Tarbela Dam (Urdu: تربیلا بند ) is a dam on the Indus River in Pakistan. It is situated 50 kilometres North-West of Islamabad near the Haripur District. It is the biggest earth filled dam in the world and it generates major portion of Pakistan's hydroelectricity. Tarbela Dam is part of the Indus Basin Project, which is an outcome from a water treaty signed in 1960 between India and Pakistan; ensuring Pakistan water supplies free of upstream control by India. Construction of Tarbela Dam started in 1968, and went on continuously until completion of the project in 1976. The dam has a volume of 138,600,000 cubic yards (106,000,000 m³). With a reservoir capacity of 11,098,000 acre-feet (13.69 km³), the dam is 469 feet (143 m) high and 8,997 feet (2,743 m) wide at its crest. This critical height is used to control the flow of the Indus during seasonal fluctuations.
A new, smaller hydro-electric power project has been constructed downstream known as the Ghazi Barotha Hydel Power Project. It is made exclusively for producing electricity and has a water channel with the highest flow in the world.
While the dam has satisfied its purpose in storing water for agricultural purpose in Pakistan, there have been environmental consequences to the Indus river delta i.e. drop of seasonal flooding and reduced water flows to the delta have decreased mangrove stands and the plenty of some fish species.
The Project – Main Features
The Project consists of a 9,000 (2,743 meters) long, 465 feet (143 meters) high (above the river bed) earth and rock fill embankment across the entire width of the river with two spillways cutting through the left bank discharging into a side valley. Its main spillway has a discharge capacity of 650,000 cusecs (18,406 cusecs) and additional spillway 850,000 cusecs (24,070 cusecs). Two supplementary embankment dams close the gaps in the left bank valley. A group of 4 tunnels (each half a mile long), through the right abutment rock have been developed for irrigation releases and power generation. During the construction phases, these tunnels were utilized primarily for river diversion. Irrigation tunnel 5 located on the left bank, for which NESPAK were the Project Consultants, was put into operation in April 1976.
A power station on the right bank near the toe of the main dam houses fourteen (14), power units, 4 units, each with installed generating capacity of 175 MW are set up on tunnel 1, 6 units (NO.5 to 10), 175 MW each on tunnel NO.2 and 4 Units (NO.11-14) of 432 MW each on Tunnel 3, thus making total generating capacity of Tarbela Power Station as 3478 MW.
The lake is 50 miles (80.5 km) long 100 square, miles (260 square kilometres) in area and has a gross storage capacity of 11.6 MAF (17.109 million cu. Meters) with a live storage capacity of 9.7 MAF (14,307 million cu. Meters). The total catchments area above Tarbela is spread over 65,000 sq. miles (168,000 sq. kilometres) which mainly brings in snowmelt water supplied in addition to some monsoon rains. Two main upstream tributaries join the Indus river, Shyok River at an elevation of 8,000 ft. (2438 meters) above sea level near Skardu and Siran River just north of Tarbela.
On May 14, 1968, the World’s largest single contract for the construction of civil works at that time, the Tarbela Dam Project was signed at a price of RS.2,965,493,217 ($ 623 Million) between the Water and Power Development Authority of Pakistan and the Tarbela Dam Joint Venture which consisted a group of three Italian and three French heavy construction contractors. Later five German and two Swiss contractors also joined the group making up a consortium of thirteen European firms led by Italian firm named Impregilo.
The construction of Tarbela Dam was carried out in three phases to meet the diversion requirements of the river. In stage-1, the river Indus was allowed to flow in its natural channel while work was continued on right bank where a 1500 feet (457 meters) long and 694 feet (212 meters) wide diversion channel was dug up and a 105 feet (32 meters) high buttress dam was constructed with its top elevation at 1, 187 feet (362 meters) The diversion channel was capable of discharging 750,000 cusecs (21,238 cusecs). Construction under stage-I lasted 2½ years.
In stage-II, the main embankment dam and the upstream blanket were constructed across the main valley of the river Indus while water remained diverted through the diversion channel. By the end of stage-II, tunnels had been built for diversion purposes. The stage-II construction took 3 years to complete. Under stage-III, the work was carried out on the closure of diversion channel and construction of the dam in that portion while the river was made to flow through diversion tunnels. The remaining portion of upstream blanket and the main dam at higher levels was also completed as a part of stage-III world.
Because the source of the Indus River is glacial melt-water from the Himalayas the river carries huge amounts of remains/residues. The annual suspended residual load is about 430 million tons per year. This means that, over time, the reservoir will fill up with these residues. The practical life of the dam and reservoir was estimated to be somewhere around fifty years, since the dam's completion in 1976, meaning that the reservoir would have been full of sediment by 2030.
Sedimentation, however, has been much lower than predicted, and it is now estimated that the useful lifespan of the dam will be 85 years, to about 2060.