Efforts to stop leaks at dam fail

Posted: Wednesday, November 24, 2010

At attempt by Alaska Electric Light & Power to stop leaks at its Bart Lake Dam has failed, but now the Federal Energy Regulatory Commission and the utility say they can live with some seepage from the lake.

It was at the insistence of the federal agency that AEL&P took the Lake Dorothy Hydroelectric Project, which included the Bart Lake Dam, off-line last summer for a $1.6 million try at stopping the leak. The lake was drained to expose a couple of hundred feet of lake bed and an impermeable membrane that extended 100 feet into the lake was extended by another 150 feet. Last summer, when AEL&P began to refill the lake, it discovered the leaks persisted.

"We think that reduced the seepage a bit, but not what we'd hoped that it would," said Scott Willis, AEL&P's generation engineer.

The water that leaks through the dam doesn't go into the penstock that takes it to a hydroelectric turbine 1,000 feet below at the water's edge.

"Whatever water that flows underneath the dam is water that we don't get to generate power with," he said.

Bart Lake is the lowest of three lakes that make up the Lake Dorothy Hydroelectric Project. Most of the project's water is held in Lake Dorothy, but all the water used for power generation has to flow to Bart Lake and then into the penstock.

Willis said that it is frustrating that the effort to control the leaks didn't fully succeed.

"We are still scratching our heads as to where it is coming from," he said.

During the lake draining process, fissures were found in the lake bed which were thought to be the source of the leaks. Those were plugged, but the leaks persisted, he said.

The leaks aren't likely undermine the safety of the rock-fill dam, he said. Unlike earth-fill dams, where erosion can weaken the structure, the 34-foot high Bart Lake dam is unlikely to erode and be threatened, he said.

"Neither we nor the FERC right now believe this seepage endangers the structural stability of the dam," Willis said.

Still, the federal regulatory agency is requiring AEL&P monitor the flow and the turbidity of the stream downstream of the dam to ensure nothing changes. The water flowing into the stream below the dam is clear, he said. The flow coming through the base of the dam is about 5.5 cubic feet per second, and amount Willis said was "relatively small."

Having the added power generation capability of the $70 million Lake Dorothy Hydroelectric Project to supplement the huge Snettisham Power Project and the city's smaller generators means Juneau hasn't had to burn diesel to meet its primary power needs since it went online.

Further, the Lake Dorothy project's turbine proved to be able to generate more power than projected with the water it carries. That extra amount is close to the amount that's being lost through the seepage, Willis said.

During the remediation project, Willis said that AEL&P was able to store inflows into Lake Dorothy for later use, and then release them into Bart Lake when the dam was again operational.

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