like a LOMA (Letter of Map Amendment) or LOMR (Letters of Map Revision) which amend the FIRM.

Risk changes over time. These flood maps are not designed to incorporate future conditions including climate change, sea-level rise, and changes in development. Additional studies are necessary when significant changes occur in order to keep up with changing conditions.

Risk Mapping, Assessment, and Planning
FEMA’s Risk Mapping, Assessment and Planning (Risk MAP) program develops non-regulatory guidance documents. Risk MAP supports community resilience by providing data, building partnerships, and supporting long-term hazard mitigation planning. 237F
238 Each Risk Map product is tailored to the needs of the individual community and may involve different steps, processes, and end results. A “whole community” approach is used throughout the development of these products and the public is provided multiple opportunities to participate. The resulting products are intended to help property owners, community planners, emergency management officials, and others make risk-informed planning and development decisions. Additionally, at the end these products should help local officials identify mitigation opportunities that will work for their communities.

There are currently no Risk MAP products available for the Operational Area (OA). However, through Risk MAP FEMA is looking to conduct flood hazard studies for all the populated coastlines in order to update their FIRMs and Flood Insurance Studies (FISs) so communities have access to the best available data on their coastal flood risk. 238F
239
National Flood Insurance Program
The NFIP makes federally backed flood insurance available to homeowners, renters, and business owners in participating communities. A federal disaster declaration is not necessary for an NFIP payout.

Insurance can also cover significantly more than FEMA’s Individuals and Household Program, which is sometimes available post-disaster. The average NFIP Claims Payment for California between 1996 and 2019 was $18,400. 239F
240 Most homeowner’s insurance does not cover flood insurance. Flood insurance is an important measure of a community’s resiliency to the flood hazard.

For most participating communities, FEMA has prepared a detailed Flood Insurance Study (FIS). The study presents water surface elevations for floods of various magnitudes, including the 1-percent annual chance (100-year) flood and the 0.2-percent annual chance (500-year) flood. Base flood elevations and the boundaries of the 100- and 500-year floodplains are shown on Flood Insurance Rate Maps (FIRMs), which are the principal tools for identifying the extent and location of the flood hazard. FIRMs are the most detailed and consistent data source available, and for many communities they represent the minimum area of oversight under their floodplain management program. Santa Clara County also has DFIRMs. This means the FIRM data is accessible to residents, local governments, and stakeholders online at FEMA’s Map Service Center and National Flood Hazard Layer websites. 238 Federal Emergency Management Association. (2021, November 4). Risk Mapping, Assessment and Planning (Risk MAP). https://www.fema.gov/flood-maps/tools-resources/risk-map 239 Federal Emergency Management Association. (2021, November 4). Risk Mapping, Assessment and Planning (Risk MAP). https://www.fema.gov/flood-maps/tools-resources/risk-map 240 Federal Emergency Management Association. (2022). Historical Flood Risks and Costs. https://www.fema.gov/data-visualization/historical-flood-risk-and-costs

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Participants in the NFIP must, at a minimum, regulate development in the SFHA in accordance with NFIP criteria. The minimum criteria for NFIP participation include, but are not limited to240F 241:
 Require permits for all proposed construction or other development in the community to determine whether such construction or development will be place in flood-prone areas.  Review proposed development to assure that all necessary permits have been received.  Require the elevation of new and substantially improved residential structures to above the base flood level.
 Require the elevation or dry floodproofing (making watertight) new or substantially improved nonresidential structures in Zone A.  With limited exception, prohibit encroachments, including fill, new construction, substantial improvements, and other development within the adopted regulatory floodway.  Adhere to additional requirements to protect buildings in coastal areas from the impacts of waves, high velocity, and storm surge.

Table 83 lists each participating municipal jurisdiction’s date of entrance into the NFIP and the effective date for its current FIRM.

Table 83: NFIP Status in the Operational Area
Community NFIP Community # NFIP Entry Date Current Effective FIRM
City of Campbell 060338 06/30/1976 02/19/2014
City of Cupertino 060339 05/01/1980 05/18/2009
City of Gilroy 060340 08/01/1980 05/18/2009
City of Los Altos 060341 07/16/1980 05/18/2009
Los Altos Hills 060342 01/02/1980 05/18/2009
Los Gatos 060343 01/17/1979 02/19/2014
City of Milpitas 060344 07/16/1980 02/19/2014
City of Monte Sereno 060345 05/18/2009 02/19/2014
City of Morgan Hill 060346 06/18/1980 05/18/2009
City of Mountain View 060347 08/15/1980 05/18/2009
City of Palo Alto 060348 09/19/1984 10/16/2012
City of San José 060349 08/02/1982 02/19/2014
City of Santa Clara 060350 07/16/1980 02/19/2014
City of Saratoga 060351 01/17/1979 02/19/2014
City of Sunnyvale 060352 05/15/1978 05/18/2009
Unincorporated County 060337 08/02/1982 02/19/2014
241 Federal Register. (2021). Request for Information on the National Flood Insurance Program’s Floodplain Management Standards. https://www.federalregister.gov/documents/2021/10/12/2021-22152/request-for-informationon-the-national-flood-insurance-programs-floodplain-management-standardsfor#:~:text=The%20minimum%20NFIP%20requirements%20for%20participating%20communities%20include%2C,im pacts%20of%20waves%2C%20high%20velocity%2C%20and%20storm%20surge.

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Building codes are an important part of developing resilient communities. All community’s that participate in the NFIP must adhere to the NFIP floodplain management criteria, including adopting a flood damage prevention ordinance. The California Department of Water Resources (DWR) has developed a 2020 model ordinance which reflects NFIP requirements and California Building Standards Code (CCR Title 24). Communities that did not use this ordinance may make amendments to their existing regulations.

This ordinance reflects the fact that the flood provisions of CCR Title 24 meet or exceed the minimum requirements for buildings and structures.2 41F
242
“Substantial improvement” refers to any reconstruction, rehabilitation, addition, or other improvement of a structure, the cost of which equals or exceeds 50 percent (or less, if defined as so in the jurisdiction’s floodplain management ordinance) of the market value of the structure before the start of construction of the improvement. “Substantial damage” means any damage of any origin sustained by a structure that would cause the cost of restoring the structure to its before damage condition to equal or exceed 50 percent of the market value of the structure before the damage occurred. The repairs to a substantially damaged structure are considered a substantial improvement and that structure would be required to meet current NFIP standards in order to protect it from future flood losses. These kind of post-disaster policies and procedures are further described in Volume 2, as applicable.

