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Flood, cataclysmic event in many religious and mythological traditions in which, in a remote time in the past, the world is destroyed or cleansed by fire or flood, often inflicted as a result of divine moral anger at the wrongdoings of humankind. In the biblical Book of Genesis, for example, human wickedness causes Yahweh (Jehovah) to regret that he created mankind and he resolves to destroy all living things. The virtuous, 600-year-old Noah, however, finds favour with Yahweh, who instructs hhim to build an ark in which both human posterity and other creatures will be preserved. The biblical Flood has its clearest antecedents in Mesopotamian mythology, recalling the geographical realities of a land subject to sudden flooding from the two great rivers Tigris and Euphrates. In the Gilgamesh Epic, a Babylonian heroic narrative ultimately of Sumerian origin, the high god Enlil sends a deluge to destroy mankind; one man, Utnapishtim, survives in a cube-shaped ark he has been instructed to bbuild and to stock with the seed of all living creatures, and the description of his preparation and voyage closely anticipate the biblical story of Noah.

The motif of the Flood also occurs in the mythology of India and Greece. In CChinese myth a hero named Yu is charged by the Emperor Shun to bring floodwater under control; this task takes him 13 years of punishing toil, but he at last solves the problem by constructing canals. He is rewarded by the emperor_s abdication in his favour. The Aboriginal Australian myth of the Great Flood, which both destroyed a previously existing world and initiated a new social order, may have a historical basis in the effects of rising sea-levels as temperatures rose after the last Ice Age. In several versions, the Flood is the work of the great serpent or rainbow-snake Yulunggul, who sends it in anger at pollution of his waterhole by the two Wawalag sisters, Waimariwi and Boaliri, whose ttravels play a major role in Aboriginal creation myths. Yulunggul devours the sisters and their two children, but after the Flood has abated he regurgitates them, creating the first inhabitants of the new world.

While Flood myths show the destructive power of water, creation myths often recount the origins of the world from a watery abyss or primal sea.

Levee, embankment along the course of a river. Natural levees are low banks that are produced by the river during floods when tthe overflowing of the river decreases the speed of the water and permits the deposit of silt. Artificial levees are considerably higher than natural ones and protect the surrounding countryside from floods. Levees are, in general, similar to the protective dykes in the Netherlands that prevent flooding by the sea.

On a large river such as the Mississippi, floods cannot be controlled by levees alone because the waters rise to heights that would overwhelm any embankment. Levees are, however, used to protect portions of the riverbank areas, such as cities and towns, that have a high economic value. The floodwaters are allowed to flow through breaks in the levees over land of low value and are drained off through supplementary channels that are sometimes equipped with secondary levees.

Flooded Cologne

In January and February 1994, the German city of Cologne was flooded when the Rhine burst its banks after an exceptional buildup of extra runoff in its mountain headwaters. Described at the time as the “flood of the century”, similar scenes were repeated along the Rhine only a year later.

Flood Control, all methods used to prevent or reduce detrimental effects of floods.

Causes of Floods

When it rains or snows, some of tthe water is retained by the soil, some is absorbed by vegetation, some evaporates, and the remainder, which reaches river channels, is called run-off. Floods occur when soil and vegetation cannot absorb all the water; water then runs off the land in quantities that cannot be carried in river channels or retained in natural ponds and constructed reservoirs held behind dams. About 30 per cent of all precipitation is run-off, and this amount may be increased by melting snow masses. Periodic floods occur naturally on many rivers, forming an area known as the flood plain. These river floods often 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 unusually high tides induced by severe winds over ocean surfaces, or by tidal waves caused by undersea earthquakes.

Effects of Floods

Floods not only damage property and endanger the lives of humans and animals, but have other effects as well. Rapid run-off causes soil erosion as well as sediment ddeposition problems downstream. Spawning grounds for fish and other wildlife habitats are often destroyed. High-velocity currents increase flood damage; prolonged high floods delay traffic and interfere with drainage and economic use of lands. Bridge abutments, bank lines, sewer outfalls, and other structures within floodways are damaged, and navigation and hydroelectric power are often impaired. Financial losses due to floods commonly amount to millions of pounds each year.

Control of Floods

The basic methods of flood control have been practised since ancient times. These methods include reforestation and the construction of levees, dams, reservoirs, and floodways (artificial channels that divert floodwater).

The ancient Chinese built levees to raise the banks of the Huang He on the supposition that the confined river would then deepen its channel to contain the maximum flow. The result, however, was a raising of the riverbed, because the sedimentary deposit of alluvial soil previously distributed over the entire flood plain during annual flooding was confined to the river bottom. In 4,000 years the level of the river rose as high as 21 m (70 ft) above the surrounding plain. In 1887 one of the worst floods in recorded history occurred when the Huang He broke through the levees, killing

more than a million people. Levees were constructed during the Middle Ages on the Po, Danube, Rhine, Rhōne, and Volga rivers and have been supplemented in modern times by reforestation and by storage reservoirs. Levees are still in extensive use, notably on the Mississippi, ...

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