Flash Floods and Urban Flood Risk

Flash floods are distinguished from ordinary river floods primarily by their speed of onset: the U.S. National Weather Service defines a flash flood as one that begins within six hours, and often within just one to three hours, of the causative rainfall or dam or levee failure, leaving little time for warning compared with the days of lead time typically available before large-river floods. Flash flooding is generated by intense, localized rainfall — frequently from slow-moving or stationary thunderstorm complexes that repeatedly regenerate over the same area, a process meteorologists term "training," in which successive storm cells pass over the same location like train cars on a track — falling onto terrain with steep slopes, thin soils, or highly impervious surfaces that cannot absorb or convey the water quickly enough. Mountainous and canyon terrain is especially prone to flash flooding because steep gradients accelerate runoff velocity dramatically; narrow canyons can funnel a wall of water, sometimes several meters high, far downstream of the actual rainfall, a dynamic responsible for incidents such as the Big Thompson Canyon flood in Colorado on July 31, 1976, in which roughly 30 centimeters of rain fell in about four hours over the canyon's upper watershed, generating a flood wave that killed 144 people, many of them unaware that clear skies at their own location offered no protection from a storm occurring miles upstream.