A flash flood is a sudden flood event caused by a hydrologic response of the drainage basin. Flash floods are normally strongly localized and associated with extreme showers or thunderstorm activity, when high rates of precipitation occur in a short period of time.

Flash floods usually arise when rain falls over a small catchment basin. The terrain may channel the extreme run-off to produce a flood peak that reaches its maximum in just a few minutes or hours. Flash floods are increasingly observed in urban areas where the surface is unable to absorb large amounts of water in a short period. Often the impetus and velocity of flood water are much more important than the associated water levels and duration in terms of potential impact on population and structures.

Flash Floods in Arroyos

An arroyo is a water-carved gully or a normally dry creek found in arid or desert regions. When storms appear in these areas, the rain water cuts into the dry, dusty soil creating a small, fast-moving river. Flash flooding in an arroyo can occur in less than a minute, with enough power to wash away sections of pavement, large boulders, cars and even houses. In mountain areas or deserts flash floods may occur in areas which actually did not receive any precipitation itself.

Flash floods and the UK

Flash floods may happen in the UK as well. On July 4, 2001 a flash flood swept through the Bryn Gwyn holiday park near Bala, north Wales leaving up to 45,000 homes without power and moving caravans several hundred yards away