What are some of the importance's of constructing a reservoir?
What are some of the importance's of constructing a reservoir?
The challenges of reservoir construction include dam failure and sediment filling.
What is soil salinization?
Reservoirs may totally alter the water resources of a country. Before the Aswan Dam was completed in Egypt, more than half of the 8 × 1010 m3 of water that flowed down the River Nile through Egypt each year ran into the sea. Most of the water can now be used in Egypt, mainly for irrigation, and instead of a single annual crop grown after seasonal flooding, more than one crop can be grown each year. However, advantages such as these must be considered in conjunction with the environmental side-effects of reservoir construction.
Reservoirs may totally alter the water resources of a country. For example, before the Aswan Dam was completed in Egypt, more than half of the water that flowed down the River Nile through Egypt each year ran into the sea. Most of the water can now be used in Egypt, mainly for irrigation, and instead of a single annual crop grown after seasonal flooding, more than one crop can be grown each year.
However, advantages such as these must be considered in conjunction with the environmental side-effects of reservoir construction. The major side-effects are detailed below.
By their very nature, reservoirs occupy large areas of land. Lake Nasser, the reservoir created by the Aswan Dam, has an area of 6000 km^2, and even in Britain some 250 km^2 of land is covered by reservoirs. The largest in the UK is Kielder Water in Northumberland, which covers about 10.5 km^2.
British reservoirs are generally in upland areas of scenic beauty that are otherwise suitable only for hiking and related pastimes, and as rough grazing for sheep, but reservoirs may cover up rich farmland or villages, or destroy sites of outstanding natural beauty or of archaeological importance. Land itself is an important resource, and drowning it under a reservoir may not be the best use of that resource, especially if the water can be supplied by alternative means.
Creation of a reservoir produces ecological changes not only to the area of the reservoir itself, by destroying the natural vegetation, but also upstream and downstream of the reservoir.
The gradient of a river upstream of a reservoir may be reduced, so the water will slow down, changing the character of the river, causing deposition of sediment, and changes to the natural vegetation and animal life.
Downstream of the reservoir the discharge will change, as well as the sediment load, also affecting the plants and animals. For example, annual flooding may cease.
Dams may collapse, releasing large amounts of water downstream, causing destruction of buildings and killing people and animals. Collapse may be caused by inappropriate construction, failure of the underlying sediments or rock, overfilling or earthquakes. The dam for the Carsington reservoir, in Derbyshire, collapsed during construction in 1984 due to failure of underlying sediments.
Earthquake damage to dams is rare but does occur. Although China, for example, has thousands of dams in earthquake areas, none has collapsed in recent years from earthquake damage, but in the UK, an area of low earthquake activity, the Earl's Burn Dam near Stirling failed following an earthquake in 1839.
The lifetime of reservoirs can vary greatly. Many reservoirs have lasted for over a hundred years, but some may be useful for only a much shorter period-fifty years or so - not because of the general deterioration of the dam as it gets older, but because sediment accumulates in the reservoir.
Rivers carry large amounts of mud, silt and sand in suspension, particularly during floods, and when a river enters a reservoir it slows down and the sediment carried in suspension is deposited on the floor of the reservoir. For example, Lake Mead, on the Colorado River, has had its storage capacity reduced by over a half since the dam was completed in 1935.
The trapping of sediment behind dams may also affect agriculture. The Nile Valley, for example, used to flood naturally once a year, and the sediment in the waters was deposited on the land, forming a fertile soil. These floodwaters and the sediment they carry are now trapped behind the Aswan Dam, and artificial fertilizers must be used down river in the valley.
Without the yearly supply of sediment in floodwater, the banks of the Nile are eroding downstream of the dam, and the Nile delta is reducing in size; the erosion by wave action no longer being counteracted by a fresh supply of sediment.
The change from annual flooding by a river to perennial irrigation that can be provided from a reservoir can cause soil salinization, if salts normally present in the river water accumulate in the soil as the water evaporates. These salts were previously washed away by the flooding, but the reduced supply of water by irrigation leaves them in the soil.
The water is taken in by plants, or evaporated by the sun, leaving the salts behind. This causes a decline in crop yields until eventually the soil becomes useless for agriculture. It can be prevented by using enough irrigation water to wash the salts through the soil, and draining this water from the fields.
Some reservoirs cause earthquakes to occur. This is perhaps not so surprising, as earthquakes are caused by stress in rocks, and the addition of a large mass of water in a reservoir on top of the rocks at the Earth's surface stresses the rocks and can trigger an earthquake.
Not all reservoirs induce earthquakes: it is in general only the larger reservoirs, or the deeper ones (over 100m deep), and only if the reservoir is built in an earthquake area, releasing stress already stored in the rocks. Induced earthquakes mainly occur during changes in water level in a reservoir, particularly during initial filling or during seasonal changes of water level.
Reservoirs that have experienced induced earthquakes include Marathon, Greece (1931), Lake Mead, USA (from 1938), Kariba, Zimbabwe (1963), Lake Nasser, Egypt (from 1965), Koyna, India (1967) and Thomson, Australia (1996). So far, none of these induced earthquakes have caused total dam failure, but the largest, at Koyna, was of considerable size (Richter magnitude 6.5, the equivalent of 100 Hiroshima atomic bombs), and led to damage of the dam.