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 melted water from ice or snow flowing over ice sheets or snowfields. Flash floods may 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 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 a barrier holding back water fails or 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 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. Flash floods are known to occur in the highest mountain ranges of the United States and are also common in the arid plains of southwestern U.S.
Hazards
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 two feet of water (60 cm) 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 increase 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, 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 canyons can be especially dangerous to hikers as they may be flooded by a storm that occurs on a mesa miles away, sweeps through the canyon, and makes it difficult to climb up and out of the way to avoid the flood.
Historical examples
1903: The Heppner Flood of 1903 killed 247 people in Oregon. U.S.
1952: The Lynmouth disaster, England: 34 dead.
1967: Flash flood in Lisbon, Portugal: 464 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, which killed 143 people in Colorado, U.S.
1990:
Ohio, U.S. flood of June 14: at least 26 dead.
1990: The
Duck Creek Floods of 1990.
1997: Flash flood kills eleven in Antelope Canyon.
1998: Flash flooding in
San Marcos, Texas, resulted from rains totaling from 15 to .
2004: Boscastle flood.
2006: Mount Rainier National Park flooding.
2006: Flash flooding kills 125 in Ethiopia.
2007: Sudan floods.
2008: The June 12–13, 2008 floods around
Duck Creek in
Davenport, Iowa.
2009: The 2009 Kentuckiana Flood resulted from 3 to 6 inches of rain falling in less than an hour.
2009: Turkish flash floods.
2009: September 21–22 in nine 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 the Cordillera Mountains, and submerging 80% of the Province of Pangasinan.
2009: In late October, a rainy nor'easter caused several flash floods in Southeast Virginia and injured over 100 people.
2009: On November 25, 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.
2010: June 11 – the Little Missouri River rose over 20 feet in only a few hours, killing at least 20 people.
2010: July 24 – Dehli lake dam on the Maquoketa River breached after 9 inches of rainfall. 15 ft. chunks of highway broke off and swept down river, and steel guard rails were flapping like paper streamers in the wind created by the rushing water. Created further flooding, farther down river from where it spilled into the Mississippi River. Downtown Davenport, Iowa Streets closed.
2010: Mid–July till Mid-August – Pakistan's three provinces were badly affected during the monsoon rains when dams, rivers and lakes overflowed killing at least 1400 people and affecting 3.5 million people.
2010: August 6, Leh, India. More than 180 people are reported to have died with 400 missing and 300 injured due to flash floods, caused probably due to cloud burst. Injured and stranded public have been airlifted by Indian army.
2010: October 4, a giant chemical flood caused by the
Ajka alumina plant accident occurred in western
Hungary. A
dam wall collapsed, freeing about one million cubic metres (35 million cubic feet) of highly alkaline liquid waste, called
red mud, from the Ajkai Timföldgyár
alumina plant in
Ajka,
Veszprém County. The mud was released as a wave, flooding several nearby localities, including the village of
Kolontárand the town of
Devecser. The flood killed seven persons and the high pH (~13) of the sludge burned several hundred people and devastated more than 40 km
2 of ground in the basin of the
Danube river.
2010: November 18, Flash flood in
St. Lucia, an island country in the
West Indies.
Flash floods occurred as part of the
2010–2011 Queensland floods.
2011: On January 26, more than 1000 people died in flash floods that swept away highways and neighborhoods in the city of
Jeddah, Saudi Arabia, which was caused by heavy rains.
See also
Freshet
Huayco
Further reading
References
External links
Scottish Flash Flood Public clip of the Fochabers flood in Moray September 9
Decision tree to choose an uncertainty method for hydrological and hydraulic modelling, choosing an uncertainty analysis for flood modelling.
Flash Flood Alley movie and video clips.
Flash Flood Safety information.
Great footage of flash floods in the arid midwest heading down dry washes after heavy rain.
Map of central Texas flash flood alley.
Workshop Proceedings Flash Flood Management
Workshop Proceedings Flash Flood Forecasting
Category:Flood
Category:Water waves
