Flash flood is a sudden, often unexpected accumulation of water on normally dry land. Flash floods develop more rapidly than other floods and occur chiefly near small rivers or streams. Most flash floods result from storms that produce heavy rains over short periods. Occasionally, a flash flood may result from another event, such as a dam break, the sudden breakup of an ice jam along a river, or rapid snowmelt caused by a volcanic eruption. Flash floods can cause injury or death because they generally occur without warning, leaving little time for escape.
The majority of flash floods happen during warm months, when thunderstorms are most frequent. Flash floods occur commonly in mountainous and hilly areas. Typically, heavy rain from a thunderstorm falls on a mountain or hill and is channeled into narrow valleys and canyons below. The water collects in these areas, overflowing small rivers or streams or causing dry streambeds to flood.
A flash flood can catch people by surprise because the flooding may take place far from its cause. Deaths often result from drivers trying to maneuver vehicles through or out of flooded areas. As little as 2 feet (0.6 meter) of rapidly moving floodwater can carry away most automobiles. Flash floods can also trigger deadly landslides. Experts recommend that people move to high ground at the first sign of a flash flood and wait for floodwaters to recede.
