Flash Flood

Abstract

Flash floods originate from a variety of sources, from convective rainfall in temperate and tropical areas affecting both rivers and surface water, from the sudden release of water from natural blockages of rivers by landslides or ice jams, from glacial lake outbursts (GLOF), and from the failure of river structures, including dams and embankments. Each of these mechanisms is described with examples from temperate rivers, mainly from Britain, and from mountain and desert environments, mainly from Pakistan. A unifying definition of flash floods is proposed to distinguish these from “normal” floods on the basis of their abrupt onset and the time from the initial perception of the flood to the occurrence of a level posing a threat to life described as a “threat response time.” A minimum rise of water level of 0.50 m in 15 minutes in rivers (0.30 m in 15 minutes in surface water floods) is proposed but in many flash floods the rise may occur as a “wall of water” with a far greater rise in a shorter time. The wavefront in rivers may steepen downstream and may persist for tens of kilometers to locations where no threat is perceived. Flash floods from intense rainfall predominate in small steep upland catchments but can occur more rarely in lowland catchments given sufficient rainfall intensity. Given their rare occurrence at individual locations or catchments, including historical records is shown to improve risk assessment.