flash flood

flash flood, an abrupt, unexpected torrent of water that rushes down a canyon, gulch, valley, or other narrow, low-lying area (including streets in urban areas). Flash floods often stem from heavy rains associated with thunderstorms or the rapid melting of snow and ice on mountains; however, they can also result from dam failures. They are relatively brief events, with waters rising and falling within hours or days.

The sudden onset of a flash flood, which typically occurs less than six hours after a heavy rainfall event, can be extremely destructive. Some flash floods produce walls of water strong enough to wash out riverbeds, pull structures off their foundations, destroy roads and bridges, and break or uproot large trees. They are among the deadliest weather-related phenomena, resulting in the deaths of more than 5,000 people worldwide each year, the majority of whom succmb to drowning.

Influencing factors

Although extreme rainfall is the primary driver of a flash flood, terrain factors can also influence how fast water levels rise and how widespread the flood becomes. A landscape characterized by steep slopes (such as mountains or hills) and large areas of impervious surfaces (including hardpan and rocks or roofs, roads, and other paved areas) can cause water to run off more rapidly and fill up low-lying areas, compared with landscapes made up of gentle slopes, thick vegetation, and water-permeable soils. Slot canyons, such as the high-walled, narrow gorges that appear in the southwestern United States, as well as areas in which recent wildfire activity has burned much of the vegetation, are also prone to flash flooding.

Climate change

A warming climate may be increasing the frequency and severity of flash floods. As average air temperatures increase, the ability of Earth’s atmosphere to hold moisture (water vapor) also increases; moisture concentrations have been shown to increase by 7 percent for every 1 °C (1.8 °F) of warming beyond preindustrial temperature levels (see also global warming). This excess moisture can yield more precipitation, which produces heavier rains that may make flash flood episodes even more common in regions already prone to them, while also enhancing their intensity.