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Driving through a flash flooded road
A flash flood after a thunderstorm in the Gobi, Mongolia

A flash flood is a rapid flooding of geomorphic low-lying areas - washes, rivers, dry lakes and basins. It may be caused by heavy rain associated with a storm, hurricane, or tropical storm or meltwater from ice or snow flowing over icesheets or snowfields. Flash floods can also occur after the collapse of an ice dam, debris dam or a human structure, such as a dam, for example, the Johnstown Flood of 1889. Flash floods are distinguished from a regular flood by a timescale less than six hours. The temporary availability of water is often utilised by plants with rapid germination and short growth cycle, and by specially adapted animal life. [1]

Contents

Causes

Flash flooding occurs when a barrier holding back water fails or when water 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 dozens of 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.

Hazards

The United States National Weather Service gives the advice "Turn Around, Don't Drown" in reference to 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. Being in 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.[2] As little as two feet of water (60 cm) can be enough to carry away most SUV-sized vehicles.[3] 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).[4]

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 rain 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 that rivers and other water channels have to handle. In addition, these regions may not have the infrastructure that wetter regions have to divert water from structures and roads, such as storm drains and retention basins, either because of sparse population, poverty or because residents believe the risk flash floods pose 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.[5] 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, sweeping through the canyon, making it difficult to climb up and out of the way to avoid the flood.

Historical examples

  • 1952 The Lynmouth disaster.
  • 1967 Flash flood in Lisbon, Portugal. 464 dead.
  • 1971 Kuala Lumpur floods, Malaysia.
  • 1976 The Big Thompson River flood, which killed 143 people in Colorado.
  • 1990 June 14, Shadyside, Ohio flooding.[6]
  • 1990 The Quad Cities Duck Creek Floods of 1990.[7]
  • 1997 Flash flood kills eleven in Antelope Canyon.
  • 1998 Flash flooding in San Marcos, Texas resulted from rains totaling from 15 to 30 inches (760 mm).
  • 2004 Boscastle flood.
  • 2006 Mount Rainier National Park Flooding.[8]
  • 2006 Flash flooding kills 125 in Ethiopia.[9]
  • 2007 Sudan floods.
  • 2008 The June 12–13, 2008 Floods around Duck Creek in Davenport, Iowa.[10]
  • 2009 The 2009 Kentuckiana Flood resulted from 20-30 inches of rain falling in 75 minutes.
  • 2009 Turkish flash floods.
  • 2009 September 21–22 in 9 Georgia Counties, Killing 10 people
  • 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.
  • 2009 October 10–13 in Northern Luzon causing major landslides in Cordillera Mountains, and submerging 80% of the Province of Pangasinan.
  • 2009 On the 25th of November, more than 100 people died in flash floods that swept away highways and neighborhoods in the city of Jeddah, Saudi Arabia, which was caused by heavy rains.[11]

See also

Further reading

  • Schmittner, Karl-Erich; Pierre Giresse (August 1996). "Modelling and application of the geomorphic and environmental controls on flash flood flow". Geomorphology 16 (4): 337–347. doi:10.1016/0169-555X(96)00002-5. 

References

  1. ^ "Definitions of flood and flash flood". National Weather Service. http://www.srh.noaa.gov/mrx/hydro/flooddef.php. Retrieved 2008-08-19. 
  2. ^ "Watches, Warnings & Advisories—Flash Flood Warning". National Weather Service. http://forecasts.weather.gov/wwamap/wwatxtget.php?cwa=usa&wwa=Flash%20Flood%20Warning. Retrieved 2007-06-25. 
  3. ^ "A Preparedness Guide to flash floods #1 weather-related killer in the United States". U.S. Department of Commerce, National Oceanic and Atmospheric Administration National Weather Service, Federal Emergency Management Agency, American Red Cross. July 1992. http://www.nws.noaa.gov/om/brochures/ffbro.htm. Retrieved 2007-06-25. 
  4. ^ "Turn Around Don't Drown". http://www.srh.noaa.gov/tadd/. Retrieved 2007-06-25. 
  5. ^ McGuire, Thomas (2004). "Weather Hazards and the Changing Atmosphere". Earth Science: The Physical Setting. Amsco School Pubns Inc. pp. 571. ISBN 0-87720-196-X. http://www.amscopub.com/images/file/File_67.pdf. Retrieved 2008-07-17. 
  6. ^ Shadyside, Ohio Floods of 1990 . NOAA. (Report). Retrieved on May 13, 2009.
  7. ^ "Flood Facts". QC Memory. http://www.qcmemory.org/Page/Flood_Facts.aspx?nt=266. Retrieved 2009-04-16. 
  8. ^ "November 2006 Flooding". NPS. http://www.nps.gov/mora/parknews/november-2006-flooding.htm. Retrieved 2008-01-08. 
  9. ^ "Flash floods kill 125 in Ethiopia". BBC. 2006-08-14. http://news.bbc.co.uk/2/hi/africa/4791813.stm. Retrieved 2007-07-03. 
  10. ^ "Duck Creek Flooding Closes Davenport Streets". Quad Cities Online. http://www.qconline.com/archives/qco/display.php?id=391015. Retrieved 2009-04-17. 
  11. ^ Arab News, Jeddah flood toll 106 . SRPC. (Report). Retrieved on December 02, 2009.

External links


Simple English

A flash flood is a very quick flooding of low-lying land. It may be caused by heavy rain or water from melted ice or snow. Flash floods can also happen after the collapse of a dam.[1]

References


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