Los Altos Hills City Council
September 19, 2024

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 11: Climate Change 220
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.21F
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

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 11: Climate Change 221
defined as extreme heat days – or days where the maximum temperature is above 92.7 °F.213F 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.214F 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.215F
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.216 F
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 217F 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 residents219 F
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 designation20F 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 County21F 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,2F
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 County23F
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 24F
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 Dams25F
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.26F
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.27F 228
Santa Clara County and its planning partners have established protocols for emergency warning and response through the County’s adopted emergency operations plan28F 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

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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.29F 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.231F 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.23 F 233
Finally, on June 27, 2017, SB 92 required an EAP for all dams that do not have a low downstream hazard potential designation.23F
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 components234F 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.235F
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 $18,400.239 F
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 to240 F 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.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 262
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.241F
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.24F 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 County24F 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 Area246F 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 Study247F 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.248F 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 areas249F 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.250F
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.25F
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 sources253F 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 watersheds25F256:
 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 Events257 F
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, 2003-2022259F 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

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 284
Flooding Source and Location
Discharge (Cubic Feet/Second)
10-
Year
50-
Year
100-
Year
500-
Year
 At U.S. Highway 101 5,900 8,300 9,100 11,000
 At U.S. Highway 237 5,900 8,300 9,100 11,000
 Downstream of Railroad 5,900 8,300 9,100 11,000
 Upstream of Westmont Avenue 2,000 2,900 3,200 4,077o
 Near Bicknell and Quito Roads 670 1,050 1,230 1,580
 Near Old Adobe and Quito Roads 730 1,150 1,350 1,720
Saratoga Creek
 At confluence with San Tomas
Aquino Creek
2,700 3,750 4,100 4,800
 At El Camino Road 2,700 3,750 4,100 4,800
 At Herriman Avenue 1,550 3,020 3,750 4,630
 At Homestead Road 2,700 3,750 4,100 4,800
 At Kiely Boulevard 2,700 3,750 4,100 4,800
 At Stevens Creek Boulevard 2,500 3,500 3,900 4,600
 At U.S. Geological Survey gage at
Springer
1,350 2,750 3,490 4,450
 At Railroad 1,760 3,230 3,950 4,800
 Downstream of Benton Street 2,700 3,750 4,100 4,800
 Downstream of Kiely Boulevard 2,700 3,750 4,100 4,800  Downstream of Warburton Avenue 2,700 3,750 4,100 4,800 Silver Creek
 At confluence with Coyote Creek 2,550 2,650 2,670 2,750  At intersection of King and McKee
Roads
2,000a 2,000a 2,000a 2,000a
 At Interstate Highway 680 2,210 2,400 2,400 2,400
 At Ocala Avenue 1,530 2,000p 2,000p 2,000p
 Downstream of confluence with
Thompson Creek
2,080 3,200 3,600 4,300
 Downstream of Cunningham Avenue 1,420p 2,150p 2,580p 2,600p  Downstream of confluence with
Miguelita Creek
2,300 2,300 2,300 2,300
 Downstream of confluence with North
Babb Creek
1,500a 1,500a 1,500a 1,500a
 Downstream of confluence with South
Babb Creek
1,940 2,600 2,700 2,700
Smith Creek
 At Railroad 200 370 440 610
 At Wedgewood Avenue 160 300 350 480

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 285
Flooding Source and Location
Discharge (Cubic Feet/Second)
10-
Year
50-
Year
100-
Year
500-
Year
 Below Smith Creek Drive 125 230 280 390
South Babb Creek
 At Clayton Road 390 760 890 1,150
 At confluence with Silver Creek 200a 200a 200a 200a
 Downstream of White Road 390a 390a 390a 390a
 Upstream of Clayton Road B b 890 B
 Upstream of Lochner Drive 400 550a 550a 550a
 Upstream of White Road 400 570a 570a 570a
South Morey Creek
 Upstream of Lions Creek B b 420 B
Stevens Creek
 At Crittenden Lane 2,350g 2,350g 2,350g 2,350g
 At Homestead Road 1,110m 4,530 5,570 7,470
 At Interstate Highway 280 1,110m 4,460 5,460 7,310
 At Stevens Creek Boulevard 1,110m 4,430m 5,430 7,240
 At U.S. Geological Survey gaging
station No. 262
1,200 2,800 5,400 7,000
 At U.S. Highway 101 3,030 5,550 5,750 5,950
 Downstream of Interstate Highway
280
1,110 4,460 5,460 7,310
 Downstream of Junipero Serra 1,550 3,200 5,580 7,650
 Downstream of Stevens Creek Dam 1,140 4,440 5,280 6,940  Downstream of Railroad 2,750 5,350g 5,350g 5,350g
 Upstream of Junipero Serra 1,500 3,150 5,500 7,500
 Upstream of Permanente Diversion 1,750 3,600 6,000 8,200  Upstream of Railroad 2,750 6,110 7,360 9,610
Sunnyvale East Channel
 Downstream of Caribbean Drive B b 1,100 B
Sunnyvale West Channel
 Downstream of Highway 237 B b 360 B
Tennant Creek
 Approximately 1,250 feet upstream of
Hill Avenue
B b 420 B
 Downstream of Maple Avenue B b 650 B
 Upstream of confluence with East
Little Llagas Creek
B b 2,015 B

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 286
Flooding Source and Location
Discharge (Cubic Feet/Second)
10-
Year
50-
Year
100-
Year
500-
Year
Thompson Creek
 2,000 feet downstream of Aborn
Road
1,440 2,550 3,000 3,700
 At Aborn Road 1,440 2,350 2,700 3,250
 At Quimby Road 1,480 1,900a 1,900a 1,900a
 Downstream of Yerba Buena Creek 1,060 1,750 1,950 2,400 Upper Penitencia Creek
 At Capitol Avenue 1,350a 1,350a 1,350a 1,350a
 At confluence with Coyote Creek 1,110 1,110 1,110 1,110  At Gridley Street 1,460 3,050 3,600 4,950
 Upstream of North Jackson Avenue 1,350a 1,350a 1,350a 1,350a  At King Road 960a 960a 960a 960b
 At Mabury Avenue 1,050a 1,050a 1,050a 1,050a
 At Upper Penitencia Road 1,460 2,810a 2,950a 2,950a
 At U.S. Geological survey gage at
Dorel Road
1,400 2,940 3,600 5,170
Uvas Creek
 At confluence with Bodfish Creek B b 10,910 B
 At confluence with Little Arthur Creek B b 8,500 B
 At downstream face of Watsonville
Road Bridge
B b 10,360 B
 At Thomas Road B b 14,000 B
 At Railroad B b 5,2003 B
 At U.S. Highway 101 B b 8,0003 B
 At Uvas Road B b 7,800 B
 Downstream of Hecker Pass Road B b 13,550 B
 Downstream of Santa Teresa
Boulevard
B b 14,000 B
Uvas Creek – East Overbank Above Highway 101
 Approximately 1,200 feet above U.S.

Highway 101
Q b 2,200 B
 At U.S. Highway 101 Q b 1,100 B
Uvas Creek – East Overbank Above Railroad
 At downstream limit of flooding Q b 3,200 B
 At upstream limit of flooding Q b 2,100 B
Watson Road Overflow Area
 At convergence with Llagas Creek B b 447 B

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 287
Flooding Source and Location
Discharge (Cubic Feet/Second)
10-
Year
50-
Year
100-
Year
500-
Year
 At divergence from West Little Llagas
Creek
B b 97 B
West Branch Llagas Creek
 Downstream of divergence from West
Branch Llagas Creek – East Split
B b 160 B
 Upstream of divergence from West
Branch Llagas Creek – East Split
B b 1,400 B
West Branch Llagas Creek – Lower Split
 At Day Road Interceptor (NRCS
PL566)
Q b 1,200 B
West Branch Llagas Creek – Middle Split
 Downstream of Highland Avenue Q q 80 Q
West Branch Llagas Creek – Upper Split
 Upstream of Highland Avenue Q q 200 Q
West Little Llagas Creek
 1,000 feet upstream of Wright
Avenue
A a 1882 A
 At Fourth Street A a 9002 A
 At U.S. Highway 101 A a 1,080b A
 Downstream of Edmundson Avenue A a 1,269 A
 Downstream of Monterey Highway A a 8132 A
 Downstream of Railroad A a 4602 A
 Upstream of Llagas Avenue A a 1,702b A
 Upstream of Monterey Highway A a 1,936 A
 Upstream of Seymour Avenue A a 1,770b A
Wildcat Creek
 Above Portos Drive 480 810 960 1,230
 At Saratoga and Los Gatos Roads 310 500 570 740
 Below Douglas Lane 430 710 840 1,070
a Decrease in flow rate based on capacity restrictions. b Data not available/computed.
c Discharge decrease due to Barron Creek Diversion.
d Flow rate accounts for upstream channel spills.
e Slow rate reflects upstream capacity restriction.
f Flow influenced by spill from adjoining watercourse.
g Flow reduction due to bridge or channel capacity restriction. h Increase in flow rate due to spills from neighboring subbasins. i Flow rate reduction due to attenuation in the floodplain. j Reduction in flood rate due to storage behind railroad. k Flow rate reduction due to attenuation in
reservoirs.
l High flows affected by Permanente Diversion.
m Decrease in flow rate due to storage along
channel.
n High flows diverted to Stevens Creek.
o Logarithm extrapolation.
p Flow rate reduction due to storage in Lake
Cunningham.
q Flooding due to spill—drainage area not
applicable.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 288

13.2.7 Warning Time
Advanced warning is essential for quick and effective response to a flood threat. Because of the sequential pattern of weather conditions needed to cause serious flooding, occurrence of a flood without warning is unusual. Warning times for floods can be between 24 and 48 hours. Flash flooding can be less predictable, but populations in potential hazard areas can be warned in advance of flash flooding danger.

NWS issues watches and warnings when forecasts indicate rivers may approach bank-full levels. Flood extent or severity categories used by NWS include minor flooding, moderate flooding, and major flooding, based on property damage and public threat261F
262:
 Minor Flooding: Minimal or no property damage, but possibly some public threat or inconvenience.
 Moderate Flooding: Some inundation of structures and roads near streams. Some necessary evacuations of people and/or transfer of property to higher elevations.  Major Flooding: Extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations.

When a watch is issued, the public should prepare for the possibility of a flood. When a warning is issued, the public is advised to stay tuned to a local radio station for further information and be prepared to take quick action if needed. A warning means a flood is imminent, generally within 12 hours, or is occurring.

Local media broadcast NWS warnings. Thresholds for flood warnings have been established on some of the major rivers in Santa Clara County, based on available stream gage information. Current stream flows are gathered from the following USGS stream gauges in the county26F 263:
 USGS 11153000 Pacheco Creek, Dunneville, CA
 USGS 11153650 Llagas Creek, Gilroy, CA
 USGS 11164500 San Francisquito Creek, Stanford University  USGS 11169025 Guadalupe River along Highway 101, San José, CA  USGS 11169500 Saratoga Creek, Saratoga, CA
 USGS 11169800 Coyote Creek, Gilroy, CA
 USGS 11173200 Arroyo Hondo, San José CA

13.3 Cascading Impacts
One of the most problematic cascading impacts of flooding is bank or coastal erosion, which in some cases can be more harmful than the actual flood. This is especially true in the upper courses of rivers with steep gradients, where floodwaters may pass quickly and without much damage, but scour the banks, edging properties closer to the floodplain or causing them to fall in.

Flooding is also responsible for hazards such as landslides or mud flows when high flows over-saturate soils on steep slopes, causing them to fail. In California, there are significant ties between wildfire, floods, and subsequently landslides as one hazard cascades into the next. 262 National Weather Service. (n.d.). Severe Weather 101 – Floods. https://www.nssl.noaa.gov/education/svrwx101/floods/faq/ 263 U.S. Geological Survey. (2023). Current Conditions for California: Streamflow. https://waterdata.usgs.gov/ca/nwis/current/?type=flow

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 289
Hazardous materials spills are also a secondary hazard of flooding if storage tanks rupture and spill into streams, rivers, or storm sewers. Debris of any kind could impact water quality and change the natural flow of water. This could potentially increase the risk next time there is a storm event.

Floods could also cause dam failure, or overtopping. The risk of dam failure is described more in Section 12. This risk is of particular concern because it may create a flood substantially larger than the original flood. Depending on the topography of the area downstream of a dam failure event, the floodwaters could remain constrained in a narrow canyon area, preventing them from slowing down before the reach urbanized flatter terrain.263F
264 Furthermore, while floodwaters will travel down and absorb into the ground in flat areas, the water will also pick up sediment and debris as it travels. The sediment and debris in the water may pose additional risks.

13.4 Exposure
The Level 2 Hazus protocol was used to assess flood risk in the OA. The model used census data at the block level and FEMA floodplain data, which has a level of accuracy acceptable for planning purposes.

Where possible, the Hazus default data was enhanced using local GIS data from local, state, and federal sources.

13.4.1 Population
Population counts of those living in the floodplain within the OA were generated by estimating percent of residential buildings in each jurisdiction within the 1-percent-annual-chance flood hazard areas and multiplying this by total population within the OA. This approach yielded an estimated population in the OA of 176,882 living within the 100-year floodplain (9.14% percent of the total OA population).Table 88 lists population estimates by jurisdiction living in the 10-percent, 1-percent and 0.2-percent annual chance flood hazard areas. The City of Gilroy, City of Santa Clara, Sunnyvale, San José, Campbell, Milpitas, and Morgan hill have the highest percentage of population exposed to 1% annual flood hazard, Table 88: Population Within the 10-Percent, 1-Percent, and 0.2-Percent Annual Chance Flood Hazard Areas
Jurisdiction
10-Percent Annual
Chance Flood Hazard
Area
1-Percent Annual
Chance Flood Hazard
Area

0.2-Percent Annual
Chance Flood Hazard
Area
Population
Exposeda
% Total of
Population
Population
Exposeda
% of Total
Population
Population
Exposeda
% of Total
Population
Campbell 2446 5.63 3685 8.48 4608 10.61
Cupertino 0 0.00 1311 2.16 1740 2.87
Gilroy 5,742 9.65 10,361 17.42 13,398 22.53
Los Altos 0 0.00 0 0.00 11 0.03
Los Altos Hills 0 0.00 0 0.00 0 0.00
Los Gatos 273 0.81 1,216 3.63 1,485 4.43
264 Santa Clara Valley Water District. (2017, October). Local Hazard Mitigation Plan. https://s3.us-west1.amazonaws.com/valleywater.org.us-west-1/s3fs-public/2021- 05/R14163%20%202017%20FINAL%20LOCAL%20HAZARD%20MITIGATION%20PLAN%20v.%2004-09- 21%20(04-12-21).pdf

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 290
Jurisdiction
10-Percent Annual
Chance Flood Hazard
Area
1-Percent Annual
Chance Flood Hazard
Area

0.2-Percent Annual
Chance Flood Hazard
Area
Population
Exposeda
% Total of
Population
Population
Exposeda
% of Total
Population
Population
Exposeda
% of Total
Population
Milpitas 2,691 3.35 5,335 6.65 9,457 11.78
Monte Sereno 0 0.00 0 0.00 0 0.00
Morgan Hill 1,758 3.90 2,979 6.61 3,412 7.58
Mountain View 1,687 2.05 4,063 4.95 6,525 7.95
Palo Alto 617 0.90 629 0.92 4,534 6.62
San José 52,257 5.15 111,750 11.02 190,009 18.74
Santa Clara (city) 10,239 8.02 16,644 13.04 35,182 27.57 Saratoga 587 1.89 1,487 4.79 1,790 5.77
Sunnyvale 2,786 1.80 13,969 9.02 21,284 13.75
Unincorporated
County 2,241 2.48 3,453 3.83 4,579 5.07
Total 83,324 4.31 176,882 9.14 298,014 15.40

13.4.2 Property
Exposed Value
Table 89, Table 90, and Table 91 summarize the estimated value of exposed buildings in the OA. This methodology estimated $8.7 billion worth of exposure to the 10-percent-annual-chance flood, representing 2.3 percent of the total replacement value of the OA, $22.4 billion worth of building-andcontents exposure to the 1-percent-annual-chance flood, representing 9.1 percent of the total replacement value of the OA, and $40 billion worth of building-and-contents exposure to the 0.2-percentannual-chance flood, representing 10.56 percent of the total.

