Flash flood

A flash flood is a fast flooding of low areas such as washes, rivers, dry lakes, and depressions. It often starts when there is heavy rain from a severe thunderstorm, hurricane, or tropical storm. Flash floods can also begin when ice or snow melts quickly, or when a natural ice or debris dam breaks apart. They are different from regular floods because they happen very quickly—often in less than six hours after the rain starts.

Flash floods can be very dangerous. In the United States, they cause more deaths each year than tornadoes or hurricanes. They can also leave behind mud and sand on floodplains, which can harm plants that are not used to flooding often.

Causes

Flash floods often happen in dry places after a lot of rain. They can also happen downhill from where the rain fell, even far away. Near volcanoes, flash floods can happen when eruptions melt glaciers. These floods are common in the high mountains of the United States and in the dry plains of the Southwestern United States. Heavy rain from hurricanes and other tropical storms, as well as the sudden melting of ice dams, can cause flash floods. Human actions, like when dams fail, can also cause flash floods.

Hazards

Flash floods can be very dangerous because they happen quickly and the water moves fast. The United States National Weather Service advises people to leave the area of a flash flood instead of trying to drive through it. Even just 2 feet of water can sweep away most vehicles. Many people have been swept away while trying to cross flooded roads.

In dry areas like deserts, flash floods can be even more dangerous. Storms there are rare but can bring a lot of water very quickly. The soil often does not absorb water well, so rivers and streams fill up fast. These areas usually do not have the same drainage systems as wetter places, making flash floods more likely. Sometimes, roads cross dry riverbeds without bridges, and a river can appear suddenly during a storm.

Deep slot canyons can also be risky for hikers. If a storm happens far away, floodwaters can rush through the canyon, making it hard to escape.

Impacts

See also: List of flash floods

Flash floods can cause big problems for cities and nature. They can damage buildings, roads, and other things people build. They can also hurt plants, animals, and people. These problems can be very hard to understand in cities.

Scientists study flash floods to better predict their effects. They have found that the quickest floods are often the most dangerous, especially at night, and many accidents happen in cars.

Flash floods can wash away soil quickly. This is why some areas, like parts of the Nile River, get a lot of dirt from flash floods. Some plants, like certain kinds of rice, can survive being flooded, but plants that like dry areas can get hurt by too much water.