Dams may lose 250,000 acre feet of water storage capacity by next year

LAHORE: The minimum water storage level of both major reservoirs, Tarbela and Mangla, may further be lowered officially, diminishing availability of irrigation supplies in Pakistan - a semi-arid country,

River flows have a paramount role in meeting water requirements of the country and any decline due to depletion of existing water storage capacity is a serious concern for all, especially the agriculture sector.

The silting of reservoirs is reducing water availability bit by bit, while officials concerned are not taking steps for removal, as advised by the consultants. Instead, they are best at just gradually increasing the level of storage in order to slow the movement of silt delta towards power houses.

Policy makers are not doing anything concrete for construction of dams upstream or downstream of Tarbela. According to experts, building of an upstream dam will help slow depositing of silt in Tarbela Dam drastically, while substantially increasing its life. Meanwhile, regulation of downstream reservoir would allow flushing of sediments from Tarbela without affecting water availability.

Despite raising of Mangla Dam by about 3MAF back in 2013, the present storage capacity of both dams stands at about 13MAF, much less than the original capacity of these reservoirs some 40 years back. Over the years, water storage capacity of both Tarbela and Mangla Dams continued to decline with no construction of any other dam; the demand of water conversely has doubled since then.

If the situation remains the same for some more years, Pakistan is surely heading towards a major disaster due to lingering water shortage and subsequent food insecurity. The latest proposal of lowering the storage capacity of Tarbela Dam formally came from Water and Power Development Authority (Wapda), suggesting to increase dead level by four feet.

Sources said, in a recent meeting with the stakeholders, including top managers of Indus River System Authority (Irsa), Wapda has indicated that minimum water conservation level of Mangla Dam needs to be lowered by 10 to 15 feet. The suggestion has been made to slow down movement of silt towards intake of power turbines witnessed during early 2017, which badly affected the operation of the turbines.

If both proposals are given go ahead by the stakeholders, more than 0.25 million acre feet water would not be available. The loss of storage capacity, according to a calculation, is estimated on both accounts of intentional increasing in dead levels to quell forming of silt near power house, and yearly depositing of sediment in the lakes of both Tarbela and Mangla Dams.

Commenting on the proposal of Wapda about increasing minimum operating level of Tarbela Dam from 1,380 feet to 1,384 feet, IRSA gives a word of caution. “Irsa recommends that minimum operating level may be restricted to 1,382 feet instead of 1,384 fee and results may be monitored,” reads a correspondent.

As far as dead level of Mangla Dam is concerned, sources said, it is still being discussed informally and stakeholders may agree over increasing minimum operating level by 10 ft to 1,050 ft.

According to an official of Wapda, on the recommendations of the International Panel of Experts (PoE), Wapda is considering to raise the minimum operating level (dead level) of Tarbela Reservoir from the existing 1,380 feet to 1,384 feet above mean sea level in order to restrict the movement of sediment’s delta towards the main dam in the reservoir.

This is being done to protect the dam and critical structures and machinery of the power house. As far as raising minimum operating level of Mangla Reservoir by 10 feet is concerned, no such decision has yet been made. Wapda keeps vigil about the sedimentation level in the reservoir and its likely impact on the dam and structure as well as machines of the power house.

Presently, various options are under discussion in view of the impact of the current year’s sediments on the generating units of Mangla Hydel Power Station. The decision will be made after thorough discussion among the quarters concerned and in consultation with all the stakeholders, the Irsa in particular.

The official said that raising the dead levels of water reservoirs was not an unusual phenomenon but normal practice throughout the world. Sediments continue to deposit in the water reservoirs and their minimum operating levels (dead levels) are revised accordingly.