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— File Photo

ISLAMABAD: An independent consultant hired by the Water and Power Development Authority (Wapda) is of the opinion that desilting of Tarbela dam is not a viable option, either economically or technically, and may damage the country’s largest power house and reservoir.

A senior government official told Dawn on Sunday that Wapda had hired the Chinese consultant to conduct a feasibility study for desilting the Tarbela reservoir to recoup over 30 per cent capacity lost to silting over the years.

One of the world’s largest earth-filled dam was completed in 1978 with World Bank’s assistance under the Indus Water Treaty signed by Pakistan and India. Owing to continuous silting and sedimentation, the dam’s storage capacity declined from 9.6 million acres feet (MAF) to about 6.6 MAF.

The official said the feasibility study concluded that instead of undertaking such a mammoth exercise it would be more economical to build a new dam of the same size and capacity. “The silt and rock deposits in the dam are huge and require massive financing almost equal to the cost of a new dam.”

He said the consultant was of the view the work of breaking rocks and boulders and flushing out silt deposits could damage the infrastructure of the power house and perhaps the dam structure. It would speed up the movement of delta and silt deposits.

The official said the river flows carried to the dam over 500,000 tons of silt annually, resulting in a build-up of over 5 km delta inside the dam’s storage area. Over 25 acres of land in the reservoir has been filled with silt, reducing its storage capacity by over 3 MAF.

A Wapda official confirmed that the Chinese consultant had submitted the technical report and it was currently under examination. He said that initial scrutiny of the report indicated that Wapda would not go for desilting of the dam, adding that the examination of the study pertained to a new project (Tarbela 4th extension) for enhancing power generation capacity of the dam by about 1,400MW.

The official said the dam had an estimated lifespan of 50 years which meant it would complete its designed life by 2029. He, however, said the process of sedimentation had slowed down in recent years which indicated the project might have a useful life of about 85 years i.e. until 2060.

The downside is that Pakistan’s per capita water availability which stood at about 5,100 cubic metres in 1960 had already gone below 975 cubic metres per person — a threshold defining Pakistan among the water-scarce nations.

This was mainly because of the reason that the 1960 treaty required Pakistan to build a dam of the size of Tarbela at least every decade but to the utter failure of planners and governments it could not take up even a single dam in more than four decades after completion of Tarbela.

The Tarbela 4th extension project is being supported by the World Bank to increase the generation capacity of Tarbela power house to about 4,900MW from 3,500MW.

The World Bank has already signed an agreement with the federal government and Wapda to provide about $840m — almost 90 per cent of the cost — for the project which is expected to be completed by 2016 and will be the first major hydropower capacity addition since completion of the 1,450MW Ghazi Barotha project in 2004.

The official said the contract for civil works of the 1,410MW extension project was likely to be awarded in March.

The contract would involve installation of three units of 470MW each at the 4th tunnel of the dam.

The final design report has put the estimated cost of the project at $930m. The remaining amount of Rs18 billion in local currency would be financed by Wapda through its own resources and federal government’s assistance.

The project is part of Wapda’s least-cost power generation strategy aimed at adding a sizeable quantity of electricity to the national grid and improving the ratio of hydel electricity in the overall system to stabilise the power tariff. It was such an economically viable project that it would be completed in four years, accrue annual benefits of about Rs30bn and pay back its cost in three years.


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Comments (4) (Closed)


Karachiwalla
Jan 07, 2013 04:08am
Original dam was built by USA engineering companies like G Atkinson etc. They should be consulted rather than china engineer. They have more understanding of Dam design and they will have more insite.
observer
Jan 07, 2013 05:16am
Can the water lake be expanded in size if recovery of lost depth due to silting is not feasible? The idea is to have equal volume of water stored in the lake as the original dam design. It would be totally desirable to use the dam itself for as long as possible.
Unbiased
Jan 07, 2013 04:17pm
Yes. The original design engineers, TAMS Consultants were consulted in 1997 for the same issue. Several options / solutions were presented, however owners decided to continue with operations of the dam with yearly rising of the lake water to stabilize the sediment delta. The original project was designed as an irrigation reservoir and not a hydropower power project, funded by the World Bank. The WB assisted in the 1960 water treaty with India to replace the lost water. The dam has performed well considering it was primarily used for power generation purposes. I think the Chinese consultant was used as they have recently worked on the Three Gorges Dam which has same type of sand coming into their dam. The issue however is twofold: Continue to use the dam as it is and it will severely impact power generation capacity of downstream Gazi Barotha and the proposed Kalabagh project. Or spent the money now to safeguard the power intakes (three right tunnels) by building an underwater dike (rock dam) to prevent silt getting into power turbines. Building the underwater dike will guarantee power generation for a long time to come. In either case Ghazi Barotha will have to deal with the future silt loads that it is not designed for. However, Kalabagh is designed (not sure if it will be built) to deal with the sediment via mid or low level sluicing/tunnels.
Reply
Jan 07, 2013 08:23am
The capacity of the dam depends upon its height (and other factors). To increase the 'size of the lake' as you are suggesting, you will need to increase the height of the dam e.g in case of Mangla Dam Raising Project, which is only possible if the conditions permit.