Flash floods result from short periods of heavy rainfall and are common near warm oceans, along steep mountain fronts in the path of moist winds, and in areas prone to thunderstorms. They are well known from the mountain and canyon lands of the U.S. desert Southwest and many other parts of the world. Some of the heaviest rainfalls in the United States have occurred along the Balcones escarpment in Texas. Atmospheric instability in this area often forms along the boundary between dry desert air masses to the northwest and warm moist air masses rising up the escarpment from the Gulf of Mexico to the south and east. Up to 20 inches (0.5 m) of rain have fallen along the Balcones escarpment in as few as three hours from this weather situation. The Balcones escarpment also seems to trap tropical hurricane rains, such as those from Hurricane Alice, which dumped more than 40 inches (102 cm) of rain on the escarpment in 1954. The resulting floodwaters were 65 feet (20 m) deep, one of the largest floods ever recorded in Texas. Approximately 25 percent of the catastrophic flash-flooding events in the United States have occurred along the Balcones escarpment. On a slightly longer timescale tropical hurricanes, cyclones, and monsoonal rains may dump several feet of rain over periods of a few days to a few weeks, causing fast, but not quite flash, flooding.
The national record for the highest, single-day rainfall is held by the south Texas region, when Hurricane Claudette dumped 43 inches (1.1 m) of rain on the Houston area in 1979. The region was hit again by devastating floods during June 8-10, 2001, when an early-season tropical storm suddenly grew off the coast of Galveston, dumping 28-35 inches (0.7-0.9 m) of rain on Houston and surrounding regions. The floods were among the worst in Houston's history, leaving 17,000 homeless and 22 dead. More than 30,000 laboratory animals died in local hospital and research labs, and the many university and hospital research labs experienced hundreds of millions of dollars in damage. Fifty million dollars were set aside to buy out the properties of homeowners who had built on particularly hazardous floodplains. Total damages exceeded $5 billion. The standing water left behind by the floods became breeding grounds for disease-bearing mosquitoes, and the humidity led to a dramatic increase in the release of mold spores, which cause allergies in some people and are toxic to others.
The Cherrapunji region in southern India at the base of the Himalaya Mountains has received the world's highest rainfalls. Moist air masses from the Bay of Bengal move toward Cherrapunji, where they begin to rise over the high Himalayas. This produces a strong orographic effect, where the air mass cannot hold as much moisture as it rises and cools, so heavy rains result. Cherrapunji has received as many as 30 feet (9 m) of rain in a single month (July 1861) and more than 75 feet (23 m) of rain for all of 1861.
Flash floods typically occur in localized areas where mountains cause atmospheric upwelling, leading to the development of huge convective thunderstorms that can pour several inches of rain per hour onto a mountainous terrain, which focuses the water into steep-walled canyons. The result can be frightening, with floodwater thundering down canyons in steep walls that crash into and wash away all in their paths. Flash floods can severely erode the landscape in arid and sparsely vegetated regions but do much less to change the landscape.
Many canyons in mountainous regions have fairly large upriver parts of their drainage basins. Sometimes the storm that produces a flash flood with a wall of water may be located so far away that people in the canyon do not even know it is raining somewhere, or that they are in immediate grave danger. Such was the situation in some of the examples described below.
A number of factors other than the amount of rainfall determine the severity of a flash flood. The shape of the drainage basin is important, because it determines how quickly rainfall from different parts of the basin converges at specific points. The soil moisture and previous rain history are important, as are the amounts of vegetation, urbanization, and slope.
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