All participating planning partners are currently in good standing with the provisions of the NFIP. In California, the DWR is the coordinating agency for floodplain management. The DWR works with FEMA and local governments by providing grants and technical assistance, evaluating community floodplain management programs, reviewing local floodplain ordinances, participating in statewide flood hazard mitigation planning, and facilitating annual statewide workshops. Compliance is monitored by FEMA regional staff and by the DWR. Maintaining compliance under the NFIP is an important component of flood risk reduction. All planning partners that participate in the NFIP have identified actions to maintain compliance and good standing. Details about participation in the NFIP are further described the individual annexes in Volume 2 of this plan.

Risk Rating 2.0
FEMA recently updated the pricing methodology for the NFIP through Risk Rating 2.0. This new methodology builds on years of investment in flood hazard information by leveraging new technology and best practices such as private sector data sets, catastrophe models, and evolving actuarial science to develop rates that are actuarily sound, equitable, easier to understand, and better reflective of risk. 24 2F 243
Previously, rates were based on relatively static measurements, emphasizing a property’s zone on the FIRM. Now, FEMA is able to incorporate additional flood risk variables into their rating calculations. This includes flood frequency, multiple flood types – river overflow, storm surge, coastal erosion, and heavy rainfall – and distance to water source, along with such property characteristics as elevation, numbers of floors, and the cost to rebuild. This reflects a significant change to their rating system. It is intended to make flood insurance more equitable. Before, policyholders were lower-valued homes were paying more than their share of the risk while policy holders with higher-valued homes were paying less than their share. With Risk Rating 2.0, FEMA is able to distribute premiums across all policyholders based on home value and a property’s unique flood risk.243F
244
Risk Rating 2.0 rates went into effect for new policies in October 2021. All remaining policies renewed on or after April 1, 2022, utilize the new rating methodology as well. The FEMA estimated first-year premium changes for existing NFIP policies in Santa Clara County is displayed in Figure 60: Risk Rating 2.0:

Project Premium Changes for Santa Clara County.
242 California Department of Water Resources. (n.d.). National Flood Insurance Program. https://water.ca.gov/nfip/ 243 Federal Emergency Management Association. (2022, April 18). Risk Rating 2.0: Equity in Action. https://www.fema.gov/flood-insurance/risk-rating
244 Federal Emergency Management Association. (2022, April 18). Risk Rating 2.0: Equity in Action. https://www.fema.gov/flood-insurance/risk-rating

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 263
Figure 61: Risk Rating 2.0: Project Premium Changes for Santa Clara County2 44F 245
The Community Rating System
The community rating system (CRS) is a voluntary program within the NFIP that encourages floodplain management activities that exceed the minimum NFIP requirements. Flood insurance premiums are discounted to reflect the reduced flood risk resulting from community actions meeting the following three goals of the CRS:
 Reduce flood damage to insurable property.
 Strengthen and support the insurance aspects of the NFIP.  Encourage a comprehensive approach to floodplain management.

For participating communities, flood insurance premium rates are discounted in increments of 5 percent.

For example, a Class 1 community would receive a 45 percent premium discount, and a Class 9 community would receive a 5 percent discount. (Class 10 communities are those that do not participate in the CRS; they receive no discount.) Previously, properties outside of the SFHA received smaller discounts: a 10-percent discount if the community is at Class 1 to 6 and a 5-percent discount if the community is at Class 7 to 9. Now, under Risk Rating 2.0, the discount is applied uniformly to all policies throughout the participating community regardless of whether the structure is located in the SFHA.

The CRS classes for local communities are based on 19 creditable activities in the following categories:
 Public information
 Mapping and regulations
245 Association of State Floodplain Managers. (2021, September). Data Visualization Dashboards for FEMA’s Risk Rating 2.0 Projected Premium Change Analysis. https://floodsciencecenter.org/projects/data-visualizationdashboards-for-fema-risk-rating-2-0-projected-premium-change-analysis/

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 264
 Flood damage reduction
 Warning and response
Figure 61 shows the nationwide number of CRS communities by class as of October 2022, when there were 1,353 communities receiving flood insurance premium discounts under the CRS program.

Figure 62: CRS Communities by Class Nationwide as of October 2022245F 246
CRS activities can help to save lives and reduce property damage. Communities participating in the CRS represent a significant portion of the nation’s flood risk; over 70 percent of the NFIP’s policy base is located in these communities. Communities receiving premium discounts through the CRS range from small to large and represent a broad mixture of flood risks, including both coastal and riverine flood risks.

Table 84: CRS Community Status in the Operational Area24 6F 247
Community NFIP
Community #
CRS Entry
Date
Current CRS
Classification
% Premium
Discount,
SFHA/non-SFHA
Cupertino 060339 10/01/2005 7 15/5
Gilroy* 060340 05/01/2007 8 10/5
Los Altos 060341 10/01/1991 8 10/5
Milpitas 060344 10/01/1991 7 15/5
Morgan Hill 060346 05/01/2003 7 15/5
246 Federal Emergency Management Association. (2022, October). Community Rating System. https://www.fema.gov/floodplain-management/community-rating-system 247 FEMA. (2023). Community Rating System. https://www.fema.gov/floodplain-management/community-ratingsystem

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 265
Community NFIP
Community #
CRS Entry
Date
Current CRS
Classification
% Premium
Discount,
SFHA/non-SFHA
Mountain View 060347 05/01/2002 8 10/5
Palo Alto 060348 10/01/1991 6 20/10
San José 060349 10/01/1991 7 15/5
Santa Clara (city) 060350 05/01/2002 7 15/5
Sunnyvale 060352 10/01/1998 7 15/5
*Gilroy CRS rating is 8 as of 5/1/2007. Starting 10/1/2023, the rating will be a 7 with a percentage premium discount of 15% of SFHA and 5% for non-SFHA.

Santa Clara Valley Water will lead the development of a separate Floodplain Management Plan specifically dedicated to meeting the CRS planning requirements, allowing planning partners that participate in the CRS program to maintain or enhance their CRS classifications. Relevant information from this Hazard Mitigation Plan will be incorporated into the Floodplain Management Plan.

