100-year flood

For other uses, see 100-year flood (disambiguation).
Mississippi River at Kaskaskia, Illinois, during the Great Flood of 1993

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.

  1. ^ Viessman, Warren (1977). Introduction to Hydrology. Harper & Row, Publishers, Inc. p. 160. ISBN 0-7002-2497-1.
  2. ^ Holmes, R.R., Jr., and Dinicola, K. (2010) 100-Year flood–it's all about chance U.S. Geological Survey General Information Product 106
  3. ^ Ries, K.G., and others (2008) StreamStats: A water resources web application U.S. Geological Survey, Fact Sheet 2008-3067 Application home page URL accessed 2015-07-12.
  4. ^ "Flood Map for Planning (Rivers and Sea)". Environment Agency. 2016. Archived from the original on 2016-09-16. Retrieved 25 August 2016.
  5. ^ "Coastal Flooding". FloodSmart. National Flood Insurance Program. Archived from the original on 2016-03-08. Retrieved 7 March 2016.