Dam leak plugged but why did water permeate wall?
While a recent leak in the dam was played down by the authorities and completely plugged, observers say the issue remains a cause for concern.
Moragahakanda Reservoir Project Director D.B. Wijeratne said the leak, observed on Christmas Day, December 25, had been completely fixed by January 14. The leak was observed 3-4m below the lip of the dam.
A senior engineer who declined to be named but who has years of experience in dam safety, said the leakage itself was not the problem. “All dams leak. Leakage in a dam is not a big thing. The problem here was that this was an unusual leak in that it happened on the downstream face of the dam,” the engineer said.
He explained that there are several safeguards in a dam to prevent seepage. The dam has what are known as “galleries”, openings or passageways in the dam’s body that intercept seepage and carry it away from the downstream face.
There are also several “waterstops” in copper, rubber and PVC movement joints placed between the concrete building blocks of the 371m-long dam that hold perforated pipes to catch water and send it down to the lowermost gallery and back into the reservoir, the source noted.
The size of the leakage was only about 250 litres a minute, which is very little. “The leak, though, should not have come out from this downstream side. It meant there was a path that goes right through the dam for water to seep through,” the engineer said.
The senior engineer stressed he did not believe there were any major defects with the construction of the dam.
While the Mahaweli Authority has full control of the dam, the project is still under the Defect Liability Period, which means the Chinese contractor, Sinohydro, has to fix any defects at no cost to the Sri Lankan government until June this year.
Senior Consultant Engineer of Uma Oya, Nihal Rupasinghe, who also worked as an engineer on dams for over 35 years at the Mahaweli Authority, confirmed the leak had been caused by a construction defect in one of the movement joints designed to absorb heat and earthquake pressures and prevent the concrete cracking.
Mr. Rupasinghe said such defects could easily occur at night when the contractors, who often work at night, might have operated in inadequate lighting conditions.
He said even a minute misplacement invisible to the eye could create leaks.
“If water levels had not been raised to the maximum level the defect would have been undetected,” he said, adding other areas also need to be inspected for defects.
Project Director Wijeratne said a team of senior engineers and consultants from the National Building Research Organisation had submitted a report to President Gotabaya Rajapaksa and were due to make a second report.
He said the leak had occurred when the dam was filled to maximum capacity for the third time: “It happened when water levels reached 184m above sea level, which is below the maximum water level of 185m”.
Mr Wijeratne said some people had presented a wrong picture of what had happened. “If we wanted to cover this up we could have brought the water level down,” he said. “We could have hidden the leak and conducted repairs. Yet we did not do so because this is a normal issue in dams. Such things could even happen in the future.”
He dismissed what he said were “silly claims” that there was limestone at the bottom of the reservoir and that there were leakages from the bottom of the dam.
He said Sri Lankan engineers had closely inspected every layer of cement used by the Chinese to see whether it met international standards.
The reservoir’s Engineerin-Charge, P.S. Bandara, said the dam had been built with roller-compacted concrete, the first dam to built in Sri Lanka using this method.
Explaining the next phase of the project, he said there was a power plant at the base of the dam which would produce 25MW. “We are currently directing water to Elahera Amuna, which provides water to Minneriya, Giritale and Kaudulla tanks,” he added. “Apart from that, the KALUGANGA Project is under construction. Water could be provided to that project as well.”
While the leak was being repaired The Sunday Times watched Chinese workers drilling the top portion of the dam to scan for further leakages before carrying out grouting. About 20 Chinese were employed on the job.