Dam

I

Introduction

Dam, barrier constructed across a stream or river to impound water and raise its level. The most common reasons for building dams are to concentrate the natural fall of a river at a given site, thus making it possible to generate electricity (see Hydro-Power); to direct water from rivers into canals and irrigation and water-supply systems; to increase river depths for navigational purposes; to control water flow during times of flood and drought; and to create artificial lakes for recreational use. Many dams fulfil several of these functions.

The first dam of which record exists was built about 4000 bc to divert the Nile in Egypt in order to provide a site for the city of Memphis. Many ancient earth dams, including a number built by the Babylonians, were part of elaborate irrigation systems that transformed unproductive regions into fertile plains capable of supporting large populations. Because of the ravages of periodic floods, very few dams more than a century old are still standing. The construction of virtually indestructible dams of appreciable height and storage capacity became possible after the development of portland cement, concrete and the introduction of earth-moving machines and materials-handling equipment.

Controlling and using water by means of dams profoundly affects the economic prospects of vast areas. One of the first stages in the progress of developing countries usually involves gaining the ability to use dams for power generation, agriculture, and flood protection.

II

Design Considerations

A dam must be impervious to water; leakage through or under a dam must be prevented to avoid excessive water loss and to prevent undermining of the structure itself. A dam must also be constructed in such a way as to withstand the forces exerted upon it. Some forces that engineers must consider when designing a dam are gravity (which tends to pull the dam down); hydrostatic pressure (from water behind the dam); uplift (vertical forces tending to reduce the weight of the dam) caused by hydrostatic pressure on the foundation; ice pressures; and earth stresses and tensions, including the effects of earthquakes.

When a site is being considered for construction of a dam, earthquake hazards must be taken into account as part of a thorough geologic analysis. In addition, geologists must determine whether the natural foundations are subject to seepage and whether they have the strength to support the weight of the dam and the water that will back up behind it.

Inadequate geologic analyses have resulted in catastrophic losses. The most famous example is the disaster that occurred at the Vaiont Dam in the Italian Alps. On October 9, 1963, 4,000 lives were lost when a rock slide falling into the water behind the dam caused a huge wave to overtop the 262-m (860-ft) concrete arch structure. The force of the huge body of water falling from this height was enough to devastate the valley below for a distance of several kilometres downstream. Various geologic conditions were responsible for the rock slide, chief among them the weakening of steep, unstable rock slopes by water impounded by the dam.

III

Height of Dams

The height of a dam is limited by the topography of the site; however, other factors may dictate a less than maximum height. If the primary purpose of a dam is power generation, the dam height is critical, for the power generated increases in direct ratio to the head (height) of water impounded. For flood-control dams, storage volume is the primary consideration. Above a certain height, increase in storage volume for various functions may not justify the greater resulting cost of construction. Other limiting factors include the value and usefulness of land that will be submerged and interference with highways and railways.

The lake or reservoir formed by a dam may be very large. For example, Lake Kariba, the reservoir behind the 128-m (420-ft) high Kariba Dam on the Zambezi River, completed in 1959 and shared by Zimbabwe and Zambia, is 282 km (175 mi) long, covers 5,180 sq km (2,000 sq mi), and contains about 18.4 million hectare m (about 149 million acre ft) of water.

IV

Spillways

After the normal operating reservoir level has been established, means must be provided to ensure that this level will not be exceeded. A spillway is therefore necessary to discharge surplus flow without damage to the dam, powerhouse, or riverbed below the dam. The most common type of spillway is the overflow. In this type, a portion of the dam has a well-rounded crest that is somewhat lower than the top of the dam. To permit maximum use of storage volume, movable gates are sometimes installed above the crest to control discharge. In dams such as those on the Mississippi River, flood discharges are of such magnitude that the spillway occupies the entire width of the dam and the overall structure appears as a succession of vertical piers supporting movable gates. Another type of spillway is the chute, a wide, gently sloping, concrete channel frequently built around the end of an embankment dam of moderate height.

High archtype dams in rock canyons usually have downstream faces too steep for an overflow spillway. In the Hoover Dam on the Colorado River in the United States, for example, a shaft spillway is used. Shaft spillways are used on dams in restricted drainage areas in which large floods are not encountered. In shaft spillways, a vertical shaft upstream from the dam drains water from the reservoir when the water level becomes too high; the vertical shaft connects to a horizontal conduit through the dam into the river below.