For the record producer, see Mark Ellis. For the album by They Might Be Giants, see Flood (album). For the baseball player, see Curt Flood. For the song by Jars of Clay, see Flood (song)
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 at Deluge (mythology).
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".
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 dikess constructed along their shores and surrounding nearby cities. Unfortunately, by restraining flood waters, these dikes can result in much greater flooding upstream and in locations where they break. 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.
Monsoon rainfalls can cause disastrous flooding in some equatorial countries, such as Bangladesh, due to their extended periods of rainfall.
Hurricanes have a number of different features which, together, can cause devastating flooding. One is waves of up to 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.
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 landslip on continental shelf) may all engender a tidal wave called a tsunami that causes destruction to coast areas. Full details of these marine floods are at Tsunami.
In some flood-prone areas with high population density, such as Holland and parts of 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.