Dams are barriers that impound water or underground streams. They retain water and generate hydropower. The Aswan High Dam in Egypt confines the Nile River and generates hydroelectric power. It flooded areas upstream and displaced many people when constructed. The Three Gorges Dam in China is the world's largest hydroelectric dam and produces significant power but flooded cultural sites and displaced over 1 million people.

1. Dams

2. What is a dam ? A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees (also known as dikes ) are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are often used in conjunction with dams to generate electricity. A dam can also be used to collect water or for storage of water which can be evenly distributed between locations.

3. Aswan High Dam Aswan Dam (Aswan High Dam) is located in the north of the border between Egypt and Sudan. It is a huge rock fill dam which confines the longest river of the world, Nile River, in the world's third largest reservoirs, Lake Nasser. The dam is known as Saad el Aali in Arabic. It was finished in 1970 and the ten years went in to the making of the dam. Since ancient times, Egypt has always depended on the water of the Nile River for various reasons. The two major tributaries of the Nile River are the White Nile and the Blue Nile. The source of White Nile is the Sobat River Bahr al-Jabal and that of the Blue Nile is in Ethiopian Highlands. The two tributaries converge in Khartoum, where they merge to form the Nile River. The Nile River has a total length of 4,160 miles (6,695 kilometers) from source to sea. The Aswan Dam which is located in Aswan, Egypt, tames the Nile River and uses the power of the river for a variety of social and economic causes. There are in fact two dams at Aswan and not just one; even though when most people talk of the Aswan High Dam they refer to the “Aswan Dam.” Collectively, these two dams run the flow of the Nile River through Egypt, and are also used to produce hydroelectric power for the Egyptians. The Aswan Dam has generated a lot of debate since it was built. A group of people fear that the dam can cause major environmental damage, and so they would not support the dam’s construction and want it removed .although doing so would have harsh consequence for eygept

4. The first dam at Aswan, the Aswan Low Dam, was built in the late 1800s by British government, and later was re done a couple of times. This dam was originally designed to manage the annual flooding of the Nile, which was important event in Egypt. Since ancient times the Egyptians have lived and cultivated near the Nile, with the help of the annual flooding to irrigate and fertilize their fields. With the increase in population the unpredictable floods became a problem. It caused loss of life and property, and the British reacted by damming the river. The first Aswan Dam was no a great success as it was not adequate for the task, and thus in the 1950s several countries like the United States decided to help build a dam which was further upstream. However, later on these countries did not abide to the deal, there by forcing Egypt to take help from the Soviet Union and thus in the 1960s, construction of the Aswan High Dam started. As a result of the construction of the Aswan High Dam, mass flooding took place upstream on the Nile, damaged people from their homes and causing huge loss to some invaluable archaeological sites. The lake which was created behind the dam is called as Lake Nasser named after the late Egyptian President Nasser. Egyptians were of opinion that the upstream flooding was a logical way to in exchange for convenient seasonal flood waters and a steady source of hydroelectric power. In the long run, quite a few problems have come up in Egypt because of the Aswan Dam. The capacity of Lake Nasser is shrinking as a result depositions of silt behind the dam, and because the silt there is water shortage in rest of Egypt.

6. Three Gorges Dam The Three Gorges Dam is a hydroelectric dam that spans the Yangtze River by the town of Sandouping , located in the Yiling District of Yichang , in Hubei province, China . The Three Gorges Dam is the world’s largest capacity hydroelectric power station with a total generating capacity of 18,200 MW . [1] The dam body was completed in 2006. Except for a ship lift , the originally planned components of the project were completed on October 30, 2008, when the 26th turbine in the shore plant began commercial operation

7. Each turbine has a capacity of 700 MW.Six additional turbines in the underground power plant are not expected to become fully operational until 2012. Coupling the dam's thirty-two main turbines with two smaller generators (50 MW each) to power the plant itself, the total electric generating capacity of the dam will eventually reach 22,500 MW. As well as producing electricity, the dam increases the Yangtze River's shipping capacity, and reduces the potential for floods downstream by providing flood storage space. The Chinese government regards the project as a historic engineering, social and economic success, with the design of state-of-the-art large turbines, and a move toward limiting greenhouse gas emissions. However, the dam flooded archaeological and cultural sites and displaced some 1.3 million people, and is causing significant

