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<br /> <br />2021 Multijurisdictional Local Hazard Mitigation Plan <br /> <br />11.6 SCENARIO <br />Historically, floods have regularly affected San Mateo County. The County can expect noteworthy flooding about <br />once a year, with a flash flood approximately every 2 years. Duration and intensity of heavy winter rains and El <br />Niño storms that cause flooding may increase due to climate change. The floodplains mapped and identified by <br />San Mateo County will continue to take the brunt of these floods. County community members prepare <br />themselves for flooding by seeking and receiving information, and by pursuing mitigation. Impacts of flood <br />events should decrease as the County, local cities, and community members continue to promote and implement <br />hazard mitigation and preparedness. <br /> <br />The worst-case scenario would be a series of heavy rains or storm events during an El Niño event or winter rainy <br />season, particularly if the rains also occur at high tide. These rains could flood numerous areas within a short <br />time. This could overwhelm the response and floodplain management capability within the planning area, as the <br />planning area would be subject immediately to flash flooding and coastal flooding, with subsequent influences on <br />the County’s streams. Major roads could be blocked, preventing critical access for many community members and <br />critical functions. High in-channel flows could cause water courses to scour, possibly washing out roads and <br />creating more isolation problems. In the event of multi-basin flooding, San Mateo County would not be able to <br />make repairs quickly enough to restore critical facilities and assets. <br /> <br />11.7 ISSUES <br />The planning team has identified the following flood-related issues relevant to the planning area: <br /> Accuracy of existing flood hazard mapping by FEMA regarding true flood risk within the planning area is <br />questionable. This is most prevalent within areas protected by levees not accredited by the FEMA <br />mapping process. <br /> Over 60 percent of the population within the 1 percent annual chance floodplain have either very high or <br />relatively high social vulnerability. <br /> Extent of flood protection currently provided by flood control facilities (dams, dikes, and levees) is not <br />known due to lack of established national policy on flood protection standards. <br /> The levee system within the planning area is not consistently adequate to mitigate effects of a 1-percent <br />annual chance flood. <br /> Risk associated with the flood hazard overlaps risks associated with other hazards such as earthquakes, <br />landslides, and coastal erosion. This provides opportunity to seek mitigation alternatives with multiple <br />objectives that can reduce risks from multiple hazards. <br /> Land-use practices are not consistent with the scope of regulatory floodplain management within the <br />planning area. <br /> How climate change will affect flood conditions in San Mateo County is uncertain. <br /> More information is needed regarding flood risk to support the concept of risk-based analysis of capital <br />projects. <br /> To determine cost-effectiveness of future mitigation projects, sustained effort is necessary to gather <br />damage reports and historical damage data such as high-water marks on structures. <br /> Ongoing flood hazard mitigation will require funding from multiple sources. <br /> <br /> <br /> <br /> <br />11-28