Tarbela Dam is a earth fill dam located on the Indus River in Pakistan. It is the largest earth-filled dam in the world and fifth-largest by structural volume. It is named after the town Tarbela, in Khyber Pakhtunkhwa, about 50 kilometres (31 mi) northwest of Islamabad.

The dam is 485 feet (148 m) high above the riverbed. The dam forms the Tarbela Reservoir, with a surface area of approximately 250 square kilometres (97 sq mi). The dam was completed in 1976 and was designed to store water from the Indus River forirrigation, flood control, and the generation of hydroelectric power.
The primary use of the dam is for electricity generation, the installed capacity of the 3,478 MW Tarbela hydroelectric power stations will increase to 6,298MW after completion of the ongoing fourth extension and the planned fifth extension financed by Asian Infrastructure Investment Bank and the World Bank


Project description
The project is located at a narrow spot in the Indus River valley, at Tarbela between Haripur District and Swabi District, approximately 60 kilometers northwest of Islamabad.

The main dam wall, built of earth and rock fill, stretches 2,743 metres (8,999 ft) from the island to river right, standing 148 metres (486 ft) high. A pair of concrete auxiliary dams spans the river from the island to river left. The dam's two spillways are located on the auxiliary dams rather than the main dam. The main spillway has a discharge capacity of 18,406 cubic metres per second (650,000 cu ft/s) and the auxiliary spillway, 24,070 cubic metres per second (850,000 cu ft/s). Annually, over 70% of water discharged at Tarbela passes over the spillways, and is not used for hydropower generation.

Five large tunnels were constructed as part of Tarbela Dam's outlet works. Hydroelectricity is generated from turbines in tunnel 1 through 3, while tunnels 4 and 5 were designed for irrigation use, although both tunnels are to be converted to hydropower tunnels to increase Tarbela's electricity generating capacity.These tunnels were originally used to divert the Indus River while the dam was being constructed.

MA hydroelectric power plant on the right side of the main dam houses 14 generators fed with water from outlet tunnels 1, 2, and 3. There are four 175 MW generators on tunnel 1, six 175 MW generators on tunnel 2, and four 432 MW generators on tunnel 3, for a total generating capacity of 3,478 MW.

Tarbela Reservoir is 80.5 kilometres (50.0 mi) long, with a surface area of 250 square kilometres (97 sq mi). The reservoir initially stored 11,600,000 acre feet (14.3 km3) of water, with a live storage of 9,700,000 acre feet (12.0 km3), though this figure has been reduced over the subsequent 35 years of operation to 6,800,000 acre feet (8.4 km3) due to silting.

The catchment area upriver of the Tarbela Dam is spread over 168,000 square kilometres (65,000 sq mi) of land largely supplemented by snow and glacier melt from the southern slopes of the Himalayas. There are two main Indus River tributaries upstream of the Tarbela Dam. These are the Shyok River, joining near Skardu, and the Siran River near Tarbela.


Background
Tarbela dam was constructed as part of the Indus Basin Project after signing of the 1960 Indus Waters Treaty between India and Pakistan. The purpose was to compensate for the loss of water supplies of the eastern rivers (Ravi, Sutlej and Beas) that were designated for exclusive use by India per terms of the Treaty. The primary objective of the dam was to supply water for irrigation by storing flows during the monsoon period and subsequently releasing stored water during the low flow period in winter.

By the mid-1970s, power generation capacity was added in three subsequent hydro-electrical project extensions which were completed in 1992, installing a total of 3,478 MW generating capacity on respectively Tunnel 1 (four turbines), Tunnel 2 (six turbines) and Tunnel 3 (four turbines).


Construction
Construction of Tarbela Dam was carried out in three stages to meet the diversion requirements of the river. Construction was undertaken by the Italian firm Salini Impregilo.


Stage 1
In the first stage, the Indus river was allowed to flow in its natural channel, while construction works commenced on the right bank where a 1500 feet (457 meters) long and 694 feet (212 meters) wide diversion channel was being excavated along with a 105 feet (32 meters) high buttress dam that was also being construction. Stage 1 construction lasted approximately 2.5 years.

Stage 2
The main embankment dam and the upstream blanket were constructed across the main valley of the river Indus as part of the second stage of construction. During this time, water from the Indus river remained diverted through the diversion channel. By the end of construction works in stage 2, tunnels had been built for diversion purposes. Stage 2 construction took 3 years to complete.

Stage 3
Under the third stage of construction, works were 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 part of stage 3 works, which were concluded in 1976.


Re-settlement of people affected by Tarbela Dam
It has a reservoir area of about 260 square kilometers and about 82,000 acres (33,000 ha) of land was acquired for its construction. The large reservoir of the dam submerged 135 villages, which resulted in displacement of a population of about 96,000 people, many of whom were relocated to a series of townships surrounding the Tarbela reservoir or in adjacent higher valleys.

For the land and built-up property acquired under the Land Acquisition Act of 1984, a cash compensation of Rs 469.65 million was paid to those affected. In the absence of a national policy, resettlement concerns of the affectees of Tarbela Dam were addressed on an ad hoc basis.

