Flash flood
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 severe thunderstorm, hurricane, tropical storm,
or meltwater
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 of
less than six hours.[1]
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 floods can occur under several
types of conditions. Flash flooding occurs when precipitation falls 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
forms a fast flowing front of water and debris. 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 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 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.[2][3]
Human activities can also cause flash floods to occur. When dams, constructed for hydro-electricity,
have failed, large quantities of water can be released and can destroy
everything within its path.[3]
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.[4]
As little as 2 feet (0.61 m) of water is enough to carry away most
SUV-sized vehicles.[5]
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).[6]
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.[7]
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.
Significant
flash floods
- 1889: Johnstown Flood, 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 Flood of 1938, Mahia Peninsula, New Zealand 21 dead
- 1952: 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.
- 1997: 11 die in flash flood in Antelope Canyon, a popular tourist attraction north of Page, Arizona.
- 2007: Sudan floods, 64 people killed.
- 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 1, Giampilieri, Messina, 37 people were
killed. See also 2009 Messina floods and mudslides.
- 2010: Madeira archipelago, 42 dead
- 2011: Lockyer Valley,
Queensland, Australia. 21 people dead, mainly in the town of Grantham.
- 2011: Philippines, Cagayan de Oro and Iligan City, 17
December 2011. At least 1200 people killed as reported by Red Cross. See
also Tropical Storm Washi
- 2012: Krasnodarskiy Kray, Russia. 172 people were killed
by a flash flood that struck at 2 A.M. local time on 7 July. Main cities
that were hit are Krymsk and Gelendzhik.[8][9]
- 2012: Uttarakhand, Uttarakhand, India: 822 dead
- 2012: Pokhara, Nepal - 32 dead + 30 missing [May 5,
2013, Nearly 3 weeks damming in Seti Gorge in Upper Seti Basin, Rock and
Avalanche fall from Western Part of Annapurna IV].
- 2013: November 17–19, Northeast Sardinia: 18 deaths,
3000 homeless. See also 2013 Sardinia floods
- 2013: Port Louis, Mauritius
- 11 dead
- 2013: Argentina floods
- 65+ dead
- 2013: Kedarnath,
Uttarakhand, India: 5000 approx dead[10]
The entire wiki link on the subject can be found at: http://en.wikipedia.org/wiki/Flash_flood
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