港航专业英语

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Hydraulic Engineering——Dams

Among mankind’s oldest works are irrigation and water supply systems;indeed,the earliest civilizations in river valleys in the Middle East were based on agriculture that depended on irrigation. Harbor facilities and canals for navigation were also early engineering accomplishments.Such systems are usually grouped together as hydraulic engineering projects. Hydraulics is the science that deals with the flow and control of water and other fluids.

Among the most impressive modern works in hydraulic engineering are such great dams as the Aswan High Dam on the Nile in Egypt and the Hoover Dam on the Colorado River in the Southwestern United States.These dams, like most modern dams, serve a number of different purposes. Among them are flood control, water storage, irrigation, navigation,and hydroelectric power.In addition, most dams are also links in a highway system, with a roadway running across them. The lakes behind them, like Lake Mead which is backed up by the Hoover Dam, also serve as recreational areas.

Before design and construction of a dam can begin,an extensive survey and study of the site must be made. This survey examines not only topographical features of the area, but also soil and rock samples to determine the geological factors that may affect it. The hydraulic features of the stream or river that is being dammed must also be determined the rate of volume of flow of the river at different seasons, and the volume of water that will be backed up by the dam. Engineers use this information to calculate potential water pressure. It is also necessary to study the site to see whether the dam can be constructed with the use of cofferdams or whether the flow of the river must be diverted. Cofferdams, are watertight piles that form an enclosure from which water can be pumped. When it is necessary to divert the river, one technique is to dig tunnels for the channel; another is to excavate a temporary channel for the river around the dam site.

Even after the site has been thoroughly investigated and the designs have been made，preliminary work is still not complete. Scale models of the dam are often made so that they can be tested under simulated conditions. Computers are also used extensively to calculate all the different stresses to which such huge structures can be subjected, including those that may be caused by earthquakes.

Basically there are two types of construction for dams, masonry and embankment. Before the invention of Portland cement, huge blocks of cut stone were ordinarily used to build dams, but today masonry dams are constructed with reinforced concrete.

Masonry dams are most often built to control swift-flowing streams in narrow valleys where there is good rock for the foundations. An excellent example is the Hoover Dam. Embankment dams are essentially great mounds of earth across a stream. In addition to compacted earth, embankment dams can be built with crushed rock or sand. This kind of dam is usually built across wider streams where the water flows rather slowly. The Aswan High Dam is a good example of