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2021 Multijurisdictional Local Hazard Mitigation Plan Principal flooding sources for San Mateo County as identified on FEMA flood maps include the following streams and water bodies:  Alpine Creek  Belmont Creek  Butano Creek  Colma Creek  Crystal Springs Channel  Denniston Creek  El Granada Creek  Holly Street Channel  La Honda Creek  Lomita Channel  Montara Creek  Pacific Ocean  Pescadero Creek  San Bruno Channel  San Francisquito Creek  San Gregorio Creek  San Vincente Creek  Woodhams Creek Over 20 creeks, channels, and water bodies, including those identified as principal flooding sources, were assessed as part of the County’s FIS. In addition to the waterways above, the FIS identified areas at risk for potential tsunami inundation. The Cities of Half Moon Bay and Pacifica are both associated with potential tsunami issues. Additional information regarding the tsunami hazard is in Chapter 15. Investigation of San Mateo County’s vulnerability to flooding can also include assessments of watershed locations. Every watershed has unique qualities that affect its response to rainfall. San Mateo County contains 34 watersheds, all of which are relatively small and drain into either the Pacific Ocean or San Francisco Bay. Unincorporated areas in the County contain 21 major watersheds. Except for Crystal Springs and San Francisquito, which both drain into the San Francisco Bay, all the rural watersheds drain into the Pacific Ocean (San Mateo County OES, 2015). 11.2.3 Principal Flood Problems The 2019 Flood Insurance Study (FIS) for San Mateo includes a description of the principal flood problems that have been noted for San Mateo County, by flooding source, as summarized in Table 11-3. Table 11-3. Summary of Flood Problems Source Description of the Flood Problem All Sources Flooding is predominantly shallow along streams on the bayside of San Mateo County. Spills from channels flow independently through the urbanized areas, usually following streets, and result in flood depths of less than 1 foot. Occasionally, railroad or highway embankments form barriers, resulting in deeper ponding or sheet flow flooding. Flooding on the ocean side of the county is predominantly confined to well-defined riverine valleys, with flood surface extending uniformly across the floodplain Colma Creek The Daly City storm drain terminates in a junction structure near the intersection of F Street and El Camino Real. Because the downstream storm drain has only one-half the waterway area of the upstream storm drain, the excess flow is forced from the storm drain through a side channel into the Colma Mobile Home Park on the northwestern side of the intersection, where it ponds. San Bruno, Crystal Springs, and Lomita Channels Shallow flooding zones between the Bayshore Freeway and the mainline of the railroad are the result of overland flows from San Bruno Channel and Crystal Springs Channel. These flows merge behind the railroad embankment and eventually cross the railroad tracks as independent flows. Approximately 220 cubic feet per second (cfs) flows into the area north and west of the Crystal Springs Channel and is pumped into the channel at a rate of 35 cfs. The Crystal Springs Channel itself has a capacity of 200 cfs and is adequate for the flows reaching it. Approximately 740 cfs flows into the area south of the Crystal Springs Channel and west of the Bayshore Freeway. This flow moves south until it reaches Lomita Channel, where it is pumped into the Millbrae (High Line) Canal and flows to San Francisco Bay. The Crystal Springs Channel (200-cfs flow) and the Belle Air storm drain (750-cfs flow) merge at San Bruno Avenue and flow northeasterly to San Francisco Bay in the San Bruno Channel (1,000-cfs flow). The shallow flooding zone adjacent to the San Bruno Channel is caused by local runoff. 11-8