Dams have two main functions. The first is to store water to compensate for fluctuations in river flow or in demand for water and energy. The second to raise the level of the water upstream to enable water to be diverted into a canal or to increase 'hydraulic head' -- the difference in height between the surface of a reservoir and the river downstream. The creation of storage and head allow dams to generate electricity (hydropower provides nearly a fifth of the world's electricity); to supply water for agriculture, industries and households; to control flooding; and to assist river navigation by providing regular flows and drowning rapids. Other reasons for building large dams include reservoir fisheries and leisure activities such as boating.
Hydropower generation capacity is a function of the amount of flow and hydraulic head. Although the head is usually related to the height of the dam, a low dam can have a high head if the powerhouse with its turbines and generators is located some distance downstream of the dam. Pipes known as 'penstocks' direct water to the turbines. Once the water has spun a turbine it flows into the 'tailwater' below the dam through a 'tailrace' pipe.
One advantage of hydro over other forms of electricity generation is that reservoirs can store water during times of low demand and then quickly start generating during the peak hours of electricity use. Thermal power plants take much longer to start up from cold than hydro plants. Hydro's suitability for generating valuable 'peaking' power has in recent years encouraged a boom in what are known as pumped-storage plants. These involve two, normally relatively small, reservoirs, one above the other. During peak hours, the water from the upper reservoir falls through turbines into the lower one, generating electricity. The water is then pumped back uphill again using cheap off-peak electricity.
Weirs and barrages are different types of 'run-of-river' dams, this means that while they raise the water level upstream they create only a small reservoir ('head pond') and cannot effectively regulate downstream flows. A weir is normally a low wall of stone, concrete or wicker. A barrage can be a huge structure ten or twenty metres high extending for hundreds of metres across the bottom reaches of a wide river. The electricity generation of a 'run-of-river' hydropower dam is proportional to the flow of the river at any one time.
While they tend to have less damaging consequences than storage dams, run-of-river dams are far from environmentally benign, and the distinction between a 'run-of-river' and a 'storage' dam is not always clear. Dam proponents have in some cases sought to downplay the impact of planned dams by claiming that they will be run-of-river. Thailand's Pak Mun Dam, for example, is repeatedly described by officials as a run-of-river project yet for much of the time the dam's gates remain closed and it operates as a storage dam. Despite years of protestations from its builders and funders that it would have minimal impacts on the river, Pak Mun managed within a couple of years to destroy one of the country's richest freshwater fisheries.
Just as every river and watershed is unique, so is every dam site and every dam. There are, however, three main types of dam design -- embankment, gravity and arch -- selected mainly according to dam-site topography and geology. Earth and rock embankments, which are usually the cheapest to build, make up more than 80 per cent of all large dams. Embankments are generally built across broad valleys near sites where the large amounts of construction material they need can be quarried. Large embankment dams are the most massive structures humanity has ever erected. The most voluminous dam in the world, Tarbela in Pakistan, contains 106 million cubic metres of earth and rock, more than 40 times the volume of the Great Pyramid.
Gravity dams are basically thick, straight walls of concrete built across relatively narrow valleys with firm bedrock. Arch structures, also made from concrete, are limited to narrow canyons with strong rock walls and make up only around four per cent of large dams. An arch dam is in form like a normal architectural arch pushed onto its back, with its curved top facing upstream and its feet braced against the sides of its canyon. The inherent strength of the shape enables the thin wall of an arch dam to hold back a reservoir with only a fraction of the concrete needed for a gravity dam of similar height.
A dam contains a number of structural features other than the main wall itself. Spillways are used to discharge water when the reservoir threatens to become dangerously high. Dams built across broad plains may include long lengths of ancillary dams and dykes. The five reservoirs of Phase 1 of the La Grande hydropower scheme in northern Quebec, for example, are impounded by eleven dams and more than 200 accompanying dykes stretching for a total length of 124 kilometres.
Extracted from: Silenced Rivers; Patrick McCully; Zed Books, 2001