SKAMOKAWA, Wahkiakum County — On the evening of Jan. 23, a Southwest Washington hillside gave way, burying a lonely stretch of Highway 4 in trees, rocks and mud. Three excavators are still on scene, scooping up debris that eventually will fill more than 3,500 dump trucks. Then, there will be more work to stabilize the slope.

“We’re looking at months, not weeks,” said Tamara Greenwell, of the state Department of Transportation, about the length of repair work.

This landslide was one of many to hit state and county roads during the intense series of storms that pummeled the region through much of January and early February and left behind trouble spots across a wide swath of Western Washington. In King County, where flooding compounded problems, at least $10.6 million will be required for repair work at 40 sites, and millions more dollars will be required for fixes elsewhere in the state.

The damage, the worst in the past half-decade, is a costly reminder of the risks posed by unstable landscapes in the Pacific Northwest, which often have been carved up with roads, logged or developed in ways that make them more prone to fail.

The winter slides also foreshadow the challenges that lie ahead in a century when climate change — driven by the build up of atmospheric greenhouse gases — is expected to result in more frequent bursts of extreme rainfall that can trigger slides. The state Department of Transportation has an inventory of some 3,000 slopes at risk to unleash landslides, debris flow and rock fall along more than 7,000 miles of highways.

Last month ranked among the top 10 wettest Januaries on record in Western Washington, according to the office of the state climatologist. And slides helped spur Gov. Jay Inslee to issue emergency proclamations that now cover 25 counties due to storm impacts.

Logging above slide

In the area where the landslide hit Highway 4, more than 25 inches of rain dumped from the skies in a little over three weeks’ time.

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The slide appeared to start at the lower edge of a clear cut about 170 feet above the highway, then continued through a small section of forested slope that had been left behind.

State timber-harvest rules are based on research showing logging sometimes can increase the likelihood of a slope failure or increase its size. And the risks to public safety posed by logging gained a higher profile in the aftermath of the 2014 Oso landslide that killed 47 people. The event triggered lawsuits  from survivors — settled in 2016 — alleging logging over the slide zone contributed to the disaster.

The toughest scrutiny is given to timber harvests that are deemed likely to have “a potential for a substantial impact on the environment,” and may require a geotechnical report and other information before logging.

The harvest site by Highway 4 did not get a geotechnical report. Dan McShane, a Washington geologist who consults on landslide risks, thinks that should have been done because the area was near a previous slope failure and was above a state highway.

Peter Hughes, Department of Natural Resources geologist who inspected the tract before the logging, came to a different conclusion. He found that unstable areas were excluded from logging, so the cut zone did not need further evaluation. Hughes, in an interview this month, said he believes the logging had an “insignificant impact” on a slide that was driven largely by the huge amounts of rain.

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As of mid-February, the taxpayer costs of fully reopening the state highway have exceeded $85,000. The final tally is expected to run much higher, and it will depend on the final engineering plan for shoring up the hillside.

“We are doing bore holes to get an idea of the materials that are present,” said Jim Struthers, a Washington state Department of Transportation geologist who expects the work will involve adding a lot of rock to hold back the slope.

Mount Rainier roads hit hard

Further north, avalanches of mud and rock — reported Feb. 6 — struck Highway 706 leading to the Nisqually Entrance to Mount Rainier National Park. The slides rerouted water flows, triggering an ongoing cleanup by the state Department of Transportation that has shut down vehicle traffic except for convoys with pilot cars arranged for some 200 people who live to the east of the damage zone.

The road is expected to reopen in March. In the meantime, the closure has been a financial blow to operators of nearby lodges, which have had to cancel some bookings.

[Update: The National Park Service tweeted Monday morning that Highway 706 leading to the Nisqually Entrance to Mount Rainier reopened. Paradise will remain closed Monday.]

“We have had to refund people for their reservations, and we can’t be reimbursed. This has not been good for us or our clients,” said Eduard Raiter, whose family operates the Storm King Spa and Cabins close to the area hit by debris flows.

State transportation officials who have surveyed the damage zone say a Forest Service road that ran above Highway 706 failed in two places and unleashed destructive debris flows. “Basically, what happened is the drainage was overwhelmed, and the (Forest Service) road failed,” Struthers said.

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The Forest Service road was part of an aging network built through these federal lands largely to access timber harvests. This road was open and used by recreationists and other motorists, according to a Forest Service official.

Another road failure occurred on a paved Pierce County route called Fairfax Forest Reserve Road that provides access to Mount Rainier’s Carbon River Ranger Station and is a vital link for residents of about a dozen homes. Water seeped some 15 feet underneath the roadbed, and also rushed over it, causing a 100-foot swath of the road to wash out.

That site is expected to be the biggest part of an estimated $2.7 million county bill for Pierce County storm damage. Geotechnical assessments have yet to be completed. If the repairs can be made with rock shoring, then they could be done in months. If a wall is needed, that will take a lot longer, according to Bruce Wegner, Pierce County Public Works road operations manager.

Highway 101 sinks

Sometimes a landslide moves slowly and causes earthquake-like cracks and settling that can be very expensive to repair.

One section of  Highway 101, which runs up the Olympic Peninsula near Cosmopolis, is undergoing an $8.8 million repair due to the impacts of one of a large, slow land failure. The project started last summer, and expected to be completed this fall, involves sinking some 600 shafts to create a buttress to support the highway.

Then on Jan. 24, another section of the highway less than a quarter mile away dropped some 5 feet and reduced traffic to a single lane. This new failure is located on another slow-moving landslide, and it will require a new geotechnical analysis to develop what is expected to be another costly fix.

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“It’s very significant. It’s continuing to drop,” said Tina Werner, a state Transportation Department official on Friday.

For heavily populated King County, slow-moving landslides also have caused serious problems. One such active slide caused cracks in the pavement of 356th Drive Southeast, a road that offers the only access to a neighborhood northeast of Fall City. Though the cracks have been sealed, an emergency evacuation route has been set up for residents, and the situation continues to be closely monitored.

Other problem areas within King County include Highway 203 near Stillwater Hill Road, where the pavement has cracks.

Broch Bender, a King County spokesperson, said that even with the clear skies of recent days the ground remains saturated. More slides are possible.

“We’re not through the woods yet,” Bender said.