Table 89: Value of Structures in the 10-Percent Annual Chance Flood Hazard Area Jurisdiction
Estimated Value within the Floodplain % of Total
Replacement
Structure Contents Total Value
Campbell $46,559,000 $43,284,000 $140,698,000 1.60
Cupertino $0 $0 $0 0.00
Gilroy $96,223,000 $165,308,000 $574,813,000 4.86
Los Altos $0 $0 $0 0.00
Los Altos Hills $0 $0 $0 0.00
Los Gatos $16,191,000 $22,610,000 $109,895,000 1.14
Milpitas $44,056,000 $47,811,000 $224,231,000 1.40
Monte Sereno $0 $0 $0 0.00
Morgan Hill $30,975,000 $18,233,000 $67,576,000 0.63
Mountain View $22,645,000 $33,678,000 $140,380,000 0.83

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 291
Jurisdiction
Estimated Value within the Floodplain % of Total
Replacement
Structure Contents Total Value
Palo Alto $7,892,000 $163,325,000 $91,985,000 0.45
San José $1,187,925,000 $2,015,872,000 $6,140,963,000 3.56 Santa Clara (city) $178,713,000 $233,150,000 $766,887,000 2.77 Saratoga $21,643,000 $16,633,000 $51,942,000 0.56
Sunnyvale $41,315,000 $68,059,000 $216,544,000 0.74
Unincorporated County $67,392,000 $67,064,000 $255,548,000 1.25 Total $1,761,529,000 $2,895,027,000 $8,781,462,000 2.30 Table 90: Value of Structures in the 1-Percent Annual Chance Flood Hazard Area Jurisdiction
Estimated Value within the Floodplain % of Total
Replacement
Structure Contents Total Value
Campbell $91,932,000 $80,508,000 $257,047,000 8.48
Cupertino $24,021,000 $18,081,000 $59,916,000 2.16
Gilroy $225,196,000 $420,220,000 $1,346,610,000 17.42
Los Altos $1,351,000 $710,000 $2,234,000 0.00
Los Altos Hills $0 $0 $0 0.00
Los Gatos $30,840,000 $42,934,000 $168,827,000 3.63
Milpitas $115,342,000 $137,652,000 $548,317,000 6.65
Monte Sereno $0 $0 $0 0.00
Morgan Hill $115,482,000 $81,114,000 $259,011,000 6.61
Mountain View $75,707,000 $109,233,000 $416,222,000 4.95 Palo Alto $18,739,000 $40,559,000 $157,789,000 0.92
San José $3,271,469,000 $5,761,915,000 $15,986,581,000 11.02 Santa Clara
(city)
$360,286,000 $476,086,000 $1,462,841,000 13.04
Saratoga $100,151,000 $73,527,000 $226,648,000 4.79
Sunnyvale $208,800,000 $260,543,000 $832,836,000 9.02
Unincorporated
County
$157,862,000 $178,277,000 $682,246,000 3.83
Total $4,797,178,000 $7,681,359,000 $22,407,125,000 9.14

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 292
Table 91: Value of Structures in the 0.2-Percent Annual Chance Flood Hazard Area Jurisdiction
Estimated Value within the Floodplain % of Total
Replacement
Structure Contents Total Value
Campbell $127,488,000 $110,733,000 $355,863,000 4.04
Cupertino $36,263,000 $29,154,000 $94,619,000 0.66
Gilroy $319,375,000 $641,660,000 $2,015,006,000 17.03
Los Altos $1,791,000 $948,000 $3,090,000 0.04
Los Altos Hills $0 $0 $0 0.00
Los Gatos $39,193,000 $56,508,000 $212,717,000 2.21
Milpitas $191,075,000 $242,248,000 $934,245,000 5.84
Monte Sereno $0 $0 $0 0.00
Morgan Hill $161,794,000 $109,502,000 $350,641,000 3.25 Mountain View $119,699,000 $166,238,000 $669,226,000 3.95 Palo Alto $76,861,000 $100,127,000 $367,056,000 1.81
San José $5,656,110,000 $10,606,664,000 $28,947,287,000 16.79 Santa Clara
(city)
$812,021,000 $1,177,901,000 $3,760,129,000 13.60
Saratoga $127,579,000 $96,493,000 $289,244,000 3.09
Sunnyvale $351,074,000 $419,774,000 $1,361,761,000 4.64 Unincorporated
County
$225,387,000 $262,239,000 $929,177,000 4.53
Total $8,245,710,000 $14,020,189,000 $40,290,061,000 10.56

13.4.3 Critical Facilities and Infrastructure
Table 92, Table 93, and Table 94 summarize the critical facilities and infrastructure in the 10-, 1-, and

0.2-percent-annual-chance flood hazard areas. Details are provided in the following sections.

Toxic Release Inventory Reporting Facilities
Toxic Release Inventory (TRI) facilities are known to manufacture, process, store, or otherwise use certain chemicals above minimum thresholds. If damaged by a flood, these facilities could release chemicals that cause cancer or other human health effects, significant adverse acute human health effects, or significant adverse environmental effects.264F 265 During a flood event, containers holding these
materials can rupture and leak into the surrounding area, disastrously affecting the environment and residents. Seventy-eight facilities within the 1-percent-annual-chance flood zone are TRI reporting facilities.
265 Environmental Protection Agency. (n.d.). Flooding. https://www.epa.gov/natural-disasters/flooding

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 293
Table 92: Critical Facilities in the 10-Percent Annual Chance Flood Hazard Area Jurisdiction
Number of Facilities in the Floodplain
Essential
Facilities Transportation Utilities Community Assets
Hazardous
Materials Total
Campbell 0 1 0 1 0 2
Cupertino 0 0 0 0 0 0
Gilroy 1 8 0 1 0 10
Los Altos 0 0 0 0 0 0
Los Altos Hills 0 0 0 0 0 0
Los Gatos 0 6 0 0 0 6
Milpitas 2 2 0 1 1 6
Monte Sereno 0 0 0 0 0 0
Morgan Hill 0 0 0 1 0 1
Mountain View 0 0 0 1 2 3
Palo Alto 0 1 0 0 3 4
San José 17 50 1 19 37 124
Santa Clara (city) 3 2 0 2 9 16
Saratoga 0 3 0 0 0 3
Sunnyvale 1 1 0 0 2 4
Unincorporated
County 0 32 0 3 0 35
Total 24 106 1 29 54 214
Table 93: Critical Facilities in the 1-Percent Annual Chance Flood Hazard Area Jurisdiction
Number of Facilities in the Floodplain
Essential
Facilities Transportation Utilities Community Assets
Hazardous
Materials Total
Campbell 1 1 0 2 0 4
Cupertino 0 2 0 1 0 3
Gilroy 5 15 0 5 1 26
Los Altos 0 0 0 0 0 0
Los Altos Hills 0 0 0 0 0 0
Los Gatos 0 7 0 2 0 9
Milpitas 3 10 0 2 1 16
Monte Sereno 0 0 0 0 0 0
Morgan Hill 0 1 0 1 0 2
Mountain View 1 1 0 3 3 8

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 294
Jurisdiction
Number of Facilities in the Floodplain
Essential
Facilities Transportation Utilities Community Assets
Hazardous
Materials Total
Palo Alto 0 1 0 0 4 5
San José 35 87 0 38 48 208
Santa Clara (city) 5 2 3 4 18 32
Saratoga 0 7 0 1 0 8
Sunnyvale 4 2 0 2 3 11
Unincorporated
County 4 49 0 3 0 56
Total 58 185 3 64 78 388
Table 94: Critical Facilities in the 0.2-Percent Annual Chance Flood Hazard Area Jurisdiction
Number of Facilities in the Floodplain
Essential
Facilities Transportation Utilities Community Assets
Hazardous
Materials Total
Campbell 1 1 0 2 0 4
Cupertino 0 4 0 1 0 5
Gilroy 7 16 0 6 1 30
Los Altos 0 0 0 0 0 0
Los Altos Hills 0 0 0 0 0 0
Los Gatos 0 8 0 2 0 10
Milpitas 4 11 0 3 3 21
Monte Sereno 0 0 0 0 0 0
Morgan Hill 1 1 0 1 0 3
Mountain View 2 1 0 4 3 10
Palo Alto 2 3 1 0 5 11
San José 52 125 7 60 62 306
Santa Clara (city) 8 7 2 7 51 75
Saratoga 1 11 0 1 0 13
Sunnyvale 7 2 0 3 6 18
Unincorporated
County 4 54 0 4 0 62
Total 89 244 10 94 131 568

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 295
Utilities and Infrastructure
It is important to determine who may be at risk if infrastructure is damaged by flooding. Roads or railroads that are blocked or damaged can isolate residents and can prevent access throughout the OA, including for emergency service providers needing to get to vulnerable populations or to make repairs. Bridges washed out or blocked by floods or debris also can cause isolation. Water and sewer systems can be flooded or backed up, causing health problems. Underground utilities can be damaged. Dikes can fail or be overtopped, inundating the land that they protect. The following sections describe specific types of critical infrastructure.

Roads
The following major roads in the OA pass through the 1-percent-annual-chance flood zone and thus are exposed to flooding:
 US 101
 Interstate 280
 Interstate 680
 Interstate 880
 State Route 9
 State Route 17
 State Route 82
 State Route 85
 State Route 87
 State Route 152
 State Route 237
Some of these roads are built above the flood level, and others function as levees to prevent flooding.

Still, in severe flood events these roads can be blocked or damaged, preventing access to some areas.

Infrastructure Lifelines
Flooding events can significantly impact critical infrastructure lifelines such as highways, bridges, airports, water and wastewater facilities and communication facilities. An analysis showed that there are 629 infrastructure lifelines (241 are bridges) that are in or cross over the 1-percent-annual-chance flood zone and 928 infrastructure lifelines in the 0.2-percent-annual-chance flood zone.

Water and Sewer Infrastructure
Water and sewer systems can be affected by flooding. Floodwaters can back up drainage systems, causing localized flooding. Culverts can be blocked by debris from flood events, also causing localized urban flooding. Floodwaters can get into drinking water supplies, causing contamination. Sewer systems can be backed up, causing wastewater to spill into homes, neighborhoods, rivers, and streams.

Levees
SCVWD constructed flood protection levees in the north, central, and southern portions of the county, some of which provide 1-percent-annual-chance flood protection. The levees along Uvas Creek, King Creek, Lyons Creek, and Coyote Creek participate in Corps’ Levee Program. Levees along the Guadalupe River do not participate. SCVWD does not believe the majority of levees could withstand intensities of a 1-percent annual chance flood. Additionally, coastal flooding from San Francisco Bay circumvents levees near the Bay. Moreover, current flood levels do not account for potential sea level rise, which would exacerbate vulnerability and further reduce the ability of the levees to prevent or reduce flooding.

The presence and effects of levee systems in the Santa Clara County OA are not reflected on the FIRM, meaning that areas, structures, and populations vulnerable to failures of those levees cannot be determined. Levee failures could place large numbers of people and great amounts of property at risk.

Unlike dams, levees do not serve any purpose beyond providing flood protection and (less frequently) recreational space for residents. A levee failure could be devastating, depending on severity of flooding and amount of land development present. In addition to damaging buildings, infrastructure, trees, and other large objects, levee failure can result in significant water quality and debris disposal issues. Severe erosion is also a consideration.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 296

13.4.4 Environment
Loss estimation platforms such as Hazus are not currently equipped to measure environmental impacts of flood hazards. The best gauge of vulnerability of the environment would be a review of damage from past flood events. Loss data that segregates damage to the environment was not available at the time of this plan. Capturing this data from future events could be beneficial in measuring the vulnerability of the environment for future updates. Habitats that are expected to be impacted by riverine flooding are shown in Table 95.

Table 95: Habitats Expected to Be Impacted by Riverine Flooding265F 266
Habitat Type FEMA 100-Year Storm
Riverine Flooding
Bay Wetlands: Coastal Salt Marsh and Coastal Brackish Marsh 1,695 acres (84%) Freshwater Wetland 2,350 acres (64%)
Grassland 3,176 acres (2%)
Riparian and Riverine 546 acres (19%)
Freshwater Lake and Pond 1,792 acres (57%)
Chaparral and Scrubland 358 acres (0.3%)
Coastal Scrubland 9 acres (0.2%)
Coniferous Forest 20 acres (0.2%)
Hardwood Forest 314 acres (0.5%)
Oak Woodland 1,341 ac (0.7%)
Redwood Forest 40 ac (0.3%)
Note: Given that vulnerability to riverine flooding is currently determined by FEMA Flood Insurance Rate Maps (FIRMS), it is possible that the flooding near the Bay could be occurring from either storm surge or riverine flooding.

Specific strategies for each will be required once source confirmation is made. Habitats with vulnerability of less than 1% were not considered highly vulnerable.
266 County of Santa Clara Office of Sustainability and Climate Action. (2015, August). Silicon Valley 2.0 Climate Adaptation Guidebook.
https://sustainability.sccgov.org/sites/g/files/exjcpb976/files/documents/1_150803_Final%20Guidebook_W_Appendic es.pdf

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 297
Figure 65: 100-Year Floodplain Area Land Cover26F
267

13.5 Vulnerability
Changes in Conditions: No change in vulnerability
 The areas vulnerable to 1% and 0.2% flood events have as mapped in FIRMs have not changed.  Population growth and building growth in the OA have been slow.  Flooding can occur in localized areas not identified in special flood hazard areas (SFHAs) during heavy rain. These are more difficult to anticipate in advance and areas at risk may not yet have been recognized.
 New development may alter local drainage patterns and change flood potential. 267 County of Santa Clara Office of Sustainability and Climate Action. (2015, August). Silicon Valley 2.0 Climate Adaptation Guidebook.
https://sustainability.sccgov.org/sites/g/files/exjcpb976/files/documents/1_150803_Final%20Guidebook_W_Appendic es.pdf

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 298
 Warming temperature from climate change may alter the frequency and intensity of storms and increase the associated runoff.

13.5.1 Population
Vulnerable Populations
A geographic analysis of demographics using the Hazus model identified populations vulnerable to the flood hazard as follows:
 Economically Disadvantaged Populations: It is estimated that 2.76 percent of the people within the 100-year floodplain are economically disadvantaged, defined as having household incomes of $20,000 or less.
 Population over 64 Years Old: It is estimated that 20,723 persons or 11.7 percent of the population in the census blocks that intersect the 100-year floodplain are over 64 years old.  Population under 16 Years Old: It is estimated that 38,185 persons or 19.9 percent of the population within census blocks located in or near the 100-year floodplain are under 18 years of age.

Commuters and visitors are also vulnerable to the flood hazard, in part because they may not be as familiar with evacuation routes and areas that typically flood. Commuters whose workplaces or major transportation routes are in or near the 1-percent-annual-chance flood zone may be especially vulnerable.

The most at-risk members of society often experience the greatest losses from disasters. Socially vulnerable populations often live in high-risk floodplains due to lack of affordable housing, historical inequitable land use and housing practices, and other societal barriers. Disasters can exacerbate preexisting racial and social disparities. According to Plan Bay Area 2050, homes in Equity Priority Communities may be over 50% more likely to experience flooding from sea level rise.267F 268
Estimated Impacts on Persons and Households
Impacts on persons and households in the OA were estimated for the 10-, 1-, and 0.2-percent-annualchance flood events through the Level 2 Hazus analysis. Table 96 summarizes the results.

