An estimated 50 residents in central Washington state need to evacuate due to a slow-moving landslide that’s expected to break loose by the end of February, authorities said.
About 80 percent of those living near Thorp Road below Rattlesnake Ridge near Yakima have taken that advice and left as experts differ over which way the slide might go.
An aerial video shows huge cracks developing at the top of the ridge, but it’s not clear how far the cracks might extend if a slide happens.
The cracks already extend into a gravel quarry on the ridge. The quarry is operated by AK Anderson Quarry.
George Machan, a senior associate geotechnical engineer with Cornforth Consultants hired by AK Anderson Quarry, said most of the material in a slide would be trapped in the quarry and not reach Interstate 82 or the Yakima River.
“There’s an extremely remote possibility that it’s going to get into the river,” Machan said.
He estimates up to 1 million cubic yards (0.76 million cubic meters) of rock and soil could move down the ridge in a landslide.
But retired geologist and independent consultant Bruce Bjornstad said the slide could end up on Interstate 82 and the Yakima River.
“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.”
Washington State Department of Transportation spokeswoman Meagan Lott said the slide is moving about 1.4 feet (0.4 meters) each week. She said the agency is prepared to close the interstate if there’s excessive movement or an increased number of rocks falling.
Tom Badger, a retired engineering geologist for the Washington State Department of Transportation, said there’s no telling what scenario to expect.
“Anytime you are approaching millions of cubic yards of material moving, there is not a deep well of experience to draw from on how it will behave,” he said. “There are a number of ways that this could develop.”