City and federal contractors originally planned to build the flood wall along the Mendenhall River much higher this summer, after it just barely protected hundreds of homes from Juneau’s largest glacial outburst flood last August. But after the project cost ballooned, leaders decided to scale it back.
Until Monday, both the U.S. Army Corps of Engineers and the City & Borough of Juneau said they planned to build the Mendenhall River flood wall to withstand a flood nearly double the size of last year. That means it would have handled a rush of water up to 90,000 cubic feet per second, versus the 48,873 cfs peak of last year’s flood.
But during a Juneau Assembly committee of the whole meeting that night, the Assembly voted 7-2 to change the plan. Now, contractors will build the wall to withstand a flood about 30% larger than last summer’s record-breaking glacial outburst flood instead.
City Manager Katie Koester said the wall will be lower because construction is far more expensive than anticipated.
“To do the project as we originally envisioned — the project that I emailed every riverfront property owner and said we were building the 90,000 (cfs) — is just not feasible with available funding,” Koester said.
Building the city’s portion of the wall, Phase 1, to that level is now estimated at a total of $19.8 million, nearly four times the initial estimate.
Koester said the city couldn’t have known that until crews started working on the HESCO barriers, which are steel cages lined with fabric and filled with sand. This spring, the ground remained frozen for much longer than last year, delaying work on the barriers for about a month.
“As staff got into this project and started really digging, they realized they had to undo large chunks of the HESCO barrier,” Koester said. “So instead of just going in and fortifying them or adding a couple barriers, it actually meant rebuilding entire sections of wall. It meant twice as many barriers as in phase one.”
Denise Koch is the city’s engineering and public works director. At the meeting, she presented new figures: it would cost another $8 million to build it against a 90,000 cfs flood, versus $3 million more to make it withstand a 63,500 cfs flood, for a total estimated cost of $14.8 million.
“This is a really, really difficult situation for the community, especially people who live in the inundation area, and we’re — similar to last year — we’re always having to make decisions quickly with incomplete information,” Koch said.
The Assembly decided to spend the additional $3 million under the assumption that the flood wall will have to be built higher again next year, and that it will be more expensive to do it then. Assembly members Christine Woll and Maureen Hall voted no.
The funding gap comes at a lean time for the city. The Assembly has proposed cutting dozens of services and closing facilities after voters passed ballot propositions that created a $10 to $12 million budget hole.
The city expects it will need to continue paying for temporary flood protection for another decade or more while the Army Corps figures out a long-term solution. So the city, the Central Council of the Tlingit & Haida Indian Tribes of Alaska, and Alaska’s congressional delegation are pursuing state loans, grants and congressionally directed spending.
A probability problem
Suicide Basin is the source of Juneau’s annual glacial outburst flood. It’s a large lake tucked between jagged mountains and the Mendenhall Glacier, which acts as an ice dam. Each summer, when the ice dam breaks, it unleashes billions of gallons of water beneath the glacier that rushes through the suburban Mendenhall Valley.
As the glacier melts due to climate change, the basin grows and so does the flood.
But eventually, scientists predict Suicide Basin won’t be a problem anymore. The glacier is expected to thin or retreat so much that the basin can’t fill up enough to create a catastrophic flood.
Mike Records is a hydraulic engineer at the Army Corps in Alaska. He estimates the basin will release damaging glacial outburst floods for roughly another 25 years.
But he said that prediction could change.
“As the glacier continues to retreat, there might be a second Mendenhall Lake, which would then cause significant calving and would accelerate the rate of retreat of the Mendenhall Glacier,” Records said.
Because glaciers retreat faster on water than on land, he said that could dramatically reduce how long the annual flood remains a problem. Scientists at the University of Alaska Southeast and the Alaska Department of Natural Resources are currently working to refine the flood lifespan estimate by measuring the depth of the glacier and how quickly it’s thinning and retreating.
Records said the Army Corps considers engineering solutions according to the probability that a flood will exceed a given size within a certain number of years. That analysis is based on what scientists know about Suicide Basin so far and the lifespans of other glacial outburst floods in Alaska, including on the nearby Taku River and the Snow River on the Kenai Peninsula.
He said the risk accumulates over time — it’s like flipping a coin repeatedly.
“Flip a coin, there’s a 50% chance of getting a head, but you know, if you keep flipping that coin, you will get a tail eventually,” Records said.
The Army Corps’ analysis, presented in tables, shows a 2% chance of a flood twice the size of last year happening this year. But a flood that large has a 40% chance of hitting the Valley at least once in the next 25 years.
HESCO barriers are not made to last that long.
John Rajek is the chief of geotechnical and engineering at the Army Corps in Alaska. He said the agency aims to start on a medium-term solution next year and is weighing a few ideas. The first idea is to simply build the HESCO barriers higher. The second idea is to rework the river channel to move floodwater out of the valley faster.
“The third concept that we’re looking at is, is a sheet pile wall system that would basically act as a flood wall,” Rajek said.
Sheet pile walls are commonly used to create seawalls. They’re thin, lightweight and driven down into the soil. They’d replace the HESCO barriers.
The next glacial outburst flood is expected this summer. It has struck in August the past three years.
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