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 dikes 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.
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.
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. See the tsunami article for full details of these marine floods.
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, England 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.
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.
Significant prehistorical floods
In prehistoric times, several great floods are known or suspected to have occurred with varying amounts of supporting evidence. These include:
See Deluge (prehistoric) for a more complete and detailed listing.
Significant modern floods