The U.S. Army Corps of Engineers has landed on a solution to put an end to the glacial outburst floods that have grown more destructive in Juneau’s Mendenhall Valley neighborhoods over the past few summers.
The agency will pursue something called a "lake tap" — essentially a tunnel through Bullard Mountain on the east side of the glacier that would steadily drain Suicide Basin, so it can’t fill to the point of bursting and send some 16 billion gallons of water through the valley.
Denise Koch, Juneau's director of engineering and public works, explained it with a metaphor on Friday.
“I just think about Suicide Basin as a proverbial bathtub,” she said. “What the lake tap is, is just leaving the drain open.”
She said the drain will empty the water from Suicide Basin into Mendenhall Lake through a conduit somewhere between the face of Mendenhall Glacier and Nugget Falls.
The decision comes after a three-day, closed-door meeting the Army Corps held with federal agencies, local officials and researchers in Juneau this week. Their main task was to discuss five options to prevent homes from flooding in the future. The Army Corps initially planned to host press briefings each day but canceled them on Tuesday.
The city announced today that city leaders, along with the U.S. Forest Service and the Central Council of the Tlingit & Haida Indian Tribes of Alaska, support the lake tap option, aligning with many of the public comments the Army Corps received last month.
Besides a lake tap, the options discussed at the meeting were a dam at the outlet of Mendenhall Lake, a permanent levee, a bypass channel through the Mendenhall River floodplain and relocating impacted residents from the Valley.
Koch said the group weighed the options based on risk to downstream residents, how quickly they could be built and the overall cost.
“Ultimately, a lake tap was seen to reduce risk the most while being able to be constructed the most quickly, for the lowest amount of cost, with the least complex and least costly operation and maintenance,” Koch said.
Koch said the tunnel could take as long as six years to excavate — the most conservative estimate. She said it could cost somewhere between $613 million and $1 billion, but that all estimates are very rough at this stage.
The Army Corps aims to finish its technical report for the lake tap in May. That will include a preliminary design, a more detailed cost estimate and a draft environmental review. There will be another public comment period once it’s complete.
To implement the solution, the Army Corps will need authorization and funding from Congress.