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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

In many arid regions of the world, the soil has very poor water retention characteristics, or the amount of rainfall exceeds the ground's ability to absorb water. When a rainfall does occur, it can sometimes result in a sudden flood of water filling dry streambeds known as a "flash flood".

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

Many rivers that flow over relatively flat land border on broad flood plains. When heavy rainfall or melting snow causes the river's depth to increase and the river to overflow its banks, a vast expanse of shallow water can rapidly cover the adjacent flood plain. Flooding deposits silt on the flood plain, improving its fertility. Throughout history, this has attracted agriculture and other human development. In order to preserve these farms and cities, some rivers prone to flooding have had extensive and elaborate systems of dikes constructed along their shores and surrounding nearby cities. Unfortunately, by restraining flood waters, these dikes can result in much greater flooding downstream and in locations where they break. Because off the dikes the difference between water-level during flood and the surface of the inland increases and the potential devastation of the flood increases. The control of annual flooding, by dikes and by dams, also prevents 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.


Main causes

Monsoon rainfalls can cause disastrous flooding in some equatorial countries, such as Bangladesh, due to their extended periods of rainfall. Heavy rain caused substantial damage across eastern Europe in the summers of 2003 and 2005. Normally riverine floods occur only in winter as a result of heavy rain in combination with melting of snow and glaciers in spring.

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 has 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

In western countries, rivers prone to flooding are often carefully managed. Defences 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 mechnical barrier across the River Thames, which is raised when the water level reaches a certain point (see Thames Barrier).
  • Venice, Italy 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 build in response to the North Sea flood of 1953 of the south western part of the Netherlands. The Dutch had allready build one of worlds largests dams in the north: the Afsluitdijk in response to a flooding in 1916.

In some flood-prone areas with high population density, such as parts of the Netherlands and England, planning laws have been used to prevent building on flood plains. In some cases, pressure from developers has caused these controls to be eroded, with an increasing number of new developments reliant on artificial defences for protection from floodwaters.

Bangladesh has not experienced catastrophic coastal flooding since 1995, but the country relies heavily on foreign support and technology to combat flooding. The United States has donated hurricane shelters to the country, and India provides the Bangladesh government with weather forecasting to give the country time to plan its response to hurricanes.

Significant prehistoric floods

In prehistoric times, several great floods are known or suspected to have occurred with varying amounts of supporting evidence. These include:

  • The flooding of the Mediterranean Sea about 5 million years ago. It had previously become a desert after continental movement had closed the Strait of Gibraltar (variously placed at 8 million or 5.5 million years ago).
  • The flooding of the Black Sea, caused by rising level of the Mediterranean as the last ice age ended (circa 5600 BC).
  • As the ice age ended in North America, there was a great flood caused by the breaking of the ice dams holding Lake Agassiz.
  • The Missoula Floods of Washington, also caused by breaking ice dams.

See Deluge (prehistoric) for a more complete and detailed listing.

Significant modern floods


Allthough nature is normally blamed for the damage, the people are at least partly responsible for the presence of human activities in areas prone tot the hazard of floods. Prevention is often aimed at containing floods with defences, often increasing the potential damage in the long run. Nowadays, strategies to deal with floods more and more include evacuation strategies to avert damage.

See also

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