A flood is an overflow of an expanse of water that submerges land, a deluge.[1] In the sense of "flowing water", the word is applied to the inflow of the tide, as opposed to the outflow or "ebb".
It is usually due to the volume of water within a body of water, such as a river or lake, exceeding the total capacity of the body, and as a result some of the water flows or sits outside of the normal perimeter of the body. It can also occur in rivers, when the strength of the river is so high it flows right out of the river channel , usually at corners or meanders. These of course, are not applicable in such instances as sea flooding.
The word comes from the Old English flod, a word common to Teutonic languages, compare German Flut, Dutch vloed from the same root as is seen in flow, float.
The Flood, the great Universal Deluge of myth and perhaps of history is treated at Deluge (in mythology).

Contents 1 Causes 2 Typical effects 3 Main mitigation strategies 4 Flood defences, planning, and management 5 Benefits of flooding 6 See also 6.1 Floods by country 7 External links

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Causes

Floods from the sea can cause overflow or overtopping of flood defences like dikes as well as flattening of dunes or bluffs. Land behind the coastal defence may be inundated or experience damage. A flood from sea may be caused by a heavy storm (storm surge), a high tide, a tsunami, or a combination thereof. As many urban communities are located near the coast this is a major threat around the world. Many rivers flow over relatively flat land border on broad flood plains. When heavy 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.
A flood occurs when an area of land, usually low-lying, is covered with water. The worst floods usually occur when a river overflows its banks. An example of this is the January 1999 Queensland floods, which swamped south-eastern Queensland. Floods happen when soil and vegetation cannot absorb all the water. The water then runs off the land in quantities that cannot be carried in stream channels or kept in natural ponds or man-made reservoirs.
Periodic floods occur naturally on many rivers, forming an area known as the flood plain. These river floods usually result from heavy rain, sometimes combined with melting snow, which causes the rivers to overflow their banks. A flood that rises and falls rapidly with little or no advance warning is called a flash flood. Flash floods usually result from intense rainfall over a relatively small area. Coastal areas are occasionally flooded by high tides caused by severe winds on ocean surfaces, or by tsunami waves caused by undersea earthquakes. There are often many causes for a flood.
Monsoon rainfalls can cause disastrous flooding in some equatorial countries, such as Bangladesh; 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 have 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. A flood can also be caused by blocked sewage pipes and waterways, such as the Jakarta flood.

Typical effects

• Physical damage- Structures such as buildings get damaged due to flood water. Landslides can also take place.
  
• Casualties- People and livestock die due to drowning. It can also lead to epidemics and diseases.
  
• Water supplies- Contamination of water. Clean drinking water becomes scarce.
  
• Crops and food supplies- Shortage of food crops can be caused due to loss of entire harvest.[2]

Main mitigation strategies

1. Mapping- of the flood prone area.
2. Land use control- Major development is restricted in areas subject to flooding.
3. Construction of engineered structures- Strong structures to withstand flood forces. Moreover the buildings should be constructed on an elevated area and if necessary should be built on stilts.

Flood defences, planning, and management

In western countries, rivers prone to floods are often carefully managed. Defences such as levées, bunds, reservoirs, and weirs are used to prevent rivers from bursting their banks. Coastal flooding has been addressed in Europe with coastal defences, such as sea walls and beach nourishment.
London is protected from flooding by a huge mechanical barrier across the River Thames, which is raised when the water level reaches a certain point (see Thames Barrier).
Venice 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 largest and most elaborate flood defenses can be found in the Netherlands, where they are referred to as Delta Works with the Oosterscheldedam as its crowning achievement. These works were built in response to the North Sea flood of 1953 of the south western part of the Netherlands. The Dutch had already built one of the worlds largest dams in the north of the country: the Afsluitdijk (closing occurred in 1932).
Currently the Saint Petersburg Flood Prevention Facility Complex is to be finished by 2008, in Russia, to protect Saint Petersburg from storm surges. It also has a main traffic function, as it completes a ring road around St Petersburg. 11 dams extend for 25.4 kilometres and stand eight metres above water level.
The New Orleans Metropolitan Area, 35% of which sits below sea level, is protected by hundreds of miles of levees and flood gates. This system failed catastrophically in the City Proper and in eastern sections of the Metro Area, resulting in the innundation of approximately 50% of the Metropolitan area, ranging from a few inches to twenty feet in coastal communities.
In China, flood diversion areas are rural areas that are deliberately flooded in emergencies in order to protect cities [1].

Benefits of flooding

This article deals principally with deleterious effects of flooding on human settlements and economic activities. However flooding can bring benefits, such as making soil more fertile and providing nutrients in which it is deficient. Periodic flooding was essential to the well-being of ancient communities along the Tigris-Euphrates Rivers, the Nile River, the Indus River, and the Yellow River, among others.