A hydroelectric plant has three components:
- A dam to create a significant waterfall. The dam also often enables the creation of a storage reservoir for water, which allows the plant to continue working even during low-water periods.
- A feeder canal, which carries water necessary for the plant’s operation. It can be an open canal, an underground river or a conduit. Some plant, where the waterfalls are not very high, do not use such canals.
- The plant itself is where the waterfall turns the turbine and hence the electricity generator (more commonly the electricity generator is known as an alternator).
Over and beyond its use for energy production, a dam also enables a river in spate to be controlled, as well as providing water storage for agricultural irrigation and sometimes even for leisure activities (beaches, water sports).
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The decision to build a hydroelectric dam depends on three conditions being fulfilled:
Good topographical situation: the ideal is a river canyon or in general a narrowing of the river. If a maximum amount of water is to be stored, the volume of the basin above the dam must also be calculated. A wide flat valley is perfect!
The right geological formations: the rocks on which the dam is supported must be stable and waterproof, both for reasons of efficiency and safety.
The right hydrological conditions: precipitation over the water catchment area that feeds the supply basin of the dam must be sufficient to fill the dam and to compensate for evaporation losses from the retaining lake.
Finally any inhabitants of the storage basin that is going to be submerged must be moved and compensated
for the disruption.
There are two main types of dam:
- Gravity dams, which are totally supported on the bedrock base. This base must be especially resistant, since it will support the whole weight of the retained water. Gravity dams are in concrete or in earth or rock fill.
- Arch dams, in the form of a convex arch, which is largely supported by the lateral rock walls. These walls must be solid and are subjected to regular inspections. This type of dam is used in narrow canyons with steep sidewalls, so that the width of the dam does not exceed 6 times its height.
Dams do not only retain water: they also hold back sediments eroded by the river that feeds the retaining basin. They therefore have a tendency to fill up with mud
over time. For example: since its commencement four years ago, the dam at Sanmenxia on the Houang Ho river in China has lost 41% of its storage capacity because of mud sedimentation.
It is therefore necessary to cater for the pumping of these sediments, or their regular emptying by means of a conduit placed at the bottom of the dam. These emptying operations are delicate. Beware of an influx of muddy water downstream from the dam; neither the inhabitants nor the fish like that!
The design of a hydroelectric dam must take account of the risk factors and reduce them to a minimum:
- The resistance to periods when the river is in spate must be studied: in 1889, the dam at Johnston in the United States gave way under the impact of flood waters, claiming 2000 victims. Today all dams are equipped with means of dealing with spate conditions.
- The resistance of the dam to earthquakes must be studied and assessed. It is also necessary to take account of the stability of the land surrounding the retaining basin: in 1963 a gigantic landslide fell into the retaining lake of the dam at Vaiont in Italy. The dam resisted, but an enormous wave of water flowed over the top of the dam, claiming 3000 victims in the valley below.
- Permanent inspection of the dam itself for: infiltration of water into the body of the dam or under the dam (the “fox effect”), deformations …
- A study of the ecological impact, particularly around and downstream of major dams. Example: the construction of the colossal Assouan dam on the Nile in Egypt (160 billion m3 of retained water capacity) has had a number of effects, amongst others a significant reduction in the silt content of the water downstream from the dam.
Result: the Nile delta which previously reached the sea, has started to retreat (at a rate of several tens of metres a year in certain places). The peasant farmers on the delta have to use more fertiliser to maintain their agricultural yields, because of the deficit in the amount of alluvial material brought down by the annual flood conditions, compared to the situation before construction of the dam.
The ecological impact of medium-sized dams is also significant. For example: the stagnant water in the retaining lake behind a dam has a tendency to be under-oxygenated: the fish that live in the impoverished water that comes out of the turbines do not appreciate the situation. On the other hand, when water from the top of the dam is suddenly released it is heavily enriched in oxygen and contains tiny air bubbles. The fish don’t like that either … It is not easy to keep the little creatures satisfied!
- Human impact study. The creation of the retaining lake for very large dams in particular, leads to the displacement of numerous people and can drown significant areas of cultivated land. For example: filling the retaining lake for the largest hydroelectric dam in the world, that of the Three Canyons on the Yang Tse Kiang in China, started in 2003 and will be completed in 2009 (the dam is 185m high, 2km long, represents 22 billion euros of investment, the plant has twenty-six 700-MW turbines producing 18 200 MW of power, the equivalent of ten nuclear generating plants!). Between 1.2 and 1.9 million people will be displaced, all of whom must be found new
homes
and given new land to cultivate! |