The U.S. Army Corps of Engineers and state Department of Ecology are reviewing an application from Puget Sound Energy to perform maintenance at its hydroelectric dam on the lower Baker River at the south end of Lake Shannon in Concrete.
The agencies are tasked with reviewing permits for construction in waterways to ensure the construction is in compliance with the federal Clean Water Act and state laws.
Puget Sound Energy proposes repairs to ensure the continued integrity and safety of the dam, according to the application.
Tom Danielson, PSE senior project manager for dam safety, said the work is needed to slow the leak of water from under and around the sides of the dam.
The project will place sand and gravel in a portion of Lake Shannon and use grout — a concrete-and-water substance similar to that used in bathroom tiling — to stop or slow the leaks.
Lower Baker Dam is a 285-foot tall arch dam that holds back Lake Shannon and generates electricity for PSE’s customers throughout the region.
The concrete dam was built in 1925.
“Essentially ever since it was constructed there has been an issue with leakage, not through the dam, but through the foundation and the sides,” Danielson said.
It’s not uncommon, he said, for old, large dams to require occasional grouting repairs.
The last time that type of work was done on Lower Baker Dam was in 1982, and before that it was done in the 1950s and 1930s.
The work in 1982 reduced the flow of leaking water from 140 cubic feet per second to 30 cubic feet per second, Danielson said. With leaks reopening and worsening over the past nearly 40 years, the flow from leaks has reached about 200 cubic feet per second.
“We’re higher than we’ve ever been,” Danielson said of the leaks. “We don’t like to see water just wasted, for electricity generation, or like this year where we have a low snowpack, and if we’re losing that water we’re not getting (the lake) as high for boaters.”
If PSE obtains the necessary permits, repair work could begin in 2020 and last two years or longer.
The corps will weigh the benefits of the project — which Danielson said is critical for long-term dam safety — against the possible impacts, including to the lake, river, wetlands and scenery, according to the project documents.
“That water that’s leaking, we’ve recently discovered it is pulling some of the foundation rock away with it over time,” Danielson said. “There isn’t an imminent threat to anybody in the valley, but over the years ... there is a stability issue with the dam.”
PSE proposes to first put 24,000 cubic yards of fill into the lake directly upstream of the dam.
The fill will include gravel and sand layers covered by a clay or manufactured membrane. In steep areas near the sides of the dam, other synthetic membranes or concrete may be used.
Danielson said that will slow the leaks, making grouting easier.
“It’s hard to put grout into something that has 200 cubic feet of water flowing through it per second,” he said.
After the fill is in place, PSE will drill about 300 holes into the bedrock surrounding the dam and inject grout.
That will involve installing drill casing and pipe through about 200 feet of water, drilling through about 70 feet of debris on the reservoir bottom, and drilling and grouting up to 250 deep in bedrock below the base of the dam, according to project documents.
To complete the work, PSE will need to build work platforms, docks and ramps to access both sides of the dam. Cranes, barges and helicopters may also be used.
Sediment barriers will be used downstream of the work to catch fill, silt or other material that gets into the river at the project site, according to project documents.
PSE plans to dispose of wastewater, material removed during drilling and excess grout at an abandoned quarry about a quarter-mile northeast of the dam.
The project is expected to permanently impact 0.2 acres of wetlands, for which PSE proposes buying mitigation credits from the Skagit Environmental Mitigation Bank in Mount Vernon, according to project documents.