Flash flood

Flash flood

Overview
A flash flood is a rapid flood
Flood
A flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water...

ing of geomorphic
Geomorphology
Geomorphology is the scientific study of landforms and the processes that shape them...

 low-lying areas—washes, river
River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

s, dry lake
Dry lake
Dry lakes are ephemeral lakebeds, or a remnant of an endorheic lake. Such flats consist of fine-grained sediments infused with alkali salts. Dry lakes are also referred to as alkali flats, sabkhas, playas or mud flats...

s and basins
Depression (geology)
A depression in geology is a landform sunken or depressed below the surrounding area. Depressions may be formed by various mechanisms.Structural or tectonic related:...

. It may be caused by heavy rain
Rain
Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...

 associated with a storm
Storm
A storm is any disturbed state of an astronomical body's atmosphere, especially affecting its surface, and strongly implying severe weather...

, hurricane, or tropical storm or meltwater
Meltwater
Meltwater is the water released by the melting of snow or ice, including glacial ice and ice shelfs over oceans. Meltwater is often found in the ablation zone of glaciers, where the rate of snow cover is reducing...

 from ice or snow flowing over ice sheets or snowfields. Flash floods may occur after the collapse of a natural ice
Ice dam
An ice dam occurs when water builds up behind a blockage of ice. Ice dams can occur in various ways.-Caused by a glacier:Sometimes a glacier flows down a valley to a confluence where the other branch carries an unfrozen river...

 or debris dam, or a human structure such as a man-made dam
Dam
A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are...

, as occurred before the Johnstown Flood
Johnstown Flood
The Johnstown Flood occurred on May 31, 1889. It was the result of the catastrophic failure of the South Fork Dam situated upstream of the town of Johnstown, Pennsylvania, USA, made worse by several days of extremely heavy rainfall...

 of 1889. Flash floods are distinguished from a regular flood by a timescale less than six hours.
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Encyclopedia
A flash flood is a rapid flood
Flood
A flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water...

ing of geomorphic
Geomorphology
Geomorphology is the scientific study of landforms and the processes that shape them...

 low-lying areas—washes, river
River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

s, dry lake
Dry lake
Dry lakes are ephemeral lakebeds, or a remnant of an endorheic lake. Such flats consist of fine-grained sediments infused with alkali salts. Dry lakes are also referred to as alkali flats, sabkhas, playas or mud flats...

s and basins
Depression (geology)
A depression in geology is a landform sunken or depressed below the surrounding area. Depressions may be formed by various mechanisms.Structural or tectonic related:...

. It may be caused by heavy rain
Rain
Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...

 associated with a storm
Storm
A storm is any disturbed state of an astronomical body's atmosphere, especially affecting its surface, and strongly implying severe weather...

, hurricane, or tropical storm or meltwater
Meltwater
Meltwater is the water released by the melting of snow or ice, including glacial ice and ice shelfs over oceans. Meltwater is often found in the ablation zone of glaciers, where the rate of snow cover is reducing...

 from ice or snow flowing over ice sheets or snowfields. Flash floods may occur after the collapse of a natural ice
Ice dam
An ice dam occurs when water builds up behind a blockage of ice. Ice dams can occur in various ways.-Caused by a glacier:Sometimes a glacier flows down a valley to a confluence where the other branch carries an unfrozen river...

 or debris dam, or a human structure such as a man-made dam
Dam
A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are...

, as occurred before the Johnstown Flood
Johnstown Flood
The Johnstown Flood occurred on May 31, 1889. It was the result of the catastrophic failure of the South Fork Dam situated upstream of the town of Johnstown, Pennsylvania, USA, made worse by several days of extremely heavy rainfall...

 of 1889. Flash floods are distinguished from a regular flood by a timescale less than six hours. The temporary availability of water is often utilized by foliage with rapid germination and short growth cycle, and by specially adapted animal life.

Causes


Flash flooding occurs when precipitation falls too quickly on saturated soil or dry soil that has poor absorption ability. The runoff collects in low-lying areas and rapidly flows downhill.
Flash floods most often occur in normally dry areas that have recently received precipitation, but may be seen anywhere downstream from the source of the precipitation, even many miles from the source. In areas on or near volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

es, flash floods have also occurred after eruptions, when glacier
Glacier
A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...

s have been melted by the intense heat. Flash floods are known to occur in the highest mountain ranges of the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 and are also common in the arid plains of southwestern United States. Flash flooding can also be caused by extensive rainfall released by hurricane
Tropical cyclone
A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...

s and other tropical storms, as well as the sudden thawing effect of ice dam
Ice dam
An ice dam occurs when water builds up behind a blockage of ice. Ice dams can occur in various ways.-Caused by a glacier:Sometimes a glacier flows down a valley to a confluence where the other branch carries an unfrozen river...

s. Human activities can also cause flash floods to occur. When dams
DAMS
Driot-Arnoux Motorsport is a racing team from France, involved in many areas of motorsports. DAMS was founded in 1988 by Jean-Paul Driot and former Formula One driver René Arnoux. It is headquartered near Le Mans, only 2 km from the Bugatti Circuit.- History :The year after its foundation,...