Table 96: Estimated Flood Impact on Persons
Jurisdiction
Number of Displaced Persons Number of Persons Requiring Short-Term Shelter
10%
Annual
Chance
Flood
1%
Annual
Chance
Flood

0.2%
Annual
Chance
Flood
10% Annual
Chance
Flood
1% Annual
Chance
Flood

0.2%
Annual
Chance
Flood
Campbell 2,297 3,796 5,047 106 204 232
Cupertino 0 1,078 1,543 0 199 284
Gilroy 5,343 9,782 13,209 304 443 581
Los Altos 0 11 21 0 1 1
Los Altos Hills 0 0 0 0 0 0
268 Plan Bay Area. (2021, October). Final Plan Bay Area 2050. https://www.planbayarea.org/finalplan2050

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 299
Jurisdiction
Number of Displaced Persons Number of Persons Requiring Short-Term Shelter
10%
Annual
Chance
Flood
1%
Annual
Chance
Flood

0.2%
Annual
Chance
Flood
10% Annual
Chance
Flood
1% Annual
Chance
Flood

0.2%
Annual
Chance
Flood
Los Gatos 571 980 1140 48 53 85
Milpitas 4,288 8,115 13,191 304 451 592
Monte Sereno 0 0 0 0 0 0
Morgan Hill 1,424 2,700 3,304 32 32 38
Mountain View 1,505 4,056 6,842 51 244 336
Palo Alto 313 392 3,872 58 72 137
San José 54,856 115,365 192,561 3,879 7,005 11,625
Santa Clara (city) 10,888 17,583 35,621 456 678 1,161
Saratoga 454 1,738 2,022 44 67 73
Sunnyvale 3,054 12,386 21,405 85 448 730
Unincorporated
County
1,965 3,522 4,839 90 123 149
Total 86,958 181,504 304,617 5,457 10,020 16,024
Public Health and Safety
Floods and their aftermath present numerous threats to public health and safety:
 Unsafe food: Floodwaters contain disease-causing bacteria, dirt, oil, human and animal waste, and farm and industrial chemicals. Their contact with food items, including food crops in agricultural lands, can make that food unsafe to eat. Refrigerated and frozen foods are affected during power outages caused by flooding. Foods in cardboard, plastic bags, jars, bottles, and paper packaging may be unhygienic with mold contamination.  Contaminated drinking and washing water and poor sanitation: Flooding impairs clean water sources with pollutants. The pollutants also saturate into the groundwater. Flooded wastewater treatment plants can be overloaded, resulting in backflows of raw sewage. Private wells can be contaminated by floodwaters. Private sewage disposal systems can become a cause of infection if they or overflow.
 Mosquitoes and animals: Floods provide new breeding grounds for mosquitoes in wet areas and stagnant pools. The public should dispose of dead animals that can carry viruses and diseases only in accordance with guidelines issued by local animal control authorities.

Leptospirosis—a bacterial disease associated predominantly with rats—often accompanies floods in developing countries, although the risk is low in industrialized regions unless cuts or wounds have direct contact with disease-contaminated floodwaters or animals.  Mold and mildew: Excessive exposure to mold and mildew can cause flood victims—especially those with allergies and asthma—to contract upper respiratory diseases, triggering cold-like symptoms. Molds grow in as short a period as 24 to 48 hours in wet and damp areas of buildings and homes that have not been cleaned after flooding, such as water-infiltrated walls, floors, carpets, toilets, and bathrooms. Very small mold spores can be easily inhaled by human bodies and, in large enough quantities, cause allergic reactions, asthma episodes, and other respiratory problems. Infants, children, elderly people, and pregnant women are considered most vulnerable to mold-induced health problems.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 300
 Carbon monoxide poisoning: In the event of power outages following floods, some people use alternative fuels for heating or cooking in enclosed or partly enclosed spaces, such as small gasoline engines, stoves, generators, lanterns, gas ranges, charcoal, or wood. Built-up carbon monoxide from these sources can poison people and animals.  Hazards when reentering and cleaning flooded homes and buildings: Flooded buildings can pose significant health hazards to people entering them. Electrical power systems can become hazardous. Gas leaks can trigger fire and explosion. Flood debris—such as broken bottles, wood, stones, and walls—may cause injuries to those cleaning damaged buildings. Containers of hazardous chemicals may be buried under flood debris. Hazardous dust and mold can circulate through a building and be inhaled by those engaged in cleanup and restoration.  Mental stress and fatigue: People who live through a devastating flood can experience longterm psychological impact. The expense and effort required to repair flood-damaged homes places severe financial and psychological burdens on the people affected. Post-flood recovery can cause, anxiety, anger, depression, lethargy, hyperactivity, and sleeplessness. There is also a long-term concern among the affected that their homes can be flooded again in the future.

Current loss estimation models such as Hazus are not equipped to measure public health impacts such as these. The best preparation for these effects includes awareness that they can occur, education of the public on prevention, and planning to deal with them during responses to flood events.

13.5.2 Property
Structures and Contents
Hazus calculates losses to structures from flooding by looking at depth of flooding and type of structure.

Using historical flood insurance claim data, Hazus estimates the percentage of damage to structures and their contents by applying established damage functions to an inventory. The analysis is summarized in Table 97, Table 98, and Table 99 for the 10-, 1-, and 0.2-percent-annual-chance flood events, respectively.

Table 97: Loss Estimates for 10-Percent-Annual-Chance Flood Jurisdiction Structures
Impacteda
Estimated Loss Associated with Flood % of Total
Replacement
Structure Contents Total Value
Campbell 278 $46,559,000 $43,284,000 $140,698,000 1.60
Cupertino 0 $0 $0 $0 0
Gilroy 382 $96,223,000 $165,308,000 $574,813,000 4.86
Los Altos 0 $0 $0 $0 0
Los Altos Hills 0 $0 $0 $0 0
Los Gatos 17 $16,191,000 $22,610,000 $109,895,000 1.14
Milpitas 234 $44,056,000 $47,811,000 $224,231,000 1.40
Monte Sereno 0 $0 $0 $0 0
Morgan Hill 373 $30,975,000 $18,233,000 $67,576,000 .63 Mountain View 112 $22,645,000 $33,678,000 $140,380,000 .83 Palo Alto 40 $7,892,000 $163,325,000 $91,985,000 .45
San José 4,541 $1,187,925,000 $2,015,872,000 $6,140,963,000 3.56

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 301
Jurisdiction Structures
Impacteda
Estimated Loss Associated with Flood % of Total
Replacement
Structure Contents Total Value
Santa Clara
(city)
918 $178,713,000 $233,150,000 $766,887,000 2.77
Saratoga 74 $21,643,000 $16,633,000 $51,942,000 .56
Sunnyvale 245 $41,315,000 $68,059,000 $216,544,000 .74
Unincorporated
County
104 $67,392,000 $67,064,000 $255,548,000 1.25
Total 7,318 $1,761,529,000 $2,895,027,000 $8,781,462,000 2.30 Note: Values shown are accurate for comparison of results in this plan. See Section 6 for discussion of data limitations.
a
Impacted structures are those with finished floor elevations below the flood event water surface elevation. These structures are the most likely to receive significant damage in a flood event.

Table 98: Loss Estimates for 1-Percent-Annual-Chance Flood Jurisdiction Structures
Impacteda
Estimated Loss Associated with Flood % of Total
Replaceme
nt
Value Structure Contents Total
Campbell 478 $91,932,000 $80,508,000 $257,047,000 2.92
Cupertino 61 $24,021,000 $18,081,000 $59,916,000 .42
Gilroy 1,116 $225,196,000 $420,220,000 $1,346,610,000 11.38 Los Altos 0 $1,351,000 $710,000 $2,234,000 .03
Los Altos Hills 0 $0 $0 $0 0.00
Los Gatos 131 $30,840,000 $42,934,000 $168,827,000 1.76 Milpitas 797 $115,342,000 $137,652,000 $548,317,000 3.43 Monte Sereno 0 $0 $0 $0 0
Morgan Hill 722 $115,482,000 $81,114,000 $259,011,000 2.4 Mountain View 414 $75,707,000 $109,233,000 $416,222,000 2.46 Palo Alto 70 $18,739,000 $40,559,000 $157,789,000 .79
San José 12,496 $3,271,469,000 $5,761,915,000 $15,986,581,000 9.27 Santa Clara
(city)
1,999 $360,286,000 $476,086,000 $1,462,841,000 5.29
Saratoga 276 $100,151,000 $73,527,000 $226,648,000 2.42 Sunnyvale 1,520 $208,800,000 $260,543,000 $832,836,000 2.84

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 302
Jurisdiction Structures
Impacteda
Estimated Loss Associated with Flood % of Total
Replaceme
nt
Value Structure Contents Total
Unincorporated
County
346 $157,862,000 $178,277,000 $682,246,000 3.33
Total 20,366 $4,797,178,000 $7,681,359,000 $22,407,125,000 5.87 Note: Values shown are accurate for comparison of results in this plan. See Section 6 for discussion of data limitations.
a
Impacted structures are those with finished floor elevations below the flood event water surface elevation. These structures are the most likely to receive significant damage in a flood event.

Table 99: Loss Estimates for 0.2-Percent-Annual-Chance Flood Jurisdiction Structures
Impacteda
Estimated Loss Associated with Flood % of Total
Replacement
Structure Contents Total Value
Campbell 666 $127,488,000 $110,733,000 $355,863,000 4.04 Cupertino 83 $36,263,000 $29,154,000 $94,619,000 0.66
Gilroy 1,469 $319,375,000 $641,660,000 $2,015,006,000 17.03 Los Altos 0 $1,791,000 $948,000 $3,090,000 0.04
Los Altos Hills 0 $0 $0 $0 0.00
Los Gatos 147 $39,193,000 $56,508,000 $212,717,000 2.21 Milpitas 1,197 $191,075,000 $242,248,000 $934,245,000 5.84 Monte Sereno 0 $0 $0 $0 0
Morgan Hill 837 $161,794,000 $109,502,000 $350,641,000 3.25 Mountain View 732 $119,699,000 $166,238,000 $669,226,000 3.95 Palo Alto 495 $76,861,000 $100,127,000 $367,056,000 1.81 San José 22,052 $5,656,110,000 $10,606,664,000 $28,947,287,000 16.79 Santa Clara
(city) 4,635 $812,021,000 $1,177,901,000 $3,760,129,000 13.60 Saratoga 357 $127,579,000 $96,493,000 $289,244,000 3.09 Sunnyvale 2,478 $351,074,000 $419,774,000 $1,361,761,000 4.64 Unincorporated
County 541 $225,387,000 $262,239,000 $929,177,000 4.53
Total 35,689 $8,245,710,000 $14,020,189,000 $40,290,061,000 10.56 Note: Values shown are accurate for comparison of results in this plan. See Section 6 for discussion of data limitations.
a
Impacted structures are those with finished floor elevations below the flood event water surface elevation. These structures are the most likely to receive significant damage in a flood event.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 303
Key results are as follows:
 There would be up to $8.7 billion of flood loss from a 10-percent-annual-chance flood event in the OA. This represents 2.3 percent of the total replacement value for the OA.  There would be up to $22.4 billion of flood loss from a 1-percent-annual-chance flood event in the OA. This represents 5.87 percent of the total replacement value for the OA.  There would be $40.29 billion of flood loss from a 0.2-percent-annual-chance flood event in the OA. This represents 10.56 percent of the total replacement value.

Structures permitted or built in the OA before the initial FIRM date are called “pre-FIRM” structures, and structures built after the FIRM date are called “post-FIRM.” The insurance rate is different for the two types of structures. Generally, it can be assumed that unmitigated pre-FIRM structures are more vulnerable to flooding than post-FIRM.

Flood-Caused Debris
Left over debris from flooding can be costly to remove and have significant consequences if not dealt with properly. The Hazus analysis estimated the amount of flood-caused debris within the OA generated by flooding, as summarized in Table 100. The model breaks debris into three general categories; Finishes (dry wall, insulation, etc.), Structural (wood, brick, etc.), and Foundations (concrete slab, concrete block, rebar, etc.).

Table 100: Estimated Flood-Caused Debris
Annual Flood
Chance
Finishes (Tons) Structure (Tons) Foundation
(Tons)
Total Debris
(Tons)
10% AnnualChance Flood24,095 3,963 3,847 31,905
1% AnnualChance Flood82,351 8,119 7,945 98,415

0.2% AnnualChance Flood162,896 14,307 14,287 191,490
Flood Insurance
Flood Insurance Statistics
Table 101 lists flood insurance statistics that help identify vulnerability in the OA. All 16 municipal planning partners participate in the NFIP, with 12,159 flood insurance policies providing $3.6 billion in insurance coverage. According to FEMA statistics, 1,556 flood insurance claims were paid between November 1978 and February 23, 2023, for a total of $22,996,576, an average of $14,779 per claim.

Properties constructed after a FIRM has been adopted are eligible for reduced flood insurance rates.

Such structures are less vulnerable to flooding since they were constructed after regulations and codes were adopted to decrease vulnerability. Properties built before a FIRM is adopted are more vulnerable to flooding because they do not meet code or are located in hazardous areas. The first FIRMs in the OA were available in 1975.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 304
Table 101: Flood Insurance Statistics268 F
269
Jurisdiction
Date of
Entry
Initial
FIRM
Effective
Date
# of Flood
Insurance
Policies
as of
2/23/2023
Total
Coverage
Total
Annual
Premium
Claims,
11/1978
to
2/23/2023
Value of
Claims
paid,
11/1978 to
2/23/2023
Campbell 06/30/1976 28 8,541,000 16,222 1 $0
Cupertino 04/18/1975 64 20,042,400 64,687 20 $812,171
Gilroy 06/04/1976 100 46,698,600 140,441 33 $287, 117
Los Altos 09/24/1976 101 31,769,400 79,333 13 $5,896
Los Altos Hills 11/26/1976 34 11,036,000 26,775 16 $45,641 Los Gatos 02/27/1976 66 20,606,800 39,411 17 $51,957
Milpitas 03/28/1975 1,025 288,494,700 1,062,641 69 $27,829 Monte Sereno 05/18/2009 9 3,150,000 5,123 4 $41,974
Morgan Hill 06/18/1980 309 93,061,100 285,955 74 $603,444 Mountain View 09/19/1975 344 117,867,000 263,363 8 $8,501 Palo Alto 09/06/1989 2,377 655, 737, 600 2,601,795 474 $8,936,790 San José 04/09/1976 4,872 1,351,586,300 4,412,960 626 $10,284,648 Santa Clara
(city)
02/11/1977 683 222,286,800 643,583 29 $264,753
Saratoga 11/28/1975 67 22,554,900 58,630 17 $53,676
Sunnyvale 12/05/1975 527 183,134,600 543,354 10 $68,655 Unincorporated
County
06/20/1978 276 74,048,500 410,772 130 $1,654,709
Unknown - 1,440 382,986,000 1,555,609 15 135,932
Total 12,322 $2,877,864,100 $12,210,654 1,556 $22,996,576 This data likely has some limitations. The “Unknown” category was not included in the 2016 dataset used in the 2017 Hazard Mitigation Plan. It also reflects a decrease in policies since the 2017 plan in every jurisdiction except Los Altos, which gained a total of 18 policies, for a total of 5,000 less policies in force across the OA. It is unclear if this is due to the rising cost of flood insurance, voluntary suspension of flood insurance, or another reason.

