Rattlesnake Ridge landslide continues to move slowly near Union Gap | Lower Valley

It’s hard not to notice the swath of disturbed earth on Rattlesnake Ridge when driving on the highway near Union Gap.

The Rattlesnake Ridge landslide was first spotted in 2017. It has been moving since then, but much more slowly than initially feared.

Up until last year, officials were meeting monthly to assess the landslide, but it is moving so slowly now, the meetings are twice a year.

Rian Skov, the chief reclamation geologist for the Washington State Department of Natural Resources, has been monitoring the site and meeting with state Department of Transportation staff and a consultant with a nearby gravel mine.

“Meeting once a month, that wasn’t any kind of meaningful change,” Skov said.

Rate of slide

Currently, the movement rate for the fastest-moving part of the slide is up to one inch per week.

By comparison, measurements taken in August 2021 showed the slab was moving 1.6 inches each week. In 2020, the slab was sliding 2 to 3 inches each week. And its maximum speed in early 2018 was 1.6 feet per week.

As it moves, it inflates and changes its structure, which is why some travelers may notice rockfall on the western part of the slide, he said. That’s also why WSDOT has shipping containers set up on Thorp Road to prevent rocks from tumbling farther.

As the slide moves forward, it has been building material and creating a “self buttress,” which will eventually stop the slide.

“It looks real nasty, but overall it’s really kind of unimpressive, as far as a landslide movement rate goes,” Skov. “It is a big ugly scar on that hill for sure.”

Skov said that the landslide still does not pose a threat to Interstate 82, based on the angle it is moving.

“It would take a massive change in the landslide morphology that we have not seen and don’t expect for any reason,” Skov said.

One of the biggest factors for changes in landslides is moisture, which the Yakima Valley is not known for, he said. It’s also not known for seismic activity, which could change the rate of the slide but not the direction.

The slide is moving on an interbed between two basalt layers, he said. A few inches of ancient soil is what is allowing the slide to move, he said.

Columbia Asphalt and Gravel has not been operating its gravel quarry near the base of the crack.

Once the slope is stable, DNR will work with the gravel company on plans to return the land back to a natural or usable state, Skov said.

Thorp Road continues to be closed due to the slide.

An inquiry about future plans for Thorp Road sent to Yakima County Engineer Matt Pietrusiewicz went unanswered by press time. 

Editor’s note: An earlier reference to biennial meetings was corrected to say officials are now meeting twice a year.

Questen Inghram is a Murrow News Fellow at the Yakima Herald-Republic whose beat focuses on government in Central Washington communities. Email [email protected] or call 509-577-7674.

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