Aswan Dam
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Aswan is a city on the first cataract of the Nile in Egypt. Two dams straddle the river at this point: the newer Aswan High Dam (Arabic: السد العالي; transliterated: as-Sad al-'Aly), and the older Aswan Dam or Aswan Low Dam. Without impoundment the River Nile would flood each year during summer, as waters from East Africa flowed down the river. These floods brought nutrients and minerals that made the soil around the Nile fertile and ideal for farming. As the population along the river grew, there came a need to control the flood waters to protect farmland and cotton fields. In a high-water year, the whole crop may be entirely wiped out, while in a low-water year there was widespread drought and famine. The aim of this water project was to prevent the river's flooding, generate electricity and provide water for agriculture.
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[edit] Construction history
The British began construction of the first dam in 1899. Construction lasted until 1902. It was opened in December 10 1902. The project was designed by Sir William Willcocks and involved several eminent engineers including Sir Benjamin Baker and Sir John Aird, whose firm, John Aird & Company, was the main contractor. A gravity dam, it was 1,900 m long and 54 m high. The initial design was soon found to be inadequate and the height of the dam was raised in two phases, 1907–1912 and 1929–1933.
When the dam almost overflowed in 1946 it was decided that rather than raise the dam a third time, a second dam would be built 6 km upriver (about 4 miles). Proper planning began in 1952, just after the Nasser revolution, and at first the USA and Britain were to help finance construction with a loan of USD $270 million. Both nations cancelled the offer in July 1956 for reasons not entirely known. A secret Egyptian arms agreement with Czechoslovakia (Eastern Bloc) and Egyptian recognition of the People's Republic of China are cited as possible reasons. Soon thereafter, Nasser nationalized the Suez Canal, intending to use its tolls to subsidize the High Dam project. This prompted Britain, France, and Israel to attack Egypt, occupying the Suez Canal and precipitating the Suez Crisis. The United Nations, USSR and US forced the invaders to withdraw and the canal was left in Egyptian hands. The Egyptian government continued to intend to finance the dam project alone by using the revenues of the Suez Canal to help pay for construction. But as part of the Cold War struggle for influence in Africa the Soviet Union stepped in in 1958, and possibly a third of the cost of the dam was paid for as a long term investment in the region. The Soviets also provided technicians and heavy machinery. The enormous rock and clay dam was designed by the Russian Zuk Hydroproject Institute.
Construction began in 1960. The High Dam, as-Sad al-'Aali, was completed on July 21, 1970, with the first stage finished in 1964. The reservoir began filling in 1964 while the dam was still under construction and first reached capacity in 1976. The reservoir raised concerns from archaeologists and a rescue operation was begun in 1960 under UNESCO. Sites were surveyed and excavated and 24 major monuments were moved to safer locations (see Abu Simbel) or granted to countries that helped with the works (such as the Debod temple in Madrid and the Temple of Dendur in New York).
[edit] Specifications
The Aswan High Dam is 3,600 m in length, 980 m wide at the base, 40 m wide at the crest and 111 m tall. It contains 43 million m³ of material. At maximum, 11,000 m³ of water can pass through the dam every second. There are further emergency spillways for an extra 5000 m³ per second and the Toshka Canal links the reservoir to the Toshka Depression. The reservoir, named Lake Nasser, is 550 km long and 35 km at its widest with a surface area of 5,250 km² and holds 111 km³.
[edit] Benefits
The dam powers twelve generators each rated at 175 megawatts, producing a hydroelectric output of 2.1 gigawatts. Power generation began in 1967. When the dam first reached peak output it produced around half of Egypt's entire electricity production (about 15% by 1998) and allowed for the connection of most Egyptian villages to electricity for the first time. The dam mitigated the effects of dangerous floods in 1964 and 1973 and of threatening droughts in 1972–73 and 1983–84. A new fishing industry has been created around Lake Nasser, though it is struggling due to its distance from any significant markets.
[edit] Environmental and cultural issues
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In addition to the benefits, however, damming the Nile caused a number of environmental issues. It flooded much of lower Nubia and over 90,000 people were displaced. Lake Nasser flooded valuable archeological sites. The silt which was deposited in the yearly floods, and made the Nile floodplain fertile, is now held behind the dam. Silt deposited in the reservoir is lowering the water storage capacity of Lake Nasser. Poor irrigation practices are waterlogging soils and bringing salt to the surface. Mediterranean fishing declined after the dam was finished because nutrients that used to flow down the Nile to the Mediterranean were trapped behind the dam.
There is some erosion of farmland down-river. Erosion of coastline barriers, due to lack of new sediments from floods, will eventually cause loss of the brackish water lake fishery that is currently the largest source of fish for Egypt, and the subsidence of the Nile Delta will lead to inundation of the northern portion of the delta with seawater, in areas which are now used for rice crops. The delta itself, no longer renewed by Nile silt, has lost much of its fertility. The red-brick construction industry, which used delta mud, is also severely affected. There is significant erosion of coastlines (due to lack of sand, which was once brought by the Nile) all along the eastern Mediterranean.
The need to use artificial fertilizers supplied by international corporations is controversial too, causing chemical pollution which the traditional river silt did not. Indifferent irrigation control has also caused some farmland to be damaged by waterlogging and increased salinity, a problem complicated by the reduced flow of the river, which allows salt water to encroach further into the delta.
Mediterranean fish stocks are also negatively affected by the dam. The eastern basin of the Mediterranean is low in fertility, and traditionally the marine ecosystem depended on the rich flow of phosphate and silicates from the Nile outflow. Mediterranean catches decreased by almost half after the dam was constructed, but appear to be recovering. The dam has been implicated in a rise in cases of schistosomiasis (bilharzia), due to the thick plant life that has grown up in Lake Nasser, which hosts the snails who carry the disease.
The Aswan dam tends to increase the salinity of the Mediterranean Sea, and this affects the Mediterranean's outflow current into the Atlantic ocean (see Strait of Gibraltar). This current can be traced thousands of kilometers into the Atlantic. Some people believe that the dam's effect on this outflow speeds up processes that lead to the next ice age.
[edit] See also
[edit] References
[edit] External links
- Sayed El-Sayed and Gert L. van Dijken 'The southeastern Mediterranean ecosystem revisited: Thirty years after the construction of the Aswan High Dam.'
- Satellite views on Google Maps
http://maps.google.com/maps?hl=en&ie=UTF8&z=14&ll=24.034001,32.865901&spn=0.056362,0.078535&t=k&om=1