NFIP Participation
As participating communities, each jurisdiction within the OA was required to adopt floodplain management criteria via local regulations that meets NFIP minimum standards. Multiple communities have adopted regulations that exceed these standards. Further detail is included in each annex in Volume II and in the following table. The special districts and fire department that participated in this plan update are not eligible to participate in the NFIP.
269 FEMA Region IX on 2/23/2023

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 305
Table 102: Date of Floodplain Management Regulation Adoption Jurisdiction Adoption Date
City of Campbell January, 2014
City of Cupertino June, 2016
City of Gilroy January, 2017
City of Los Altos March, 2009
City of Milpitas November, 1995
City of Morgan Hill September, 2017
City of Mountain View April, 2023
City of Palo Alto 2004
City of San Jose April, 2009
City of Santa Clara June, 2023
City of Saratoga 1996
City of Sunnyvale November, 1994 – anticipated update Oct. 2023 County of Santa Clara April, 2009
Town of Los Altos Hills November, 2001
Town of Los Gatos 2003
Repetitive Loss and Severe Repetitive Loss
A repetitive loss property is defined by FEMA as an NFIP-insured property that has experienced any of the following since 1978, regardless of any changes in ownership:
 Four or more paid losses in excess of $1,000.
 Two paid losses in excess of $1,000 within any rolling 10-year period.  Three or more paid losses that equal or exceed the current value of the insured property.

A severe repetitive loss property is further defined as follows:
 Four or more paid losses in excess of $5,000 each, with the cumulative amount of such claim payments exceeding $20,000.
 At least two separate claim payments made, with the cumulative amount of the building portion of such claims exceeding the market value of the building.  At least two of the above referenced claims occurred within any rolling 10-year period and must be more than 10 days apart.

Repetitive loss properties make up only 1 percent of flood insurance policies in force nationally, yet they account for 25 to 30 percent of the nation’s flood insurance claim payments. According the draft 2023 State of California Hazard Mitigation Plan, 34.8 percent of the repetitive properties in the state are located outside of the SFHA. The government has instituted programs encouraging communities to identify and mitigate the causes of repetitive losses. The key identifiers for repetitive loss properties are the existence of flood insurance policies and claims paid by the policies. This doesn’t include properties which have flooded repeatedly but don’t have insurance. Over 50 percent of severe repetitive loss structures nationally are estimated to be without NFIP coverage.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 306
FEMA-sponsored programs, such as the CRS, require participating communities to identify repetitive loss areas. A repetitive loss area is the portion of a floodplain holding structures that FEMA has identified as meeting the definition of repetitive loss. Identifying repetitive loss areas helps to identify structures that are at risk but are not on FEMA’s list of repetitive loss structures because no flood insurance policy was in force at the time of loss.

FEMA’s list of repetitive loss properties identifies four such properties in the OA as of February 23, 2023.

The breakdown of the properties by jurisdiction is presented in Table 103. With the potential for flood events annually, all of the mapped floodplain is considered to be susceptible to repetitive flooding.

Table 103: Repetitive Loss Properties
Jurisdiction
Number of
Repetitive
Loss
Properties
Type of Repetitive
Loss Properties
Number of
Severe
Repetitive
Loss
Properties
Type of Severe
Repetitive Loss
Properties
Cupertino 3
Single Family (1)
Other Nonresidential (2) - -
Los Gatos 1 2–4 Family (1) - -
Morgan Hill 2 Single Family (2) 4
Single Family (2)
Business (1)
Other Nonresidential (1)
Palo Alto 4
Single Family (3)
Other Nonresidential (1) 1 Business (1)
San José 6 Single Family (6) 1 Single Family (1)
Sunnyvale 1 Other Nonresidential (1) - -
Unincorporated
County 8 Single Family (8) 3 Single Family (3)
Unknown 1 2-4 Family (1) - -
Total 25
Single Family (20)
Other Nonresidential (4)
2-4 Family (2)
9 Single Family (6)
Business (2)
Other Nonresidential (1)
Note: Based on FEMA Region IX Report of Repetitive Losses, 2/23/2023.

Substantial Improvement/Substantial Damage
In order to be a part of the NFIP, participating jurisdictions commit to implementing substantial improvement / substantial damage provisions of their floodplain management regulations after an event.

According to FEMA, these terms are defined as:
 Substantial Improvement means any reconstruction, rehabilitation, addition or other improvement to a structure, the total cost of which equals or exceed 50 percent of the market value of the structure before the state of construction of the improvement.  Substantial damage means damage of any origin sustained by a structure whereby the cost of restoring the structure to its before damaged condition would equal or excel 50 percent of the market value of the structure before the damage occurred.

Multiple jurisdictions within the OA have adopted higher standards, including a repetitive loss or cumulative damage. These higher standards are described further in Volume II in addition to additional information on how these provisions are implemented.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 307
Table 104: Substantial Damage/Substantial Improvement Implementation Jurisdiction Responsible Party Higher Standard?
City of Campbell Floodplain Administrator No
City of Cupertino Building Official Yes
City of Gilroy Floodplain Administrator Yes
City of Los Altos Floodplain Administrator, Building
Official
No
City of Milpitas City Manager, Floodplain
Administrator
Yes
City of Morgan Hill Floodplain Administrator Yes
City of Mountain View Floodplain Administrator or
designee
Yes
City of Palo Alto Floodplain Administrator No
City of San Jose Floodplain Manager, other
departments/divisions including
public works as necessary
No
City of Santa Clara Floodplain Administrator, Building
Official
No
City of Saratoga City Manager or designee, usually
someone from Public Works or
Community Development
No
City of Sunnyvale Floodplain Administrator, Building
Official
No
County of Santa Clara Yes
Town of Los Altos Hills Floodplain Administrator No
Town of Los Gatos Floodplain Administrator No

13.5.3 Critical Facilities and Infrastructure
Percentage of damage and functional down-time estimates were not generated for critical facilities and infrastructures in the flood scenarios. However, a count of facilities within the flood hazard boundaries was obtained using the Hazus facility inventory (Tables 92-94). Replacement values provided in Hazus were used to estimate the flood loss potential to critical facilities exposed to the flood risk. The Hazus critical facility results are presented in Table 105, Table 106, and Table 107.

Table 105: Estimated Damage to Critical Facilities and Infrastructure from the 10% Annual Chance Flood
Type of Critical Facility Number of
Facilities Affected Estimated Replacement Cost
Essential Facilities 24 $236,048,440
Transportation 106 $474,684,230
Utilities 1 $1,030,643,100
Community Assets 29 Not Available

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 308
Type of Critical Facility Number of
Facilities Affected Estimated Replacement Cost
Hazardous Materials 54 Not Available
Total/Average 214 $1,741,375,770
Note: N/A = Not Applicable.

Table 106: Estimated Damage to Critical Facilities and Infrastructure from the 1% Annual Chance Flood
Type of Critical Facility Number of
Facilities Affected Estimated Replacement Cost
Essential Facilities 58 $871,412,940
Transportation 185 $959,350,260
Utilities 3 $1,070,055,100
Community Assets 64 Not Available
Hazardous Materials 78 Not Available
Total/Average 388 $2,900,818,300
Note: N/A = Not Applicable.

Table 107: Estimated Damage to Critical Facilities and Infrastructure from the 0.2% Annual Chance Flood
Type of Critical Facility Number of Facilities
Affected Estimated Replacement Cost
Essential Facilities 89 $3,478,003,120
Transportation 244 $1,348,101,860
Utilities 10 $2,057,661,430
Community Assets 94 Not Available
Hazardous Materials 131 Not Available
Total/Average 568 $6,883,766,410
Note: N/A = Not Applicable.

These tables indicate that a significant number of important assets are currently located within the 10-, 100-, and 500-year floodplains. A breakdown of the number of critical facilities at risk for each jurisdiction can be found in Table 92 through Table 94. Flooding could impact response facilities such as fire and policies stations which may directly damage facilities and equipment within buildings, as well as impact their ability to provide response activities in the immediate aftermath of a flood. Several schools are also in flood hazard zones. Transportation networks such as roadways and railways may be damaged or become temporarily impassable during a flood, which may provide further difficulty for people displaced from their homes and seeking shelter. Numerous other community facilities included in the community assets category are also in hazard zones, which indicates a variety of community support services may disrupted by flooding. Facilities with hazardous materials on site may be compromised by flooding, which may result in contamination of floodwaters and dispersal of harmful materials. The City of San Jose, the City of Gilroy, and the unincorporated county have the highest number of critical facilities and infrastructure at risk to flooding.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 309

13.5.4 Environment
Flooding is a natural event, and floodplains provide many natural and beneficial functions. Nonetheless, flooding can impact the environment in negative ways. Migrating fish can wash into roads or over dikes into flooded fields, with no possibility of escape. Pollution from roads, such as oil, and hazardous materials can wash into rivers and streams. During floods, these can settle onto normally dry soils, polluting them for agricultural uses. Human development such as bridge abutments and levees, and logjams from timber harvesting can increase stream bank erosion, causing rivers and streams to migrate into non-natural courses.

Additionally, while the vulnerability assessment typically focuses on human vulnerability to flood events, the opposite is also worth noting. Floodplains have many natural and beneficial functions; however, due to negative impacts of floods, many structural and other measures have been devised to limit how far a floodplain can extend. Disruption of natural systems can have long-term consequences for entire regions; however, this potential impact has only recently been noted. Some well-known, water-related functions of floodplains include the following269 F
270:
 Natural flood and erosion control.  Filter nutrients and impurities from runoff.  Provide flood storage and conveyance.  Process organic wastes.  Reduce flood velocities.  Moderate temperatures of water.  Reduce flood peaks.  Groundwater recharge.
 Reduce sedimentation.  Promote infiltration and aquifer recharge.  Surface water quality maintenance.  Reduce frequency and duration of low surface flows
Areas within the floodplain that typically provide these natural functions are wetlands, riparian areas, sensitive areas, and habitats for rare and endangered species. In the northern end of the county, wetlands (both Bay Wetlands and Freshwater Wetlands) will be impacted by flooding adjacent to the coastline. In the southern end of the county, flooding of the Pajaro River watershed would impact Grassland, Riparian, and Freshwater Wetland habitats.270F 271 Riparian habitat would be impacted throughout
the county; however, this habitat type is naturally adapted to withstand some degree of seasonal flooding.

13.5.5 Economic Impact
Locations of flooding will experience the heaviest economic impact. Within these areas, renovations of commercial buildings may be necessary, disrupting associated services. Additionally, significant damage within agricultural areas may occur with destruction of crops and other agricultural products. The tourism industry may also be affected by major flood events, as popular vacation areas tend to overlap flood hazard zones. Finally, flooding can cause extensive damage to public utilities and disruptions to delivery of services. Loss of power and communications may occur; and drinking water and wastewater treatment facilities may be temporarily out of operation.
270 FEMA. (2022, April 1). Benefits of Natural Floodplains. https://www.fema.gov/floodplain-management/wildlifeconservation/benefits-natural 271 Santa Clara Valley Water District. (2017, October). Local Hazard Mitigation Plan. Microsoft Word - R14163 2017 FINAL LOCAL HAZARD MITIGATION PLAN v. 04-09-21 (amazonaws.com)

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 310

13.6 Future Trends in Development
Pre-pandemic, Santa Clara County had been one of the state’s fastest growing counties, averaging a

1.21-percent increase in population per year from 2005 through 2015. The Silicon Valley job market grew, and many young tech employees elected to live in an urban environment rather than commute from the suburbs. The area has not recovered economically since the pandemic. The population decreased slightly from 2021 to 2022, and employment levels for almost all major areas of the economy were still below pre-pandemic levels at last indication.271F
272 Almost 90,000 tech industry employees were laid off in 2022 alone. The Silicon Valley housing market has cooled, but not crashed. A decrease in population and employment, particularly in high-income industries, may reduce the amount of new development in the SFHA. The Santa Clara County Planning Department Website did not show any significant development being proposed in the SFHA in the unincorporated areas of the county at the time of this writing. It is unclear if these trends will continue over the next five years.

Any areas of growth could be impacted by the flood hazard if located within the identified hazard areas.

The planning partners have appropriate policies, plans, and programs in place to address future growth within flood hazard areas. The SCVWD intends to discourage development within vulnerable areas and/or to encourage higher regulatory standards on the local level. Additionally, all municipal planning partners are participants in the NFIP and have adopted flood damage prevention ordinances which regulate development in high-hazard areas. All municipal planning partners also have general plans that address frequently flooded areas in their safety elements. All partners have committed to linking their general plans to this hazard mitigation plan. More information on planning partners’ development is available in Volume 2 of this plan.

With around 60 percent of communities in the OA participating in the CRS program, there is incentive to adopt consistent, appropriate, higher regulatory standards in areas with the highest degree of flood risk.

Additionally, there are many active regional partners involved in land use planning and risk reduction in the Bay Area. Plan Bay Area 2050 is the nine-county San-Francisco Bay Area plan for long-term development. This plan includes 35 strategies for housing, the economy, transportation, and the environment. It maintains urban growth boundaries in order to curb urban sprawl and identifies other strategies to reduce risk from flooding. The 2015 Stronger Housing, Safer Communities report led by ABAG developed a series of strategies for developing safe, smart growth in the Bay Area, including recommendations for flood protection measures.

The County of Santa Clara’s Office of Sustainability and Climate Action has also developed the Silicon Valley 2.0 Climate Adaptation Guidebook which provides additional recommendations and examples.

Jurisdictions in the OA can expand upon these and other resources to make risk-informed land use and flood mitigation decisions in light of future growth.

13.7 Scenario
Historically, floods have regularly affected the Santa Clara County OA. The OA can expect noteworthy flooding about once a year, with a flash flood every 2 to 3 years. Duration and intensity of heavy winter rains and atmospheric river events that cause flooding may increase due to climate change. The floodplains mapped and identified for the Santa Clara County OA will continue to take the brunt of these floods. OA residents prepare themselves for flooding by seeking and receiving information, and by pursuing mitigation. Impacts of flood events should decrease as the OA continues to promote and implement hazard mitigation and preparedness.
272 Silicon Valley Indicators. (n.d.). Data about Silicon Valley’s Economy and Community Health. https://siliconvalleyindicators.org/

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 311
The worst-case scenario would be a series of heavy rains or storm events during an atmospheric river event, particularly if the rains also occur at high tide. These rains could flood numerous areas within a short time. This could overwhelm the response and floodplain management capability within the OA, as the OA would be subject immediately to flash flooding and coastal flooding, with subsequent influences on the County’s streams. Major roads could be blocked, preventing critical access for many residents and critical functions. High in-channel flows could cause water courses to scour, possibly washing out roads and creating more isolation problems. In the event of multi-basin flooding, Santa Clara County would not be able to make repairs quickly enough to restore critical facilities and assets.

13.8 Issues
Important issues associated with floods in the OA include the following:
 The extent of the flood-protection currently provided by flood control facilities (dams, dikes, and levees) is not known due to the lack of an established national policy on flood protection standards.
 The levee system within the OA is not consistently adequate to mitigate effects of a 1-percent annual chance flood.
 The risk associated with the flood hazard overlaps the risk associated with other hazards such as earthquake, landslide, mud slides and fishing losses. This provides an opportunity to seek mitigation alternatives with multiple objectives that can reduce risk for multiple hazards.  There is no consistency of land-use practices and floodplain management scope within the OA.  How climate change will affect flood conditions in the OA is uncertain.  More information is needed on flood risk to support the concept of risk-based analysis of capital projects.
 There needs to be a sustained effort to gather historical damage data, such as high-water marks on structures and damage reports, to measure the cost-effectiveness of future mitigation projects.  Ongoing flood hazard mitigation will require funding from multiple sources.  There needs to be a coordinated hazard mitigation effort between jurisdictions affected by flood hazards in the OA.
 Floodplain residents need to continue to be educated about flood preparedness and the resources available during and after floods.
 The concept of residual risk should be considered in the design of future capital flood control projects and should be communicated with residents living in the floodplain.  The promotion of flood insurance as a means of protecting private property owners from the economic impacts of frequent flood events should continue.  Existing floodplain-compatible uses such as agricultural and open space need to be maintained.

