2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather; Natural Disasters

A flood (in Old English flod, a word common to Teutonic languages; compare German Flut, Dutch vloed from the same root as is seen in flow, float) is an overflow of water, an expanse of water submerging land, a deluge. In the sense of "flowing water", the word is applied to the inflow of the tide, as opposed to the outflow or "ebb". The Flood, the great Universal Deluge of myth and perhaps of history is treated at Deluge in mythology.

Flooding in Asheville, North Carolina in July 1916
Flooding in Asheville, North Carolina in July 1916

Main causes

Since prehistoric times people have lived by the seas and rivers for the access to cheap and quick transportation and access to food sources and trade; without human populations near natural bodies of water, there would be no concern for floods. However fertile soil in a river delta is subject to regular inundation from normal variation in precipitation.

Floods from the sea can cause overflow or overtopping of flood-defenses like dikes as well as flattening of dunes or bluffs. Land behind the coastal defence may be inundated or experience damage. A flood from sea may be caused by a heavy storm ( storm surge), a high tide, a tsunami, or a combination thereof. As many urban communities are located near the coast this is a major threat around the world.

Rock River floodwaters in downtown Fort Atkinson, Wisconsin.
Rock River floodwaters in downtown Fort Atkinson, Wisconsin.

Many rivers that flow over relatively flat land border on broad flood plains. When heavy the deposition of silt on the rich farmlands and can result in their eventual depletion. The annual cycle of flood and farming was of great significance to many early farming cultures, most famously to the ancient Egyptians of the Nile river and to the Mesopotamians of the Tigris and Euphrates rivers .

A flood happens when an area of land, usually low-lying, is covered with water. The worst floods usually occur when a river overflows its banks. An example of this is the January 1999 Queensland floods, which swamped south-eastern Queensland. Floods happen when soil and vegetation cannot absorb all the water. The water then runs off the land in quantities that cannot be carried in stream channels or kept in natural ponds or man-made reservoirs.

Periodic floods occur naturally on many rivers, forming an area known as the flood plain. These river floods usually result from heavy rain, sometimes combined with melting snow, which causes the rivers to overflow their banks. A flood that rises and falls rapidly with little or no advance warning is called a flash flood. Flash floods usually result from intense rainfall over a relatively small area. Coastal areas are occasionally flooded by high tides caused by severe winds on ocean surfaces, or by tidal waves caused by undersea earthquakes. There are often many causes for a flood.

Monsoon rainfalls can cause disastrous flooding in some equatorial countries, such as Bangladesh, Hurricanes have a number of different features which, together, can cause devastating flooding. One is the storm surge (sea flooding as much as 8 metres high) caused by the leading edge of the hurricane when it moves from sea to land. Another is the large amounts of precipitation associated with hurricanes. The eye of a hurricane has extremely low pressure, so sea level may rise a few metres in the eye of the storm. This type of coastal flooding occurs regularly in Bangladesh.

In Europe floods from sea may occur as a result from heavy Atlantic storms, pushing the water to the coast. Especially in combination with high tide this can be damaging.

Under some rare conditions associated with heat waves, flash floods from quickly melting mountain snow have caused loss of property and life.

Undersea earthquakes, eruptions of island volcanos that form a caldera, (such as Thera or Krakatau) and marine landslips on continental shelves may all engender a tidal wave called a tsunami that causes destruction to coastal areas. See the tsunami article for full details of these marine floods.

Floods are the most frequent type of disaster worldwide. Thus, it is often difficult or impossible to obtain insurance policies which cover destruction of property due to flooding, since floods are a relatively predictable risk.

Flood defenses, planning, and management

Autumn Mediterranean flooding in Alicante (Spain), 1997.
Autumn Mediterranean flooding in Alicante (Spain), 1997.

In western countries, rivers prone to flooding are often carefully managed. Defenses such as levees, bunds, reservoirs, and weirs are used to prevent rivers from bursting their banks. Coastal flooding has been addressed in Europe with coastal defenses, such as sea walls and beach nourishment.

