Tarbela Dam is in Pakistan and is the world’s largest fill-type dam. It is built over the River Indus near the small town of Tarbela in the Haripur District of the country and is also the second largest dam in the world in terms of reservoir capacity, which is 11.62 million acre-feet (14.3 billion cubic metres).
The project was executed by Water and Power Development Authority (WAPDA) on behalf of the Government of Pakistan. Construction was carried out under a single civil works contract. Construction on the primary elements of the project was started in 1968 and initial operations of the dam started in 1978. The project was fully completed in 1984, at a cost of $1.49bn. The origins of the project date back to development plans promoted under the patronage of the World Bank in the early 1960s. The Indus Water Treaty was a complex international cooperation program whose goal was the development of Pakistan via the creation of infrastructure that would allow for the exploitation of the immense reserve of water resources constituted by the Indo basin, especially for irrigation purposes.
Major financial contributions for the Tarbela Dam Project came from the World Bank in the form of a bank loan and two International Development Association (IDA) credits. The bank loan was approved in 1968 and the two IDA credits were approved by 1978. The loan and credits were fully paid out by September 1981.
Since 1968 Impregilo has been at the helm of a European consortium that was commissioned to construct Tarbela’s system of dams, an open-air powerhouse and the tunnels required by the main plant. The size of the project was of a scale that had never been experienced by the Italian company: the length of the main dam at its crest was almost 3 km, and the construction site required the constant presence of complex and diversified machinery and was five times the size of those of the largest construction sites hitherto managed by Impregilo. Throughout the project, as many as 45,000 workers from 26 different countries worked at Tarbela; this also meant building a close-knit network of dwellings and auxiliary services.
In the early 1980s, the Tarbela consortium was also commissioned to enhance the plant’s hydroelectric capacity by building a new power station. In addition to fulfilling the original purpose of the dam (i.e. supplying water for irrigation), since the start of the twenty-first century the plant has been generating over 23% of the hydroelectric output of the West Pakistan Water and Power Development Authority.
Need of the Project
Pakistan was entirely dependent on unregulated flows of the Indus River and her tributaries even after 20 years of independence. The agricultural yield was on a lower side due to deficiency of water during the critical growing season and the main reason behind this problem was the seasonal variations in the river flow. Additionally, there were no storage reservoirs to conserve surplus flows. So, to strengthen the irrigation system, the Pakistani Government, with assistance from the World Bank, embarked on the Indus Basin Project, which comprised of two large dams. The first dam built under this water control project was the Mangla Dam on the Jhelum River, while the other was Tarbela on the Indus River.
The primary purpose of the Tarbela Dam Project was to regulate the flows of the Indus River for irrigation use. Other objectives were to achieve substantial generation of hydroelectric power and flood control by conserving snow melt and monsoon flows of the Indus River.
Components of the Dam
The Tarbela Dam Project involved construction of an earth and rock-fill dam on the River Indus and a power plant. Maximum height of the main embankment from the lowest foundation point is 470 ft and length is 9,000 ft at its crest.
The main embankment is flanked by two auxiliary embankments on the left bank. The maximum height of one of the auxiliary embankments is 345 ft, while that of the other is 220 ft. The lengths at crest are 2,340 ft and 960 ft respectively.
The two auxiliary embankments carry two spillways, the service spillway and the auxiliary spillway. The service spillway is fitted with seven gates and has a discharge capacity of 650,000 cusec. The auxiliary spillway with its nine gates can discharge 850,000 cusec.
The valley wall on the right bank, at the upstream end, was cut through to build a group of four tunnels. Three of these tunnels were intended for hydro-power generation, while the fourth tunnel is used for irrigation releases. A fifth tunnel was constructed on the left bank of the river. It became operational in 1976 and is basically used for irrigation purposes.
Catchment area of the reservoir created by the dam is 169,600 square kilometres. The reservoir, with a maximum depth of more than 450 ft, can impound up to 11.62-million-acre foot (MAF) of water at the maximum lake elevation of 1,550 ft. Net usable capacity of the reservoir is 9.68 MAF. The water is stocked during the months of June, July and August, when the river flow is at its maximum.