13.2 Hazard Profile
The following information is extracted from the Santa Clara County Flood Insurance Study 247F 248:
 The mountains and foothills in northern Santa Clara County are the sources of the watercourses that flow through the north portion of the OA. Near San José, the major waterways include Los Gatos, Guadalupe, and Alamitos Creeks flowing out of the Santa Cruz Mountains; Coyote Creek and a host of tributaries, including Upper Penitencia and Silver Creeks, flowing out of the Diablo Range; and Fisher Creek with headwaters on the western side of the Coyote Creek Valley. The 75-mile-long Coyote Creek is the primary natural drainage facility for the eastern side of the Santa Clara Valley.
 Permanente and Stevens Creeks, which flow north through the OA near Mountain View, are the primary runoff drainage channels in that area. In addition to providing flood control, these creek beds provide gravel lenses that penetrate the impervious underground clay layers. These lenses allow rain runoff to percolate down to replenish the underground water supply.  The principal watercourses in the south portion of the OA are Llagas, Uvas, and Coyote Creeks.

Edmundson (Little Llagas), Church, Center, Tennant, Maple, and Foothill Creeks also flow through the area. The area is unusual in that creeks originate in both the Diablo Range, to the east, and the Santa Cruz Mountains, to the west. Waters originating in the area are conveyed to Monterey Bay via the Pajaro River.
 Drainage-ways in the OA are a combination of natural channels (creek beds) and channels altered by human activity.
 Drainage patterns in the OA have been altered by urbanization, and the runoff, which has increased, is a greater flood threat than in previous years. The construction of water-conservation flood retention facilities has also altered the drainage pattern.  A variety of conditions cause flooding in the Santa Clara County OA. In smaller drainage basins, flooding is usually the result of intense storms. In larger basins, flooding results from storms of long duration. Shallow overland flooding often occurs due to the small capacity of the creeks. 248 Federal Emergency Management Association. (2014, February 19). Flood Insurance Study Santa Clara County. https://www.milpitas.gov/_pdfs/FISReport.pdf

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 266

13.2.1 Types of Flood-Related Hazards
Flooding in the Santa Clara County OA typically occurs during the rainy winter season. Four types of flooding primarily affect the County: stormwater runoff, riverine, flash floods, and tidal floods.

Stormwater Runoff Floods
Stormwater runoff is generated from rain and snowmelt events that flows over land or impervious surfaces, such as parking lots and roads, without being absorbed. 2 48F 249 Stormwater flooding is a result of
local drainage issues and high groundwater levels. Locally, heavy precipitation, especially during high lunar tide events, may induce flooding within areas other than delineated floodplains or along recognizable channels. If local conditions cannot accommodate intense precipitation through a combination of infiltration and surface runoff, water may accumulate and cause flooding problems.

Flooding issues of this nature generally occur within areas with flat gradients, and generally increase with urbanization, which speeds accumulation of floodwaters because of impervious areas 249F 250. Pump stations
may be unable to pump stormwater during a large precipitation event due to insufficient channel capacity or submergence of the pump station, rendering it inoperable. Shallow street flooding can occur unless channels have been improved to account for increased flows. Numerous areas within the County undergo stormwater flooding that contributes to street and structure inundation.

Urban flooding is by increased water runoff due to urban development and drainage systems. Drainage systems are designed to remove surface water from developed areas as quickly as possible to prevent localized flooding on streets and within other urban areas. These systems utilize a closed conveyance system that channels water away from an urban area to surrounding streams, and bypasses natural processes of water filtration through the ground, containment, and evaporation of excess water. Because drainage systems reduce the amount of time surface water takes to reach surrounding streams, flooding in those streams can occur more quickly and reach greater depths than prior to development within that area.25 0F
251
Riverine Floods
Riverine flooding is overbank flooding of rivers and streams. Natural processes of riverine flooding add sediment and nutrients to fertile floodplain areas. Flooding in large river systems typically results from large-scale weather systems that generate prolonged rainfall over a wide geographic area, causing flooding in hundreds of smaller streams, which then drain into the major rivers. Shallow area flooding is a special type of riverine flooding. FEMA defines shallow flood hazards as areas inundated by the 100-year flood with flood depths of only 1 to 3 feet. These areas are generally flooded by low-velocity sheet flows of water. Two types of flood hazards are generally associated with riverine flooding:
 Inundation: Inundation occurs when floodwater is present, and debris flows through an area not normally covered by water. These events cause minor to severe damage, depending on velocity and depth of flows, duration of the flood event, quantity of logs and other debris carried by the flows, and amount and type of development and personal property along the floodwater’s path.  Channel Migration: Erosion of banks and soils worn away by flowing water, combined with sediment deposition, causes migration or lateral movement of a river channel across a floodplain.

A channel can also abruptly change location (termed “avulsion”); a shift in channel location over a large distance can occur within as short a time as one flood event. 249 Environmental Protection Agency. (2023, February 2). National Pollutant Discharge Elimination System (NPDES) Stormwater Program. https://www.epa.gov/npdes/npdes-stormwater-program 250 FEMA. (1997, January). Multi Hazard Identification and Risk Assessment: The Cornerstone of the National Mitigation Strategy. https://babel.hathitrust.org/cgi/pt?id=pst.000032338492&view=1up&seq=5 251 Environmental Protection Agency. (n.d.). Flow Alteration. https://www.epa.gov/caddis-vol2/flow-alteration

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 267
Natural stream channels in rural parts of the Santa Clara County OA typically can accommodate average rainfall amounts and mild storm systems; however, severe floods occur in years of abnormally high rainfall or unusually severe storms. During those periods of severe floods, high-velocity floodwaters carry debris over long distances, block stream channels, and create severe localized flooding.

Flash Floods
The National Weather Service defines a flash flood as a rapid and extreme flow of high water into a normally dry area, or a rapid water level rise in a stream or creek above a predetermined flood level. Such floods generally begin within 6 hours of the rain event that causes them. Ongoing flooding can intensify to flash flooding in cases where intense rainfall results in a rapid surge of rising flood waters. 251F 252
Flash floods can tear out trees, undermine buildings and bridges, and scour new channels. In urban areas, flash flooding is an increasingly serious problem due to removal of vegetation and replacement of ground cover with impermeable surfaces such as roads, driveways, and parking lots. The greatest risk from flash floods is occurrence with little to no warning. Major factors in predicting potential damage are intensity and duration of rainfall, and steepness of watershed and streams.

Coastal Floods
Coastal flooding of normally dry land by the coastline is caused by abnormally high tides, storm surge, and persistent onshore winds and waves. Rising and falling water levels, breaking waves, and shifting sands are common sights along the shore. Typically, they are a normal part of life on the coast, however when they strengthen intensity they can threaten life, property, and livelihoods of coastal populations.

