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 of Indus Basin Project. The Dam is located on the river Indus known as the “Abbasin” or the father of the river at a distance of 130 KM (50 Miles) to the North West of Islamabad, in District Swabi, KPK.

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. meters) before falling 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.

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). 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. 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.

History

As a consequence of partition of the Indo-Pakistan Sub-Continent in 1947, India and Pakistan became two independent sovereign states. The Irrigation System which existed at the time of partition in 1947 was divided between the two countries without any consideration given to the irrigation boundaries, resulting in an international water dispute which was resolved by signing of the Indus Water Treaty in 1960 under the aegis of World Bank. The Treaty assigned three Eastern rivers (Ravi, Beas and Sutlej) to India and three Western rivers (Indus, Jhelum & Chenab) to Pakistan. It also provided construction of replacement works called Indus Basin Project (IBP) to compensate for perpetual loss of Eastern river’s water. The works proposed under the Treaty included two multipurpose dams, one of which is Tarbela Dam on Indus river, having the provision of power generation.

Award of contract May 14, 1968
Construction 1968 – 1976

Main Dam

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 gravel cobbles and finally large sized riprap on the outer slopes. An impervious blanket, 12.8 meters at the dam and tailing to 1.52 meters at the upstream end, covers 1737 meters 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.

Reservoir

The 50-miles (81 kilometers) long reservoir created by the project has a gross storage capacity of 11.6 million acre feet at the maximum lake elevation of 1550 feet (472 meters) a residual capacity of 1.9 MAF at the assumed level of maximum drawdown elevation 1300 feet (396 meters) and a net useable capacity of 9.7 MAF.

Main Features of the Reservoir:

Main Spillways

Service Spillways:

Auxiliary Spillways:

Power Stations

General:

The Barrage

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 No. standard bays, 8 No. under sluices and 8 No. head regulator bays in addition to rim embankments, fuse plug and dividing island.
The Barrage can pass the design flood of 18,700 cumecs, 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 equaling 46,200 cumecs.

Tarbela 4th Extension Hydro Power Project

Project Benefits

In addition to fulfilling primary purpose of the dam i.e. supplying 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 3605 MW, recorded during the year.