inundation of land by the rise and overflow of a body of water. Floods occur most commonly when water from heavy rainfall, from melting ice and snow, or from a combination of these exceeds the carrying capacity of the river system, lake, or the like into which it runs. Usually the combined flow of several water-swollen tributaries causes flooding along a river bank or shoreline. Accounts of floods that destroyed nearly all life are found in the mythology of many peoples (see Deluge). Not all floods are destructive, however. The annual floodwaters of the Nile and some other larger rivers historically deposited fertile soil along the surrounding floodplain, which is used extensively for agriculture. The damming of the Nile and other rivers in modern times, however, often has greatly reduced this deposition.
The rise and fall of the water level in a river is called the flood wave. Its highest point, or crest, travels progressively downstream. In the upstream portions of a river the flood crest passes quickly. Further downstream the greater volume of water causes slower passage of the flood crest, resulting in floods of longer duration. In many regions, annual floods follow the thaws and rains of spring; flooding also may occur because of thawing ice jamming narrower and shallower parts of a river. In the Arctic regions, especially in the basins of northward flowing rivers, the floods are caused by the thawing of the southern portion of the basin before the ice blocking the lower course of the river melts. Less predictable are floods resulting from ocean waves, called storm surges, pushed onshore by an advancing hurricane, and from sudden torrential flows, called flash floods, following a brief, intense rainstorm or the bursting of a natural or constructed dam or levee. In addition to the duration and quantity of rainfall, the nature of the soil (permeability; state of saturation) of an area affects the frequency of floods.
Generally, flood control measures along a river are attempted at both its headwaters and its low-lying floodplains. Runoff can be detained in the headwaters by planting ground cover on the slopes, building terraces to increase soil infiltration and prevent soil erosion, and building small check dams or retaining ponds to reduce the flow of water. Flood control on the lower floodplains involves building levees to contain the flow and straightening or dredging the channel to improve flow characteristics. Concern over the affects of channelization on rivers in floodplains has led to the development of flood-control approaches that attempt to combine the way floodplains naturally handle floodwaters with traditional methods that restrict those waters greater spread. Such an approach might involve increasing the distance of levees from a river's channel along with the creation of wetlands to absorb floodwaters. Among the chief flood-control projects in the United States are the flood control works along the Mississippi River, the installations of the Tennessee Valley Authority, the Glen Canyon and Hoover dams on the Colorado River, and the systems of dams in the Columbia River basin (including Grand Coulee Dam) and in the Missouri River basin.
A flood of the Tiber was recorded in 413 B.C. Records of floods on the Danube date from A.D. 1000. In China some of the world's most disastrous floods have been caused by the unstable Huang He (Yellow River). The river, which flows at or above the level of the bordering land, is contained in part by levees; however, because its channel has gradually become filled with deposited sediment, any appreciable increase in its volume causes the river to overflow and flood the surrounding area. The Netherlands, dependent on its dikes for protection from inundation, has suffered many disastrous floods from the sea and the Rhine and Meuse rivers. In 1970, 1985, and 1991, hundreds of thousands of people in Bangladesh were killed when the combination of high tides and a tropical cyclone (see hurricane) storm surge caused widespread flooding of the low-lying delta of the Ganges and Brahmaputra rivers.
In the United States the Johnstown, Pa., flood of 1889, in which thousands of lives were lost, was caused by the breaking of an earth dam above the city. Even greater loss of life occurred (1900) in Galveston, Tex., when tide and storm surges engulfed the city after a hurricane. The hurricanes of 1938 on the New England and Long Island coasts and Hurricane Donna in 1960 along the Atlantic coast from Florida to the Long Island Sound were also followed by storm surges. In June, 1972, extremely heavy rainfall associated with a tropical storm inundated the basins of the Chemung and Susquehanna rivers of New York and Pennsylvania, causing severely damaging floods in Corning and Elmira, N.Y., and Wilkes-Barre and Harrisburg, Pa. In July, 1979, Hurricane Claudette deposited a U.S. record of 43 in. (109 cm) of rain in Alvin, Tex., in 24 hours. Hurricane Katrina in Aug., 2005, led to extensive and devastating storm-surge flooding along the Louisiana and Mississippi coasts, and the failure of several levees in the New Orleans area resulted in hundreds of deaths. The worst floods in the United States from river overflow were in 1913 on the Miami River (a tributary of the Ohio), in 1927, 1937, 1973, and 2011 on the Mississippi River and some of its tributaries, in 1935–36 on several New England rivers, and in 1993 on the Missouri, Mississippi, and some of their tributaries.
- See Rampage (1973). ,
- Flood (1982). ,
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