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 mass 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.
Floods from the sea can cause overflow or over topping of flood defenses like construction 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, 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.
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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, as happened in 2007 with the Sudan floods. 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 meters high caused by the leading edge of the hurricane when it moves from sea to land. Another is the large amounts of meteorology associated with hurricanes. The eye of a hurricane has extremely low pressure, so sea level may rise a few meters 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 Ocean 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.
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