Leaks from Alaska Electric Light & Power's Bart Lake Dam are continuing to plague the Lake Dorothy Hydroelectric Project, utility officials say.
Now, a federal agency may require the lake to be drained below dam level to fix the leaks.
Bart Lake is part of the Lake Dorothy Hydroelectric Project, which went online earlier this fall and provides about 20 percent of Juneau's power.
Scott Willis, AEL&P's power generation manager, said the seepage is not likely to jeopardize the stability of the dam.
"All of the engineers that have looked at it don't believe the dam is unsafe," Willis said.
Still, the Federal Energy Regulatory Commission has told the Juneau-based utility that it needs to develop a plan to reduce or eliminate the seepage.
Most of the water used in the Lake Dorothy Project is held in Lake Dorothy itself, but it flows through Bart Lake on the way to the hydroelectric generator at sea level on Taku Inlet south of Juneau.
The Bart Lake dam itself is modest, only 34 feet high, Willis said. It is known as a rock-fill dam on a gravel and boulder foundation, with a concrete face on the upstream and top.
One solution to the seepage, Willis said, may be to drill into the rock and inject grout into fissures to seal them. Some exploratory drilling has already been done, but found no water and the holes were then sealed with grout, Willis said.
Another possibility would be to place an impervious layer of material on the inside of the dam after the water level is lowered, he said. One or both of those solutions will likely be recommended to FERC, Willis said.
With the water drained below Bart Lake Dam, the Lake Dorothy Project would not be able to produce power, but Willis said they hope to produce more power from it before the drawdown. That will save water at the Snettisham Power Project, the city's largest power source, to be used later.
That should enable AEL&P to meet the city's power needs without relying on diesel generators this summer, Willis said. That may not be the case for a nearby silver mine.
"We're not sure how this would impact deliveries to Greens Creek (Mine), we're still doing projections," he said.
The Hecla Corp. mine on Admiralty Island produces its own diesel power when surplus AEL&P power is not available.
"They really like being on our hydro, and we're still hopeful we'll be able to continue deliveries to Greens Creek," Willis said.
Willis said that given the construction method, some seepage is expected but the amount has been more than was anticipated.
Seepage can undermine a dam by washing away its foundation, but is less likely in a dam constructed of boulders, such as Bart Lake. Also, AEL&P has been taking daily turbidity measurements in the water flowing from the dam, which indicates that materials making up the dam are not being carried away, he said.
Lowering the level of Bart Lake will be done using a combination of siphons and pumps, Willis said.
Contact reporter Pat Forgey at 523-2250 or firstname.lastname@example.org.