Alaska Department of Transportation crews moved the material and reopened the road Sunday. Another slide occurred Monday evening. Several earthquakes, with magnitudes ranging from 2 to 3, occurred Sunday morning as well. Photo courtesy of Lynette Campbell.

Last Sunday and Monday, large mudslides in the upper valley flowed down the steep mountainsides and across the Haines Highway, forcing it to close while state workers cleared the road with heavy machinery.

An Alaska Department of Transportation engineer expects those to continue, and for other debris flows to increase, because the slides are caused by melting and calving permafrost on the Chilkoot Mountains’ ridgeline.

About six years ago, DOT materials engineer Bob Trousil studied the mountainsides above 19 and 23 Mile in an effort to create a model that could predict such slides. While funding for the studies dried up and such a model was never built, they did learn what was causing the slides.

“It’s just frozen material that’s very similar to glacial remnants that would include gravel, silt, some old soils that accumulated up there and are frozen in enough thickness that when it thaws it breaks off, like a calving glacier, from a couple feet to four or five or eight feet or more in some places,” Trousil said. “It thaws and breaks off and tumbles down the valley to the point where it accumulates and stacks up.”

The bulk of the debris consists of rock, mud, sand, gravel and boulders. When the sediment settles in the valley, it reaches what geologists call its “angle of repose”-the point at which dry materials remain stable on a slope. The angle of repose decreases relative to how saturated the material becomes. When it rains, the sediment loses its strength. “There’s a point where it can’t keep itself together and it just starts to flow,” Trousil said.

According to Trousil, the flows can reach up to 25,000 cubic yards of mud and other debris, sliding down the mountain at a rate of 20 to 30 feet per second. “A river at flood stage, like the Chilkat [River], on a day when it’s flowing pretty good is five to eight feet per second, so it’s fast.” The flows are so dense that large rocks and boulders float in the mud when it settles on the road.

Over several years, with several slides a year, DOT crews move more than 100,000 cubic yards of material. Crews typically pile it up along the road. Periodically, they will dump some of it into the Chilkat River, which they’re permitted to do.

Trousil said while the flows at 19 and 23 Mile are common, other smaller areas are “waking up.” He said they’re much smaller, but they’re a problem.

“This same kind of activity that we see has been accelerated up high with the melting affecting some of these other higher areas on these smaller drainages,” Trousil said. “The permafrost seems to be warming things up a little bit creating this supply of materials. These smaller ones are beginning to become more active. All along [the ridge] there is a corridor of these debris ow drainages. I think it’s safe to say it’s not going to go away over time.”

Local DOT station manager Matt Boron said the smaller slides cost between $10,000 to $50,000 to clean up and often go unnoticed by the public. The larger slides, which requires DOT to hire contractors and typically result in road closures, can cost upwards of $200,000, Boron said. He said bigger slides often occur during fall rain events after hot, dry summers.

DOT engineers have plans to curb such closures in the area when they begin phase two of the reconstruction project next summer. The plans call for elevating the road by about 20 feet in some areas and building concrete box culverts underneath the highway. The flows would cascade into the culverts, where DOT crews could move the material without obstructing traffic. While unrelated to the slides, the road will be elevated in other areas by several feet, which some worry will reduce access to the river.

Alaska Mountain Guides owner Sean Gaffney served on the Chilkat River Access committee, which met to discuss such issues last winter. On Thursday he told the planning commission, who has the opportunity to weigh in on the highway reconstruction plans, that losing access at 19 Mile will have “tremendous impact.”

“If you lose 19 Mile, from the commercial rafting perspective, you lose the Tsirku river,” Gaffney said. “That’s the takeout that works for the Tsirku, that allows that to be used. Then you’re going to stack up all of the remaining traffic.”

If 19 Mile becomes unavailable, Gaffney said commercial tours will pull out at 14 Mile, which is already congested.

Borough manager Debra Schnabel also addressed the planning commission, and made them aware of the river access committee’s concerns. The planning commission will consider DOT’s draft plans for approval at a Nov. 8 meeting. “Between now and Nov. 8, [the borough planner] and I and others will be dogging the staff at DOT to ensure that that access issue is addressed,” Schnabel said.

Planning commissioner Donnie Turner said while the road construction contractor worked with residents to provide access during construction, he was surprised to find out nothing in the contract guaranteed river access.

“There’s nothing in the contract that says they had to do anything about maintaining access to the river,” Turner said. “The contractor was awesome and they worked around us, but they didn’t have to. If they wanted to say no they could have just blocked us off the river.”

DOT public information officer Aurah Landau said the department has considered and maintained most river access points, but that the river bank around 19 Mile has an unstable slope and is “not a prudent spot for public access.”

Landau said DOT will improve 23 pullouts and parking areas to access Chilkat River recreation areas, shoulders along the highway and keep a section of highway at 20.5 Mile to offer river access.

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