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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.
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.
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.
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
- New Orleans, Louisiana, USA was flooded in August 2005 in the aftermath of Hurricane Katrina.
- Flooding in Mumbai India in July 2005 left over 700 dead.
- One of Canada's most devastating floods happened in southern Alberta, Canada in June 2005. The flooding affected many major metropolitan areas including Calgary. After 3 weeks of flooding, 72 people died.
- The 2002 European flood was a flooding disaster that affected many states including Czech Republic, Germany and Poland. Historical cities like Prague and Dresden were partly flooded. In Germany the so called "Jahrhundertflut" (flood of century) caused a 22,6 billion Euro damage.
- 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 Red River Flood of 1997 was a major flood that occurred in April and May 1997, along the Red River of the North in North Dakota, Minnesota, and Manitoba. It was the most severe flood of the river since 1826.
- The Great Flood of 1993 was the greatest flooding disaster in United States history.
- In 1982 the river Jucar (Valencia, Spain) broke Tous's reservoir provoking an avalanche of 16.000 m3/seg. and more than 30 dead men. The flood was called "La pantanada".
- In 1975 a freak typhoon destroyed over sixty dams in China's Henan Province, killing over 200,000 people. (see Banqiao Dam)
- In 1972 Hurricane Agnes caused 122 deaths, mostly from the overflowing of rivers in New York and Pennsylvania.
- In 1965, the storm surge of Hurricane Betsy flooded downtown New Olreans, 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 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 flood caused 61 deaths in central Queensland due to a small but intense cyclone.
- The storm early January 1916 in the Zuiderzee caused severe damage along the coast of Noord-Holland, Utrecht, Gelderland, Overijssel and Friesland in the Netherlands.
- On September 8, 1900, a storm surge associated with an unnamed hurricane killed between 6,000 and 8,000 people in Galveston, Texas.
- Johnstown Flood disaster (or Great Flood of 1889), May 31, 1889. Heavy rains and the failure of the South Fork Dam near Johnstown, Pennsylvania released 20-million gallons of water, causing the death of over 2,200 people and $17 million (USD)in damage. It was the first major disaster relief effort handled by the new American Red Cross, led by Clara Barton. Support for victims came from all over the United States and 18 foreign countries. It remains one of the greatest disasters in U.S. history.
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.