Flood

For the record producer see Mark Ellis. For the album by They Might Be Giants see Flood (album).

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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."

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 the fertility, which has throughout history 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.

Table of contents

1 Main Causes 2 Flood defenses, planning and management 3 Great floods and global flood myths 4 External link

Main Causes

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 case flooding, and together they can be devistating. One is from the leading edge of the hurricane when it hits land causing waves of up to 8 metres high. Another is from large amounts of precipitation associated with hurricanes. The eye of hurricanes have 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 freakish events associated with heat waves, flash floods from quickly melting mountain snow has caused loss of property and life.

Flood defenses, planning and management

In western countries river flooding rarely does considerable damage, as rivers at risk of flooding are carefully managed. Defenses, such as levees, reservoirs and wiers prevent rivers from bursting their banks. In countries such as England and Holland which have high population densities there is pressure from developers to allow building on flood plains, and planning laws are used to make sure that flood plains are left usused, so flood water can do little damage. London, England is protected from the flooding of the River Thames by a huge mechanical barrier, called the Thames Barrier, which is raised when the water level reaches a certain point. Venice, Italy is also protected from flooding by a similar arrangement.

Coastal flooding has been successfully tackled in Europe with coastal defenses, such as sea walls and beach nourishment.

Bangladesh has not experienced catastrophic coastal flooding since 1995 when 8 metre coastal banks, but the country relies heavily on foreign support and technology to combat flooding. The United States have donated hurricane shelters to the country, and India provides the Bangladesh government with weather forcasting to give the country time to plan their response to hurricanes.

Great floods and global flood myths

In geological or 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 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.

Ancient mythology makes several references to a great flood, of which the most well known is probably the Noachian deluge of Genesis. Another very similar version is given in the Babylonian account of the Epic of Gilgamesh, which apparently derives from a similar Sumerian account. The Sumerians also referred to a great flood in other texts, such as the Sumerian king list, and another remarkably similar to the Biblical version, suggesting that the Genesis account has drawn influence from the older Sumerian depiction.

A large percentage of the world's cultures have stories of a "great flood". The strong likelihood is: a global climatic change in recent geological time brought about some large deluge. Another theory, although not strongly supported, suggests some of the major floods may have been caused by platology, the drifting apart of continents. Evidence is mounting from ice-cores in Greenland that the switch from a glacial to an inter-glacial period can occur over just a few months, rather than over the centuries earlier research suggested.

See also: Drought, Hydrography, Trasvasement, Meteorology

External link

• Catastrophic flood database