Coastal floods come with a unique range of concerns including storm, waves, and erosion. All of these can contribute to extensive damage to homes, businesses, and infrastructure. The flooding of bay adjacent SFHAs is likely in the Santa Clara County OA, particularly where land is at or slightly above sea level. Areas mapped as Zone V and Zone VE are considered at high-risk from coastal flooding and are subject to stricter building requirements because of the likelihood of damage from strong waves.

High Tide Floods
It no longer takes a strong storm or hurricane to flood coastal areas. Tidal floods are characterized by inundation of normally dry lands by the coast, often caused by extreme high tide events that result in shallow flooding of low-lying coastal areas. Colloquially known as “King Tides,” these tides exceed the highest water level reached at high tide on an average day and normally occur once or twice per year.

King Tide events are the leading cause of flooding by bay waters.

Tidal flooding is becoming increasingly exacerbated by sea level rise as a result of climate change or tectonic activity.2 52F
253 Average daily water levels are rising along with the oceans. As a result, high tides are reaching higher and extending further inland than in the past. Additional information regarding the impacts and exposure of the OA to sea level rise is presented in Section 11, Climate Change. 252 National Weather Service. (n.d.). Flood Related Hazards. https://www.weather.gov/safety/flood-hazards 253 National Oceanic and Atmospheric Administration. (n.d.). Flooding on a Sunny Day? Here’s How. https://oceantoday.noaa.gov/flooding-sunny-day/

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 268

13.2.2 Principal Flooding Sources
FEMA’s Flood Insurance Study for Santa Clara County assessed over 50 creeks, channels, and water bodies, including the following principal flooding sources 253F 254:
 Adobe Creek
 Alamitos Creek
 Alviso Slough
 Arastradero Creek
 Arroyo Calero
 Barron Creek
 Berryessa Creek
 Calabazas Creek
 Canoas Creek
 Concepcion Drain
 Coyote Creek
 Daves Creek
 East Little Llagas Creek
 East Penitencia Creek
 Evergreen Creek
 Fisher Creek
 Fisher Creek Overbank
 Flint Creek
 Fowler Creek
 Guadalupe River
 Guadalupe Slough
 Hale Creek
 Lions Creek
 Llagas Creek
 Llagas Overbank
 Los Gatos Creek
 Lower Penitencia Creek
 Matadero Creek
 Miguelita Creek
 Miller Slough
 North Morey Creek
 Permanente Creek
 Permanente Diversion
 Purissima Creek
 Quimby Creek
 Ronan Channel
 Ross Creek
 Ruby Creek
 San Francisco Bay
 San Francisquito Creek
 San Joaquin River
 Santa Teresa Creek
 San Tomas Aquino Creek
 San Tomas Aquino Creek
Reach 2
 Saratoga Creek
 Silver Creek
 Smith Creek
 South Babb Creek
 South Morey Creek
 Stevens Creek
 Sunnyvale East Channel
 Sunnyvale West Channel
 Thompson Creek
 Upper Penitencia Creek
 Upper Penitencia Creek Reach 2
 Upper Penitencia Creek Reach 2
Overflow
 Uvas Creek
 West Branch Llagas Creek
 West Little Llagas Creek
 Wildcat Creek
Understanding watershed conditions can help clarify the OA’s vulnerability to flooding. A watershed is the area of land that drains to a common waterway. Every watershed has unique qualities that affect its response to rainfall. Stormwater discharge is influenced by the watershed’s soil type, geology, typography, vegetation, shape, and land use. 254F
255 The Santa Clara County OA contains five watersheds 255F256:
 Coyote Watershed is the OA’s largest watershed, with 322 square miles. It contains Coyote Creek, which is the longest creek in the county.
 Guadalupe Watershed drains the Guadalupe River and its tributaries through downtown San José.
 Lower Peninsula Watershed is a small-creek watershed that feeds tidal wetlands along the San Francisco Bay’s southwest shoreline.
 Uvas-Llagas Watershed is mainly agricultural land and natural areas. This is the only watershed in the county in which waterways flow southward.
254 Federal Emergency Management Association. (2014, February 19). Flood Insurance Study Santa Clara County. https://www.milpitas.gov/_pdfs/FISReport.pdf
255 USGS. (2019, June 8). Surface Runoff and the Water Cycle. https://www.usgs.gov/special-topics/water-scienceschool/science/surface-runoff-and-water-cycle 256 Santa Clara Valley Water District. (n.d.). Watersheds of Santa Clara Valley. https://www.valleywater.org/learningcenter/watersheds-santa-clara-valley

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 269
 West Valley Watershed is the smallest watershed in the county, covering 85 square miles of numerous small creeks.

Figure 63: Watersheds in the Operational Area256F
257

13.2.3 Past Events
Based on NOAA’s National Centers for Environmental Information, 141 flood events in the OA were recorded between 1950 and 2022, as summarized in Table 85: History of Flood Events, . These events include flash floods, lakeshore floods, coastal floods, and flooding from heavy multi-day rain events.

Since 1954, 10 presidential-declared flood events in the OA have caused in excess of $4.468 billion in property damage throughout the region.

According to the USDA’s Risk Management Agency, Santa Clara County received $13,131,222 in payments for insured crop losses on 5,031 affected acres as a result of excessive moisture and flood events between 2003 and 2022. Table 86: Crop Insurance Claims Paid from Excessive Moisture and Flood, 2003-2022 summarizes these payments. The highest damaging year was 2016. Additionally, 39 flood-related federally declared disasters or emergencies have occurred in California since 1953. This equates to a major flood event impacting the state around once every 2 years. 257 Santa Clara Valley Water District. (2023). Watersheds of Santa Clara Valley. https://www.valleywater.org/learningcenter/watersheds-santa-clara-valley

