1% Annual Chance Flood

Find a description of this common term used in floodplain management.

The 1% annual chance flood (also known as the 100-year flood) has a 1 in 100 probability of being equaled or exceeded in any given year. Areas subject to the 1% annual chance flood are identified on the Federal Emergency Management Agency’s (FEMA) Flood Insurance Rate Maps (FIRMs) and National Flood Hazard Layer available on the FEMA Flood Map Service Center and are referred to as special flood hazard areas.

Special flood hazard areas include A Zones (A, AE, AH, and AO Zones) and Velocity Zones (V and VE Zones). The V Zones and Coastal A Zones are also referred to as coastal high hazard areas because they are subject to high-velocity wave action from storms. For flood zones, FIRMs typically indicate a Base Flood Elevation (BFE), which is the elevation the water is expected to reach in a 1% annual chance flood. For details on the criteria used to map the FIRM flood zones and information on how to read flood maps, see Interpreting Federal Emergency Management Agency Flood Maps and Studies in the Coastal Zone from the Massachusetts Office of Coastal Zone Management (CZM).

It is important to note that the use of “100-year flood” can be misleading, since floods of this magnitude can occur more frequently than once every 100 hundred years. The 100-year flood is a recurrence interval, which reflects a frequency analysis of past flood events (over 10 or more years) and corresponds to the 1 in 100 probability of occurrence in any given year (the annual exceedance probability).

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