There is constant pressure to convert these existing uses to more intense uses within the OA during times of moderate to high growth.
 The economy affects a jurisdiction’s ability to manage its floodplains. Budget cuts and personnel losses can strain resources needed to support floodplain management.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 13: Flood 312
Table 108: EMAP Consequence Analysis: Flood
Subject Ranking Impacts/Flood
Public Minimal to
severe
The localized impact is expected to be severe for persons living within the inundation area. Residents of the SFHA are most at risk from flooding. Flooding can result in injury, loss of life, and the loss of property and livelihood. Daily life of residents, visitors, and commuters would be disrupted,
particularly if evacuations are necessary. Flood waters could carry containments which impact public health. Water that is slow to recede could act as a habitat for disease-carrying insects. The public would be exposed to risk during flood clean-up, including mold. Vulnerable populations may be disproportionally impacted by a flood event and have unique response and recovery needs. There may long-term public health consequences of a flood event.

Responders Minimal to
moderate
Responders may be responsible for supporting evacuations, closing roads, assisting injured members of the public, and managing the overall incident. Depending on the event,
responders may play a significant role in locating and
assisting survivors after the flood. During the course of their duties, responders will likely face increased risk of personal injury. They may be directly exposed to the flood water, including any containments.

Continuity of
Operations (including
continue delivery of
services)
Minimal to
severe
Temporary relocation may be necessary if inundation affects government facilities. Delivery of services could be affected if there is any disruption to facilities, roads, and/or utilities due to the inundation or cascading impacts.

Property, Facilities, and
Infrastructure
Minimal to
severe
The localized impact could be severe for property, facilities and infrastructure that are inundated. Additional damage or disruption could be caused by debris, road closures, and stormwater issues. Water and wastewater treatments plants may be overloaded.

Environment Minimal to
severe
Flooding can provide benefits to environment. However, it also can potentially expose the environment to containments, hazardous materials, silt, and debris. Flooding can damage or destroy natural habitats and wildlife like fish caught up in the flood water can die. Floods can additionally cause erosion, landslides, and changes to the watershed.

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 flood water to recede. A major flood event could be costly. There would be emergency response needs, disaster cleanup,
delays and disruption in services and transportation, and potential closure to local businesses due to direct flood losses or lack of employees or customers.

Public Confidence in
the Government
Minimal to
severe
The public’s confidence will vary, depending on the perception of the warning time, the information shared, and the time it takes for response and recovery. Accurate and timely
distribution of information before, during, and after the event will influence public trust.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 14: Landslide/Mass Movement 313
14 Landslide/Mass Movement
Definitions
 Landslide: The movement of masses of loosened rock and soil down a hillside or slope. Slope failures occur when the strength of the soils forming the slope is exceeded by the pressure, such as weight or saturation, acting upon them.
 Mass Movement: A collective term for landslides, debris flows, and sinkholes.  Mudslide (or Debris Flow): A river of rock, earth, organic matter, and other materials saturated with water. Mudslides develop in the soil overlying bedrock on sloping surfaces when water rapidly accumulates in the ground, such as during heavy rainfall or rapid snowmelt. Water pressure in the pore spaces of the material increases to the point that the internal strength of the soil is drastically weakened. The soil’s reduced resistance can then easily be overcome by gravity, changing the earth into a flowing river of mud or “slurry.”

14.1 General Background
The U.S. Geological Survey defines landslides to include a wide range of ground movement, such as rock falls, deep failure of slopes, and shallow debris flows. Although gravity acting on an over-steepened slope is the primary reason for a landslide, there are other contributing factors.

Landslides and mudslides can be initiated by storms, earthquakes, fires, volcanic eruptions, or human modification of the land. They can move rapidly down slopes or through channels and can strike with little or no warning at avalanche speeds, posing a serious hazard to properties on or below hillsides.

When landslides occur—in response to such changes as increased water content, earthquake shaking, addition of load, or removal of downslope support—they deform and tilt the ground surface. The result can be destruction of foundations, offset of roads, breaking of underground pipes, or overriding of downslope property and structures.

The USGS defines land subsidence as the loss of surface elevation due to the removal of subsurface support. In California, the two principal causes for land subsidence are aquifer compaction due to excessive groundwater pumping and decomposition of wetland soils exposed to air after wetland conversion to farmland.

14.1.1 Landslide Types
Landslides are commonly categorized by the type of initial ground failure. Common types of slides are shown in Figure 65. The most common is the shallow colluvial slide, occurring particularly in response to intense, short-duration storms. The extent of landslides can range from mild to severe and are dependent on several factors, including the type of event. The largest and most destructive are deep-seated slides (greater than 10 to 15 feet deep), although they are less common than other types.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 14: Landslide/Mass Movement 314
Figure 66: Common Types of Landslides
Mudslides (or debris flows) are rivers of rock, earth, organic matter, and other soil materials saturated with water. They develop in the soil overlying bedrock on sloping surfaces when water rapidly accumulates in the ground, such as during heavy rainfall or rapid snowmelt. Water pressure in the pore spaces of the material increases to the point that the internal strength of the soil is drastically weakened. The soil’s reduced resistance can then easily be overcome by gravity, changing the earth into a flowing river of mud.

A debris avalanche (see Figure 66) is a fast-moving debris flow that travels faster than about 10 miles per hour (mph). Speeds in excess of 20 mph are not uncommon, and speeds in excess of 100 mph, although rare, can occur. The slurry can travel miles from its source, growing as it descends, picking up trees, boulders, cars, and anything else in its path. Although these slides behave as fluids, they pack many times the hydraulic force of water due to the mass of material included in them. They can be among the most destructive events in nature.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 14: Landslide/Mass Movement 315
Figure 67: Typical Debris Avalanche Scar and Track27F
273
Landslides also include the following:
 Rock Falls: Blocks of rock that fall away from a bedrock unit without a rotational component.  Rock Topples: Blocks of rock that fall away from a bedrock unit with a rotational component.  Rotational Slumps: Blocks of fine-grained sediment that rotate and move down slope.  Transitional Slides: Sediments that move along a flat surface without a rotational component.  Earth Flows: Fine-grained sediments that flow downhill and typically form a fan structure.  Creep: A slow-moving landslide often only noticed through crooked trees and disturbed structures.
 Block Slides: Blocks of rock that slide along a slip plane as a unit down a slope.

14.1.2 Landslide Modeling
Two characteristics are essential to conducting an accurate risk assessment of the landslide hazard:
 The type of initial ground failure that occurs, as described above  The post-failure movement of the loosened material (“runout”), including travel distance and velocity
All current landslide models—those in practical applications and those more recently developed—use simplified hypothetical descriptions of mass movement to simulate the complex behavior of actual flow.

The models attempt to reproduce the general features of the moving mass of material through measurable factors, such as base shear, that define a system and determine its behavior. 273 California Department of Conservation. (n.d.). Hazards from “Mudslides” …Debris Avalanches and Debris Flows in Hillside and Wildfire Areas. https://www.conservation.ca.gov/cgs/Pages/Publications/Note_33.aspx

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 14: Landslide/Mass Movement 316
Due to the lack of experimental data and the limited current knowledge about the behavior of the moving flows, landslide models use simplified parameters to account for complex aspects that may not be defined. These simplified parameters are not related to specific physical processes that can be directly measured, and there is a great deal of uncertainty in their definition. Some, but not all, models provide estimates of the level of uncertainty associated with the modeling approach.

Run-out modeling is complicated because the movement of materials may change over the course of a landslide event, depending on the initial composition, the extent of saturation by water, the ground shape of the path traveled and whether there is additional material incorporated during the event.273F 274

14.1.3 Landslide Causes
Mass movements are caused by a combination of geological and climate conditions, as well as encroaching urbanization. Vulnerable natural areas are affected by residential, agricultural, commercial, and industrial development and the infrastructure that supports it. The following factors can contribute to landslide: change in slope of the terrain, increased load on the land, shocks and vibrations, change in water content, groundwater movement, frost action, weathering of rocks, and removing or changing the type of vegetation covering slopes.

Excavation and Grading
Slope excavation is common in development of home sites or roads on sloping terrain. Grading can result in slopes that are steeper than the pre-existing natural slopes. These steeper slopes can be at an increased risk for landslides. The added weight of fill on slopes can also result in an increased landslide hazard. Small landslides can be common along roads, in either the road cut or the road fill. Landslides below new construction sites are indicators of the potential impacts stemming from excavation.

Drainage and Groundwater Alterations
Water flowing through or above ground is often the trigger for landslides. Any activity that augments the amount of water flowing into landslide-prone slopes can increase landslide hazards. Broken or leaking water or sewer lines can be especially problematic, as can water retention facilities that direct water onto slopes. However, even lawn irrigation and minor alterations to small streams in landslide-prone locations can result in damaging landslides. Ineffective stormwater management and excess runoff can also cause erosion and increase the risk of landslide hazards. Drainage can be affected naturally by the geology and topography of an area. Development that results in an increase in impervious surface impairs the ability of the land to absorb water and may redirect water to other areas. Channels, streams, flooding, and erosion on slopes all indicate potential slope problems.

Road and driveway drains, gutters, downspouts, and other constructed drainage facilities can concentrate and accelerate flow. Ground saturation and concentrated velocity flow are major causes of slope problems and may trigger landslides.

Changes in Vegetation
Removing vegetation from very steep slopes can increase landslide hazards. Areas that have experienced wildfire and land clearing for development may experience long periods of increased landslide hazard. In addition, woody debris in stream channels (both natural and man-made from logging) may cause the impacts from debris flows to be more severe. 274 McDougall, S. (2016). 2014 Canadian Geotechnical Colloquium: Landslide runout analysis — current practice and challenges. Canadian Geotechnical Journal. 54(5): 605-620. https://doi.org/10.1139/cgj-2016-0104

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 14: Landslide/Mass Movement 317

14.1.4 Landslide Management
While small landslides are frequently a result of human activity, the largest landslides are often naturally occurring phenomena with little or no human contribution. The sites of large landslides are typically areas of previous landslide movement that are periodically reactivated by significant precipitation or seismic events. These naturally occurring landslides can disrupt roadways and other infrastructure lifelines, destroy private property, and cause flooding, bank erosion, and rapid channel migration.

Landslides can create immediate, critical threats to public safety. Engineering solutions to protect structures on or adjacent to large active landslides are often extremely or prohibitively expensive. Despite their destructive potential, landslides can serve beneficial functions to the natural environment. They supply sediment and large wood to the channel network and can contribute to complexity and dynamic channel behavior critical for aquatic and riparian ecological diversity. Effective landslide management should include the following elements:
 Continuing investigation to identify natural landslides, understand their mechanics, assess their risk to public health and welfare, and understand their role in ecological systems.  Regulation of development in or near existing landslides or areas of natural instability through the Santa Clara County Code and City ordinances.
 Preparation for emergency response to landslides to facilitate rapid, coordinated action among Santa Clara County, local cities, and state and federal agencies, and to provide emergency assistance to affected or at-risk citizens.
 Evaluation of options including landslide stabilization or structure relocation where landslides are identified that threaten critical public structures or infrastructure.

14.1.5 Land Subsidence Effects
Subsidence is one of the most diverse forms of ground failure, ranging from small or local collapses to broad regional lowering of the earth’s surface. The causes of subsidence, mostly associated with human activities, are as diverse as the forms of failure, and include dewatering (oxidation) of peat or organic soils, dissolution in limestone aquifers, first-time wetting of moisture-deficient low-density soils, natural compaction, liquefaction, crustal deformation, subterranean mining, and withdrawal of fluids (groundwater, petroleum, geothermal).

The compaction of susceptible aquifer systems caused by excessive groundwater pumping is the single largest cause of subsidence in California. The second largest cause of subsidence in California is the oxidation (decomposition) of organic soils.274F
275 Alteration to the Sacramento-San Joaquin Delta in the late 1800s through the creation of levees and ground water pumping is known as the single largest human alternation of the Earths’ surface topography, but it left over 5,200 square miles of areas susceptible to subsidence, primarily from decomposition of organic carbon in the peat soils.275F 276
275 U.S. Geological Survey. (2018, October 18). Decomposition of Organic Soils in the Sacramento-San Joaquin Delta. https://www.usgs.gov/centers/land-subsidence-in-california/science/decomposition-organic-soils-sacramentosan-joaquin 276 U.S. Geological Survey. (2018, November 29). Land Subsidence Along the California Aqueduct in West-central San Joaquin Valley, California. https://www.usgs.gov/publications/land-subsidence-along-california-aqueduct-westcentral-san-joaquin-valley-california

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 14: Landslide/Mass Movement 318
Aquifer Compaction
Aquifer compaction due to groundwater pumping affects both manmade infrastructures and natural systems. The greatest effects are on infrastructure that traverses a subsiding area. In the San Joaquin Valley, the main problems reported are related to water conveyance structures. Many water conveyance structures, including long stretches of the California Aqueduct, are gravity driven through the use of very small gradients; even minor changes in these gradients can cause reductions in designed flow capacity.

Managers of the canals, such as the California Department of Water Resources, the San Luis DeltaMendota Authority, the Bureau of Reclamation, and the Central California Irrigation District, have to repeatedly retrofit their canals to keep the water flowing, even at reduced amounts. Subsidence also affects roads, railways, bridges, pipelines, buildings, and wells.

Compaction of an aquifer system may permanently decrease the aquifer’s capacity to store water. Even when water levels rise, sediments can remain compacted; most compaction that occurs as a result of historically low groundwater levels is irreversible.

Additionally, as the topography of the land changes by varying amounts in different places, low areas, such as wetlands, change size and shape, migrate to lower elevations, or even disappear. Rivers may change course or erosion/deposition patterns to reach a new equilibrium.

Decomposition of Wetland Soils
The Sacramento-San Joaquin Delta of California was once a great tidal freshwater marsh. It is blanketed by peat and peaty alluvium deposited where streams that originate in the Sierra Nevada, Coast Ranges, and South Cascade Range enter San Francisco Bay. In the late 1800s, levees were built along the stream channels, and the land thus protected from flooding was drained, cleared, and planted. The leveed tracts and islands help to protect water-export facilities in the southern Delta from saltwater intrusion by displacing water and maintaining favorable freshwater gradients. However, the decomposition of organic carbon in the peat soils causes land subsidence in the Delta and increases stresses on the levees. Ongoing subsidence behind the levees, where the land has been drained, exposed to the atmosphere, and planted, increases stresses on the levee system, making it less stable.

This threatens to damage agricultural and developed lands and degrade water quality in the massive water-transfer system.

14.2 Hazard Profile

14.2.1 Past Events
Losses from landslides are typically lower than those from flooding. However, in the El Niño storms of early 1998, the USGS documented $150 million in losses due to approximately 300 landslides in the Bay Area and Santa Clara County. The slides ranged from a 25-cubic-meter failure of engineered material to reactivation of the 13 million-cubic-meter Mission Peak earth flow complex in Alameda County.

Landslides have occurred in conjunction with earthquakes and heavy rains events in Santa Clara County.

Table 109 lists known landslide events that affected Santa Clara County between 1980 and 2023. Other landslides around the Bay Area near the OA, particularly in the Santa Cruz Mountains, are documented by the California Geological Survey.276F
277
277 California Geological Survey. (2011). Susceptibility to Deep-Seated Landslides in California. https://www.conservation.ca.gov/cgs/Documents/Publications/Map-Sheets/MS_058.pdf

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 14: Landslide/Mass Movement 319
Table 109: Landslide Events in Santa Clara County27F
278,
278F
279,
279F
280,
280F
281,
281F
282
Dates of Event Event Type FEMA
Declaration Location Losses/Impacts
12/19/1981 to
1/08/1982
Severe storms, flood,
mudslides, high tide
651 San Francisco
Bay area
Prolonged heavy rains and saturated soils caused
numerous slope failures and mud flows on steep and
unstable slopes throughout the San Francisco Bay area.
1/21/1983 to
3/30/1983
Coastal storms, floods, slides,
tornadoes
677 San Francisco
Bay area
A landslide restricted Clayton Road to one lane just east of the community of Alum Rock. Another, on the east side of Milpitas, resulted in vertical and horizontal offset of a roadway.
4/24/1984 Morgan Hill Earthquake None Calaveras fault
east of San
José.

