The pace of the 20-acre Rattlesnake Ridge landslide near Union Gap is continuing to slow, officials say.
Measurements taken in August show the slab is moving less than 2 inches each week, Yakima County emergency services director Tony Miller said. In 2020, the slab was sliding 2 to 3 inches each week.
The slab is expected to continue sliding south, with debris depositing in a nearby quarry.
“It’s just going to continue moving real slow,” Miller said. “It’s moving a little bit, crumbling and falling right into the pit.”
Start of the slide
State and county officials, as well as scientists, have been monitoring the slide since a crack was first spotted on the ridge in 2017.
At first, officials said the slide could pose an immediate danger and 60 people living near the site were evacuated. Some of the residents returned after an assessment firm hired by the state confirmed the landslide was not likely to rapidly slip.
Yakima County commissioners agreed to keep Thorp Road closed on the south side of the ridge due to falling rocks, and the state Department of Transportation positioned cargo containers loaded with concrete road barriers nearby to help keep any major debris from hitting U.S. Highway 12.
A slow slide
Geologists still don’t expect a rapid slide. One researcher, Stephen Reidel at Washington State University, calls the landslide “constipated.”
Reidel, a research faculty member at WSU in the Tri-Cities, said the slab is sliding on a sedimentary layer between the basalt flows and doesn’t expect it to speed up.
“As the basalt slides into the old quarry there, we’re going to see the basalt stacking up and probably forming a buttress for the landslide,” he said.
The rock collecting in the quarry should eventually build up enough to act as a barrier, stopping the landslide, he said.
“It’ll just stabilize itself and then stop moving,” he said.
Rattlesnake Ridge today
The Washington State Department of Transportation, Washington State Department of Natural Resources and Yakima County are still monitoring the slide.
Miller, with Yakima County Emergency Management, said in August the slab was reported to be moving about 1.6 inches each week.
DNR spokesperson Jim Smillie said the landslide’s speed has been consistent over the past six months.
The recent measurement is slightly slower than the landslide’s speed in 2020, which was 0.15 feet each week in the areas moving the slowest. The slab was moving about 2 to 3 inches each week.
The landslide’s maximum speed, recorded in the period from January to April 2018, was 1.6 feet, or about 19 inches per week.
Thorp Road remains closed and the cargo containers are still in place, and the cause of the landslide has not been identified, Miller said.