A roadblock near the intersection of Highway 530 and Oso Loop Road in Oso on Saturday. (Ian Terry / The Herald)

A roadblock near the intersection of Highway 530 and Oso Loop Road in Oso on Saturday. (Ian Terry / The Herald)

Slide hazard prompts closure of Oso highway until Thursday

OSO — State transportation officials have extended the precautionary closure of Highway 530 until at least Thursday because of mudslide concerns, keeping the Darrington area’s main transportation link severed from the rest of Snohomish County.

The highway has been shut down in both directions since Friday evening between Oso Loop Road and C-Post Road. The area lies about 1.5 miles west of the massive slide that killed 43 people in 2014.

State Department of Transportation officials said Sunday that geologists want more time to study whether the unstable slope on the south side of the highway has the potential to develop into a larger mudslide. Part of the hillside moved an estimated 4 feet since a geologist first recorded cracks along an unpaved road nearly a week ago. No additional movement was detected over the weekend.

“If the entire 24 acre slide were to give way, it could completely cover Highway 530 and potentially reach Whitman Road,” said Dave McCormick, an assistant regional administrator, in a news release. “We know this is a huge disruption to the lives of those who live and work in the Stillaguamish River Valley, but there’s still a heightened risk that this slide could still move.”

A day earlier, state officials had expected to make an announcement Monday about whether to keep the road closed.

For now, drivers who need to travel between Oso and Darrington must use Highway 20 in Skagit County, adding at least an hour to a one-way trip. Emergency vehicles and property owners inside the road closure will be allowed through.

About a dozen households are under a voluntary evacuation warning.

County Councilman Nate Nehring, who represents the area, said he’s been in close contact with elected leaders in Darrington.

“While the road closure does pose an inconvenience to folks up in Darrington, the biggest issue is safety,” Nehring said. “We want to make sure we’re being careful and taking every precaution to make sure people are safe. That’s the No. 1 priority.”

The Mountain Loop Highway, much of which is unpaved, isn’t likely to open until later this spring.

The deadly mudslide three years ago left Darrington temporarily isolated. It took a month to open an emergency bypass road to limited public use. The bypass was built to help disaster crews access the scene when piles of mud and debris covered the highway. Highway 530 did not return to full function until six months after the disaster.

The latest concern arose after state transportation crews were notified a week ago about ground cracks on private property near Montague Creek, about a half mile from Skaglund Hill. Four days later, Department of Natural Resources geologists documented more earth movement in the area, which sits about 300 feet above the highway. The cracks formed after bouts of heavy rain.

State geologists, along with county and Seattle City Light employees, returned Saturday to conduct surveys. The slope has a documented history of slides.

Noah Haglund: 425-339-3465; nhaglund@heraldnet.com. Twitter: @NWhaglund.

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