This 6.2 magnitude earthquake caused minor landslides
throughout the region.
10/17/1989 Loma Prieta Earthquake 845 San Andreas
fault near Loma
Prieta.

Landslides and rockslides in Santa Clara County on steep slopes in the Santa Cruz Mountains blocked roads,
damaged structures, and caused at least two deaths.
1/03/1995 to
2/10/1995
Severe winter storms,
flooding, landslides, mud
flows
1044 San Francisco
Bay area
Minor landslide damage in Santa Clara County was
attributed to heavy rains and saturated soils.
2/13/1995 to
4/19/1995
Severe winter storms,
flooding, landslides, mud
flows
1046 San Francisco
Bay area
Minor landslide damage in Santa Clara County was
attributed to heavy rains and saturated soils.
278 Association of Bay Area Governments. (2011). 2011 Bay Area Hazard Mitigation Plan. https://abag.ca.gov/2011-bay-area-hazard-mitigation-plan 279 U.S. Geological Survey. (1987). The Morgan Hill, California, Earthquake of April 24, 1984. U.S. Geological Survey Bulletin 1639. https://pubs.usgs.gov/bul/1639/report.pdf
280 U.S. Geological Survey (1989). The Severity of an Earthquake. https://pubs.usgs.gov/gip/earthq4/severity_text.html 281 NOAA. (2023). Storm Events Database. https://www.ncdc.noaa.gov/stormevents/ 282 California Department of Conservation. (n.d.). Reported California Landslides Database. https://www.conservation.ca.gov/cgs/landslides

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 14: Landslide/Mass Movement 320
Dates of Event Event Type FEMA
Declaration Location Losses/Impacts
2/02/1998 to
4/30/1998
Severe Winter Storms and El
Nino Rainstorm
1203 San Francisco
Bay region
$7.6 million in Santa Clara County landslide damage
occurred mostly in the northern county, along the range front of the Santa Clara Valley. $6.1 million in damage was attributed to reactivation of three local landslides. The rest was attributed to small debris flows along road cuts or narrow canyon walls. In Alum Rock, the Penitencia Creek landslide caused extensive damage to water and sewer
lines and closed roads. Another landslide closed Clayton Road east of Alum Rock area. The third, near Old
Piedmont Road on the east side of Milpitas, had a
displacement near the toe of about 20 cm.
1/10/2017 Debris Flow, Heavy Rain None Santa Clara,
Montalvo
The third and final system in a string of Atmospheric River events between January 2 to 11. This system resulted in widespread roadway flooding and debris flows across the CWA. Black road at Gist Road is impassable due to major mudslide or rockslide.
1/18/2017 Debris Flow, Heavy Rain None Santa Clara,
Redwood
Estates
Three storm systems swept through the region between
January 18-23. The first occurred on January 18 as a cold front moved through. Heavy rain, widespread flooding, and debris flows were observed. Mud/rock/dirt slide blocking one lane heading towards Highway 17 from Bear Creek
Rd.
2/07/2017 Debris Flow, Heavy Rain None Santa Clara,
Los Gatos
An atmospheric river swept through the Bay Area
beginning on the night of Feb 6. This system produced
widespread roadway flooding, debris flows, and strong
winds. Mud slide reported at 18500 Limekiln Canyon Road. 2/09/2017 Debris Flow, Heavy Rain None Santa Clara,
Los Altos Hills
A cold front passed over the area Thursday Feb 9. There were strong winds ahead of the front and heavy rains
associated with the frontal passage that produced roadway flooding and debris flows. Mud/dirt/rockslide blocking south bound lanes Skyline Dr at Alpine.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 14: Landslide/Mass Movement 321
Dates of Event Event Type FEMA
Declaration Location Losses/Impacts
2/09/2017 Debris Flow, Heavy Rain None Santa Clara,
Saratoga
A cold front passed over the area Thursday, Feb 9. There were strong winds ahead of the front and heavy rains
associated with the frontal passage that produced roadway flooding and debris flows. Second mudslide in area and a tree down partially blocking 23600 SR9 near SavannahChanelle Vineyard. 2/20/2017 Debris Flow, Heavy Rain None Santa Clara,
Los Gatos
Potent AR brought copious amounts of rain to the region causing widespread flooding, debris flow, accidents, and over topping of reservoir spillways. Highway 17
southbound shut down due to a rockslide just north of
Lexington Reservoir.
1/06/2019 Debris Flow, Heavy Rain None Santa Clara,
Los Gatos
A vigorous cold front swept through California on January 5th bringing widespread rainfall and gusty winds. Unstable air behind the frontal passage and sufficient low level shear allowed shallow thunderstorms to develop over the coastal waters, some of which contained rotating cells. Two
waterspouts developed and made landfall as tornadoes on the 6th. Additionally, heavy rainfall and strong winds
caused roadway flooding, minor debris flows, and
numerous downed trees across the region. This storm
system caused two fatalities; one caused by a downed tree in Berkeley and another man died in Santa Rosa Creek
due to rising waters. Mud slide reported at Cats/Lexington south side on Hwy 17.
1/17/2019 Flood, Heavy Rain None Santa Clara,
Saratoga
A moderate to strong atmospheric river impacted much of California in the middle of the month. A weak surface low developed off the coast on January 15th bringing moderate to heavy rainfall to portions of the region. Over the next 24 to 36 hours a second strong low pressure system moved to the north and east bringing heavy rain, destructive winds, high surf, flooding, and thunderstorms to the Bay Area.

Numerous reports were received of downed trees and
power lines. Winds were recorded between 60 and 100
mph. Downed trees resulted in two fatalities. Mud/dirt/rock at Mt Eden Rd and Orchard Meadow Dr blocking most of
roadway.

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Dates of Event Event Type FEMA
Declaration Location Losses/Impacts
2/04/2019 Debris Flow, Heavy Rain None Santa Clara,
Redwood
Estates
A mid/upper low with a very cold airmass moved through in early February bringing snow to lower elevation peaks
across the region prompting a rare Winter Weather
Advisory. Junipero Serra Peak received around a foot of snow. Rainfall just ahead of this system also brought
roadway flooding and minor debris flows. Mud, rock, and debris covered Hicks Road resulting in a multiday closure.

Highway 35 closed due to sinkhole.
2/10/2019 Debris Flow, Heavy Rain None Santa Clara,
Bells Station
A cold front moved through the region into February 10th lowering snow levels below 1500 ft, per the Bodega Bay
Profiler. The areas peaks saw another dusting to several inches of snow as a result with Mount Hamilton recording almost 6 inches. Additionally, showers ahead of the front the previous evening caused some minor roadway
flooding. Mud and rocks in slow lane on WB HWY 152.
2/11/2019 Landslide None Santa Clara 160 meters east of intersection of Hicks Road and Pheasant Road. Mud, rock, and debris covered Hicks Road resulting in multiday closure
2/13/2019 Debris Flow, Heavy Rain None Santa Clara,
Robertsville
An atmospheric river with an associated cold front moved through the region from February 12th to the 15th bringing widespread flooding and debris flows. Multiple mainstem rivers flooded prompting evacuations from local officials.

Strong wind gusts caused downed trees, power outages,
and structural damage. Additionally, a tree fell on a car causing one fatality and one serious injury on Highway 17 while another downed tree caused a serious multi-car
traffic accident that resulted in another fatality as well as major injuries. The areas peaks received upwards of 10
inches of rainfall and widespread wind gusts of 50 to 60 mph were observed. Mount Saint Helena recorded wind
gusts of 80 mph. Mudslide blocking both lanes of Hicks Rd in south San José.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 14: Landslide/Mass Movement 323
Dates of Event Event Type FEMA
Declaration Location Losses/Impacts
2/13/2019 Flood, Heavy Rain None Santa Clara,
Saratoga
An atmospheric river with an associated cold front moved through the region from February 12th to the 15th bringing widespread flooding and debris flows. Multiple mainstem rivers flooded prompting evacuations from local officials.

Strong wind gusts caused downed trees, power outages,
and structural damage. Additionally, a tree fell on a car causing one fatality and one serious injury on Highway 17 while another downed tree caused a serious multi-car
traffic accident that resulted in another fatality as well as major injuries. The areas peaks received upwards of 10
inches of rainfall and widespread wind gusts of 50 to 60 mph were observed. Mount Saint Helena recorded wind
gusts of 80 mph.

Mud in NB lane of CA-9 1 mile south of Redwood Gulch.
2/14/2019 Debris Flow, Heavy Rain None Santa Clara,
Saratoga
An atmospheric river with an associated cold front moved through the region from February 12th to the 15th bringing widespread flooding and debris flows. Multiple mainstem rivers flooded prompting evacuations from local officials.

Strong wind gusts caused downed trees, power outages,
and structural damage. Additionally, a tree fell on a car causing one fatality and one serious injury on Highway 17 while another downed tree caused a serious multi-car
traffic accident that resulted in another fatality as well as major injuries. The areas peaks received upwards of 10
inches of rainfall and widespread wind gusts of 50 to 60 mph were observed. Mount Saint Helena recorded wind
gusts of 80 mph. Mud slide blocking lanes at SR 9 and
Booker Creek Rd.

Santa Clara County Multijurisdictional Hazard Mitigation Plan Section 14: Landslide/Mass Movement 324
Dates of Event Event Type FEMA
Declaration Location Losses/Impacts
1/27/2021 Debris Flow, Heavy Rain None Santa Clara,
Coyote
A plume of moisture from the tropical Pacific brought an Atmospheric River to the Bay Area January 26–29. This
system generated heavy rain rates causing flooding and
debris flows over area burn scars as well as 15 to 20
inches of rain in the Santa Lucia Mountains. Mudflows near the River Fire burn scar in Monterey County caused
damage to homes, covered roadways, and trapped animals
at local ranches. Debris flows near the Dolan Fire burn scar caused an entire section of Highway 1 near Rat Creek to collapse into the Pacific Ocean. This was an unusually cold system for an Atmospheric River resulting in lower snow levels and allowing for accumulating snow as low as 1300 feet in elevation. Additionally, strong south to southeast winds gusted to 60-70 mph across area peaks with Mt
Diablo reaching 80 mph. Valley locations were gusting up to 40 mph. Numerous trees fell across the region including into homes and onto cars. Multiple power outages were
also reported. It is estimated that the storm caused millions of dollars in damage across Santa Cruz County.

Mud/dirt/rocks in roadway at Metcalf Rd and Monterey
Hwy.
3/10/2021 Flood, Heavy Rain None Santa Clara,
San José
A cold upper low moved through the region in early March bringing widespread showers and isolated thunderstorms
to the Greater Bay Area. This system caused roadway
flooding, debris flows, lightning, and small hail. Snow was also reported on some of the area’s peaks throughout the region as snow levels dropped down to 2,000 ft. A
mudslide occurred along the River Fire burn scar in
Monterey County sending mud and debris into nearby
homes. Roadway flooding northbound 280 & 87 due to
heavy rain.

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Dates of Event Event Type FEMA
Declaration Location Losses/Impacts
3/10/2021 Hail None Santa Clara,
San José
International
Airport
A cold upper low moved through the region in early March bringing widespread showers and isolated thunderstorms
to the Greater Bay Area. This system caused roadway
flooding, debris flows, lightning, and small hail. Snow was also reported on some of the area’s peaks throughout the region as snow levels dropped down to 2,000 ft. A
mudslide occurred along the River Fire burn scar in
Monterey County sending mud and debris into nearby
homes. Image on social media showing small hail near the San José International Airport.

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Figure 68: A Santa Clara County Debris Flow Triggered by Winter Storms Following the Loma Fire, 201728 F
283
According to the Santa Clara Valley Water District,283F 284 Santa Clara County has experienced as many as
13 feet of subsidence caused by excessive pumping of groundwater in the early 1900s. The SCVWD was created in the early 1930s to protect groundwater resources and minimize land subsidence. To reduce the demand on groundwater and minimize subsidence, the SCVWD uses a combination of imported surface water (from the State Water Project and San Francisco’s Hetch-Hetchy system), recycled water, and groundwater. Figure 68 shows the history of land surface elevation, groundwater elevation, and the estimated population of Santa Clara County from 1900 up to 2020. The SCVWD started importing water in the 1960s when the groundwater elevation reached its lowest elevation. 283 Swanson, B. (n.d.). A Santa Clara County debris flow triggered by winter storms following the Loma Fire, 2017 [Photograph]. California Geologic Survey.
284 SCVWD. (n.d.). Subsidence. https://www.valleywater.org/your-water/where-your-watercomes/groundwater/subsidence#:~:text=Land%20subsidence%20is%20a%20settling%20of%20the%20Earth%27s,J ose%20to%20southern%20San%20Francisco%20Bay%20were%20impacted.

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Figure 69: SCVWD Historic Groundwater Conditions284F
285

14.2.2 Location
In general, landslide hazard areas are where the land has characteristics that contribute to the risk of the downhill movement of material, such as the following:
 A history of landslide activity or movement during the last 10,000 years.  A steep slope.
 Stream or wave activity, which has caused erosion, undercut a bank, or cut into a bank to cause the surrounding land to be unstable.
 Recent wildfires, as debris flows often occur in areas that experienced wildfires the previous year.285F
286
 Recent construction, construction debris, or erosion due to construction.  The presence of an alluvial fan (geologic features built by runoff spreading out in a wide fan-like area), indicating vulnerability to the flow of debris or sediments. 285 Santa Clara Valley Water District. (2021). Imported Water: Vital to Santa Clara Valley [Photograph]. https://www.valleywater.org/your-water/where-your-water-comes/imported-water 286 USGS California Water Science Center. (2018, October 31). Post-Fire Flooding and Debris Flow. https://ca.water.usgs.gov/wildfires/wildfires-debris-flow.html

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 The presence of impermeable soils, such as silt or clay, which are mixed with granular soils such as sand and gravel.

The best available predictor of where movement of slides and earth flows might occur is the location of past movements. Past landslides can be recognized by their distinctive topographic shapes, which can remain in place for thousands of years. Most landslides recognizable in this fashion range from a few acres to several square miles. Most show no evidence of recent movement and are not currently active. A small proportion of them may become active in any given year, with movements concentrated within all or part of the landslide masses or around their edges.

The recognition of ancient dormant mass movement sites is important in the identification of areas susceptible to flows and slides because they can be reactivated by earthquakes or by exceptionally wet weather. Also, because they consist of broken materials and frequently involve disruption of groundwater flow, these dormant sites are vulnerable to construction-triggered sliding.

The California Landslide Hazard Identification Act directs the State Geologist to identify and map hazardous landslide areas for use by municipalities in planning and decision-making on grading and building permits. Three factors that characterize landslide hazard areas include significant slope, weak rocks, and heavy rains. This program focuses on urban areas and growth areas that exhibit these characteristics. The OA includes both high- and low-risk landslide areas.

The Association of Bay Area Governments Resilience Program286F 287 provides more detailed mapping for the
Bay Area through use of USGS Summary of Distribution of Slides and Earth Flows (1997) and Map Showing Principal Debris-Flow Source Areas (1997). The County of Santa Clara overlaid these data on its jurisdictional boundaries to develop Geological Hazard Zones to suggest areas specific geologic hazards may be present. Additional geologic reports are required for construction in areas where a specific geologic hazard may be present.
287 Association of Bay Area Governments. (n.d.). Resilience. https://abag.ca.gov/our-work/resilience

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Figure 70: Deep-Seated Landslide Susceptibility in Santa Clara County

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14.2.3 Frequency
There are over 75,000 active and dormant landslides mapped in the Bay Area and other natural hazards such as earthquakes, heavy rain, floods, or wildfires continue to trigger landslides so there will likely be more landslide activity in the OA on an annual basis.287F 288 In the OA, landslides typically occur where
landslides and earth flows have occurred in the past. These previous locations may not show any evidence of recent movement and may not be currently active, but some portion of them may become active in any given year from natural hazard events. As shown in Table 109, damage from the El Niño rainstorm event in 1998 was mainly attributed to reactivation of landslide locations and because of sequential severe storms that saturated steep, vulnerable soils. Landslide events occurred during the severe storms of 1983, 1995, and 1998. Atmospheric rivers that occurred along the west coast in 2021 and 2022 also caused landslide events in California. As more frequent storms hit the west coast and wildfires become more frequent due to climate change, there will likely be increased landslide incidents.

