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Flood Table 11-7. Summary of Still-Water Elevations the Pacific Ocean Still-Water Elevationa (feet) 10-Year 50-Year 100-Year 500-Year Low 9.1 10.2 10.7 11 Mean 20.2 23.5 24.6 26.9 High 31.3 36.8 38.5 42.7 a. Elevation in 1988 North American Vertical Datum Source: FEMA Flood Insurance Study Number 06081C0290E, San Mateo County Unincorporated Areas, October 16, 2012 11.2.8 Warning Time 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. Potential flood warning time depends on the time between the rainfall and the first occurrence of flooding. Flash flooding can be less predictable, but populations in potential hazard areas can be warned in advance of flash flooding danger. The National Weather Service (NWS) issues watches and warnings based on river flow forecasts. NWS uses the following flood extent or severity categories, based on property damage and public threat:  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. A warning means a flood is imminent, generally within 12 hours, or is occurring. Local media broadcast NWS warnings. Thresholds for flood warnings on rivers in San Mateo County are as follows:  Lower Crystal Springs Reservoir at Dam:  Action state, minor flooding/initial flood stage, and major flood stage data are not available.  Moderate flooding is 284 feet.  San Francisquito Creek At Stanford University:  Action state is 8 feet.  Moderate flooding is 9.5 feet.  Minor flooding/initial flood and major flood stages are not available. 11.3 EXPOSURE A quantitative assessment of exposure to the flood hazard was conducted using the flood mapping shown in Figure 11-2 and the asset inventory developed for this plan. Population exposure was estimated by calculating the number of buildings in the FEMA-mapped floodplain as a percent of total planning area buildings, and then applying this percentage to the estimated planning area population. Detailed results by municipality are provided in Appendix E; results for the total planning area are presented below. 11-19