A flash flood is a rapid flooding of low-lying areas: washes, rivers, dry lakes and depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, tropical storm, or meltwater from ice or snow flowing over ice sheets or snowfields. Flash floods may also occur after the collapse of a natural ice or debris dam, or a human structure such as a man-made dam, as occurred before the Johnstown Flood of 1889. Flash floods are distinguished from regular floods by having a timescale of fewer than six hours between rainfall and the onset of flooding. The water that is temporarily available is sometimes used by certain plants for growth. However, plants that thrive in drier areas can be harmed by flooding, as the plants can become stressed by the large amount of water.
Flash floods can occur under several types of conditions. Flash flooding occurs when it rains rapidly on saturated soil or dry soil that has poor absorption ability. The runoff collects in gullies and streams and, as they join to form larger volumes, often form a fast flowing front of water and debris.
Flash floods most often occur in dry areas that have recently received precipitation, but they may be seen anywhere downstream from the source of the precipitation, even many miles from the source. In areas on or near volcanoes, flash floods have also occurred after eruptions, when glaciers have been melted by the intense heat. Flash floods are known to occur in the highest mountain ranges of the United States and are also common in the arid plains of the Southwestern United States. Flash flooding can also be caused by extensive rainfall released by hurricanes and other tropical storms, as well as the sudden thawing effect of ice dams. Human activities can also cause flash floods to occur. When dams fail, a large quantity of water can be released and destroy everything in its path.
The United States National Weather Service gives the advice "Turn Around, Don't Drown" for flash floods; that is, it recommends that people get out of the area of a flash flood, rather than trying to cross it. Many people tend to underestimate the dangers of flash floods. What makes flash floods most dangerous is their sudden nature and fast-moving water. A vehicle provides little to no protection against being swept away; it may make people overconfident and less likely to avoid the flash flood. More than half of the fatalities attributed to flash floods are people swept away in vehicles when trying to cross flooded intersections. As little as 2 feet (0.61 m) of water is enough to carry away most SUV-sized vehicles. The U.S. National Weather Service reported in 2005 that, using a national 30-year average, more people die yearly in floods, 127 on average, than by lightning (73), tornadoes (65), or hurricanes (16).
In deserts, flash floods can be particularly deadly for several reasons. First, storms in arid regions are infrequent, but they can deliver an enormous amount of water in a very short time. Second, these rains often fall on poorly absorbent and often clay-like soil, which greatly increases the amount of runoff that rivers and other water channels have to handle. These regions tend not to have the infrastructure that wetter regions have to divert water from structures and roads, such as storm drains, culverts, and retention basins, either because of sparse population or poverty, or because residents believe the risk of flash floods is not high enough to justify the expense. In fact, in some areas, desert roads frequently cross a dry river and creek beds without bridges. From the driver's perspective, there may be clear weather, when a river unexpectedly forms ahead of or around the vehicle in a matter of seconds. Finally, the lack of regular rain to clear water channels may cause flash floods in deserts to be headed by large amounts of debris, such as rocks, branches, and logs.
Deep slot canyons can be especially dangerous to hikers as they may be flooded by a storm that occurs on a mesa miles away. The flood sweeps through the canyon; the canyon makes it difficult to climb up and out of the way to avoid the flood.
- 1889: Johnstown Flood, Pennsylvania, U.S.: more than 2,200 people dead
- 1903: Heppner Flood of 1903; Oregon, United States: 247 dead, 25% of the city
- 1938: Los Angeles Flood of 1938, California, U.S.: 115 dead
- 1938: Kopuawhara flash flood of 1938, Mahia Peninsula, New Zealand: 21 dead
- 1952: Lynmouth disaster, England: 34 dead
- 1963: Petra Flash Flood, Jordan: 23 dead
- 1963: Vajont dam disaster, Italy: 1910 dead
- 1967: Flash flood in Lisbon, Portugal: 464 dead
- 1969: Nelson County, Virginia, US: 123 dead
- 1971: Kuala Lumpur floods, Malaysia: 32 dead
- 1972: The Black Hills flood, South Dakota, U.S.: 238 dead
- 1976: The Big Thompson River flood, Colorado, U.S.: 143 dead
- 1997: Antelope Canyon, a popular tourist attraction north of Page, Arizona:11 dead
- 2003: Bukit Lawang in Indonesia 239 people (5 of them were tourists) were killed
- 2006: Surat flood
- 2006: Jember Regency in Indonesia 59 people dead
- 2007: Sudan floods, 64 dead.
- 2009: September 26 in Metro Manila primarily Marikina city, Taguig City, and Pasig City; and many municipalities of the provinces of Rizal, Bulacan and Laguna leaving more than 100 dead and thousands homeless. It also submerged several municipalities under feet of deep water for several weeks.
- 2009: October 1, Giampilieri, Messina, 37 dead. See also 2009 Messina floods and mudslides.
- 2010: Madeira archipelago, 42 dead
- 2011: Lockyer Valley, Queensland, Australia. 21 dead, mainly in the town of Grantham.
- 2011: Philippines, Cagayan de Oro and Iligan City, 17 December 2011. At least 1200 dead
- 2012: May 5, Nearly three weeks of damming left 72 dead in the Seti Gorge in Upper Seti Basin. Rock and avalanche fall from the western part of Annapurna IV mountain in Pokhara, Nepal.
- 2012: Krasnodarskiy Kray, Russia. 172 dead following a flash flood that struck at 2 A.M. local time on 7 July. Main cities that were hit are Krymsk and Gelendzhik.
- 2013: Uttarakhand, Uttarakhand, India: 822 dead
- 2013: November 17–19, Northeast Sardinia: 18 dead, 3000 homeless. See also 2013 Sardinia floods
- 2013: Port Louis, Mauritius: 11 dead
- 2013: Argentina floods: 99+ dead
- 2013: Kedarnath, Uttarakhand, India: ~5000 dead
- 2014: Srinagar, Jammu & Kashmir, India: approximately 300 dead.
- 2014: Serbia, Bosnia and Croatia: 30+ dead
- 2015: May 25, Central Texas floods: 25+ dead
- 2016: June 25, West Virginia floods: 24+ dead
- 2016: July 30, Ellicott City, Maryland: 2 dead
- 2016: August 13, South Louisiana floods: 13 dead.
- 2016: September 20, Garut Regency in Indonesia floods: 33 dead
- 2017: November 15, Νότια Αττική (West Attiki), Greece floods: 23 dead
- 2018: April 26, Tzafit Canyon, Israel flood: 10 dead
- 2018: May 27, Ellicott City, Maryland: Flood was considered worse than the one that occurred just under two years prior: 1 dead 
- 2018: August 20, Raganello Canyon, Italy flood: 10 dead
- 2018: September 12, parts of Spain, Italy, France flood: 15 dead
- 2018: October 15–16, Aude region, France flood, 15 dead and 99 injured
- 2018: October 25, Dead sea flash flood: 21 dead
- 2019: Flash Flood in Southern Iran: at least 23 dead; also in Northern Afghanistan: at least 16 dead
- Storm surge
- Coastal flood
- Flash flood watch
- Flash flood warning
- Flash Flood Guidance Systems
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