Until better data is generated specifically for landslide hazards, this severe storm frequency is appropriate for the purpose of ranking risk associated with the landslide hazard. Subsidence is hard to predict. Given that it is a generally slow and gradual process which develops over time including in areas like the San Joaquin River Delta, it can be assumed the OA is at continuous risk from this hazard.

Further information on the impact of climate change on the probability of landslide/mass movement is included in Section 11.

Probability in OA: Occasional

14.2.4 Severity
Landslides destroy property and infrastructure and can take the lives of people. Slope failures in the United States result in an average of 25 to 50 lives lost per year and are estimated to cost society billions of dollars in damages. Landslides can pose a serious hazard to properties on or below hillsides. When landslides occur—in response to such changes as increased water content, earthquake shaking, addition of load, or removal of downslope support—they deform and tilt the ground surface. The result can be destruction of foundations, offset of roads, breaking of underground pipes, or overriding of downslope property and structures.

14.2.5 Warning Time
The speed of mass movements may range from inches per year to many feet per second, depending on slope, material, and water content. Some monitoring methods can provide an idea of the type of movement and the amount of time prior to failure. It is also possible to determine what areas are at risk during general time periods. Assessing geology, vegetation and predicted precipitation can help in predictions. Landslide early warning systems (LEWS) have gained more attention from researcher, government officials, and other decision makers in recent years.28F 289 The San Francisco Bay region was
once home to the to the first public debris-flow hazard advisory in the United States however, it had to be shut down due to lack of resources. There is currently no practical warning system for individual landslides. The current standard operating procedure is to monitor situations case-by-case and respond after the event has occurred. Warning signs for landslide activity include the following:
 Springs, seeps, or saturated ground in areas that have not typically been wet before 288 Santa Clara Valley Water District. (2017). Local Hazard Mitigation Plan. https://www.valleywater.org/floodingsafety/local-hazard-mitigation-plan 289 Guzzetti, F., & Gariano, S. L., & Peruccacci, S., & Brunetti, M. T., & Marchesini, I., & Rossi, M., & Melillo, M. (2020, January). Geographical Landslide Early Warning Systems. Earth-Science Reviews. https://www.sciencedirect.com/science/article/pii/S0012825219304635

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 New cracks or unusual bulges in the ground, street pavements or sidewalks  Soil moving away from foundations
 Ancillary structures such as decks and patios tilting and/or moving relative to the main house  Tilting or cracking of concrete floors and foundations  Broken water lines and other underground utilities
 Leaning telephone poles, trees, retaining walls or fences  Offset fence lines
 Sunken or down-dropped roadbeds
 Rapid increase in creek water levels, possibly accompanied by increased turbidity (soil content)  Sudden decrease in creek water levels though rain is still falling or just recently stopped  Sticking doors and windows, and visible open spaces indicating jambs and frames out of plumb  A faint rumbling sound that increases in volume as the landslide nears  Unusual sounds, such as trees cracking or boulders knocking together

14.3 Cascading Impacts
Landslides can cause secondary effects such as blocking access to roads, which can isolate residents and businesses and delay transportation. This could result in economic losses for businesses. Other potential problems resulting from landslides are power and communication failures. Vegetation or poles on slopes can be knocked over, resulting in possible losses to power and communication lines.

Landslides also have the potential of destabilizing the foundation of structures, which may result in monetary loss for residents. The damage of destruction of culverts, levees, dams, or other flood mitigation infrastructure during a landslide can result in increased likelihood and damage from flooding. They also can damage rivers or streams, potentially harming water quality, fisheries, and spawning habitat.

14.4 Exposure

14.4.1 Population
Population could not be examined by landslide hazard area because the boundaries of census block groups do not coincide with the hazard area boundaries. However, population was estimated using the building count from the National Structure Inventory multiplied by the most recent average household size from the U.S. Census Bureau. Using this approach, the estimated population living in a moderate to high landslide risk area is 182,752. Table 110 shows the population residing in landslide hazard areas by city.

Table 110: Population Exposed to Landslide Hazard
Jurisdiction Population in hazard
area
% of Total City
Population
Campbell 629 1.45
Cupertino 4,530 7.47
Gilroy 2,578 4.33
Los Altos 1,082 3.40

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Jurisdiction Population in hazard
area
% of Total City
Population
Los Altos Hills 2,986 35.33
Los Gatos 7,527 22.45
Milpitas 2,217 2.76
Monte Sereno 575 16.67
Morgan Hill 4,583 10.18
Mountain View 337 .41
Palo Alto 620 .90
San Jose 39,694 3.91
Santa Clara 614 .48
Saratoga 6,639 21.39
Sunnyvale 611 .39
Unincorporated 16,158 17.90
County Total 91,379 4.72

14.4.2 Property
There are 506,562 acres of land in the OA in a moderate to high landslide risk area. There is also a high number of existing structures and personal vehicles in the OA with an estimated value of $23 billion.

Table 111 shows the number and replacement value of structures exposed to the landslide risk and Table 112 shows the land by acreage exposed to moderate to high landslide hazard in the OA.

Table 111: Exposure and Value of Structures in Moderate to High Landslide Risk Areas Jurisdiction
Estimated Value within the Landslide Risk Area % of
Total
Replace
ment
Value Structure Contents Vehicle Total
Campbell $119,347,275 $93,490,164 $17,091,000 $229,928,440 1.45 Cupertino $642,077,740 $352,803,132 $46,269,000 $1,041,149,872 7.47 Gilroy $426,639,497 $241,668,405 $24,615,000 $692,922,903 4.33 Los Altos $183,084,928 $112,059,655 $11,295,000 $306,439,583 3.40 Los Altos Hills $551,842,500 $29,164,233 $30,159,000 $611,165,733 35.33 Los Gatos $1,329,885,950 $802,145,351 $97,668,000 $2,229,699,302 22.45 Milpitas $280,013,499 $146,512,485 $22,212,000 $448,737,984 2.76 Monte Sereno $102,860,017 $52,144,778 $6,264,000 $161,268,795 16.67 Morgan Hill $636,319,187 $339,329,715 $46,368,000 $1,022,016,903 10.18 Mountain View $50,017,254 $26,790,401 $6,714,000 $83,521,656 0.41 Palo Alto $278,075,584 $207,755,004 $33,669,000 $519,499,589 0.90 San José $5,567,073,749 $3,165,037,570 $450,567,000 $9,182,678,320 3.91

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Jurisdiction
Estimated Value within the Landslide Risk Area % of
Total
Replace
ment
Value Structure Contents Vehicle Total
Santa Clara
(city) $84,759,092 $49,960,448 $14,184,000 $148,903,541 0.48 Saratoga $1,136,966,663 $661,435,350 $78,399,000 $1,876,801,014 21.39 Sunnyvale $104,153,879 $87,361,931 $8,604,000 $200,119,810 0.39 Unincorporated
County $2,611,823,403 $1,557,860,929 $220,905,000 $4,390,589,332 17.90 Total $14,104,940,226 $7,925,519,558 $1,114,983,000 $23,145,442,784 4.72 Table 112: Acreage in Moderate to High Landslide Hazard Areas Jurisdiction
Moderate to High Hazard Area
Area (acres) % of Total
Campbell 143.29 3.7%
Cupertino 2616.51 36.3%
Gilroy 3880.9 36.6%
Los Altos 247.32 5.9%
Los Altos Hills 2321.63 40.0%
Los Gatos 2954.63 39.6%
Milpitas 1572.02 18.1%
Monte Sereno 222 21.4%
Morgan Hill 1996.03 24.2%
Mountain View 105.14 1.3%
Palo Alto 4605.85 27.7%
San José 24693.71 21.3%
Santa Clara (city) 121.54 1.0%
Saratoga 2999.47 36.7%
Sunnyvale 144.7 1.0%
Unincorporated County 457938.24 75.9%
Total 506562.98 60.7%

14.4.3 Critical Facilities and Infrastructure
Table 113 summarizes critical facilities exposed to the landslide hazard in moderate, high, and very high risk areas. No loss estimation of these facilities was performed due to the lack of established damage functions for the landslide hazard. A significant amount of infrastructure, under the Infrastructure Lifeline category, can be exposed to mass movements:

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 Roads: Access to major roads is crucial after a disaster event. Landslides can block roads, causing neighborhood isolation and transportation delays. This can result in economic losses for businesses.
 Bridges: Landslides can damage road bridges. Mass movements can knock out bridge abutments or significantly weaken the soil supporting them, making them hazardous for use.  Power Lines: Power lines are generally elevated above steep slopes; but the towers supporting them can be subject to landslides. A landslide could trigger failure of the soil underneath a tower, causing it to collapse and ripping down the lines.

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Table 113: Critical Facilities and Infrastructure in Moderate to Very High Landslide Risk Areas Jurisdiction Essential Facilities Transportation Utilities Community Assets
Hazardous
Materials Total
Campbell 0 5 0 1 0 6
Cupertino 0 10 0 2 0 12
Gilroy 0 0 0 1 0 1
Los Altos 1 3 0 0 0 4
Los Altos Hills 2 10 0 0 0 12
Los Gatos 3 12 0 0 0 15
Milpitas 0 5 0 0 0 5
Monte Sereno 0 0 0 0 0 0
Morgan Hill 1 1 0 0 0 2
Mountain View 0 9 0 0 0 9
Palo Alto 0 5 1 3 0 9
San José 11 110 1 10 0 132
Santa Clara (city) 0 11 0 1 0 12
Saratoga 5 14 0 3 0 22
Sunnyvale 0 2 0 0 0 2
Unincorporated County 12 73 4 30 0 119
Total 35 269 6 49 0 359

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14.4.4 Environment
Environmental problems as a result of mass movements can be numerous. Landslides that fall into streams may significantly impact fish and wildlife habitat, as well as affecting water quality. Hillsides that provide wildlife habitat can be lost for prolong periods of time due to landslides. Some habitat types support more diverse species than others. In California, riparian areas contain the greatest diversity of species. For an area such as Santa Clara County, where steeps slopes, landslide potential, and other related geologic hazards are prevalent, erosion control is important to minimize the related adverse effects of a mass-movement event.289F
290

14.5 Vulnerability
Change in conditions: Increase in vulnerability
 As shown in section 14.2.1, an increasing number of landslide events have been documented since 2017.
 There has been a recent increase in the number of heavy rainfall events. Increased precipitation contributes to saturated soils which may stimulate landslide movement.  Landslide risk is highest in the foothills. Roads here are often narrow, or may have limited access such as dead-end spur roads. This makes ingress and egress difficult if roadways become blocked by slides.

14.5.1 Population
All of the estimated 91,379 persons exposed to high landslide risk areas are considered to be vulnerable.

Los Altos Hills, Los Gatos, and Saratoga have the highest percentage of population residing in areas at high risk to landslides. Increasing population and the fact that many homes are built on view property atop or below bluffs and on steep slopes subject to mass movement, increases the number of lives endangered by this hazard. Landslides losses don’t tend to be as heavily felt by socially vulnerable populations as other hazards. Everyone that lives in high-risk areas is at risk. There has been little research on human vulnerability to landslide as most research focuses on structural vulnerability however, some studies suggest that human behavior can play an important role in predicting potential losses.290F
291 While the size and timing of some landslides mean that injury or death is highly likely, advanced warning and immediate action (such as moving to a higher floor) can sometimes make all the difference. Education and outreach could help mitigate this risk to the public.

Some landslides onset too quickly for protective action to be taken, which can affect all demographic groups. However, residents who are older, those who are young, those who are visually or hearing impaired, or those who have mobility difficulties due to physical impairments may be particularly at risk to landslide. In cases where warning time is possible prior to a landslide, people with limited English proficiency may have difficulty receiving or understanding warning from a television, radio emergency warning system or cell phone alert to take appropriate action. 290 County of Santa Clara. (1994). Santa Clara County General Plan. https://stgenpln.blob.core.windows.net/document/GP_Book_A.pdf 291 Pollock, W. & Wartman, J. (2020, October 4). Human Vulnerability to Landslides. National Library of Medicine. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7567151/

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14.5.2 Property
Although complete historical documentation of the landslide threat in the OA is lacking, the mountainous terrain surrounding the Santa Clara Valley indicates potential for landslides. Loss estimations for the landslide hazard are not based on modeling utilizing damage functions, because no such damage functions have been generated. Instead, loss estimates were developed representing 10 percent, 30 percent, and 50 percent of the replacement value of exposed structures. This allows emergency managers to select a range of economic impact based on an estimate of the percent of damage to the general building stock. Table 114 shows the general building stock loss estimates in the aggregate of all landslide risk areas.

Table 114: Loss Potential Based on All Building Stock in Aggregated Landslide Areas Jurisdiction Exposed Value
Estimated Loss Potential from Landslide
10% Damage 30% Damage 50% Damage
Campbell $229,928,440 $22,992,844 $68,978,532 $114,964,220 Cupertino $1,041,149,872 $104,114,987 $312,344,962 $520,574,936 Gilroy $692,922,903 $69,292,290 $207,876,871 $346,461,452 Los Altos $306,439,583 $30,643,958 $91,931,875 $153,219,792 Los Altos Hills $611,165,733 $61,116,573 $183,349,720 $305,582,867 Los Gatos $2,229,699,302 $222,969,930 $668,909,791 $1,114,849,651 Milpitas $448,737,984 $44,873,798 $134,621,395 $224,368,992 Monte Sereno $161,268,795 $16,126,880 $48,380,639 $80,634,398 Morgan Hill $1,022,016,903 $102,201,690 $306,605,071 $511,008,452 Mountain View $83,521,656 $8,352,166 $25,056,497 $41,760,828 Palo Alto $519,499,589 $51,949,959 $155,849,877 $259,749,795 San José $9,182,678,320 $918,267,832 $2,754,803,496 $4,591,339,160 Santa Clara
(city) $148,903,541 $14,890,354 $44,671,062 $74,451,771 Saratoga $1,876,801,014 $187,680,101 $563,040,304 $938,400,507 Sunnyvale $200,119,810 $20,011,981 $60,035,943 $100,059,905 Unincorporated
County $4,390,589,332 $439,058,933 $1,317,176,800 $2,195,294,666 Total $23,145,442,784 $2,314,544,278 $6,943,632,835 $11,572,721,392

14.5.3 Critical Facilities and Infrastructure
There are 398 critical facilities exposed to the landslide hazard to some degree. A more in-depth analysis of the mitigation measures taken by these facilities to prevent damage from mass movements should be done to determine if they could withstand impacts of a mass movement.

Several types of infrastructure are exposed to mass movements, including transportation, water and sewer and power infrastructure. Highly susceptible areas of the OA include mountain roads and transportation infrastructure. At this time all infrastructure and transportation corridors identified as exposed to the landslide hazard are considered vulnerable until more information becomes available.

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14.5.4 Environment
Landslides and other mass movements impact the environment through changing the earth’s topography; diverting or reducing the quality of rivers, streams, and groundwater; destroying forests and vegetation; and the destruction of natural habitats particularly around at-risk water bodies. Erosion from landslide can significantly degrade the potability water and its capacity to serve as a habitat for fish and other wildfire.

Landslides are difficult to predict. The environment most vulnerable to landslide hazard is the same as the environment exposed to the hazard.

