Flash flood

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flash flood

Flooding caused by rapidly rising water level in streams, creeks, rivers, or other waterways, normally dry stream beds, or in urban areas, usually as a result of intense rainfall over a relatively small area or for moderate to intense rainfall over highly saturated or impervious land surfaces, and generally occurring within minutes to several hours of the rainfall event.

Steep terrain tends to concentrate runoff into streams very quickly and is often a contributing factor. Changes in soil properties (e.g., burn areas from wildfires), hydrophobic or impervious soils, removal of surface vegetation, and excess runoff from warm rainfall on significant snowpack can also be important contributors. Additional causes of flash floods include ice jams and levee and dam failures.

Term updated 24 April 2017.

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