WSDOT completes project that reduces risks of debris flow onto U.S. 2 near Bolt Creek Fire burn scar

SKYKOMISH, Wash. - Nearly 15,000 acres of the Mount Baker-Snoqualmie National Forest were destroyed by the Bolt Creek Fire in 2022. It has been more than a year since the fire finally died out, but its more than 8-mile-long burn scar remains a landslide danger to drivers and locals alike.  

Experts said it will take several years for the burn scar to recover. Washington State Department of Transportation just completed an important step in the recovery, which also helps increase safety along the U.S. 2 Highway.

"It’s one of our mountain passes, there’s limited detours available. It’s an important route. We don’t have too many mountain passes that stay open in the winter and we have communities up there who rely on it. So, we want to make sure we do our best to keep the highway open to reduce the impact to everybody who uses the route," said Cody Chaussee, a geotech with WSDOT’s Engineering Geology Office.

WSDOT crews recently finished countless hours of construction along the highway, installing flexible fences and an earthen berm the catch debris flows, which are like fast-moving landslides of trees, limbs, rocks or mud from the burn scar. The barriers are necessary, as officials said rain or snow could trigger a slide disaster.

"You can’t really predict debris flows. They’re inherently unpredictable. But what we can predict is where one might go if one is occurring," said Chaussee.

He explained right after the fire was out, members of the engineering geology office shifted their focus to debris flows to learn the locations of potential risks.

"We went out there on foot, looked at all the potential debris flow areas and then tried to get an idea of what type of catchment we’d have available at each of these areas," said Chaussee. "In the event of debris flow, if debris comes down, they typically follow channels, like an existing channel network. So, there’s plenty of channels for them to follow, but not all of them lead to a catchment area. And a catchment area is essentially low-lying typograph. So, is there a ditch there, is there a natural low-lying valley, or a wetland or somewhere that is essentially going to stop the debris or prevent it from reaching the roadway."

Chaussee said staff studied reports from the Washington Geologic Survey and a burn severity report from the U.S. Forest Service Burned Area Emergency Response. He explained WSDOT geotechs used the reports to detect two areas near Skykomish where there was a higher risk of debris flows that could impact U.S. 2. This information was used to help them design protections against the dangers of debris flows.

"If a debris flow occurs, we know it’s going to go one of three ways in this location: it’s going to go to the west where there’s at least a catchment area, it’s going to the east where there’s another catchment area. Or it’s going to come right down the middle," said Chaussee.

Right down the middle is where crews detected risks and installed two flexible fences along the highway by milepost 44 near Skykomish. One fence is 60 feet long and the other is 110 feet long, located above each culvert. 

"We needed to build those fences up to make sure that if debris comes down, it’s going to get caught by something and reduce the risk of reaching the roadway," said Chaussee.

Crews also built an earthen berm, six feet high and 94 feet long, designed to deflect trees or limbs before they reach the road.

"There’s a catchment area right next to where the earthen berm was constructed. So, if debris comes down, we’re hoping it essentially hits the earthen berm and then is deflected into the catchment area," explained Chaussee.

During the most intense days of the fire, U.S. 2 closed several times east of Gold Bar because fire and debris fell onto the highway. Chaussee said he hopes he and his team designed the best tools to protect the road.

"This is all basically counting on the debris flow is following the existing channel network. So that’s what we used as guidance to see where debris might go should one occur," said Chaussee. "Hopefully the fences never have to get tested."

WSDOT crews will continue monitoring areas along the highway and burn scar for any down trees, clearing ditches and culverts, and any potential for debris flows. In spring 2024, crews will add native plant seeds to help stabilize and regrow the area.