Flood Control Dams

One way to avoid floods is to take the obvious precaution of living where there is no danger of high waters. It has always been convenient and often necessary to build homes and factories on the floodplains along rivers and streams and on 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 cities, flood damage has become a serious national problem. One of the basic approaches to flood control is to minimize the extent of flooding by building dams, reservoirs, levees, and other engineering works.

Engineers in ancient times built earthen mounds to keep back floodwater. Such artificial embankments, called levees, held Chinese rivers in check for centuries. This method was followed in colonial America.

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. Floodways and spillways divert excess water from the main river channel and carry it off by a different route.

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 for gradual release later. Even if the reservoir is nearly full, it acts like a safety valve. The amount of water which would add 3 meter to the height of a river 30 meter wide would add only one meter to a reservoir or lake 90 meter 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. During excessive rains, water collects in the storage reservoirs and is released in controlled amounts to the channel below the dams. The water carries with it large quantities of the richest topsoil, which muddies the rivers and is ultimately lost in the ocean. American rivers, for example, carry off an estimated 280 million cubic meters of solid matter each year. Agricultural experts propose to return the steepest hills along the headwaters of American rivers to forest. By means of terracing, contour plowing, and a wise choice of plants, runoff and erosion are checked on gentler slopes. By damming gullies, runoff is slowed, and silt from above slowly rebuilds the eroded spots. Thus flood prevention and erosion control go hand in hand. Preventing soil erosion also aids flood control by slowing the rate at which silt fills the reservoirs behind flood-control dams. Steps to lessen the effects of drought also aid in flood control. Some lakes, swamps, and marshes that were once drained to make farm lands are being restored to preserve the level of underground water in time of drought. This program also reduces floods by increasing evaporation and by the safety-valve action of wide lakes or swamps on narrow rivers. Thus the prevention and control of floods are tied with drought measures as well as with waterpower, navigation, soil conservation, and land use.

0 0

Post a comment