  • London is protected from flooding by a huge mechanical barrier across the River Thames, which is raised when the water level reaches a certain point (see Thames Barrier).
  • Venice has a similar arrangement, although it is already unable to cope with very high tides, and will become increasingly inadequate if anticipated rises in sea level occur.
  • The biggest and most elaborate flood defences can be found in the Netherlands, where they are referred to as Delta Works with the Oosterscheldedam as its crowning achievement. These works were built in response to the North Sea flood of 1953 of the south western part of the Netherlands. The Dutch had already build one of worlds largest dams in the north of their country: the Afsluitdijk (closing occurred in 1932) in response to flooding in many rivers, including the Danube and the Elbe.
  • Ethopia saw one of its worst floods in August 2006.
  • The Mid-Atlantic States flood of 2006 in the eastern United States is considered to be the worst in that region since the flooding caused by Hurricane David in 1979.
  • Korea saw one of its worst floods in May 2006.
  • In the Indian states of Tamil Nadu and Andhra Pradesh, many villages were isolated due to heavy November rains caused by low-pressure areas in the Bay of Bengal.
  • Record rain across eastern Europe in August 2005 caused very severe flooding.
  • 80% of New Orleans, Louisiana, USA was flooded due to the failure of several floodwalls in August 2005 in the aftermath of Hurricane Katrina.
  • Flooding in Mumbai India in July 2005 left over 700 dead. Some areas went under 15 feet of water.
  • One of Canada's most devastating floods occurred in southern Alberta in June 2005. The flooding affected many major metropolitan areas including Calgary. 72 deaths resulted from the three-week flood.
  • In January 2005, flooding on the Rivers Eden, Kent, Derwent, Greta and Cocker as well as others in Cumbria, England, flooded about 2000 properties and caused about £500 *In June 2001, floods from Tropical Storm Allison killed over 30 people in the Houston, Texas, area.
  • The 2000 Mozambique flood, caused by heavy rains followed by a cyclone, covered much of the country for three weeks, killing thousands, leaving the country devastated for years afterwards.
  • The 1998 Yangtze River Floods left 14 million people homeless.
  • The Red River Flood of 1997 occurred in April and May 1997 along the Red River of the North in North Dakota, Minnesota, and Manitoba. It was the most severe flooding of the river since 1826.
  • The May 8th 1995 Louisiana Flood caused extensive damages to the New Orleans, Louisiana, area.
  • Summer 1993 severe monsoon flooding in the American southwest.]
  • The Great Flood of 1993 was one of the most destructive floods in United States history.
  • In Alaska from May to September 1992 unusally wet conditons, plus snow melt caused the 100 year flood in areas of Alaska.
  • January 1992 saw severe floods in South America.
  • In 1983 the Pacific Northwest saw one of their worst winter floods. And the some of the Northwest states saw their wettest winter yet. The damage was estimated at 1.1 billion dollars.* In 1965 Hurricane Betsy flooded large areas of New Orleans for up to 10 days, drowning around 40 people.
  • In 1957, storm surge flooding from Hurricane Audrey killed about 400 people in southwest Louisiana.
  • The Hunter Valley floods of 1955 in New South Wales destroyed over 100 homes and caused 45,000 to be evacuated.
  • The North Sea Flood of 1953 caused over 2,000 deaths in the Dutch province of Zeeland and the United Kingdom and led to the construction of the Delta Works and the Thames Barrier.
  • The Lynmouth flood of 1952 killed only 34 people, however it was very destructive and destroyed over 80 buildings.
  • 1940 saw severe floods in Queensland, Australia.
  • The 1931 Huang He flood caused between 800,000 and 4,000,000 deaths in China, one of a series of disastrous floods on the Huang He.
  • The Great Mississippi Flood in 1927 was one of the most destructive floods in United States history.
  • The 1916 Clermont
On the left is a photo taken during the 1998 floods in Swifts Creek in Australia. On the right is the same location 8 years later
On the left is a photo taken during the 1998 floods in Swifts Creek in Australia. On the right is the same location 8 years later

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