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 270
Table 85: History of Flood Events257F
258,
258F
259
Date Declaration # Type of Event Estimated Damage
2/5/1954 15 Flood & Erosion Not available
12/23/1955 47 Flood Coyote Creek, Stevens Creek, Matadero Creek, San Francisquito Creek, and
Guadalupe River flooded
4/4/1958 82 Heavy Rainstorms and
Flood
Penitencia Creek, Guadalupe River, San
Tomas Aquinas Creek, Stevens Creek,
Permanente Creek, Matadero Creek, and
San Francisquito Creek flooded. $20 million,
plus $4 million agricultural damage
3/6/1962 122 Floods Not available
10/24/1962 138 Severe Storms and
Flooding
$4 million in regional flooding
2/25/1963 145 Severe Storms, Heavy
Rains and Flooding
Not available
1/16/1973 None Severe Storms and
Flooding
$86,207 in damage
1/7/1982 651 Severe Storms, Flood,
Mudslides and High
Tide
$273 million, 256 homes and 41 businesses
destroyed; 6,259 homes and 1,276
businesses damaged.
2/9/1983 677 Coastal Storms,
Floods, Slides and
Tornadoes
$523 million
2/21/1986 758 Severe Storms and
Flooding
$407 million; 1,382 homes and 185
businesses destroyed; 12,447 homes and
967 businesses damaged.
2/11/1992–
2/14/1992
None Severe Storms and
Flooding
$20,000 in damage
1/13/1993 None Severe Storms and
Flooding
$112,000 in damage
1/10/1995 1044 Severe Winter Storms,
Flooding, Landslides,
Mud Flows
$741 million total; 11 deaths
3/12/1995 1046 Severe Winter Storms,
Flooding Landslides,
Mud Flow
Approx. $1.1 billion total; damage to homes:
major 1,322; minor 2,299; destroyed 267.
12/10/1996 None Flood Not available
1/01/1997 None Flash Floods Not available
258 National Oceanic and Atmospheric Administration. (2017). Storm Events Database. https://www.ncdc.noaa.gov/stormevents/
259 Association of Bay Area Governments. (2010). Data and Research. https://abag.ca.gov/our-work/resilience/dataresearch

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 271
Date Declaration # Type of Event Estimated Damage
1/4/1997 1155 Severe Storms,
Flooding, Mud and
Landslides
$1.8 billion total; 23,000 homes; 2,000
businesses damaged or destroyed.
1/25/1997 None Flash Flood Not available
2/3/1998 None Flash Flood Not available
2/7/1998 None Flash Flood Not available
2/8/1998 None Flash Flood Not available
2/9/1998 1203 Severe Winter Storms
and Flooding
$550 million; 17 deaths
2/13/2000 None Flash Flood Mainly on Coyote Creek
10/13/2009 None Heavy Rain and
Flooding
$400,000
1/18/2010–
1/20/2010
None Heavy Rain and
Flooding
Localized flooding, roads closed, damage
estimate not available.
12/23/2012 None Heavy Rain and
Tornado
Localized flooding, levee overtopped in East
Palo Alto.
2/28/2014 None Heavy Rain and
Flooding
Flooding of urban areas, small streams and
creeks, and a few localized mud and
rockslides.
12/2/2014 None Flood Not available
12/3/2014 None Flood Not available
12/11/2014 None Heavy Rain and
Flooding
Flooding and mudslides
2/06/2015 None Heavy Rain and
Flooding
Multiple off ramps from I-280 flooded.
10/28/2016 None Flood Not available
12/10/2016 None Flood Not available
2/7/2017 None Flash Flood Not available
2/9/2017 None Flood Not available
2/14/2017 4301 Severe Winter Storms,
Flooding, and
Mudslides
34 of 57 CA Counties declared for flooding
events that occurred from January 3 to
January 12, 2017
2/20/2017 None Flood, Flash Flood Not available
2/21/2017 None Flood Not available
1/8/2018 None Flood Not available
1/9/2018 None Flood Not available
1/25/2018 None Flood Not available
3/1/2018 None Flood Not available
4/6/2018 None Flood Not available
11/22/2018 None Flood Not available
11/23/2018 None Flood Not available
11/29/2018 None Flood Not available

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 272
Date Declaration # Type of Event Estimated Damage
12/17/2018 None Flood Not available
1/6/2019 None Flood Not available
1/16/2019 None Flood Not available
1/17/2019 None Flood Not available
2/4/2019 None Flood Not available
2/13/2019 None Flood Not available
2/14/2019 None Flood Not available
2/27/2019 None Flood Not available
5/15/2019 None Flood Not available
11/26/2019 None Flood Not available
12/1/2019 None Flood Not available
12/2/2019 None Flood Not available
1/16/2020 None Flood Not available
1/27/2021 None Flood Not available
3/10/2021 None Flood Not available
12/27/2022–
1/31/2023
4683 Severe Winter Storms,
Flooding, Landslides,
and Mudslides
Damage estimates are ongoing at the time of
this writing. Damage was sufficient for a
Presidential Disaster Declaration.

Note: N/A = Not Applicable.

Table 86: Crop Insurance Claims Paid from Excessive Moisture and Flood, 2003-202225 9F 260
Crop Year Commodity Acres
Affected Indemnity Amount
2003 None None None
2004 None None None
2005 All Other Crops 79 $13,144
2006 All Other Crops 83 $6,937
2007 None None None
2008 None None None
2009 None None None
2010 None None None
2011 Walnuts, Cherries, Processing Apricots 910 2,706,413 2012 Cherries 239 $113,052
2013 None None None
2014 Cherries 18 $29,015
260 United States Department of Agriculture. (n.d.). Cause of Loss Historical Data Files. https://www.rma.usda.gov/Information-Tools/Summary-of-Business/Cause-of-Loss

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 273
Crop Year Commodity Acres
Affected Indemnity Amount
2015 Cherries, Processing Apricots, All Other Crops 322 $1,053,095 2016 Cherries, Processing Apricots 1,059 $4,279,020
2017 Cherries, Processing Apricots 119 $69,405
2018 None None None
2019 Cherries, Cultivated Wild Rice, All Other Crops 1,725 $3,975,874 2020 Cherries, Processing Apricots, All Other Crops 245 $337,118 2021 Cherries, Grapes 80 $122,467
2022 Cherries 152 $425,682
Total 5,031 $13,131,222

13.2.4 Location
Flooding that has occurred in portions of the OA has been extensively documented by gage records, high water marks, damage surveys, and personal accounts. This documentation was the basis for the 2014 FIRMs generated by FEMA for the Santa Clara County OA. The 2014 current effective Flood Insurance Study is the sole source of data used in this risk assessment to map the extent and location of the flood hazard, as shown in Figure 63: Mapped Flood Hazard Areas in the Operational Area.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 274
Figure 64: Mapped Flood Hazard Areas in the Operational Area

13.2.5 Frequency
Recurrence intervals and average annual numbers of events in the Santa Clara County OA were calculated based on data from 1996 to 2022 in NOAA’s Storm Events Database. Santa Clara County has experienced one hundred fourteen significant events since 1996 classified as “flood” in the database.

