Diamer-Bhasha dam: Life without any sediment management measure estimated at 70 years

LAHORE: The life of Diamer Bhasha dam without any sediment management measure is estimated 70 years with annual capacity reduction of 1.16% and evaluation of different flushing scenarios which suggests that the life of reservoir could be extended to more than 140 years through sediment flushing, said sources.

They have proposed to carry out flushing not earlier than 30 year of reservoir operation in order to restrict sediment delta towards the dam and also to prevent Tarbela reservoir from heavy sedimentation during its useful life.

According to sources, watershed management for Bhasha dam can reduce the sediment inflow up to 50% but it requires serious investment. They said dredging is another popular method of sediment removal but it is useful in shallow reservoirs or at habours. Infrastructure of roads and other transport system e.g. railway and roads are not well developed in the area to carry out such a huge dredging works. The site for disposal of evacuated material is also an issue. So, as far as the facilities available at the site, dredging will not be a viable option in case of Diamer Bhasha Dam.

Siphoning dredging is also not possible in case of this reservoir because it is very deep. As there are very high mountains on both sides of the reservoirs, constructing a diversion channel for sediment diversion is also not possible. Therefore, keeping in view this preliminary analysis, flushing is the best option considering the site conditions, they added.

The dam will loss 80% of its designed capacity nearly in about 70 years of operation which means its useful storage will finish after 70 years of operation if any necessary sediment management measures are not adopted. The original capacity at the beginning of reservoir operation is computed as 9.99 BCM that will deplete to 1.92 BCM after 70 years of operation due to sedimentation. This means that the annual reduction in the reservoir capacity of Bhasha Dam due to sedimentation is 1.16%.

Reservoir sedimentation is a major problem worldwide. Large reservoirs in Pakistan have lost 33% of their design capacities due to sedimentation. The reduction in the storage will effect hydropower production, irrigation, water supply, ecosystem, recreation and navigation. About 70% population of Pakistan is linked to agriculture, which engages about 45% of the labour force of the country. According to the economic survey, contribution of agriculture in GDP is 21% (Agriculture in Pakistan, 2014). Most of the agriculture land in Pakistan is fed by two major reservoirs Tarbela and Mangla, both of which have lost about 1/3 of their capacities due to sedimentation.

Loss in reservoir storages of Pakistan due to sedimentation was reported as 7.43 BCM, 33%, which was equal to Mangla Dam back in 2010. Diamer Bhasha Dam is located on Indus River 40 km downstream of Chilas, the district headquarters of Diamer in Northern Areas. The Indus River at Bhasha has total catchments of about 153,200 km² draining Karakorum, Himalayas and Hindukush. Diamer Bhasha Dam will be also amongst the largest water storages of Pakistan and would have a gross storage capacity of about 10 billion cubic meters (BCM). Other benefits, including electricity, flood control, fish production and aesthetic values, can be attributed to the project.

Copyright Business Recorder, 2020