Work on $55 million bridge over slumping part of Denali Park Road could start next year

Drilling occurred at Pretty Rocks in 2018. Core samples from the drilling indicate how much ice underlies this part of the Denali Park Road. (NPS Photo/Mary Lewandowski)

The National Park Service hopes to start work on a bridge over a slumping portion of the 92-mile road into Denali National Park next summer. The affected stretch of the gravel road is at mile 45 in an area known as Pretty Rocks in Polychrome Pass, where subterranean ice is melting due to climate warming. 

The melt-caused landslide is accelerating, prompting action to maintain the route used to transport thousands of visitors through the park every summer. Denali National Park officials addressed the landslide and proposed $55 million bridge to span over it during an online meeting with the Denali Citizens Council.

Park science and resources team leader Dave Shirokauer says that when the road was cut into a mountainside 90 years ago, the section that is now slumping was constructed over a then-unknown underground rock glacier.

“We recently determined that the mechanism here is a rock glacier, so there’s a large ice component,” Shirokauer said.

Schirokauer says warming temperatures have accelerated melting of that ice and slumping of the roadbed over the last 3 years.

“In ’18 and ’19 it went from inches a month to inches per day,” he said. “And in 2021, this August, it kicked up to over half an inch an hour.”

Time lapse of the Pretty Rocks slump, from July 21 to Aug. 25, 2021. During this time, the road displacement was about 21 feet. (NPS Geology Team)

Schirokauer says it was taking more than a hundred dump truck loads of gravel per week to maintain the road this summer, and that still wasn’t enough. 

“The landslide has moved far enough now that the ice is actually exposed at the surface,” he said.

Unable to keep up, the park service closed the road at mile 42 in August. 

Refreezing during the winter should make the section useable again for a portion of next season, but the plan is to span the trouble spot with a bridge anchored in solid rock on either side.

Following analysis by the Park Service and the Federal Highway Administration, park Deputy Superintendent Brooke Merrell said the bridge proposal was selected as the best option “in all respects — in terms of timeline to put the option into place, cost, impacts to resources.”

Environmental analysis of the bridge project and other Polychrome Pass area road work will be conducted this winter on an accelerated timeline, according to park planner Miriam Valentine.

“If we are successful in getting funding, and Federal Highways can get the contracting lined up, we can actually start work in the end of summer in ’22,” she said.

Valentine says there will be opportunities for public comment in October and again in January. She says the Federal Highway Administration anticipates that it will take between two and three seasons to complete the bridge and other area road repairs, which are estimated to cost a total of $118 million.

Park superintendent Don Striker says the $55 million bridge portion of the funding is in President Biden’s budget, working its way through Congress. 

“The money for everything is in queue right now and looking promising,” he said.

Striker has been at Denali for 9 years but will not be overseeing the bridge project. The National Park Service announced this week that he’s been selected to be the new superintendent of North Cascades National Park in Washington State. 

Striker is scheduled to leave Denali in November. Deputy Superintendent Brooke Merrell will be at the helm until a new superintendent is named.  

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Dan Bross is a reporter at KUAC in Fairbanks.

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