, constructed for hydro-electricity, have failed, large quantities of water can be released and can destroy everything within its path.

Hazards


The United States National Weather Service
National Weather Service
The National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...

 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 foot (0.6096 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
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...

 (73), tornado
Tornado
A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...

es (65), or hurricanes (16).

In desert
Desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...

s, 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 increase the amount of runoff
Surface runoff
Surface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, meltwater, or other sources flows over the land. This is a major component of the water cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source...

 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 basin
Retention basin
A retention basin is used to manage stormwater runoff to prevent flooding and downstream erosion, and improve water quality in an adjacent river, stream, lake or bay. Sometimes called a wet pond or wet detention basin, it is an artificial lake with vegetation around the perimeter, and includes a...

s, either because of sparse population, poverty, or because residents believe the risk by flash floods is not high enough to justify the expense. In fact, in some areas, desert roads frequently cross dry river and creek beds without bridges. From the driver's perspective, there may be clear weather, when unexpectedly a river 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 canyon
Slot canyon
A slot canyon is a narrow canyon, formed by the wear of water rushing through rock. A slot canyon is significantly deeper than it is wide. Some slot canyons can measure less than one metre across at the top but drop more than 30 m to the floor of the canyon.Most slot canyons are formed in...

s can be especially dangerous to hikers as they may be flooded by a storm that occurs on a mesa
Mesa
A mesa or table mountain is an elevated area of land with a flat top and sides that are usually steep cliffs. It takes its name from its characteristic table-top shape....

 miles away, sweeps through the canyon, and makes it difficult to climb up and out of the way to avoid the flood.

Significant flash floods



  • 1889: Johnstown Flood
    Johnstown Flood
    The Johnstown Flood occurred on May 31, 1889. It was the result of the catastrophic failure of the South Fork Dam situated upstream of the town of Johnstown, Pennsylvania, USA, made worse by several days of extremely heavy rainfall...

    , more than 2,200 people dead
  • 1903: Heppner Flood of 1903
    Heppner Flood of 1903
    The Heppner Flood of 1903 was a major flash flood along Willow Creek responsible for destroying a large portion of Heppner, Oregon, United States, on June 14, 1903...

    ; Oregon
    Oregon
    Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...

    , United States
    United States
    The United States of America is a federal constitutional republic comprising fifty states and a federal district...

    : 247 dead, 25% of the city
  • 1952: Lynmouth disaster, England: 34 dead
  • 1967: Flash flood in Lisbon, Portugal: 464 dead
  • 1971: Kuala Lumpur floods
    1971 Kuala Lumpur floods
    The 1971 Kuala Lumpur floods was a major flash flood disaster in Malaysia that occurred in January 1971. The flooding was the result of heavy monsoon rains, which swelled the Klang, Batu, and Gombak rivers. 32 people were killed and 180,000 people were affected...

    , Malaysia: 32 dead
  • 1972: The Black Hills flood
    Black Hills flood
    The Black Hills Flood of 1972, in the Black Hills of Western South Dakota, USA, occurred on June 9, 1972. The extreme rainfall of around of rain in 6 hours sent Rapid Creek and other creeks overflowing and flooded many residential and commercial properties around the Black Hills...

    , South Dakota, U.S.: 238 dead
  • 1976: The Big Thompson River flood, which killed 143 people in Colorado, U.S.
  • 2007: Sudan floods
    2007 Sudan floods
    On 3 July 2007, flash floods started to devastate many parts of Sudan, including some areas in conflict-battered Darfur and war-torn Southern Sudan.-Damage:...

  • 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 taking more than a hundred lives and leaving thousands of affected residents homeless. It also submerged several municipalities under feet deep of water for several weeks.

See also

  • Coastal flood
  • Freshet
    Freshet
    A freshet can refer to one of two things:* A flood resulting from heavy rain or a spring thaw. Whereas heavy rain often causes a flash flood, a spring thaw event is generally a more incremental process, depending upon local climate and topography...

  • Huayco
    Huayco
    A huayco or huaico is a Peruvian term that refers to a flash flood caused by torrential rains occurring high in the mountains, especially during the weather known as El Niño....

  • Flash flood watch
    Flash flood watch
    A Flash flood watch is issued by the National Weather Service when conditions are favorable for flash flooding in flood-prone areas where grounds are already saturated from recent rains....

  • Flash flood warning
    Flash Flood Warning
    A Flash Flood Warning is issued when a flash flood is imminent or occurring in the warned area. A flash flood is a sudden, violent flood after a heavy rain...


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