Srisailam dam: National Institute of Oceanography urged to study plunge pool pit

Soft rocks, eroding river bed add to plunge pool concerns; AP seeks National Institute of Oceanography’s help

By   |  Published: 17th Aug 2019  12:47 am

Hyderabad: The issue of safety of Srisailam dam and the potential impact on the dam from the plunge pool pit is something that has been of concern for some years now.

The dam’s safety related issues once again came to the fore this year with the Andhra Pradesh government asking the National Institute of Oceanography (NIO) to study the extent of the plunge pool pit and submit a report. Two teams of NIO, one from Visakhapatnam and another from Goa, examined the pit in May.

The continuous growth of the plunge pool size over the years has been a matter of concern which was raised several times by the National Committee on Dam Safety at its annual meetings. In fact, at its 38th meeting in January 2018 at Thiruvananthapuram, in response to a query from the NCDS Member Secretary, the Andhra Pradesh representative informed NCDS that “the plunge pool opposite block nos. 6 and 8 was getting eroded every year. All the remedial measures suggested till date have been unable to arrest the erosion and need a re-look for the safe operation of the dam”.

During this year’s NCDS meeting at Bhubaneswar in February, Andhra Pradesh informed the committee that it entrusted NIO with the task of studying erosion of the plunge pool opposite blocks four to 16 of the dam. NCDS has also been concerned over the need to increase the Srisailam dam spillway capacity.

AP had informed NCDS that this issue was studied and referred to a dam safety panel of experts under the chairmanship of former Central Water Commission chairman AB Pandya. Asked if the safety of the dam was the joint responsibility of AP and Telangana, a senior Telangana government functionary said the dam belongs to Andhra Pradesh and its safety is that State’s responsibility.

While the depth of the plunge pool was 68 feet in 2012, NIO is learnt to have found that its depth increased to more than 200 feet this year. And, this was before the current spell of release of an average of eight lakh cusecs of water from the dam because of Krishna river’s heavy inflows into Srisailam reservoir.

A 2006 study by AquaVision Engineering of Switzerland published in the International Journal of Hydropower and Dams, which took into account 20 years of scour data of the riverbed after the spillway, found that the bed itself had eroded by more than 10 m over the years.
Further adding to the concern of the dam’s safety are the established findings of the bedrock at the dam site, which according to researchers, is associated with many joints, layers of soft shale rock, weak pockets, sheared and fractured zones, as well as some cavities.

Two gates to be used as last resort

Hyderabad: Srisailam dam has 12 crest gates, but only 10 are used for discharging water from the reservoir. The two gates that are not opened are Gates No. 1 and 12, the first on the right bank of the river on Andhra Pradesh’s side and the other on the left on Telangana side of the river.

These two gates, officials say, are to be used only as a last resort if it ever comes to a situation to save the dam from stresses of excessive floods. While opening Gate No. 1 could flood the right bank hydel power house, an open Gate No. 12 could mean possible erosion of the hillside on the left bank. The left bank is where the ghat road leading to Srisailam temple town is laid. Opening Gate No. 12 could also cause flooding of the underground left bank hydel power station.

The entire dam structure, from its right to left, is divided into 22 vertical ‘blocks’ for the purpose of the dam’s maintenance and repairs. The 12 crest gates extend from block 4 to block 16.

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