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 twi
A．Y
B．N
C．NG