Smaller floods may occur more frequently and be categorized as a different event type, typically “flash flood” or “winter storm.” Based on these data, floods have a 158 percent chance of occurring in any given year, flash floods have a 33 percent chance, and coastal floods have a 4 percent chance. The total estimated percent chance of occurrence for any type of flood in a given year is 100 percent, meaning that flooding will likely continue to be an annual hazard.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 275
The frequency and magnitude of floods will likely be influenced by climate change. Climate change may cause an increase in the number of intense rainfall events, resulting in increased flood risk. Sea level rise, which may exacerbate the risk of flooding in shore areas, is also of concern.

Further information on the impact of climate change on the probability of flooding is included in Section 11.

Probability in OA: Occasional

13.2.6 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. This is especially true when a channel migrates over a broad floodplain, redirecting high velocity flows and transporting debris and sediment. Another element that characterizes the community’s flood threat is length of time floodwaters remain above flood stage.

Although jurisdictions can implement mitigation and take preventative actions to significantly reduce severity and threat of flood events, some type of residual risk will always exist (i.e., risk of a hazard event occurring despite technical and scientific measures applied to reduce/prevent it). Threats associated with residual risk could include failure of a reservoir, a dam breach, or other infrastructure failure, or a severe flood event that exceeds flood design standards or drainage capacity.

Flood severity is often evaluated by examining peak discharges. Table 87 lists peak flows used by FEMA to map the floodplains of the OA as found in the effective Santa Clara County Flood Insurance Study.

Table 87: Summary of Peak Discharges Within the Operational Area260F 261
Flooding Source and Location
Discharge (Cubic Feet/Second)
10-
Year
50-
Year
100-
Year
500-
Year
Adobe Creek
 Above Railroad (At El Camino Real) 1,350 2,500 2,700a 2,700a  At East Charleston Road 1,400a 1,400a 1,400a 1,400a
 At East Meadow Drive 1,350 1,350 1,350 1,350
 At Edith Road 1,000 1,830 2,140 2,700
 At El Monte Avenue 690 1,340 1,700 2,370
 At corporate limits 890 1,650 1,920 2,400
 At Foothill Expressway 1,070 2,120 2,320 2,690
 At Middlefield Road 1,020a 1,020a 1,020a 1,020a
 At Moody Road 590 1,150 1,430 1,930
 At Old Altos Road 960 1,760 2,050 2,490
 At Pine Lane 1,110 2,150 2,360 2,730
261 Federal Emergency Management Agency. (2014, February 19). Flood Insurance Study. https://map1.msc.fema.gov/data/06/S/PDF/06085CV002B.pdf?LOC=65c0f78954006b048e415150264ffe4b

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 276
Flooding Source and Location
Discharge (Cubic Feet/Second)
10-
Year
50-
Year
100-
Year
500-
Year
 At Railroad 1,350 1,450a 1,450a 1,450a
 At U.S. Highway 101 1,660 1,780 1,780 1,780
 At Van Buren Road 1,060 1,890 2,220 2,810
 Below Alma Street 1,450 1,700 1,700 1,750
 Below Purissima Creek 1,040 1,980 2,200 2,510
Alamitos Creek
 Downstream of confluence with
Arroyo Calero
2,150 5,180 6,750 11,000
 Downstream of confluence with Golf
Creek
3,530 7,020 8,680 12,700
 Downstream of confluence with
Greystone Creek
2,940 6,200 7,800 11,800
 Downstream of confluence with
Randol Creek
2,660 5,800 7,380 11,400
 Upstream of confluence with Arroyo
Calero
1,430 3,580 4,750 7,900
 Upstream of confluence with
Guadalupe River
3,630 7,180 8,860 12,900
Alamitos Creek By-Pass Channel b B 3,250 b
Alamitos Creek Overflow Area b B 140 b
Arastradero Creek
At Page Mill Road 140 300 360 460
Arroyo Calero
 Downstream of confluence with Santa
Teresa Creek
1,020 1,820 2,180 3,010
 Upstream of confluence with Alamitos
Creek
1,180 1,980 2,330 3,110
 Upstream of confluence with Santa
Teresa Creek
660 1,120 1,320 1,770
Arroyo De Los Coches
 At confluence with Berryessa Creek b B 1,420 b
Barron Creek
 At El Camino Real 270 270 270 270
 At Foothill Expressway 176 364 453 640
 At Foothill Expressway 320 630 760 1,100
 At Laguna Avenue 180b 180b 180b 180b
 At Lower Fremont Road 96 208 268 390
 At mouth 320 430 430 430
 At Ramona Street 320 430a 430a 430a

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 277
Flooding Source and Location
Discharge (Cubic Feet/Second)
10-
Year
50-
Year
100-
Year
500-
Year
 At Railroad 320 675 675 675
 At Upper Fremont Road 32 77 98 143
 Downstream of El Camino Real 270 270 270 270
 Upstream of Barron Creek Diversion b B 740 b
 Upstream of Fabian Way b B 250 b
 Upstream of Laguna Avenue b B 1,603 b
 Upstream of Railroad 320 820 920 1,080
Berryessa Creek
 At confluence with Calera Creek b B 3,600a b
 At confluence with Sierra Creek 1,230 2,250 2,580 1,230  At confluence with Tularcitos Creek b B 2,500a b
 At confluence with Wrigley Ditch b B 2,000a b
 At Morrill Avenue 1,230 1,7001 1,750a 1,230
 At Piedmont Road b B 1,600 b
 Downstream of confluence with
Arroyo De Los Coches
b B 2,000a b
 Downstream of Montague
Expressway
800a 800a 800a 800a
Calabazas Creek
 Above Prospect Road b B 1,800 b
 Above Railroad and Prospect Creek b B 1,140 b
 At Coffin Road 3,000 4,100 4,600 5,800
 At El Camino Real 2,090d 2,290d 2,340d 2,360d
 At Grant Road 1,200 1,600 1,800 2,300
 At Interstate Highway 280 1,950 2,490 2,700 3,360
 At Junipero Serra 2,000 2,700 3,100 3,900
 At Kifer Road 2,600 3,600 4,000 5,200
 At Lawrence Expressway 2,100 3,000 3,300 4,200
 At Rainbow Drive Below La Mar
Court
750 1,070 1,310 1,370
 Below Miller Avenue 1,670 2,050 2,210 2,670
 Below Tantau Avenue/Upstream of
Pruneridge Avenue
1,700a 1,900a 1,950a 2,000a
 Downstream of confluence with
Rodeo Creek
1,170 1,700 1,950 2,610
 Downstream of Prospect Road 7501 1,000e 1,180e 1,220e  Downstream of U.S. Highway 101 2,760d 3,200f 4,780f 5,510f  Through box culvert at Miller Avenue 1,400a 1,550a 1,600a 1,600a

