Magazine article UNESCO Courier

Large Dams

Magazine article UNESCO Courier

Large Dams

Article excerpt

"When the garden is full of fruit, it's time to bottle some for the winter," says Jacques Lecornu. managing director of the International Commission on Large Dams (ICOLD). "In the same way. dams retain water reserves for use in the dry season."

Floods claim some 100,000 victims per year, and according to ICOLD's 1995 Position Paper on Dams and Environment, flood control "has always been a particularly significant motive for dam construction." Dams also "mitigate droughts, adjust natural runoff with its seasonal variations and climatic irregularities to meet the pattern of demand for irrigated agriculture, power generation, domestic and industrial supply and navigation. They provide recreation, attract tourism, promote aquaculture and fisheries, and can enhance environmental conditions." Swiss engineer Nicholas Schnitter, author of A History of Dams (1994), adds that they also protect estuaries against the ravages of flood tides.

This positive view is increasingly being questioned, however, by environmentalists and scientists who advance strong arguments in support of their case. Concern was expressed as early as 1973 at an ICOLD congress in Madrid in the following terms: "The real problem to be solved is the question whether dams are useful or detrimental, whether they improve our environment as a whole and man's well-being or whether they spoil it, and appreciating in each case whether they should be built and according to what characteristics".

Past and present

The first dam for which there is reliable evidence was built in Jordan 5,000 years ago to supply the city of Jawa with drinking water. Around 1800 B.C., during the reign of the Pharaoh Amenemhet III, the Egyptians constructed a reservoir with the amazing storage capacity of 275 million [m.sup.3] in Al Fayyum Valley, some 90 km southwest of Cairo. Known as Lake Moeris (today Lake Qarun), it was used for 3,600 years.

According to Jacques Lecornu, more than half of ICOLD's nearly 40,000 registered large dams (higher than 15 metres) have been built in the last 35 years.

Dams can be divided into two broad categories according to their use: those designed to regulate river flow in the interests of navigability, and those which confine and check the flow of water for a variety of other needs. Each type has a specific design. Most water storage dams (83%) are embankment (earth fill) dams, a less costly technique than masonry. The materials used may be rock (Egypt's Aswan High Dam), earth (the Nurek dam in Tajikistan) or a mixture. There are various types of concrete or masonry dams: gravity-dams consisting of a massive triangular wall (the Grande-Dixence dam, Switzerland), arch dams built in a convex arch facing the reservoir (Zimbabwe's Kariba dam), and buttress dams (e.g. the Alcantara II dam in Spain). More than 52% of the world's dams are located in China, 16% in the United States, and 6% in Japan.

Costly savings

Reservoir-dams are mainly need for irrigation and drinking-water supply. Although waterfalls have been used for centuries to generate energy, hydroelectric production has increased over the past thirty years and now meets about 20% of world electricity demand (but only 7% of global energy demand). By economizing on coal and oil, hydroelectricity limits the air pollution caused by fossil fuel combustion. Hydroelectric power could be developed to great advantage if only it were not so often associated with dams that have dire consequences on the environment, especially in dry ecosystems.

The effects that dams can have on the environment are described in Freshwater Resources, a booklet produced by UNESCO and the United Nations Environment Programme (UNEP). They include changes in the ecosystem, loss of agricultural land, the spread of water-related diseases (especially bilharzia and malaria), regulation of river flow, change in water quality and sediment regime, flood risks, changed conditions for fisheries, agriculture, transport and other economic activities, population displacements, loss of recreational areas and increases in the area's seismic potential because of the reservoir's weight. …

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