The Rattlesnake Ridge landslide has slowed its descent down the ridge’s western slope from a peak speed of 1.6 feet per week in the early months of last year to 0.7 foot per week, according to a geological report released earlier this month.
The slide was discovered in October 2017 and reached its maximum speed from January to April of 2018 before beginning to slow, the report by the state Department of Natural Resources said.
“Everything we’re seeing is (showing that) the landslide is slowing down — slowly, but it is slowing down,” said Stephen Slaughter, landslide hazards program coordinator for the DNR.
“Landslides are a very mysterious thing, and I won’t expect anything (like) if it were to plateau for a little bit,” he added. “It’s really unknown.”
Live cameras courtesy of the University of Nevada's seismology lab
What about the snow?
Recent heavy snows should not threaten the stability or speed of the 8-million-ton landslide, he said.
“Weight (added by the snow) is not a concern. Considering that this is a landslide that is extremely heavy, the snow is not a factor contributing to the mass,” Slaughter said. “As far as melt-off, water for landslides is usually a driving factor, but the Rattlesnake Ridge landslide is entirely different because it’s entirely based in bedrock.”
Unlike shallow landslides consisting of a soil layer above bedrock — which can become saturated with water and become dangerous — water struggles to penetrate bedrock like that in Rattlesnake Ridge.
If any changes were to happen, officials would know immediately, Slaughter said.
“We’ve got a lot of stuff up there,” he said of monitoring equipment on the ridge. “There’s several of us who look at the data every day. It’s what we do, and I’ll do it this weekend.”
It is not just the DNR that is keeping close tabs on the site.
Rigorous monitoring of the 20-acre, 200-foot-deep landslide began when it was discovered. At the time, geologists believed it could pose an immediate danger, and 60 people living at the base of the mountain were evacuated. When an assessment firm hired by the state confirmed geological reports that the landslide was unlikely to rapidly slip — instead, gradually falling over years or decades — some residents returned.
Various agencies have offered help in monitoring the landslide. For instance, the state department of transportation conducts regular surveys on the ridge. In late 2017, Thorp Road, which is positioned on the south side of the ridge east of Union Gap, was closed.
GPS data is also monitored, Slaughter said.
Steve Malone, a University of Washington emeritus professor, was part of another group monitoring seismometers, which would show shifts and tiny earthquakes on the ridge, beginning in January 2018.
“We haven’t seen anything out of the ordinary,” Malone said. “We had set up a bunch of different monitoring techniques to process the data as it came in, and it was really consistent.”
Along with the increase in the slide’s speed, the data points became more active in the spring and early summer before gradually declining through the fall. In December, Malone’s group decided to remove its gear after having multiple issues with cords being sliced by falling rock or chewed through by wildlife.
Around that same time, Columbia Asphalt, which operates the quarry at the bottom of the ridge, hired a contractor to install new monitoring equipment on the western slope of the ridge as part of an agreement with the DNR.
Slaughter said a report on the equipment and its findings would be shared Monday with the DNR.
“It’s still probably the most heavily monitored landslide in the country,” Slaughter said.