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 278
Flooding Source and Location
Discharge (Cubic Feet/Second)
10-
Year
50-
Year
100-
Year
500-
Year
 Upstream of Benton Street 2,100d 2,170a 2,170a 2,200a  Upstream of Kifer Road 2,550d 2,820d 3,000d 3,340d
 Upstream of Lawrence Expressway 2,050d 2,310d 2,370d 2,540d  Upstream of Pomeroy Avenue 2,190d 2,200d 2,200d 2,200d  Upstream of U.S. Highway 101 2,760d 3,020d 3,200d 3,550d  Upstream of State Highway 237 3,010d 3,420d 5,000d 5,100d Calera Creek
 At confluence with Berryessa Creek b B 920 b
 Upstream of Interstate Highway 680 b B 850 b
Canoas Creek
 At Blossom Hill Road 1,320 1,390 1,400 1,420
 At Capitol Expressway 1,850 1,910 1,960 2,000
 At confluence with Guadalupe River 1,900a 1,950a 1,970a 2,000a  At Cottle Road 480 500 510 530
 At Santa Teresa Boulevard 780 810 830 850
 Upstream of Nightingale Drive 1,990 2,250 2,350 2,500 Concepcion Drainage
 At Alto Verde Lane 22 51 68 102
Coyote Creek
 At Interstate Highway 280 3,880 10,180 12,630 14,700
 At U.S. Geological Survey gage near
Edenvale
4,050 10,940 13,670 14,700a
 At U.S. geological Survey gage near
Madrone
4,500 12,000 15,000 24,000
 Downstream of Anderson Reservoir 4,500 11,000 15,000 23,500  Downstream of confluence with
Berryessa Creek
7,300 10,500 12,800 15,000
 Downstream of confluence with Silver
Creek
6,200 10,300 12,500 15,000
 Downstream of Silver Creek
Diversion
4,000 10,680 13,330 14,700
 Upstream of confluence with Fisher
Creek
4,410 12,010 14,830 16,400a
 Upstream of confluence with Silver
Creek
3,790 9,920 11,400a 11,400a
 Upstream of Silver Creek Diversion 4,000 10,680 13,330 14,700

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 279
Flooding Source and Location
Discharge (Cubic Feet/Second)
10-
Year
50-
Year
100-
Year
500-
Year
Daves Creek
 At Los Gatos Creek 130 230 270 370
East Little Llagas Creek
 Approx. 1,500 ft. upstream of
Sycamore Avenue
b B 2,211 b
 At confluence of Church Creek b B 5,355 b
 At confluence of San Martin Creek b B 3,712 b
 At U.S. Highway 101 700 1,200 1,300 1,700
 At Tenant Creek confluence b B 2,881 b
 Upstream of Seymour Avenue 330 430 460 490
East Penitencia Creek
 Downtown of Trimble Road 280 340a 340a 340a
 Upstream of confluence with Lower
Penitencia Creek
480 970h 1,080h 1,280h
 Upstream of Trimble Road 280 400 450 540
Fisher Creek
 At confluence with Coyote Creek 700a 700a 700a 700a
 At Kalana Avenue 470 960 1,130 1,500
 At Miramonte Avenue 300 600 710 930
 At Richmond Avenue 450 700 700 700
 At Willow Springs Road 270 460 560 810
 Downstream of Bailey Avenue 1,000 1,810 2,160 2,950
 Upstream of Bailey Avenue 620 900 900 900
 Upstream of Railroad 1,260 2,310 2,560 3,530
Fisher Creek Overbank
 500 feet downstream of Richmond
Avenue
250 630 900 1,540
 At Bailey Avenue 220b 680 970 1,670
Guadalupe River
 At Blossom Hill Road 3,500 8,500 11,500 19,000
 At Coleman Avenue 7,000 13,500a 15,500a 15,500a
 At Hedding Street 7,500 9,800a 9,800a 9,800a
 At Hobson Avenue 7,000 11,400a 11,400a 11,400a
 At Interstate Highway 280 6,000 7,000a 7,000a 7,000a
 At Malone Road 5,600 11,500 11,900a 11,900a
 At Railroad 5,800 10,900a 10,900a 10,900a

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 280
Flooding Source and Location
Discharge (Cubic Feet/Second)
10-
Year
50-
Year
100-
Year
500-
Year
 Downstream of confluence with
Canoas Creek
5,500 11,000 12,800 12,800
 Downstream of confluence with Los
Gatos Creek
7,000a 10,000a 10,000a 10,000a
 Downstream of confluence with Ross
Creek
4,500 9,000 12,500 20,000
 Downstream of State Highway 17 7,500 12,000a 13,000a 17,000a  Upstream of confluence with Canoas
Creek
4,500 9,500 12,000a 12,000a
Hale Creek
 At Berry Avenue 510 1,020 1,120 1,580
 At confluence with Permanente Creek 710 880 900 960
 At Cuesta Drive/North Springer Road 595 750 760 810
 At Foothill Expressway 460 970 1,060 1,490
 At Interstate Highway 280 101 218 284 440
 At Rosita Avenue 595 700a 700a 700a
 At Summer Hill Avenue 177 370 472 735
Lions Creek
 Upstream of West Branch Llagas
Creek
b b 1,840 b
Llagas Creek
 At Rucker Avenue 4,900i 9,700i 10,200i 12,700i
 At Railroad 2,200 3,900 5,300 8,500
 Downstream of Buena Vista Creek 5,200 10,400 11,000 11,500a  Downstream of Chesbro Reservoir 900 3,100 3,900 6,000  Downstream of East Little Llagas
Creek
5,000 9,800 10,400 12,900
 Downstream of Hayes Creek 1,800 3,800 4,800 7,500
 Downstream of Leavesley Road 5,200d 5,200d 5,200d 5,200d  Downstream of Live Oak Creek 5,500 9,700 9,800 10,300  Downstream of Machado Creek 1,400 3,600 4,500 7,000
 Downstream of Panther Creek 5,300 9,700a 9,800a 10,100a  Downstream of Princevalle Drain b b 18,800 b
 Downstream of West Branch Llagas
Creek
b b 17,800 b
 Upstream of East Little Llagas Creek 2,500 4,300 5,400 8,600  Upstream of Jones Creek b b 18,800 b
 Upstream of Panther Creek 5,200 9,400a 9,400a 9,400a

