For thousands of years, dam and water storage technologies have allowed civilizations to flourish in parts of the world where dry climates would otherwise limit human settlement. As early as 3000 BC, civilizations along the Tigris, Euphrates, Ganges, and Nile Rivers constructed earth and stone dams across these large rivers. These structures allowed them to store water for agriculture and create complex societies on that basis.
A dam consists of a mass of earth, timber, rock, concrete, or any combination of these materials that obstructs the flow of water. A dam can either divert water or store it in a reservoir, the artificial body of water that a dam creates. Diversion dams (weirs) raise the elevation of a river and divert water into a canal for transport to a mill, power plant, or irrigated field. Storage dams impound water in a reservoir.
There are three major types of dams--gravity, arch, and buttress. Gravity dams rely for stability on their weight to resist the hydrostatic, or water, pressure exerted by the reservoir. Arch dams, built along arcs that curve upstream into reservoirs, are most commonly found in narrow canyons with hard rock foundations. The arch dam transmits the horizontal water thrust to the abutments. Multiple-arch dams consist of a number of single arches supported by buttresses. Like gravity dams, buttress dams rely on gravity for stability, but require less material than standard gravity structures. They resist hydrostatic loads by using the same engineering principles of the flying buttresses that braced the high walls of Gothic cathedrals.
Engineers of the late Roman Empire built the first known arch dams and several buttress dams. In medieval times, Moorish engineers harnessed water from the Sierra Nevada and constructed massive irrigation systems in southern Spain. Persia's Ilkhanid Mongols built the world's tallest arch dams at that time as well. In the sixteenth century, Spanish engineers initiated a new era of dam building by applying mathematical principles to dam design. From the late eighteenth to early nineteenth centuries, French and English engineers, including Bernard Belidor, Charles Bossut, Charles Augustin Coulomb, and Henry Moseley, designed dams further based on mathematical principles. By the end of the 1800s, Europeans had initiated large dam projects around the world, particularly in areas of British control. British engineers completed Egypt's Aswan Dam in 1902, enlarged it in 1934, and rebuilt it upstream in 1960. The dam resulted in increased agricultural and industrial capacity for the region's growing population.
The twentieth century heralded the development of large, multipurpose water projects that could control floods, store water for irrigation, municipal, and industrial use, improve the navigability of waterways, and provide water for the generation of hydroelectric power. Indeed, one of the century's great advances came with the development of technologies that could generate electricity from the motion of falling water. The introduction of concrete as a construction material allowed dam designers from the 1910s to the 1930s to consider thin shells and complex curved shapes to minimize the volume of concrete required and the overall cost of arch dams. The face of a double-curvature (dome) arch dam is curved in two directions: from side to side, transferring force into the canyon walls, and from top to bottom, transferring force into the canyon floor. Post-tensioned steel rods or cables commonly reinforce concrete arch dams and gravity dams, reducing the cross-section (and volume of materials) for a conventional gravity dam of the same height. Vertical steel rods, stressed by jacks and securely anchored into the rock foundation, resist the tendency of the thin shells to overturn.
The U.S. Bureau of Reclamation's Hoover Dam (originally called Boulder Dam) (a curved gravity dam) served as an early prototype for the century's multipurpose water projects. Built along the Colorado River in 1935, Hoover Dam stands nearly 222 meters high--the tallest dam in the world at that time. The dam created Lake Mead, the largest man-made lake in the U.S. Hoover Dam Power Plant's 17 large turbines generate a total capacity of 2,074,000 kilowatts of electrical power for Nevada, Arizona, and California.
Hoover Dam served as a model for multipurpose water projects around the world, including government projects in Pakistan and India. Other examples of major late twentieth century dam projects include Syncrude Tailings Dam in Alberta, Canada; Rogun Dam in Tajikistan; Tehri Dam in the Indian Himalayas; and the Itaipu Dam on the Parana River between Brazil and Paraguay. By harnessing large rivers' water and energy, these dams allow for massive agricultural and industrial development in areas with natural limitations.
Perhaps the twentieth century's most ambitious dam project was Three Gorges Dam in China, the largest construction scheme in China since the Great Wall. Started in 1992 and completed through the turn of the twentieth century, the dam was built in order to rid the fabled Yangtze River of its deadly floods, provide an important source of electrical energy for China, and open up the inland Chongdong region to commerce. Plans were first developed in the 1920s. They were revived in the mid-1950s, when devastating floods along the Yangtze raised the recurring need for new water policy measures. The project was finally started in 1993. The dam, almost 2.5 kilometers wide and 180 meters high, created a reservoir 66 kilometers long and hundreds of meters deep, at a cost of approximately $24 billion. On its completion in 2009, the first set of generators will produce an amount of power expected to create as much electricity as 18 nuclear power plants.
Controversy over Three Gorges Dam reflects the larger social, environmental, and economic questions of dam building. In the late twentieth century, the desire to protect rivers, riparian habitat, and human health hindered the construction of large dams and even mandated the removal of some. In China, critics of Three Gorges Dam claim that slowing the flow of the Yangtze River creates an environmental hazard by allowing sources of pollution to collect. Indeed, the inevitable buildup of silt behind large dams poses a technological problem that has not yet been solved by engineers. Cultural and social concerns also raise questions about the viability of large dams. The reservoir created by Three Gorges Dam drowned valuable antiquities sites, including evidence of the ancient Ba people. It also flooded more than 100 towns, forcing the resettlement of 1.13 million people--the largest resettlement in the history of dam building. Three Gorges Dam, like all major dam projects, reflects the need to balance industrial growth with environmental and social concerns.
Jessica B. Teisch
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