The Hydropower Plant
The Tarbela hydro power plant is constructed on the right side of the main embankment. It is installed with 14 power generating units and the water is fed from outlet tunnels one, two and three. The plant has a total installed capacity of 3,478 MW.
Four generators of 175 MW each on tunnel one were put into production in 1977. There are six 175 MW generators on tunnel two, four of which were commissioned in 1982 and the remaining two in 1985. Four generators of 432 MW each were installed in tunnel four in 1992.
Tarbela Dam is one of the world’s largest earth and rock filled Dam and greatest water resources development project, which was completed in 1976 as a component of Indus Basin Project. The Dam is located on the river Indus at 130 km to the North West of Islamabad, in Distt Sawabi KPK. Emerging from the land of glaciers on the northern slopes of Kailash ranges, some 17,000 feet (5,182 metres) above sea level, the river Indus has its source near the Lake Mansrowar in the Himalyan catchment area. It flows over 1,800 miles (2,900 km) before it outfalls into the Arabian Sea draining an area of about 372,000 square miles (964,261 sq.km).
The World Bank accepted Tarbela Dam Project as a part of the Settlement (Replacement) plan under Indus water treaty in 1965. WAPDA was entrusted with its execution on behalf of the Government of Pakistan. The construction of Tarbela Dam was carried out in three stages to meet the diversion requirements of the river. In stage-I, the river Indus could flow in its natural channel while work was continued on right bank where 1,500 feet (457 metres) long and 694 feet (212 metres) wide diversion channel was excavated and 105 feet (32 metres) high buttress dam was constructed with its top elevation at 1,187 feet (362 metres). Construction under stage-I lasted 2.5 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 part of stage-III works.
The main embankment of the Dam is a carefully designed, zoned structure composed of impervious core, bounded on both sides by gradually increasing sized material including coarser sands, gravels, cobbles and finally large sized riprap on the outer slopes. An impervious blanket, 12.8 metres at the dam and tailing to 1.52 metres at the upstream end, covers 1,737 metres of the alluvial foundation on the upstream side. The dam crosses this essentially alluvial valley and connects the last points to high ground before the mountains give way to the plains. 7.32 metres thick filter drain mattress under embankment together with nearly vertical chimney drain provides the necessary facility to collect the seepage.
The 50-miles (81 km) long reservoir created by the Project has a gross storage capacity of 11.6 million-acre feet at the maximum lake elevation of 1,550 feet (472 metres) a residual capacity of 1.9 MAF at the assumed level of maximum drawdown elevation 1,300 feet (396 metres) and a net useable capacity of 9.7 MAF.
The Barrage located 7 km downstream of Tarbela Dam provides a pond, which re-regulates the daily discharge from Tarbela by diverting the flow into the Power Channel. The principal features include 20 standard bays, 8 under sluices and 8 head regulator bays in addition to rim embankments, fuse plug and dividing island. The Barrage can pass the design flood of 18,700 cumec, equivalent to the flood of record, through the standard bays and under sluices at the normal pond level of El. 340 m. The fuse plug has been provided to pass the extreme flood up to the capacity of Tarbela’s spillway and tunnels equalling 46,200 cumec.
In addition to fulfilling primary purpose of the dam i.e. , supply water for irrigation, Tarbela Power Station has generated 438.978 Billion kWh of cheap Hydel energy since commission. A record annual generation of 16.463 Billion kWh was recorded during 1998-99. Annual generation during 2013-14 was 15.181 Billion kWh while the station shared peak load of 3,605 MW was recorded during the year.
The Tarbela Dam Project was built by Tarbela Dam Joint Venture, which was led by Italian contractor Impregilo. The joint venture was a consortium of three Italian and three French heavy civil engineering contractors. Five German and two Swiss companies joined the consortium in 1969.
Members of the joint venture led by Impregilo included Costruzioni generali Farsura, Impresa Astaldi Estero, Compagnie de Constructions Internationales, Compagnie Francaise d’Entreprises, Societe de Construction de Batignolles, Hochtief, Philipp Holzmann, Strabag Bau, Ed Zueblin, C. Baresel, Conrad Zschokke and Losinger.
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