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Flood 11.1.2 FEMA Regulatory Flood Zones FEMA defines flood hazard areas through statistical analyses of records of river flow, storm tides, and rainfall; information obtained through consultation with the community; floodplain topographic surveys; and hydrologic and hydraulic analyses. Flood hazard areas are delineated on Digital Flood Insurance Rate Maps (DFIRMs), which are official maps of a community on which the Federal Insurance and Mitigation Administration has delineated both special flood hazard areas (SFHAs) and risk premium zones. DFIRMS identify the following:  Locations of specific properties in relation to SFHAs  Base flood (1-percent annual chance flood) elevations at specific sites  Flood magnitudes in specific areas  Undeveloped coastal barriers where flood insurance is not available  Regulatory floodways and floodplain boundaries (1-percent and 0.2-percent annual chance floodplain boundaries). The SFHA is the land area on a DFIRM covered by floodwaters of the base flood. In SFHAs, National Flood Insurance Program (NFIP) floodplain management regulations must be enforced and purchase of flood insurance is mandatory. The NFIP defines the base flood elevation as the floodwater elevation during a base flood event (a flood that has a 1-percent chance of occurring in any given year). A structure within a 1-percent annual chance floodplain has a 26-percent chance of undergoing flood damage during the term of a 30-year mortgage. The 1-percent annual chance flood is a regulatory standard adopted by federal agencies and most states to administer floodplain management programs. The 1-percent annual chance flood is used by the NFIP as the basis for insurance requirements nationwide. DFIRMs also depict 0.2-percent annual chance flood designations (500-year events). DFIRM, FIRMs, and other flood hazard information identify the expected spatial extent of flooding from a 1- percent or 0.2-percent annual chance event, defining specific areas as follows:  Zones A1-30 and AE—SFHAs that are subject to inundation by the base flood, determined using detailed hydraulic analysis. Base flood elevations are shown within these zones.  Zone A (Also known as Unnumbered A-zones)—SFHAs where no base flood elevations or depths are shown because detailed hydraulic analyses have not been performed.  Zone AO—SFHAs subject to inundation by types of shallow flooding where average depths are between 1 and 3 feet. These are normally areas prone to shallow sheet flow flooding on sloping terrain.  Zone VE, V1-30—SFHAs along coasts that are subject to inundation by the base flood with additional hazards due to waves with heights of 3 feet or greater. Base flood elevations derived from detailed hydraulic analysis are shown within these zones.  Zone B and X (shaded)—Zones where the land elevation as been determined to be above the base flood elevation, but below the 500-year flood elevation. These zones are not SFHAs.  Zones C and X (unshaded)—Zones where the land elevation has been determined to be above both the base flood elevation and the 500-year flood elevation. These zones are not SFHAs. Coastal SFHAs are of concern to San Mateo County, particularly along the areas of the coastline at or slightly above sea level. DFIRMS depict two coastal flood hazard zones: 11-3