The temporary flood wall along the Mendenhall River just barely protected hundreds of homes from Juneau’s largest glacial outburst flood last summer. Now, city and federal contractors are racing to repair the damage and bolster it for the flood expected this summer.
Dump trucks roared down the suburban streets of the Mendenhall Valley this week hauling massive boulders. Workers in neon vests tended to the flood wall in residents’ backyards.
The wall is composed of HESCO barriers, which are blocks made of steel cages lined with fabric and filled with sand. They’re stacked and tied together to form a temporary levee.
Zachery Howdeshell is a foreman at Admiralty Construction, a local contractor helping to take down and rebuild parts of the flood wall. His team uses a contraption called a recovery frame that hooks into the steel cages to carefully pull them apart.
“Since all of the barriers have an open bottom … when you pick it (up), all the material will eventually just come out of the bottom and release,” he said, pointing to a pile of sand and a stack of folded steel cages that they’ll reuse.
Then, they’ll level out the ground and stack them up again — this time three HESCO barriers high instead of two — along Riverside Drive across from Melvin Park.
Last summer was the first test of the flood wall. It reached a maximum of three barriers high, and in some areas — especially sharp bends in the river — floodwater came within inches of flowing over. Water also scoured the large boulders placed along the bank and leaked through the barriers, flooding a couple dozen crawl spaces. Some barriers lost sand or slumped toward the river.
This year, the City & Borough of Juneau and the U.S. Army Corps of Engineers are armoring the banks with more boulders, building the flood wall higher and wider and extending it along a greater stretch of the river. Officials say they’re building for the worst-case scenario and hoping for the best.
Denise Koch is the city’s director of engineering and public works. She said along some streets, HESCO barriers could be stacked four or more high.
“Most places where those HESCO barriers are used for flood fighting, they are typically used at a max of three high,” she said. “After that, it’s less certain — you get concerns with the weight of the HESCO barriers.”
Areas that could see HESCOs stacked four high include the bend from Riverside Drive to Killewich Drive, Betty Court and Dimond Park. Meander Way could be stacked four-and-a-half HESCOs high, based on a preliminary map the city published.
Koch said when they build higher, they usually also have to build wider to create a broad foundation that supports the weight.
“With HESCOs, we’re trying to get through a slightly worse flood the next year,” Koch said. “But they are not a forever solution.”
The city is using $5 million pulled from the proposed Capital Civic Center project to repair and bolster Phase 1 of the flood wall, which goes from the end of Marion Drive to Postal Way.
The city decided not to move forward with Phase 2 before the flood last year. But this year, the Army Corps stepped in to build it and cover the estimated $19 million price tag. Phase 2 will extend the wall along both sides of the river — upstream to Back Loop Bridge and downstream to just before Juneau International Airport. Much of the undeveloped floodplain on the west side of the river will still be left open.
The Army Corps has also shipped in 41 industrial-sized pumps to bail out any water that pools behind the flood wall.
Herschel Deaton is the chief of emergency management at the Army Corps in Alaska. At a flood response training at Dimond Park earlier this month, he said the flood wall should be completed by July 15 and he’s optimistic it will protect hundreds of homes again this year.
“The CBJ leadership has asked us to heighten it, to make it higher, to basically withstand a 50-year flood, which is 90,000 CFS, which is cubic feet per second of water,” he said.
That’s a measurement of how fast the floodwater could rush through the river. Last August, the record-breaking flood peaked at 48,873 cubic feet per second, according to stream gage data finalized by the U.S. Geological Survey.
Aaron Jacobs is the senior service hydrologist at the National Weather Service in Juneau. He said the 90,000 cubic feet per second scenario is pretty unlikely — the flood would have to nearly double in size.
“So that’s a 2% chance of it getting to that height in the next 10, 15 years,” Jacobs said.
That’s based on statistics the Army Corps came up with in an unpublished report the agency declined to share with KTOO.
Jacobs said that the flood this summer is more likely to be closer to what residents saw last year. The annual flood has grown from cresting at nearly 15 feet in 2023 to 16.65 feet last year.
The size of the flood depends largely on the volume of Suicide Basin — the glacial lake responsible for producing the flood — and the rate that floodwater rushes through the river. Later this summer, researchers will use drones to update the Suicide Basin volume estimate. The speed of the water can’t be measured until the flood is underway.
The Army Corps plans to present HESCO designs at the Juneau Committee of the Whole meeting on Monday.