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 281
Flooding Source and Location
Discharge (Cubic Feet/Second)
10-
Year
50-
Year
100-
Year
500-
Year
Los Gatos Creek
 At Leigh Avenue 1,680 6,510 7,440 11,340
 At Meridian Avenue 1,770 6,620 7,570 11,500
 At Park Road 1,580 6,140 6,990 10,630
 At State Highway 17 1,540k 6,370 7,300 11,200
 Below Lexington Dam 1,610 5,850 6,650 9,630
 Below Vasona Dam 1,550 6,100 6,950 10,600
 Upstream of confluence with
Guadalupe River
2,130 7,000 7,980 11,900
Lower Penitencia Creek
 At Capitol Avenue 740 1,200 1,210 1,220
 At confluence with Berryessa Creek 2,550 3,700 3,700 3,700  At Nimitz Freeway 1,750a 3,500a 3,500a 3,500a
 At Redwood Avenue 850 1,150j 1,150j 1,150j
 At South Main Street 7003 1,120j 1,120j 1,120j
 Downstream of confluence with
Berryessa Creek
2,550 2,600a 2,600a 2,600a
 Downstream of confluence with East
Penitencia Creek
800 1,670 2,150 2,840
 Downstream of Trimble Road 320 1,060h 1,510h 1,620h
Madrone Channel
 At East Dunne Avenue b b 600 b
 Upstream of East Little Llagas Creek b b 1,200 b
Matadero Creek
 Above confluence with Arastradero
Creek
194 392 506 690
 Approximately 270 feet upstream of
U.S. Highway 101
b b 2,800 b
 At Alma Street 1,380 2,000a 2,000a 2,000a
 At corporate limits 402 795 970 1,300
 At El Camino Real 1,100 2,100 2,280 2,690
 At Louis Road 1,380 1,500b 1,500b 1,500b
 At Middlefield Road 1,380 1,900b 1,500b 1,900b
 At Railroad b b 2,435 b
 At U.S. Highway 101 1,660 1,775 1,775 1,775
 Below confluence with Arastradero
Creek
325 660 790 1,030
 Downstream of Foothill Expressway b b 1,900 b

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 282
Flooding Source and Location
Discharge (Cubic Feet/Second)
10-
Year
50-
Year
100-
Year
500-
Year
 Downstream of Park Boulevard b b 2,700 b
 Downstream of U.S. Highway 101 b b 3,100 b
 Upstream of Railroad 1,220 2,170 2,520 2,810
Mayfield Slough
At Embarcadero Road 10.00 b 10.5 10.8
Miller Slough
 At U.S. Highway 101 b b 760 b
Middle Road Overflow Area
 At convergence with Llagas Creek b b 39 b
 At divergence from West Little Llagas
Creek
b b 658 b
North Morey Creek
 Upstream of Lions Creek b b 485 b
Pajaro River
 At U.S. Highway 101 b b 30,500 b
Permanente Creek
 At confluence with Hale Creek 780l 1,650l 1,780l 1,980l  At El Camino Real 1,150 1,310 1,310 1,310
 At Railroad 1,270 1,470 1,600 1,600
 Downstream of confluence with Hale
Creek
1,000a 1,000a 1,000a 1,000a
 Downstream of East Charleston Road 1,390n 1,400a 1,400a 1,400a  Downstream of Miramonte Avenue 370 760 890 1,030
 Downstream of Permanente Road 760 1,260 1,480 1,960
 Downstream of Portland Avenue 1,340 2,050 2,050 2,050  Downstream of U.S. Highway 101 1,350 1,400a 1,400a 1,400a  Upstream of confluence with Hale
Creek
440l 840l 980l 1,110l
 Upstream of Interstate Highway 280 1,250 2,160 2,570 3,480  Upstream of Portland Avenue 1,340 2,220 2,700 3,440
 Upstream of Tributary, 700 feet
upstream of Highway 280
860 1,460 1,720 2,310
 Upstream of U.S. Highway 101 1,350 2,250f 4,000f 7,100f Permanente Diversion
 At confluence with Stevens Creek 1,230 1,280 1,390 1,550  At Grant Road 1,200 1,240a 1,340a 1,490a
 Downstream of Carmel Terrace 1,075a 1,075a 1,075a 1,075a

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 283
Flooding Source and Location
Discharge (Cubic Feet/Second)
10-
Year
50-
Year
100-
Year
500-
Year
 Downstream of Diversion Structure 1,190 1,610 1,610 1,610 Prospect Creek
 Upstream of confluence with
Calabazas Creek
b b 635 b
Purissima Creek
 At corporate limits 147 320 402 588
 At Interstate Highway 280 37 82 104 153
 At Viscaino Road 88 182 227 320
San Francisco Bay
At confluence of Guadalupe Slough
and Coyote Creek
b b 10.8 b
At crossing of Railroad and Alviso
Slough
b b 11.3 B
At Milpitas b b 11.4 B
At Mountain View 10.2 b 10.7 11.0
At Palo Alto 9.9 b 10.5 10.8
At Sunnyvale 3.7 b 10.7 B
San Francisquito Creek
 At Alma Street 4,350 7,050 8,280 9,850a
 At U.S. Geological Survey gage 4,050 6,700 7,860 10,500  Downstream of Chaucer Road 4,350 6,000a 6,000a 6,200a  Downstream of Middlefield Road 4,350 6,350a 6,690a 7,410a  Near Pasteur Drive 4,200 6,850 8,070 10,400
 Upstream of Middlefield Road 4,350 7,100 8,330 9,850a San Francisquito Creek – Overflow
 At Chaucer Street b b 563 B
 At Middlefield Road b b 752 B
 Combined Middlefield/Chaucer
Overflows
b b 1,080 B
San Thomas Aquino Creek
 At Cabrillo Avenue 2,560f 2,920f 2,920f 2,920f
 At confluence with Saratoga Creek 5,900 8,300 9,100 11,000  At El Camino Real 3,570 3,610 3,610 3,610
 At Homestead Road 3,450f 3,450f 3,450f 3,450f
 At Pruneridge Avenue 3,460 3,820f 3,820f 3,820f
 At Saratoga and Los Gatos Roads 620 990 1,140 1,480
 At Stevens Creek Boulevard 3,300 3,820f 3,820f 3,820f