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Chapter 7 – Flood Hazards: Risks and Mitigation

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CHAPTER 7–FLOOD HAZARDS CHAPTER 7 – FLOOD HAZARDS: RISKS AND MITIGATION CHAPTER CONTENT 7.1 Riverine, Stream, and Alluvial Flood Hazards, Vulnerability and Risk Assessment 7.1.1 Identifying Riverine, Stream, and Alluvial Flood Hazards 7.1.2 Profiling Riverine, Stream, and Alluvial Flood Hazards 7.1.3 Assessment of State Flood Vulnerability and Potential Losses 7.1.4 Assessment of Local Flood Vulnerability and Potential Losses 7.1.5 Current Riverine, Stream, and Alluvial Flood Hazard Mitigation Efforts 7.1.6 Additional Flood Hazard Mitigation Opportunities 7.2 Sea-Level Rise, Coastal Flooding, and Erosion Hazards, Vulnerability and Risk Assessment 7.2.1 Identifying Sea-Level Rise, Coastal Flooding, and Erosion Hazards 7.2.2 Profiling Sea-Level Rise, Coastal Flooding, and Erosion Hazards 7.2.3 Assessment of Sea-Level Rise, Coastal Flooding, and Erosion Vulnerability and Potential Losses 7.2.4 Current Sea-Level Rise, Coastal Flooding, and Erosion Hazard Mitigation Efforts 7.3 Tsunami and Seiche Hazards, Vulnerability and Risk Assessment 7.3.1 Identifying Tsunami and Seiche Hazards 7.3.2 Profiling Tsunami Hazards 7.3.3 Assessment of State Tsunami Vulnerability and Potential Losses 7.3.4 Assessment of Local Tsunami Vulnerability and Potential Losses 7.3.5 Current Tsunami Hazard Mitigation Efforts 7.3.6 Additional Tsunami Hazard Mitigation Opportunities 7.4 Levee Failure and Safety, Vulnerability and Risk Assessment 7.4.1 Identifying Levee Hazards 7.4.2 Profiling Levee Hazards 7.4.3 Assessment of Levee Failure Vulnerability and Potential Losses 7.4.4 Current Levee Failure Hazard Mitigation Efforts 7.5 Dam Failure and Safety Hazards, Vulnerability and Risk Assessment 7.5.1 Identifying Dam Hazards – Failures and Overtopping 7.5.2 Profiling Dam Hazards – Failures and Overtopping 7.5.3 Assessment of Dam Failure Vulnerability and Potential Losses 7.5.4 Current Dam Failure Hazard Mitigation Efforts 7.5.5 Additional Dam Failure Hazard Mitigation Opportunities About Chapter 7 Chapter 7 assesses hazards and risks related to flooding. Flooding is considered one of the three primary hazards in California (along with earthquake and wildfire), as explained in Section 1.2.3 of the 2018 SHMP (see that section for a discussion of the hazard classification system and information on the criteria used for hazard risk assessments). Flood hazards discussed in this chapter include riverine, stream, and alluvial flooding and coastal flooding, erosion, and sea level rise, all of which are influenced by climate and weather. Levee and dam failure are identified as secondary hazards, because they may be triggered by primary hazard events and also by flooding and inundation resulting from tsunamis. California’s geographic diversity represents a difficult challenge to planning for flood mitigation. California has a 1,100-mile-long coastline; prominent coastal and inland mountain ranges, including the Sierra Nevada; a large riverine Central Valley; the Sacramento and San Joaquin Delta; and extensive and highly varied deserts. These geographical factors combine to create various types of floods, specifically defined in the SHMP as: Riverine—flooding that occurs along river and stream channels and that can range from slow-rise gradual inundation to flash floods from high velocity flows. CALIFORNIA STATE HAZARD MITIGATION PLAN │ SEPTEMBER 2018 SECTION 7.1 - PAGE 375 CHAPTER 7–FLOOD HAZARDS Alluvial fan—flows of shallow depths and high velocities often containing sediment and rocks along uncertain flow paths on the surface and at the toes of alluvial fans. Coastal—inundation of locations normally above high tide, often caused by storm surge occurring with high tide and exacerbated over time with climate change-induced sea-level rise. Increased coastal erosion can also result from these conditions. Engineered structure failure—flooding resulting from dam or levee failure. Tsunami—high-speed seismic ocean waves triggered by earthquakes and underwater landslides. For more information on the criteria and template used for hazard risk assessments and a discussion of the hazard classification system, see Chapter 1: Introduction, Section 1.2.3. RIVERINE, STREAM, AND ALLUVIAL FLOOD HAZARDS, VULNERABILITY AND RISK ASSESSMENT Floods represent the second most destructive source of hazard, vulnerability, and risk, both in terms of recent state history and the probability of future destruction at greater magnitudes than previously recorded. In addition to causing tragic loss of life, flooding in California can have a serious impact on the state’s economy and environmental resources. With California representing one of the world’s largest economies, a major flood here will have an unprecedented