Flood Control.

Flood Control. I INTRODUCTION Flood Control, all methods used to prevent or reduce detrimental effects of floods. II 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 stream channels, is called runoff. Floods occur when soil and vegetation cannot absorb all the water; water then runs off the land in quantities that cannot be carried in stream channels or retained in natural ponds and constructed reservoirs (see Dam). About 30 percent of all precipitation is runoff, 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 tsunamis caused by undersea earthquakes (see Earthquake). III EFFECTS OF FLOODS Floods not only damage property and endanger the lives of humans and animals, but have other effects as well. Rapid runoff causes soil erosion as well as sediment deposition problems downstream. Spawning grounds for fish and other wildlife habitat 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 are commonly millions of dollars each year. IV CONTROL OF FLOODS The basic methods of flood control have been practiced 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 (Yellow River) 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 4000 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, Rhône, 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 (see Levee). 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 over $10 billion in damage. At its height, the Missouri river crested almost 15 m (almost 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 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 runoff and released gradually during dry seasons (see Water Supply and Waterworks). The Hoover Dam on the Colorado River, the reservoirs in the Miami Conservancy District, 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. In other countries, one remarkable flood-control project 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 (Escaut) 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 River 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 runoff. 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, such as crop rotation and contour plowing. See Conservation; Erosion. 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 depend for their continued fertility on periodic flooding because the soil deposited by sedimentation from floodwater is very rich. V FLOOD-CONTROL LEGISLATION In the 20th century the problem of flood control in the United States has assumed national importance because of the increasing frequency and intensity of floods in all of the great river valleys as a result of deforestation. In addition, agricultural and industrial development in these valleys has necessitated a coordinated program of flood control. Federal legislation has been passed to aid the states in effecting adequate control measures. Federal action in this field was long hampered by many constitutional limitations. As late as 1879 the problem of Mississippi River flood control was left to the 31 states in that river's drainage system. In that year, the federal government, under the guise of improving navigation (a power constitutionally granted to it), helped finance the construction of levees. The federal government has played an increasingly important role in the problem of flood control. When scientific research into the causes of floods showed that the construction of levees was insufficient as a method of control, the first steps were made to provide for reforestation and soil conservation. The Clarke-McNary Act of 1924, the Mississippi Flood Control Act of 1928, and the McSweeney-McNary Act of 1928 were all directed toward that end. In 1935 the Soil Conservation Service was established by the Congress of the United States under an act declaring a policy of permanent provision for control and prevention of soil erosion, and for control of floods. In the same year the Fulmer Act was passed, authorizing the secretary of agriculture to enter into cooperative agreements with the states for better forest land management. In 1937 further authority was granted to the secretary of agriculture in the Bankhead-Jones Farm Tenant Act to control soil erosion and to mitigate floods. Another act of Congress provided for an investigation of the material resources in the California watersheds. Similar acts covered the watersheds of the Río Grande and the Pecos River. The period of the 1930s is notable for the many national forests that were established to assist in the soil-conservation program. The most important single act directed at the problem of flood control was the establishment of the TVA in 1933. By 1940 the Supreme Court of the United States had passed on the constitutionality of almost all federal activities in the field of flood control and had upheld actions that related to almost all waters of the United States, without limiting these actions to the navigable streams, as had been done earlier. In the Flood Control Act of 1946, Congress authorized construction of 123 projects to aid river regulation, flood control, and power development. In 1972 Congress moved to reduce the hazards from the 28,000 nonfederal dams in the country by passing a bill providing for a dam-inspection program. The action was spurred by two disastrous dam failures during the year. At Buffalo Creek, West Virginia, a makeshift dam at a coal mine site collapsed with a loss of more than 100 lives; and at Rapid City, South Dakota, two earthen dams gave way, resulting in more than 200 deaths. Many aspects of flood control can be handled by individual states or groups of states acting on a regional basis. One such project, completed in 1922, is the Miami Conservancy District, a system of reservoirs in the Miami River valley in Ohio that works to reduce flood hazards. Such organizations as the Los Angeles County Flood Control District, the TVA, and others relating to specific areas have made important studies of hydrologic and meteorological conditions to aid in the forecasting of flood dangers and to plan flood control. 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