Many affectees of the Tarbela Dam have still not been settled and have not been given land as compensation by the government of Pakistan, in accordance with its contractual obligations with the World Bank. However, some of the affectees have been settled at Khalabat Township and other places across Pakistan.


Lifespan
Because the source of the Indus River is glacial meltwater from the Himalayas, the river carries huge amounts of sediment, with an annual suspended sediment load of 200 million tons.

Live storage capacity of Terbela reservoir had declined more than 33.5 per cent to 6.434 million acre feet (MAF) against its original capacity of 9.679 MAF because of sedimentation over the past 38 years. The useful life of the dam and reservoir was estimated to be approximately fifty years. However, sedimentation 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.

Pakistan also plans to construct several large dams upstream of Tarbela, including the Diamer-Bhasha Dam. Upon completion of the Diamer-Bhasha dam, sediment loads into Tarbela will be decreased by 69%.


Project Benefits
In addition to fulfilling the primary purpose of the dam, i.e. supplying water for irrigation, Tarbela Power Station has generated 341.139 billion kWh of cheap hydro-electric energy since commissioning. A record annual generation of 16.463 billion kWh was recorded during 1998–99. Annual generation during 2007–08 was 14.959 billion kWh while the station shared peak load of 3702 MW during the year, which was 23.057% of total WAPDA system peak.


Tarbela-IV Extension Project
In September 2013, Pakistan's Water and Power Development Authority signed a Rs.26.053 billion contract with Chinese firm Sinohydro and Germany's Voith Hydro for executing civil works on the 1,410 MW Tarbela-IV Extension Project. Construction commenced in February 2014, and is expected to be completed by June 2017. Upon completion, the project will augment the Early Harvest Project of the China–Pakistan Economic Corridor.

The project is being constructed at tunnel 4 of the dam which was originally intended for irrigation, but was later taken up for power generation as country’s electricity demand increased. As much of the infrastructure for the project already exists, conversion of the irrigation tunnel into a hydroelectric tunnel was regarded as a cost-effective way to ease Pakistan's energy shortfall.

Annual benefits of the project were estimated at about Rs. 30.7 billion. On an annual basis, over 70% of water passing through Tarbela is discharged over spillways, while only a portion of the remaining 30% is used for hydropower generation.

As of February 2016, the Government of Pakistan states that 49% of works have been completed on the 4th phase extension project.


Financing
The project's cost was initially estimated to be $928 million,but the cost was revised downwards to $651 million. The World Bank had agreed to provide a $840 million loan for the project in June 2013. The loan had two components: The first component is a $400 million International Development Association loan,which will be lent as aconcessional loan at low interest rates.The second portion consists of a $440 million from the World Bank's International Bank for Reconstruction and Development. Pakistan's Water and Power Development Authority was to provide the remaining $74 million required for construction, before the project's cost was downwardly revised by $277 million. Interest costs for the loans are estimated to cost $83.5 million.

Because of revised lower costs to $651 million from $928 million, the World Bank permitted Pakistani officials to expedite completion of the project by 8 months at a cost of an additional $51 million. Pakistani officials were also permitted to divert $126 million towards the Tarbela-V Extension Project.

Tarbela-V extension project
The Tarbela Dam was built with five original tunnels, with the first three dedicated to hydropower generation, and the remaining two slated for irrigation use; the fourth phase extension project utilizes the first of the two irrigation tunnels, while the 5th phase extension will use the second irrigation tunnel. Pakistan's Water and Power Development Authority sought expressions of interest for the Tarbela-V Extension Project in August 2014, and was given final consent for construction in September 2015.

Construction is yet to begin, but will require an estimated 3.5 years for completion. The project will require installation of three turbines with a capacity of 470 MW each in Tarbela's fifth tunnel which was previously dedicated to agricultural use. Upon completion, the total power generating capacity of Tarbela Dam will be increased to 6,298 MW.

The hydro power project of tunnel no. 5 has two major components: power generation facilities and power evacuation facilities. The major works included under the project are: modifications to tunnel no. 5 and building a new power house and its ancillaries to generate about 1,800GWh of power annually, a new 50 km of 500kV double-circuit transmission line from Tarbela to the Islamabad West Grid Station for power evacuation, and a new 500kV Islamabad West Grid Station.


Financing
In November 2015, the World Bank affirmed that it would finance at least $326 million of the project's estimated $796 million cost which includes $126 million of funding that was diverted from the $840 million fourth phase extension project after costs for that project were revised downwards.On September 2016, the World Bank approved an additional financing of $390 million for the fifth extension hydropower project of Tarbela dam that will support the scaling up of the power generation capacity by adding 1,410 megawatts to the existing tunnel no. 5 of the dam. The project will be financed by the International Bank for Reconstruction and Development (IBRD), with a variable spread and 20-year maturity, including a six-year grace period. This will be the first World Bank-supported project in South Asia to be jointly financed with the Asian Infrastructure Investment Bank (AIIB) which will be providing $300m and the Government of Pakistan $133.5m. The total cost of the project is $823.5m.

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