9. Assiut Barrage The Assiut Barrage is a dam on the Nile in the city of Assiut in Upper Egypt (250 miles to the south of Cairo ). It was designed by the famous British engineer Sir William Willcocks who also designed and built the Aswan Dam . The Assiut dam was constructed between 1898 and 1903 across the Nile, about 350 miles down-stream from the Aswan Dam in order to divert the water of the river at low water into Egypt ’s largest irrigation canal, the Ibrahimiya Canal . The dam was estimated to cost £525,000 but by the time of its completion it actually cost £870,000. The main contractor for the project was the British contractor Messrs & Aird & Co. The project’s massive inventory involved 2,400,000 cubic yards of earthwork, 125,000 cubic yards of concrete, 85,000 cubic yards of masonry, 125,000 cubic yards of pitching and more than 4,000 tons of cast iron pipes. Completed in 1902 The dam consists of a masonry dam about 2,769 feet long extended on both sides by earthen banks, making a total length of about 3,937 feet. There are 111 arched openings of 5 meter (16 feet 4 inches) span in the masonry dam. They can be closed by steel sluice-gates 16 feet high. The piers and arches are founded upon a masonry platform 87 feet wide by 10 feet thick. This platform is protected on its up-stream and down-stream sides by a continuous and impermeable line of cast iron tongued and grooved sheet-piling with cemented joints.

10. This piling extends into the sand bed of the river to a depth of 23 feet below the upper surface of the platform and prevents it from being undermined. The river bed is protected against erosion for a width of 67 feet upstream by a stone paving laid on a clay puddle to check infiltration, and on the downstream side for the same width by a stone paving having an inverted filter bed underneath, so that any springs that may be caused by the water above the sluices shall not carry sand with them from beneath the paving. The piers between the openings have a length of 51 feet up and down stream and are 6.56 feet wide with the exception of every thirtieth pier, which has double this width. The roadway is 41 feet above the top of the masonry platform. The dam has a maximum height of about 48 feet, the maximum head of water retained being about 33.5 feet. It is constructed of granite, the foundation platform mentioned above being of concrete. A lock 262.5 feet long by 52.8 feet wide and capable of passing the largest Nile cargo ships and barges was constructed at the dam. The Ibrahimiya head regulator structure, which was built at the same time when the Assiut Barrage was being constructed, was of similar design to the barrage except having only nine 5 m wide sluices, and a 9 m wide lock.

12. Koyna Dam The Koyna Dam is one of the largest dams in Maharashtra , India . It is a rubble-concrete dam constructed on Koyna River . It is located in Koyna Nagar , Satara district , nestled in the Western Ghats on the state highway between Chiplun and Karad , Maharashtra. The main purpose of dam is to provide hydroelectricity with some irrigation in neighboring areas. The Koyna hydro-electric project is one of the largest hydroelectric project in India having a total installed capacity of 1,920 MW . The project is actually composed of four dams, with the Koyna Dam having the largest catchment area. The project consists of 4 stages of power generation. All the generators are located in the underground powerhouses excavated deep inside the surrounding mountains of the Western Ghats . The catchment area dams the Koyna River and forms the Shivasagar Lake which is approximately 50 km (31 mi) in length

13. Completed in 1963, it is one of the largest civil engineering projects commissioned after Indian independence. The Koyna hydro-electric project is run by the Maharashtra State Electricity Board . The dam has contributed to earthquakes in the recent past, including the devastating 1967 Koynanagar earthquake that almost razed the dam, resulting in the dam developing major cracks. After the disaster Grouting of the cracks was done. Also internal holes were drilled to relieve the hydrostatic pressures in the body of the dam. In 1973 non overflow portion of the dam was strengthened followed by strengthening of spillway section in 2006. Now the Dam expected to be safe against any future earthquake even of more intensity than that of 1967.

15. Advantage of dam Advantage of building dam: -prevent flooding in lower course -produce hydroelectric power for economic and industrial development -better for irrigation -can store water from rain and directly from the rivers so we have water during times of droughts -improve water transport -attract tourists Disadvantage of building dams: -it's very expensive -many people have to move -upper-middle course will be flooded -historical sites will be flooded -collapse of the dam may cause great damage -the living environment of animals and fishes will be destroyed in the lower course

16. Disadvantage of a dam 1. Dams are extremely expensive to build and must be built to a very high standard. 2. The high cost of dam construction means that they must operate for many decades to become profitable. 3. The flooding of large areas of land means that the natural environment is destroyed. 4. People living in villages and towns that are in the valley to be flooded, must move out. This means that they lose their farms and businesses. In some countries, people are forcibly removed so that hydro-power schemes can go ahead. 5. The building of large dams can cause serious geological damage. For example, the building of the Hoover Dam in the USA triggered a number of earth quakes and has depressed the earth’s surface at its location. 6. Although modern planning and design of dams is good, in the past old dams have been known to be breached (the dam gives under the weight of water in the lake). This has led to deaths and flooding. 7. Dams built blocking the progress of a river in one country usually means that the water supply from the same river in the following country is out of their control. This can lead to serious problems between neighbouring countries. 8. Building a large dam alters the natural water table level. For example, the building of the Aswan Dam in Egypt has altered the level of the water table. This is slowly leading to damage of many of its ancient monuments as salts and destructive minerals are deposited in the stone work from ‘rising damp’ caused by the changing water table level