UPDATE: Geologist Warns Yakima-area landslide could be worse than officials expect

The fissure on Rattlesnake Ridge is seen in a drone video made last Sunday. One geologist says officials should be prepared for the worst.  (Steve Mack)

An expert in the geology of Central Washington warned Friday that a landslide from Rattlesnake Ridge near Yakima could be much more destructive than government officials project, slamming into Interstate 82 and the Yakima River.
Bruce Bjornstad, a retired geologist from Pacific Northwest National Laboratory and an independent consultant, said a past landslide nearby appears to have spread quickly and covered much of the valley floor.
“This situation has all the elements needed for a big landslide,” he said. “When, and if, that thing goes, it could wipe out dozens of cars on the interstate, it could potentially dam the river and affect that whole valley.”
But other geologists, including an engineering firm hired by the quarry where a large crack first appeared in October, say any slide is likely to be slow-moving and mostly contained by the mined-out pit, sparing the freeway and river.
The difference in scenarios hinges on which direction the hillside will fall when it collapses, which seems increasingly likely as the crack continues to grow and widen.
In addition to threatening roadways and the river, a slide could potentially bury a cluster of homes housing more than 50 people, most of whom have evacuated.
Cornforth Consultants, a geotechnical engineering firm hired by quarry operator Columbia Asphalt and Gravel, says the hillside is likely to slump to the south, which would mean much of the debris would be trapped in the pit.
That projection is based on monitoring, computer modeling — and years of experience with landslides, said Charles Hammond, an engineering geologist for Cornforth.
Rattlesnake Ridge is made up of thick layers of basalt, interspersed in some places with weaker sediment layers, Hammond explained. The rock layers are tilted to the south, toward the quarry pit, and the angle is a fairly shallow 10 degrees — which is why his team doesn’t believe the slide will roar off at high speed.
“It’s not like it’s a pile of soil or material that will readily fall apart, like sand or silt or gravel,” Hammond said Friday. “It’s solid rock.”
Based on measurements of the widening gash that now cuts across the ridge, Cornforth estimates the slide will break off in early February. The mass, which is moving about 1.4 feet a week, contains about 4 million cubic yards of rock and soil.

Cracks opened up on a ridge above Interstate 82 near Yakima, and has forced some evacuations. (Courtesy Steven Mack)

Bjornstad, however, who has studied slides in the area for decades, is convinced the western flank of the slope will break off and follow a trajectory straight into the freeway and river.
He points to the huge fissure that has propagated up from the quarry — and which he predicts will continue to grow. Those cracks extend deep into the hillside and cut across the basalt layers, he said.

A similar landslide occurred a scant 20 miles away, when Toppenish Ridge slumped into Toppenish Creek thousands of years ago, he said. “It looks like it occurred very quickly,” Bjornstad added.“When, and if, that landslide occurs, all the material to the west of that (fissure) is going to come down … and it could extend all the way across the river.”
A fast-moving slide that crosses the freeway and river is a worst-case scenario, Bjornstad acknowledged — but one he thinks officials should plan for. It’s also possible the hillside will slump, then stabilize, until a trigger like heavy rain or an earthquake causes it to move, he said.


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