Washout

A washout is the sudden erosion of soft soil or other support surfaces by a gush of water, usually occurring during a heavy downpour of rain (a flash flood) or other stream flooding. These downpours may occur locally in a thunderstorm (particularly in summer, when they are often nearly stationary), or over a large area, such as following the landfall of a tropical cyclone. If a washout occurs in a crater-like formation, it is called a sinkhole, and usually involves a leaking or broken water main or sewerage pipes. (Other types of sinkholes, such as collapsed caves, are not washouts.)

Widespread washouts can occur in mountainous areas after heavy rains, even in normally dry ravines. A severe washout can become a landslide, or cause a dam break in an earthen dam. Like other forms of erosion, most washouts can be prevented by vegetation whose roots hold the soil and slow the flow of water. Deforestation increases the risk of washouts. Retaining walls and culverts may be used, although particularly severe washouts may even destroy these if they are not large or strong enough.