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2021 Multijurisdictional Local Hazard Mitigation Plan several ways: reducing a stream’s capacity to contain flows; increasing flow rates or velocities downstream; and allowing waves to extend further inland. Human activities can interface effectively with a floodplain as long as steps are taken to mitigate the activities’ adverse impacts on floodplain functions. 11.1.4 Secondary Hazards The most problematic secondary hazard for riverine flooding is bank erosion, in some cases more harmful than actual flooding. This is especially true in the upper courses of rivers with steep gradients, where floodwaters may pass quickly and without much damage, but scour banks, edging properties closer to the floodplain or causing them to fall in. Flooding is also responsible for hazards such as landslides when high flows over-saturate soils on steep slopes, causing them to fail. Hazardous materials spills are also a secondary hazard of flooding if storage tanks rupture and spill into streams, rivers, or storm sewers. 11.2 HAZARD PROFILE 11.2.1 Federal Flood Program Participation National Flood Insurance Program (NFIP) Table 11-1 lists flood insurance statistics that help identify vulnerability within the planning area. More than 5,200 policies are in force providing more than $1.6 billion in insurance. According to FEMA statistics, flood insurance claims were paid between January 1, 1978, and November 30, 2020, for a total of $10.3 million, an average of $11,580 per claim. Properties constructed after adoption of a FIRM or DFIRM are considered less vulnerable to flooding because they were constructed after adoption of regulations and codes to decrease vulnerability. Properties built before adoption of a FIRM or DFIRM are more vulnerable to flooding because either they do not meet code or are within hazardous areas. The first flood maps of the planning area became available as early as 1971; however, most FIRMs were not available until the 1980s. All municipal partners to this plan participate in the NFIP. Their current standing under the NFIP is described in the capability assessment section of their annexes in Volume 2 (Chapters 1 to 21). Community Rating System Five planning partners currently participate in the CRS program. Table 11-2 summarizes the CRS status of each. Many of the mitigation actions identified in this plan are creditable activities under the CRS program. Therefore, successful implementation of this plan offers the potential to enhance the CRS classification. 11.2.2 Principal Flooding Sources Natural stream channels in rural parts of San Mateo County 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. To control these floodwaters when they reach more urban areas, the County and its cities have developed various flood control districts and flood improvements, such as culverts, bridges, levees, channel alterations, and underground storm drains (San Mateo County OES, 2015). 11-6