Removal of rock barriers: Wapda plans to shut down two Tarbela tunnels for 85 days

ISLAMABAD: Water and Power Development Authority (WAPDA) has planned to shut down tunnel 3 and tunnel 4 of Tarbela Hydropower Project’s 4th extension for 85 days starting from February 20, 2022, aimed at removing rock barriers in front of both tunnels.

According to WAPDA, the work on raised intake of T3 and T4 was substantially completed during the year 2020-21. The operation of the units depends on the water level above the rock barriers. The removal of rock barriers was carried out keeping in view the constraints/ directives of IRSA for completion of works till May 25, 2021, in the wake of a rise in temperature in Northern Areas, resulting in an increase in inflows in the reservoir.

Due to these constraints and as per National Power Control Centre (NPCC) requirements for early completion of works, WAPDA had to increase the level of removal of rock barriers from planned E1 1398 ft. to 1417 ft.

The rock barrier removal up to E1 1417 ft was achieved on May 28, 2021. The tunnels were made operational and units started on June 10, 2021. The approved shut down of T3 and T4-powerhouses was up to June 30, 2021.

With the present rock barrier at EI.1417 Ft operation of T3 or T4 intakes below reservoir level at EI.1437 ft is not permissible.

The constraint will be reduced from El. 1437 to 1417 t. upon removal of rock barrier up to El. 1398 ft. for the partial operation of single intake (preferably T3). The full operational constraint of T3 & T4 will be reduced to El. 1430 ft. from El 1450 ft. (20 ft. lower from the position of rock barrier).

According to Wapda, the partial operation of single intake (preferably T3) starts at 6 meters (20 feet) above rock barrier level and the full operation of both the tunnels is at 10 meters (33 feet) above rock barrier level. With the lowering of the rock barrier from El.1417 ft. to 1398 ft., the SOP’s regarding operations of units remain the same except that units will be operated at lower levels.

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The operation of units will be as follows: (i) two units of T3 will be operated at reservoir level 20 feet or above rock barrier;(ii) third unit of T3 will be operated at reservoir level 26.5 feet or above rock barrier; and (iii) full operation of T3 and T4 at reservoir level 33 feet or above rock barrier.

The removal of the remaining rock barrier is planned during the lean period of reservoir operation 2021-22. Work Plan has been prepared and shared with IRSA. During the preparation of the Work Plan, the Annual Canal Closure being observed every year has been considered.

Similarly, the average per day quantity of rock material already removed during 2021 has been considered for evaluating the number of days required for the removal of remaining rock barriers. It is estimated that 85 days will be required for actual work and is dependent upon the availability of required reservoir levels from IRSA. Previously, the approval of shut down of the units was achieved after correspondence from WAPDA with NPCC and concerned Ministries. As per the criteria circulated the required reservoir levels will likely be available from 20-2-2022. The period of shut down (as per operating criteria circulated) will start from February 20, 2022for 85 days. However, units will be operated before and after the period of shut down as per SOPs including the availability of required reservoir level (submergence above rock barrier).

Ministry of Water Resources has taken up the matter with the Power Division, (Ministry of Energy) for approval of shut down as being demanded by NPCC and as per previous practice.

Copyright Business Recorder, 2022