100-year flood

A 100-year flood is a flood event that has on average a 1 in 100 chance (1% probability) of being equaled or exceeded in any given year.^[1]

A 100-year flood is also referred to as a 1% flood.^[2] For coastal or lake flooding, a 100-year flood is generally expressed as a flood elevation or depth, and may include wave effects. For river systems, a 100-year flood is generally expressed as a flowrate. Based on the expected 100-year flood flow rate, the flood water level can be mapped as an area of inundation. The resulting floodplain map is referred to as the 100-year floodplain. Estimates of the 100-year flood flowrate and other streamflow statistics for any stream in the United States are available.^[3] In the UK, the Environment Agency publishes a comprehensive map of all areas at risk of a 1 in 100 year flood.^[4] Areas near the coast of an ocean or large lake also can be flooded by combinations of tide, storm surge, and waves.^[5] Maps of the riverine or coastal 100-year floodplain may figure importantly in building permits, environmental regulations, and flood insurance. These analyses generally represent 20th-century climate.