14.6 Future Trends in Development
Santa Clara County has been one of the state’s fastest growing counties from 2010-2020, but more recently has experienced a decrease in population. It can be assumed the OA will continue to grow, though at a slower pace. Mass movements are becoming more of a concern as development moves outside of urban centers and into areas less developed in terms of infrastructure. The planning partners are equipped to handle future growth within landslide hazard areas. Landslide risk areas are addressed in the safety elements of local general plans. All planning partners have committed to linking their general plans to this hazard mitigation plan. This will create an opportunity for wise land use decisions as future growth impacts landslide hazard areas.

Additionally, the State of California has adopted the International Building Code (IBC) by reference in its California Building Standards Code. The IBC includes provisions for geotechnical analyses in steep slope areas that have soil types considered susceptible to landslide hazards. These provisions assure that new construction is built to standards that reduce the vulnerability to landslide risk. California real estate disclosure laws will also help ensure the public is aware of their risk.

14.7 Scenario
Major landslides could occur in the OA as a result of soil conditions that have been affected by severe storms, groundwater, or human development. The worst-case scenario for landslide hazards in the OA would generally correspond to a severe storm such which includes heavy rain and causes flooding in the OA. Landslides are most likely during late winter when the water table is high. After heavy rains from November to December, soils become saturated with water. As water seeps downward through upper soils that may consist of permeable sands and gravels and accumulates on impermeable silt, it will cause weakness and destabilization in the slope. As rains continue, the groundwater table rises, adding to the weakening of the slope. Gravity, poor drainage, a rising groundwater table and poor soil exacerbate hazardous conditions. An intense storm could cause saturated soil to move, resulting in landslides.

Continued heavy rains and flooding will complicate the problem further. As emergency response resources are applied to problems with flooding, it is possible they will be unavailable to assist with landslides occurring all over the OA. In this scenario, it is probable that private and public property, including infrastructure, could be affected. Members of the public could be injured or killed. Road obstructions caused by mass movements would create isolation problems for residents and businesses in sparsely developed areas. Mass movements could affect bridges that pass over landslide prone ravines and knock out rail service through the OA. Landslides carrying vegetation such as shrubs and trees may cause a break in utility lines, cutting off power and communication access to residents. The impacts to the immediate vicinity of a landslide would be severe while the impacts to the OA would depend on what public infrastructure, including transportation routes, are involved.

14.8 Issues
Important issues associated with landslides in the OA include the following:
 There are existing homes in landslide risk areas throughout the OA. The degree of vulnerability of these structures depends on the codes and standards the structures were constructed to.

Information to this level of detail is not currently available.

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 Future development could lead to more homes in landslide risk areas.  Mapping and assessment of landslide hazards are constantly evolving. As new data and science become available, assessments of landslide risk should be reevaluated.  The impact of climate change on landslides is uncertain. If climate change impacts atmospheric conditions, then exposure to landslide risks is likely to increase.  Landslides may cause negative environmental consequences, including water quality degradation.
 The risk associated with the landslide hazard overlaps the risk associated with other hazards such as earthquake, flood, and wildfire. This provides an opportunity to seek mitigation alternatives with multiple objectives that can reduce risk for multiple hazards.

Table 115: EMAP Consequence Analysis: Landslide/Mass Movement Subject  Ranking  Impacts/Landslide/Mass Movement
Public  Minimal to
moderate
The public is at risk from injury, loss of life, and loss of property from a landslide, debris flow, or other mass
movement event. Because of the localized nature of these events, the damage would be severe in the immediate
vicinity and less impactful further away from the site.

Responders  Minimal  Responders may be called upon in the event of a landslide/mass movement event. They may be asked to
assess the situation, rescue survivors, divert traffic, and manage the overall incident.

During the course of their duties, they may be exposed to an increased risk of personal injury. Responders
responding to another hazard event could also be at risk from a landslide caused by an event like an earthquake’s aftershock or flood.

Continuity of
Operations (including
continued delivery of
services)
Minimal to
moderate
Continuity of operations and continued delivery of
services would depend on if a facility was in the
immediate vicinity of the landslide/mass movement event.

There also may be transportation delays if key roadways are impacted.

Property, Facilities, and
Infrastructure
Minimal to
severe
Landslides are a serious hazard to property, facilities, and infrastructure. They can result in damage and destruction to property through the destruction of foundations, offset of roads, destruction of bridges, breaking of underground pipes, or overriding of downslope property and structures.

Environment  Minimal to
moderate
Landslides and other mass movement events can alter
the earth’s topography, change water courses and water
quality, remove forests and vegetation, destroy natural habitats, and cause erosion. Landslide areas are prone to repeated slides.

Economic Conditions  Minimal to
moderate
Impacts to the economy will depend on the location of the event. Disruptions to impacted facilities, critical
infrastructure, or transportation routes could have a longterm effect on the local economy.

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Subject  Ranking  Impacts/Landslide/Mass Movement
Public Confidence in
the Government
Minimal to
severe
The public’s confidence will depend on the government’s ability to manage and properly message the event.

Prompt and accurate information will influence public
trust.

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15 Tsunami
Definitions
 Tsunami: A series of traveling ocean waves of extremely long wavelength usually caused by displacement of the ocean floor and typically generated by seismic or volcanic activity or by underwater landslides.
 Seiche: A standing wave in an enclosed or partially enclosed body of water such as bays and lakes. Seiches are typically caused when strong winds and rapid changes in atmospheric pressure or an earthquake push water from one end of a body of water to the other.

15.1 General Background

15.1.1 Tsunami
A tsunami consists of a series of high-energy waves that radiate outward like pond ripples from an area where a generating event occurs. Earthquakes may produce displacements of the sea floor that can set the overlying column of water in motion, initiating a tsunami, depending on the magnitude of the earthquake and the type of faulting. Landslides, including from glaciers melting, and underwater volcanos are also sources of these events.

Tsunamis are typically classified as local or distant. Locally generated tsunamis have minimal warning times, leaving few options except to relocate to high ground. They may be accompanied by damage resulting from the triggering earthquake due to ground shaking, surface faulting, liquefaction, or landslides.

Distant tsunamis may travel for hours before striking a coastline, giving a community a chance to implement evacuation plans. In the open ocean, a tsunami may be only a few inches or feet high, but it can travel with speeds approaching 600 miles per hour. Tsunami waves arrive at shorelines over an extended period. Figure 70 shows likely travel times across the Pacific Ocean for a tsunami generated along the California coastline near the San Francisco Bay Area.

As a tsunami enters the shoaling waters near a coastline, its speed diminishes, its wavelength decreases, and its height increases greatly. The first wave usually is not the largest. Several larger and more destructive waves often follow the first one. As tsunamis reach the shoreline, they may take the form of a fast-rising tide, a cresting wave, or a bore (a large, turbulent wall-like wave). The bore phenomenon resembles a step-like change in the water level that advances rapidly (from 10 to 60 miles per hour).

The configuration of the coastline, the shape of the ocean floor, and the characteristics of advancing waves play important roles in the destructiveness of the waves. Offshore canyons can focus tsunami wave energy and islands can filter the energy. The orientation of the coastline determines whether the waves strike head-on or are refracted from other parts of the coastline. A wave may be small at one point on a coast and much larger at other points. Bays, sounds, inlets, rivers, streams, offshore canyons, islands, and flood control channels may cause various effects that alter the level of damage. It has been estimated, for example, that a tsunami wave entering a flood control channel could reach a mile or more inland, especially if it enters at high tide.

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The first visible indication of an approaching tsunami may be recession of water (draw down) caused by the trough preceding the advancing, large inbound wave crest. Rapid draw down can create strong currents in harbor inlets and channels that can severely damage coastal structures due to erosive scour around piers and pilings. As the water’s surface drops, piers can be damaged by boats or ships straining at or breaking their mooring lines. The vessels can overturn or sink due to strong currents, collisions with other objects, or impact with the harbor bottom.

Figure 71: Potential Tsunami Travel Times in the Pacific Ocean, In Hours291 F 292
Conversely, the first indication of a tsunami may be a rise in water level. The advancing tsunami may initially resemble a strong surge increasing the sea level like the rising tide, but the tsunami surge rises faster and does not stop at the shoreline. Even if the wave height appears to be small, 3 to 6 feet for example, the strength of the accompanying surge can be deadly. Waist-high surges can cause strong currents that float cars, small structures, and other debris. Boats and debris are often carried inland by the surge and left stranded when the water recedes.

At some locations, the advancing turbulent wave front will be the most destructive part of the wave. In other situations, the greatest damage will be caused by the outflow of water back to the sea between crests, sweeping all before it and undermining roads, buildings, bulkheads, and other structures. This outflow action can carry enormous amounts of highly damaging debris with it, resulting in further destruction. Ships and boats, unless moved away from shore, may be dashed against breakwaters, wharves, and other craft, or be washed ashore and left grounded after the withdrawal of the seawater. 292 National Oceanic and Atmospheric Administration. (n.d.). Estimated Tsunami Travel Times to Coastal Locations. https://www.ncei.noaa.gov/maps/ttt_coastal_locations/

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15.1.2 Seiche
A seiche is a standing wave in an enclosed or partially enclosed body of water, such as San Francisco Bay.

Seiches are typically caused when strong winds and rapid changes in atmospheric pressure or an earthquake push water from one end of a body of water to the other. The largest seiche that was ever measured in the San Francisco Bay, following the 1906 earthquake, was 4 inches high. The Bay Area has not been adversely affected by seiches.29F
293 However, the OA may see seiches on creeks if there was a local earthquake event. These kinds of seiches could have devastating environmental impacts.

15.2 Hazard Profile

15.2.1 Past Events
According to the 2023 California State Hazard Mitigation Plan,293F 294 over 80 tsunamis have been observed
or recorded along the coast of California in the past 150 years. The National Centers for Environmental Information294F
295 has recorded the California coastline being impacted by tsunami wave events on six dates since 2005: November 15, 2006, February 27, 2010, March 11, 2011, September 16, 2015, January 1, 2022, and January 15, 2022. Together these events caused approximately $55 million in property damage and cost one life. The Santa Clara County OA has never been impacted by a tsunami. The closest tsunami to affect the OA was the tsunami event on March 10, 2011, that occurred in Japan and traveled across the Pacific Ocean to create wave surges that damaged coastal areas in nearby Santa Cruz and Monterey Counties. These counties were included in FEMA-1968-DR-CA declaration. The largest impact of this tsunami experienced in the San Francisco Bay was at Berkeley Marina, where the tsunami caused about $50,000 in damages.

15.2.2 Location
Although the OA has not been significantly impacted by tsunami or seiche events before, the recent tsunamis due to disaster events around the world – including earthquakes and an underwater volcano eruption - has underscored how vulnerable the California coastline is to this type of hazard. Following these events, there have been a number of new studies and reports expanding on the State of California’s understanding of its exposure to tsunami risk. Cal OES, in conjunction with the University of Southern California and FEMA, developed Tsunami Hazard Areas Maps to assist cities and counties in identifying their tsunami hazard for tsunami response planning utilizing the best available scientific information. The risk areas are not defined in terms of human-created geographic features, however, according to the 2022 California Geological Survey Tsunami Hazard Area Map295 F 296 – County of Santa
Clara, approximately the area of the OA around the San Francisco Bay including parts of Sunnyvale, San José, Palo Alto, and Mountain View are in the Tsunami Hazard Area.

While only a relatively small portion of the OA may be directly impacted by a tsunami wave, additional tsunami impacts could be felt in the OA along area creeks that would rise with floodwaters from a San Francisco Bay tsunami caused by a local earthquake.
293 Alameda County Community Development Agency. (2022, March 17). Safety Element of the Alameda County General Plan. https://www.acgov.org/cda/planning/generalplans/documents/SafetyElementupdateapprovedbyBOS31722-FINAL.pdf 294 California Office of Emergency Services. (2023). Hazard Mitigation Planning. https://www.caloes.ca.gov/office-ofthe-director/operations/recovery-directorate/hazard-mitigation/state-hazard-mitigation-planning/ 295 National Centers for Environmental Information. (n.d.). Storm Events Database. https://www.ncdc.noaa.gov/stormevents/
296 California Department of Conservation. (2022, October 7). California Tsunami Maps and Data. https://www.conservation.ca.gov/cgs/tsunami/maps

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Figure 71 shows potential tsunami inundation areas on the southern portion of the San Francisco Bay and Coyote Creek, which is the northern portion of the Santa Clara County OA.

Figure 72: Tsunami Hazard Area

15.2.3 Frequency
The frequency of tsunamis is related to the frequency of the events that cause them, so it is similar to the frequency of seismic, volcanic activities, or landslides around the Pacific Basin. Generally, four or five tsunamis occur every year in the Pacific Basin.

It is unclear what impact future conditions such as climate change may have on the frequency of tsunami events. It is possible that increased severe flood events, wildfires, and warming conditions could impact

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the frequency of landslides, which are one known sources of tsunamis. However, they make up only a relatively small percentage of the root causes of tsunamis. Earthquakes are the most common cause of tsunamis. Current data indicates that there is not a statistically significant impact of climate change as we know it on earthquakes. This doesn’t necessarily mean that climate change does not have impact.

Current science may simply lack the capability to distinguish a direct correlation between these hazards.

For example, earthquakes can be triggered by changes to the amount of stress on a fault. According to NASA’s Jet Propulsion Laboratory at the California Institute of Technology, changing conditions, such as surface water levels, due to hazards such as flooding and droughts may impact earthquakes, but we have no way of knowing by how much. It is unlikely that the OA would notice a significant change in the frequency of tsunami events due to climate change.

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

Probability in OA: Unlikely

15.2.4 Severity
Tsunamis are a threat to life and property to anyone living or visiting near the ocean. The full extent of a tsunami event can be severe, and is often measured by magnitude, similar to earthquakes. The higher the magnitude, the more severe and destructive the tsunami may be.

From 1950 to 2007, 478 tsunamis were recorded globally. Fifty-one of these events caused fatalities, to a total of over 308,000 coastal residents. The overwhelming majority of these events occurred in the Pacific basin. Recent tsunamis have struck Nicaragua, Indonesia, and Japan, killing several thousand people.

Property damage due to these waves was nearly $1 billion. The San Francisco Bay faces the greatest threat from distant tsunamis originating in areas such as Alaska, Washington, and Japan.

It is general consensus that the Santa Clara County OA would not likely see significant impacts from a tsunami originating in the Pacific Ocean, given the area’s primarily inland location. The extent of the damage would be limited to the immediate area. Boats, docks, and property near the bay may suffer some impacts if they were to be hit directly. Additionally, the OA would likely see minor tsunami impacts on creeks from a local earthquake event, with any floodwaters flowing up creeks impacting people visiting the creeks. A local earthquake tsunami can occur any time, and the resulting floodwater waves can carry damaging debris.

Some studies suggest that future conditions such as climate change are likely to increase the severity of tsunamis. Sea-level rise is one of the main reasons why. These additional impacts will most severely be experienced in low-elevation Pacific islands however, all coastal communities are at risk of the dangers associate with sea-level rise. Scientists studying the probability of an Alaska-Aleutian subduction zone earthquake and the subsequent impact to southern California supported the conclusion that rising sea levels can increase tsunami impacts. If the sea continues to rise as predicted, someday the impacts of a small tsunami will be similar to a large tsunami of today. The risk to the OA may change over time.

15.2.5 Warning Time
Typical signs of a tsunami hazard are earthquakes and/or sudden and unexpected rise or fall in coastal water. The large waves are often preceded by coastal flooding and followed by a quick recession of the water. Tsunamis are difficult to detect in the open ocean; with waves less than 3 feet high. The tsunami’s size and speed, as well as the coastal area’s form and depth, affect the impact of a tsunami; wave heights of 50 feet are not uncommon. In general, scientists believe it requires an earthquake of at least a magnitude 7 to produce a tsunami.