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  3. Flood Control As long as people live on the Earth they suffer from floods. Stories of great floods in ancient times--for exa

Flood Control As long as people live on the Earth they suffer from floods. Stories of great floods in ancient times--for exa

admin2012-12-13  87
问题                            Flood Control
    As long as people live on the Earth they suffer from floods. Stories of great floods in ancient times--for example, the Bible story of Noah and the Ark--have come down from many early peoples.
    Although floods are not increasing in size or frequency in the United States, damage from them, however, is increasing to such an extent that they are a major menace (威胁) to the national welfare (社会安全). Most large cities and industries are situated on the seacoasts, on the shores of large lakes or on rivers. In the fiver valleys lie the richest farmlands and the easiest routes for railways and highways. Land subjected to flooding in the United States has been estimated at 50 million acres. As fast as they are supposedly protected by flood-control works, people move into flood plains. The population density on flood plains is more than twice the national average. Yet these areas cannot be given complete protection. The average annual property loss is in excess of one billion dollars. A single great flood in 1955 in six northeastern states caused property damage amounting to a half billion dollars. A flood induced by Hurricane Agnes in June 1972 caused property damage in the eastern United States that totaled about 3 billion dollars.
    One way to avoid floods is to take the obvious precaution of living where there is no danger of high waters. The Bible story of The Tower of Babel tells of an attempt to escape flood damage in this way. To follow such a program, however, would compel people to leave many of their richest regions.
    It has always been convenient and often necessary to build homes and factories on the floodplains along rivers and streams and 6n the seacoasts. American pioneer settlers depended upon the streams for drinking water, transportation, and power to run their mills and factories. Floodplains, deep with the silt laid down by overflowing rivers, are fertile farmlands. The earliest towns and farms, therefore, were established along the riverfronts, and large portions of them were built on land that was subject to periodic flooding.
    While the communities were small, the damages suffered from floods were limited. With the great population and industrial growth of the United States, flood damage has become a serious national problem.
    Two Approaches to Flood Control
    There are two basic approaches to flood control. One is to minimize the extent of flooding by building dams, reservoirs, levees (防洪堤), and other engineering works. The other is to prevent floods by conservation practices designed to hold the water where it falls in the headstreams and watersheds.
    Engineers in ancient times built earthen mounds to keep back floodwater. Such artificial embankments (堤防), called levees, held Chinese rivers in check for centuries. This method was followed in colonial America. New Orleans built a levee for protection from the Mississippi River early in the 1700s. The states intensified levee construction on the Mississippi in about 1850.
    Modern Levee Building
    Because a levee at one point confines the water there and raises the peak of flood waters upstream and downstream, levees once started usually have to be built at all the low points of a river system. Furthermore, a system of levees is only as strong as its weakest spot. Thus uniform height and strength are required. Only a government, which controls the river from end to end, can safely supervise levee building. The Mississippi River has probably the longest continuous levees in the world. One extends from Pine Bluff, Ark., southward for 380 miles.
    To keep the current from eating away the levee surfaces, long-rooted Bermuda grass is thickly sown on them. They may also be covered with mats of willow branches, with asphalt (沥青), or with a flexible mat of concrete blocks connected by reinforcing fabric and twist wire. Such coverings are called revetments (铺面,堑壕). They are also used to stabilize a river’s banks and keep the stream within its old channel.
    Floodways and Spillways
    Floodways (分洪河道) divert excess water from the main river channel and carry it off by a different route. From Cairo, Ill., to New Madrid, Mo., for example, the high east-bank bluffs and the levees along the west bank create a narrow channel for the river. A setback levee was built about five miles west of the river front levee. The strip between the setback and the riverfront levee is known as the Birds Point-New Madrid Floodway. At extremely high flood stages water enters the floodway through fuse plugs in the old riverfront levee near Cairo and reenters the main river just above New Madrid. The floodway was operated during the 1937 flood and was valuable in protecting Cairo from high water. There are three other floodways on the lower Mississippi.
About 35 miles above New Orleans flood waters can be diverted through the bonnet Carre Spillway (泄洪道). This is an emergency channel, controlled by flood gates, to Lake Pontchartrain, which empties into the Gulf of Mexico.
    Jetties and Cutoffs
    Jetties (防波堤) are piers or embankments designed to narrow the channel, and thus increase the scouring action of the current. They are particularly valuable at the mouth of the fiver to carry silt-laden waters out to sea.
    Cutoffs shorten and straighten the river’s channel. They speed up the river’s flow and thus lower flood stages upstream. On the Mississippi, cutoffs have reduced the river distance from Memphis, Tenn., to Vicksburg, Miss., by 170 miles and lowered the flood water high stage more than 12 feet at Arkansas City, Ark., and 6 feet at Vicksburg. Other channel improvements, such as widening and deepening, clearing out vegetation, and paving the banks, also increase the capacity of a river to carry off flood waters. In addition, these projects improve navigation.
    Dams and Reservoirs
    Dams and the reservoirs behind them help control floods. By emptying a dam before a flood is expected, storage space is obtained in which the flood waters can be impounded (贮存) for gradual release later. Even if the reservoir is nearly full, it acts like a safety valve. An amount of water, which would add ten feet to the height of a river 100 feet wide, would add only one foot to a reservoir or lake 1,000 feet wide. Moreover, evaporation from the broad surface of a reservoir or lake is far greater than evaporation from the narrow surface of a river. Thus less water flows on to swell floods downstream.
    Flood-control dams are built to create big storage capacity and are planned for rapid filling and emptying. Their value has been proved by the experience of the Miami Conservancy District in Ohio. Following severe floods that occurred in 1913, five dams and retarding basins were built across streams in the valley of the Miami River. During excessive rains, water collects in the storage reservoirs and is released in controlled amounts to the channel below the dams.
The fiver distance between Memphis and Vicksburg is shorten by______ because of the cutoffs.
选项
答案170 miles
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