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 him 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 build 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 Chinese 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 travels 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 the 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 CologneIn 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 the 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 deposition 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, where the river has been confined to a narrow channel to provide the depth necessary for navigation. Maintaining that depth has required repeated dredging of the channel, adding to the already large cost of sustaining the levee system.

Floods in the Mississippi Valley have demonstrated that levees alone do not provide sufficient protection against flooding on a large river, and other methods of flood control, including dams and floodways, are now in use on the Mississippi River. However, flood-control measures failed to contain the great flood of the summer of 1993, one of the worst in United States history. Swelled by record spring rains, the Mississippi and Missouri rivers and many of their tributaries overflowed their banks, inundating an estimated 8 million acres by early August. The raging floodwaters also inflicted major damage on levees, dams, and floodways, ruined an additional 12 million acres of cropland, and caused about $12 billion in damage. At its height, the Missouri river crested almost 15 m (50 ft) above its banks; south of Illinois and Missouri, where the riverbed is very wide and lined with levees, there was only minor damage.Although dams have been used for many centuries, their primary purposes were to build up water reservoirs for irrigation and other domestic uses and to create power. Only recently have they been constructed specifically for flood control. An effective method of controlling floodwaters, and water supply generally, is to construct coordinated groups of dams and reservoirs on the headwaters of the streams that lead into the main rivers, so that water can be stored during periods of heavy run-off and released gradually during dry seasons. The Hoover Dam on the Colorado River, the reservoirs of the La Grande hydroelectric project in Quebec, and dams of the Tennessee Valley Authority (TVA) have demonstrated the value of this method. When the tributaries on which these dams are located are at their normal level, the dams operate solely to produce power and provide water for various purposes. During time of high water the dams operate to slow down the flow. The dams closest to the origins of the tributaries restrain the floodwaters while the dams farther down slowly release their normal reservoirs and are drained. Then the floodwaters are released to each succeeding dam and are finally emptied into the main river, the capacity of which has been increased by straightening and deepening.
One remarkable project aimed at controlling coastal rather than riverine flooding is the Delta Plan, a Netherlands effort begun in 1958 and completed in 1985. The project consists of a series of giant dams that link islands in the deltas of the Rhine, Maas, and Schelde rivers. A huge storm-surge barrier 9 km (5.6 mi) long is lowered only when a sea flood is anticipated; at other times, tides move freely through the passage. Another such project, on a somewhat smaller scale, was completed across the Thames a short distance below London in 1983.Through the centuries people have created a flood problem by cutting down trees and digging up the vegetable cover of the soil, thus increasing soil erosion. Cultivation decreases the ability of the soil to retain water and increases run-off. Vast land areas along the headwaters of rivers throughout the world have been laid waste by intensive cultivation and subsequent erosion. Flood control in these areas has been directed to restoring vegetation and instituting efficient methods of soil management and conservation, such as crop rotation and contour ploughing.Another method of flood control is the construction of floodways on the lower reaches of rivers to divert floodwaters. The rivers are widened at certain points and allowed to overflow. Inundation of certain confined areas prevents the flooding of other areas. The Egyptians have used regulated flooding for thousands of years. Many areas in the Nile Valley have historically depended for their continued fertility on periodic flooding because the soil deposited by sedimentation from floodwater is very rich. Since the 1960s, however, the dramatic reduction in sediment reaching the river_s lower course due to the creation of Lake Nasser behind the Aswan High Dam, has underlined how easy it is for human action to upset the balance of a dynamic natural system such as a regularly flooding river.
Flash FloodFlash floods, such as this one in Nevada, often occur in mountainous, continental regions where water rushes rapidly downhill or in desert regions where storm water may exceed the infiltration capacity of the ground.

Thames Flood Barrier

The Thames Flood Barrier is a spectacular engineering accomplishment designed to protect London from flooding caused by tidal surges until at least the year 2030. About 51,000 tonnes of steel and 210,000 cu m (71 million cu ft) of concrete went into the building of the structure, which is the world’s largest movable flood barrier.

Scapa Flow, sea basin, northern Scotland, in the Orkney Islands. It is about 24 km (15 mi) long, 13 km (8 mi) wide, and contains several islets. Scapa Flow was the principal naval base of the British during World War I and World War II. The German fleet that surrendered to the Allies in November 1918 was interned at Scapa Flow. On June 21, 1919, one week before the Versailles Treaty was signed, the German crews scuttled their ships to prevent their subsequent use by the Allies. On October 14, 1939, during World War II, a German U-boat penetrated the supposedly secure harbour of Scapa Flow and sank the battleship HMS Royal Oak.