Full Srisailam Dam may Add to Stability Problems: Experts

Hyderabad: Serious doubts are being raised whether the Srisailam dam can be filled to its capacity this rainy season, particularly with the National Dam Safety Authority (NDSA) already expressing serious doubts over the dam’s safety.

The NDSA had flagged critical issues related to the safety of the dam which, it said, needed to be addressed urgently. In a report given to the dam authorities, the NDSA had last month said the problems at the dam were “serious” and had the “potential to compromise the dam’s safety.”

Concerns were flagged in February this year by the Telangana irrigation department which wrote to the NDSA saying the “ongoing deterioration in the plunge pool area of the Srisailam dam, poses a significant threat to the dam’s safety.”

Following this, a team from the NDSA visited the dam this April and issued a set of instructions for taking up remedial measures and urgent studies required for addressing issues related to the plunge pool — a huge cavern created by falling water with great force from the dam’s spillway immediately next to the structure.

The dam can store 214.8 tmc ft of water. The gates are lifted to release additional inflows, and the force of the falling water into the plunge pool area can result in further expansion of the cavity because of fresh scouring.

“The plunge pool cavity is already extending in the direction under the dam foundations. The NDSA must take note of the possibility of heavy inflows, and heavy discharges and how that may further impact dam safety because of the plunge pool problem,” an irrigation engineering expert said.

According to the NDSA’s inspection note on the dam in April, in addition to the plunge pool concerns, the dam faces another significant problem – that of blocked drainage holes in the galleries. These holes are designed to relieve the ‘uplift pressure’ by allowing water to drain from the bottom as the dam gets filled up.

The NDSA also noted that temporary repairs to stop the plunge pool’s expansion towards the dam — by installing steel cylinders filled with concrete —have started giving away. Of the 62 such cylinders, the NDSA found five opposite the dam’s 9 and 10 blocks have been washed away. It said that the National Institute of Oceanography in 2018 after a hydrographic survey of the plunge pool, recommended that a bathymetric survey, and underwater videography of the cylinders must be carried out, but both were not carried out by the dam authorities.

It is only this month that underwater videography of the damaged cylinders has been taken up.

The limits on storage must be looked at by the NDSA as at present the drainage holes are not working. And no steps, even temporary, have been taken up to fill the plunge pool either with CC tetrapods or blocks that can reduce the impact of water crashing down from the spillway into the void and at least reduce scouring, if not stop it altogether till permanent repairs are completed, an irrigation engineer with decades of experience in dam safety said.

“When the dam is filled, it experiences two types of pressure – horizontal, and uplift. What this means is that the dam is being pushed by the stored water and the water pressure from below also tries to lift the structure at the same time. If gates are opened and water is let out, then the uplift thrust pressure, and the horizontal push can come down. But in view of the already existing serious problems, a limit on the storage till all repairs are done, needs to be considered by the NDSA,” the expert said.

What NDSA found:

Plunge pool’s maximum depth is below the level of the deepest foundation of the dam.

Plunge pool scouring is creeping towards the dam.

Drainage holes in dam galleries choked, can cause instability issues.

NDSA’s must list:

Geophysical investigations on the apron to assess the situation of cavities, bedrock levels, nature of fractures underneath apron/old cofferdam.

Geophysical report and results must be available at the latest by end June.

Immediate rehabilitation works for restoration replacement of damaged cylinders 1 to 39.

Minimise operation of Gates 5 and 6 in view of severe damage to protection cylinders and possible scour of the old cofferdam.

Long term recommendations:

Stability analysis as per lS provisions from Block 8 to 12 as a minimum.

Analysis must consider the plunge pool in dewatered condition, consider horizontal shear zones reported during dam construction.

Take up and complete geological mapping of plunge pool and downstream abutment areas, carry out seismic analysis of the area.