Excerpted from Chapter 1 of Patrick McCully’s:
Silenced Rivers: The Ecology and Politics of Big Dams
Consultant into the Irrigation division of Sri Lanka, 1991
Dams have two primary functions. The very first is to store liquid to pay for changes in river movement or perhaps in need for liquid and energy. The 2nd to improve the amount of the water upstream make it possible for liquid to-be diverted into a canal or to increase ’hydraulic mind’ –– the real difference high between your surface of a reservoir together with river downstream. The creation of storage and head enable dams to create electricity (hydropower provides nearly a fifth associated with the world’s electrical energy); to supply water for agriculture, sectors and homes; to control floods; and help river navigation by providing regular flows and drowning rapids. Other cause of building huge dams include reservoir fisheries and leisure tasks such as boating.
Hydropower generation capacity is a purpose of the amount of circulation and hydraulic head. Even though mind is normally pertaining to the height of the dam, a minimal dam might have increased head in the event that powerhouse having its turbines and generators is located some length downstream associated with the dam. Pipes generally ’penstocks’ direct liquid to the turbines. After the water has actually spun a turbine it moves in to the ’tailwater’ underneath the dam through a ’tailrace’ pipe.
One advantageous asset of hydro over other types of electricity generation is that reservoirs can store liquid during times of reasonable need and quickly begin generating through the maximum hours of electrical energy usage. Thermal power flowers just take much longer to start out up from cool than hydro flowers. Hydro’s suitability for generating valuable ’peaking’ energy features recently encouraged a boom with what tend to be generally pumped–storage flowers. These involve two, typically relatively small, reservoirs, one above the various other. During top hours, water from upper reservoir drops through turbines to the lower one, creating electricity. The water is then pumped right back uphill once more using cheap off–peak electrical energy.
Weirs and barrages are different kinds of ’run–of–river’ dams, this means as they improve the water-level upstream they create only a small reservoir (’head pond’) and cannot effortlessly manage downstream flows. A weir is generally a decreased wall surface of stone, tangible or wicker. A barrage could be a massive construction ten or twenty metres high extending for hundreds of metres throughout the bottom achieves of a broad river. The electricity generation of a ’run–of–river’ hydropower dam is proportional into flow of the river at any one time.
As they are apt to have less damaging effects than storage space dams, run–of–river dams are not even close to environmentally harmless, and also the distinction between a ’run–of–river’ and a ’storage’ dam is certainly not always obvious. Dam proponents have sometimes desired to downplay the effect of in the offing dams by claiming that they'll be run–of–river. Thailand’s Pak Mun Dam, as an example, is over and over explained by officials as a run–of–river project however for much of the time the dam’s gates stay closed therefore runs as a storage dam. Despite years of protestations from its designers and funders so it might have minimal effects on the lake, Pak Mun handled within after some duration to destroy one of many nation’s richest freshwater fisheries.