In a new report, engineer Norman Norrish estimated the odds of a fast-moving slide at less than 5 percent. And he said a slide that spills into the Yakima River is “highly remote.”

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The slumping hillside at Rattlesnake Ridge is likely to continue creeping slowly downhill — perhaps for years — rather than letting loose in a rush, says an independent consultant hired to provide a second opinion on the unfolding geologic drama near Yakima.

In a report released late Tuesday, engineer Norman Norrish estimated the odds of a fast-moving slide at less than 5 percent. And if the slide accelerates to the point where it poses a risk to Interstate 82, instruments will pick up that motion in plenty of time to close the highway, evacuate people and set up detour routes, the report concludes.

A slide that spills into the Yakima River is “highly remote,” Norrish wrote, nor is the slide likely to reach a small cluster of homes near the base of the slope.

However, if the worst-case scenario — a fast-moving slide — does occur, the analysis estimates that soil and rock could reach the northbound lanes of I-82.

Rattlesnake Ridge

Rattlesnake Ridge fissure, Dec. 31, 2017.
Rattlesnake Ridge fissure, Dec. 31, 2017.

More likely, though, is an intermittent rain of rock onto the lower slopes and Thorp Road, which skirts the base of the hill. As debris sloughs off, the result could be a buttress at the toe of the slide and in the quarry that could slow or stop the slide motion, the report says.

The new analysis is generally consistent with the conclusions reached by Cornforth Consultants, hired to monitor the slope by the operator of a quarry at the base of the hill.

Like Cornforth, Norrish finds little evidence that the west side of the hill is weakening and likely to collapse into the freeway and river. Another geologist had raised the possibility, but later said he was less worried because the cracks did not appear to be extending across the western flank.

In a joint letter to the public and the dozens of agencies and local and tribal governments with a stake in the outcome, Gov. Jay Inslee and Public Lands Commissioner Hilary Franz vowed to continue monitoring the slope.

The report notes that it will probably not be practical or affordable to keep up the current, intensive level of monitoring. But even at a lower level, it should be possible to detect any significant changes or warning signs that warrant increased surveillance, the report says.

State Geologist Dave Norman, of the Washington Department of Natural Resources, said agencies are discussing how to keep tabs on an event that could go on for years or decades.

“That’s a tough one,” he said. “I don’t know if we’ve got the solution yet.”

Norrish recommends that an automated warning system be set up to send alerts when motion reaches levels of concern.

Cracks in the slope were first spotted in October, and portions of the slope have moved 12 feet since then.

Initially, the slide was accelerating and engineers predicted up to 4 million cubic yards of debris could come toppling down sometime in early March. But since then, the slope has moved at a steady rate of about 1.5 to 1.7 feet per week, and officials have stopped issuing dates.

Norrish describes the slope as “one of the best-documented landslides” he’s ever seen. “However, even with this comprehensive investigative effort,” he cautions, “there are no absolutes when come to the prediction of future landslide behavior.”