Flash flood

A flash flood is a rapid flooding of low-lying areas such as washes, rivers, dry lakes, and depressions. It often happens when there is heavy rain from a severe thunderstorm, hurricane, or tropical storm. Flash floods can also start 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 fast—often in less than six hours after the rain begins.

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

Causes

Flash floods often happen in dry places that have just had a lot of rain, but they can happen anywhere downhill from where the rain fell, even far away. In areas near volcanoes, flash floods can happen after eruptions melt glaciers with their heat. 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 also cause flash floods. Human actions, like when dams fail, can release a huge amount of water and 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. Many people think their cars will protect them, but even just 2 feet of water can sweep away most vehicles. In fact, 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. For example, in 2015, a sudden storm in southern Utah caused flash floods that trapped hikers in a slot canyon, leading to several fatalities.

Impacts

See also: List of flash floods

Flash floods can cause big problems for both cities and nature. They can damage buildings, roads, and other things people build. They can also hurt plants, animals, and people. These problems are especially 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.