Hydropower extension projects at Tarbela Dam

Overview

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Tarbela Dam on the River Indus was commissioned in the mid-1970s primarily as a water resource reservoir for irrigation and was at that time the largest rock and earth-fill dam in the world. During Tarbela’s subsequent operation, its role in providing hydropower generation has become more significant. As a result, successive hydropower stations have been added to the water release tunnels over the last few decades. Power stations on tunnels 1 to 3 provide 3,478MW of installed capacity with the 1410MW 4^th hydropower extension recently being completed on the 4^th tunnel bring this to 4,888MW. These power plants utilise tunnels through the right abutment of the dam which can discharge directly to the Ghazi Barotha headpond – a headpond formed by the downstream hydropower scheme which effectively forms a constant tailwater level for the 4 right-bank power stations.

Unlike the other tunnels, the fifth release tunnel (T5) is situated within the left abutment of the dam (see photograph) and was never envisaged for power generation. Thus, it was situated at a higher level than the other tunnels and discharges into the Dal Dara channel. This channel also receives the discharge from Tarbela’s two spillways (with a combined design flow of 47,000m3/s) and conveys this water, along with any T5 release, back to the Ghazi Barotha headpond, to join the rest of the Indus flow.

Construction of a hydropower station on the end of T5, so that it would discharge directly into the Dal Dara channel, would effectively reduce the scheme’s net head by around 10% when compared to the other right-bank power schemes due to the raised tailwater level in the Dal Dara channel. It is not possible to reduce the Dal Dara channel water levels, since this would impact the spillway plunge pool water depths. An innovative solution has been adopted that provides a tailwater culvert and canal system which allows lower tailwater levels – similar to the other right-bank schemes - giving the T5 project an installed capacity of 1530MW.

The presenters will discuss:

• The arrangement for the 5^th Extension Hydropower Project
• Ongoing sediment management and how raised intakes are being adopted to extend the life of the reservoir
• Construction and commissioning of the T3 and T4 raised intakes plus the plans for the T5 raised intake
• Construction challenges and lessons learned from the 4^th Extension Project and how these are being applied to the 5^th Extension Project
• ow Physical and CFD models of the Dal Dara channel have been used conjunctively to confirm the operability of the tailrace culvert and canal system whilst ensuring the safety of the existing spillway plunge pools
• Recent photos from site showing the construction stage of the 5^th Extension Project and comparing these to the 4^th Extension Project