Tarbela Dam | |
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 part of Indus Basin Project. The Dam is built on one of the World’s largest rivers – the Indus known as the “Abbasin” or the father of rivers.
Emerging from the land of glaciers on the northern slopes of Kailash ranges, some 17,000 feet (5182 meters) above sea level, the river Indus has its source near the Lake Mansrowar in the Himalyan catchment area. It flows over 1800 miles (2900 k. metres) before it outfalls into the Arabian sea draining an area of about 372,000 square miles (964,261 sq.kms).
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. HARZA ENGINEERING COMPANY International who were the General Consultants of WAPDA, carried out the review studies of the Project. In February, 1960 Tippetts – Abbett - McCarthy – Stratton of USA commonly known as TAMS were appointed the Project Consultants, and were entrusted with the task of investigation, preparation of detailed designs, and contract documents for the project and also the supervision of construction work during its execution.
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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 cumecs) and auxiliary spillway 850,000 cusecs (24,070 cusecs). Two auxiliary 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 constructed for irrigation releases and power generation. During the construction operations, these tunnels were used initially for river diversion. Irrigation tunnel 5 situated 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 housesfourteen(14), power units, 4 units, each with installed generating capacity of 175 MW are installed 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 reservoir is 50 miles (80.5 km) long 100 square, miles (260 square kilometers) 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 catchment area above Tarbela is spread over 65,000 sq. miles (168,000 sq. kilometers) which largely brings in snowmelt 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 seal level near skardu and Siran river just north of Tarbela.
Main Dam
The principal element of the project is an embankment 9,000 feet (2743 meters) long with a maximum height of 465 feet (143 meters). The total volume of earth and rock used for the project is approximately 200 million cubic yards (152.8 million cu. Meters) which makes it the largest man made structure in the world , except for the Great Chinese Wall which consumed somewhat more material. The main embankment 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, 42 feet (12.8 meters) thick at the dam and tailing to 5 feet (1.52 meters) at the upstream end, covers 5,700 feet (1737 meters) of the alluvial foundation on the upstream side. These deposits in the valley are upto 700 feet (213 meters) deep and in places consist of open work gravels. The dam crosses this essentially alluvial valley and connects the last points to high ground before the mountains give way to the plains. A 24 feet (7.32 meters) thick filter drain mattress under the embankment together with nearly vertical chimney drain provides the necessary facility to collect the seepage.
Auxiliary Dams
The auxiliary dams resembling the main embankment dam in design close the gaps in the left periphery of reservoir. The smaller of the two auxiliary dams, however, has a vertical core extending down to the underlying rock, and the larger auxiliary dam has a short upstream blanket terminating in a cut off to rock.
Spillways 
On the left bank , two spillways discharges into a side channel. The total spillway capacity is 1,500,000 cusecs (42,476 cumecs) which constitutes the peak outflow resulting from routing the probable maximum flood. The service spillway having 44 percent of the total capacity is sufficient to pass all but very rare floods. Its maximum discharge capacity is 650,000 cusecs (18, 406 cumecs).
The auxiliary spillway is similar in design to the service spillway. It has nine radial gates with crest elevation of 1492 feet (455 meters) and flip bucket at elevation 1220 feet (372 meters) A longitudinal drainage gallery along with a network of drainage pipes under the channel and the head works has been provided to release pore water pressure in both the spillway foundations.
Reservoir
The 50-miles (81 kilometers) long reservoir created by the Project has a gross storage capacity of 11.6 million acre feet (MAF) (17,109 m.cu. meters) at the maximum lake elevation of 1550 feet (472 meters) a residual capacity of 1.9 MAF (2,802 m cu. Meters) at the assumed level of maximum drawdown elevation 1300 feet (396 meters) and a net usable capacity of 9.7 MAF (14,307 m cu. Meters). The Tarbelareservoir stores water during the summer months of June, July and August when water either causes disaster by flooding in the surrounding areas or goes waste into the sea. It is to be noted that more dams can be constructed on Indus since its annual flow is substantially more than is being stored at present. Kalabagh Dam on River Indus is in its advanced stages of design, while investigations are underway for the upstream Basha Dam.
Tunnels
The four, each of half mile long, tunnels through the right (rock) abutment initially served for the diversion of water during the final phases of construction of the Project. Now they are being used for Power generation (tunnels 1, 2, 3 and eventually 4). The discharge capacity of each irrigation tunnel at higher reservoir elevations is approximately 90,000 cusecs (2,549 cumecs). The discharge pass through energy dissipator structures and the water returns to the river. A fifth tunnel on the left bank designed to augment irrigation releases upto 80,000 cusecs (2,265 cumecs) at high reservoir level, has also been added to the project.
Power Station
According to the original plan, four (4) power units of 175 MW generating capacity each were to be installed on each of the tunnels 1, 2 and 3 located on the right bank with the ultimate installed capacity of 21,00 MW. Of these, four (4) units on tunnel 1 were commissioned in the year 1977. Due to increasing prices of the fossil fuel, the Govt of Pakistan has been laying greater emphasis on generation of cheap Hydel power. In pursuance of this policy, WAPDA carried out studies to tap the maximum power potential of Tarbela. As a result, it has been found possible to install six (6) units, instead of four (4) only on tunnel NO.2. Units 5 to 8 on tunnel NO.2 were commissioned in 1982, and units 9 and 10 in 1985. Based on studies, four power units of 432 MW capacity each were installed on tunnel NO.3. Thus the total ultimate power potential of the project enhanced from 2100 MW as originally planned to 3478 MW.
Project Implementation
On May 14, 1968, the World’s largest single contract for the construction of civil works of the Tarbela Dam Project was signed at a price of RS.2,965,493,217 ($ 623 Million) between the Water and Power Development Authority and the Tarbela Dam Joint Venture which comprised 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 namely Impregilo.
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 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 excavated 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 cumecs). 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 works.
Salient features/Principal data of main components of Tarbela Dam and Power House Tarbela Dam
Auxiliary Dam NO.1
Length at Crest EI. 1565 ft (477 M) 2,340 feet (713 M) Maximum Height 345 feet (105 M) Volume including Blanket 18,000,000 cu.yds (13,752,000 cu meters)
Auxiliary Dam No.2
Length at Crest EI, 1565 ft (477 M) 960 feet (293 M) Maximum Height 220 feet (67 M) Volume 2,000,000 cu. Yds (1,528,000 cu.meters)
Tunnels at Right Bank
Number 4 Length 2400 feet each (732 M) Diameter (u/s gate shafts) 45 feet (13.72 M) Diameter (u/s of gate shafts) 36 feet (10.97 M)
Tunnel at Left Bank
Length 2760 feet (841 M) Diameter (13.26 M)
Project Benefits
In addition to fulfilling primary purpose of the Dam i.e. supplying water for Irrigation, Tarbela Power Station has generated 341.139 Billion KWh of cheap hydel 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. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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