impact on the national economy as well. When California floods: Critical infrastructure is damaged and could be out of service for long periods Vital services become isolated or are closed Jobs are lost or put at risk when businesses are dislocated or closed Water supplies and water quality are affected Vulnerable communities are displaced and/or personal property is lost Natural resources and public access are damaged or eliminated. To manage flood risk, California has a complex system of flood infrastructure consisting of approximately 20,000 miles of levees, more than 1,500 dams and reservoirs (1,250 of which are under state jurisdiction), and more than 1,000 debris basins. Federal and state facilities in the Central Valley include approximately 1,600 miles of project levees, several bypasses and appurtenant weir and control structures, seven dams, and other associated facilities. IDENTIFYING RIVERINE, STREAM, AND ALLUVIAL FLOOD HAZARDS This section addresses floods as one of three primary hazards in the classification system introduced in Chapter 1, Section 1.2.3 and includes information identifying the following dimensions of this hazard: Its location within the state (i.e., geographic area affected) Previous occurrences within the state The probability of future events (i.e., chances of recurrence) Floods represent a significant concern for the State of California for several reasons. First, California has a chronic and destructive flooding history. Second, California has widespread flooding vulnerability as indicated by the Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map (FIRM) designations, which show flood hazard zones being common in populated areas. Third, most local governments that have FEMA-approved Local Hazard Mitigation Plans (LHMPs) have identified flooding as an important hazard. CALIFORNIA STATE HAZARD MITIGATION PLAN │ SEPTEMBER 2018 SECTION 7.1 - PAGE 376 CHAPTER 7–FLOOD HAZARDS PROFILING RIVERINE, STREAM, AND ALLUVIAL FLOOD HAZARDS Every county in the state experiences floods, although the nature of these events varies due to the state’s diverse climatology and geography. Disparate climatological patterns present challenges to flood mitigation planning in California. These patterns include: El Niño conditions La Niña conditions Desert monsoons Northwest coastal conditions Tropical storms Gulf of Alaska storms Atmospheric river patterns Flooding, erosion, and debris flows can also occur in California in the months and years following large hot fires. High severity wildfires greatly reduce the amount of vegetation, which can reduce the amount of rainwater absorption, allowing excessive water runoff that often includes large amounts of debris. Structures located anywhere near a severe burn area are susceptible to flooding. Periods of high-intensity rainfall are of particular concern, but post-fire flooding can also occur during a normal rainy season. For more information, see: https://www.water.ca.gov/LegacyFiles/sfmp/resources/Computer_View_Highlight.pdf. Hydrologic Regions California’s ten hydrologic regions present disparate flood mitigation planning challenges. The following is a brief description of the ten regions, shown in Map 7.A. North Coast Hydrologic Region The North Coast hydrologic region runs along the Pacific Coast from the California-Oregon border to the mouth of the Russian River. This region is sparsely populated, with the majority of settlement in the Humboldt Bay area. The area receives larger rain totals than any other region and has historically experienced some of the state’s most spectacular and devastating flood events. Tsunamis also pose a very real threat, particularly to the community of Crescent City in Del Norte County. San Francisco Bay Hydrologic Region The San Francisco Bay hydrologic region runs along the north central coast and encompasses most of the Bay Area counties. It reaches to just north of Ukiah in Mendocino County, south to the Coyote Creek watershed in Santa Clara County, and inland to just east of the Sacramento-San Joaquin Delta. The area around San Francisco Bay is heavily populated, and the entire region is marked by hills, river valleys such as those along the Russian River, and marshlands. The region is most vulnerable to classic stream flooding, landslides, and some urban flooding. Flooding along the coastal and bay shorelines can be severe when winter storms coincide with high tides. Sonoma County, most of which is located in this region, records the most National Flood Insurance Program (NFIP) repetitive losses of any area in California. Central Coast Hydrologic Region The Central Coast hydrologic region reaches from Año Nuevo Point in San Mateo County down the Pacific Coast to near the crest of the coast range in Santa Barbara County. The region is mountainous with
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