NDSA flags concerns over stability of Srisailam dam
The NDSA team found that deep scour zones up to 5 metres below the original bed level were observed, particularly between spillway blocks 9 to 13 of the Srisailam dam
The National Dam Safety Authority (NDSA) has warned of a potential threat to the structural stability of the 40-year-old Srisailam dam built on Krishna river if Andhra Pradesh government doesn’t take damage control measures, both long-term and short-term.
A team of top officials of the NDSA led by its chairman Anil Jain visited Srisailam project site on April 29 and 30 to inspect the damages caused to the project over the years.
In a letter to special chief secretary (water resources) to the Andhra Pradesh government G Sai Prasad on May 5, the NDSA chairman pointed out various expert committees had inspected the Srisailam dam in the last 6-7 years, the last one being a joint inspection by a team of officials by NDSA, Central Water Commission and Central Soil and Materials Research Station (CSMRS) in February 24.
“Unfortunately, the state government has failed to comply with the recommendations made by these expert committees. The scouring of the river bed and rock profile on the downstream side of the dam has the potential to compromise the dam’s safety. No one is aware about the conditions of the measures to protect the rock face against scouring,” Jain said in the letter, which was seen by HT.
The technical review report of the NDSA, which was reviewed by HT, has identified several critical issues about the safety of Srisailam dam. According to the report, the inspection revealed significant damage to the plunge pool and downstream protection systems.
The plunge pool is the area where the water released from the dam and hits the rock bed in the apron (front portion of the dam). “The water gushing out of the gates located at such a height has tremendous kinetic energy and when it falls on the plunge pool, it rises up again to a great height before falling down again,” Rao said.
The water gushing out of the crest gates when the reservoir is filled to the brim, resulted in a huge crater due to the massive impact of the water pressure. In October 2009, Krishna River witnessed unprecedented floods at Srisailam. The dam, designed for a maximum flood discharge of 19 lakh cusecs, had witnessed a flow of 25.5 lakh cusecs. The irrigation engineers had to fully open all the 12 radial gates to the maximum extent to release the entire water at a high speed. This had resulted in the disturbance of the plunge pool and the crater in the apron had become huge.
The NDSA team found that deep scour zones up to 5 metres below the original bed level were observed, particularly between spillway blocks 9 to 13 of the Srisailam dam. Steel cylinders designed to protect the apron, particularly from 26th cylinder to 45th one – have either been severely damaged or washed away.
“Despite warnings and recommendations by earlier expert panels including the Dam Safety Review Panel (DSRP), no essential surveys have been carried out since 2018,” the NDSA report said, adding that scour-induced undercutting of the concrete apron could compromise spillway toe stability and possibly lead to structural distress or progressive failure.
The inspection team also found a high degree of clogging in the foundation drainage system and most parts of the foundation gallery remains largely unmaintained. The team also noticed seepage of water to the extent of 757 litres per minute in the gallery, which is significant given the potential uplift pressures it could induce.
In his letter to the state government, the NDSA chairman suggested short-term and long-term measures to ensure safety of the Srisailam dam. As a short-term measure, the government should immediately address the issue of the plunge pool before the onset of the monsoon season of 2025.
“The irrigation department must carry out suitable geophysical investigation on the apron at the earliest to assess the situation of cavities and bedrock levels before June end. Topographical survey needs to be carried for the apron, right and left banks of the plunge pool,” he said.
He suggested immediate restoration and replacement of the damaged cylinders from No 1 to 39. The exposed, damaged and eroded surface of the cylinder may be filled with concrete. “Gate operations may be minimized for Gate Nos. 5 and 6 in view of the severe damaged condition of the cylinders and possible scour of the old cofferdam,” he said, adding that as a long-term measure, the government may take the help of the CWC.
Former Union secretary EAS Sarma on Saturday urged Andhra Pradesh chief minister N Chandrababu Naidu to take up the restoration works of 40-year-old Srisailam dam built on Krishna river while pointing out the threats highlighted by the National Dam Safety Authority (NDSA) in its report.
“If there is any damage to the Srisailam dam, it will have a cascading effect on Nagarjunasagar, Pulichintala and Prakasam barrages on Krishna river and result in a catastrophe not only to the people of Vijayawada and Guntur but also the new capital city of Amaravati,” Sarma said in his letter to chief minister N Chandrababu Naidu.
Social activist Inaganti Ravi Kumar said the government should also initiate stringent action against the officials of the irrigation department, who have failed to take up adequate measures to prevent damages to the Srisailam dam in the last six years. “Both the previous TDP and YSRCP governments are responsible for this present situation in Srisailam dam,” he said.
