One year later: Rattlesnake Ridge landslide moving slower than ever

UNION GAP, Wash. - About a year ago, a local pilot noticed a big crack running through Rattlesnake Ridge, which towers over the busy Interstate 82 just south of Union Gap.

Many grew panicked when they learned the crack was spreading because the hillside was sliding at a steady rate. Drone footage released in January 2018, which showed the significant progress of the slide in a matter of a few months, convinced a number of people that a disaster seemed imminent.

Some 50 people living at the base of the ridge evacuated for about a month, and many locals speculated that it was only a matter of time before the hillside failed and came crashing down onto the freeway that connects Yakima to the Tri-Cities.

A lot of that fear came from the fact that so much was unknown about the slide, even to the experts monitoring it, says Horrace Ward, senior emergency planner for the Yakima County Office of Emergency Management.



The data observed since then indicates that the landslide will stabilize itself, Ward says, adding that the slide has been moving southerly – not toward the freeway, but toward a nearby quarry – since monitoring began last year.  

The YVOEM started monitoring Rattlesnake Ridge on Oct. 26, and it took several weeks to figure out that the 20-acre mass was actually sliding.

Dozens of state and local agencies joined the effort to predict what would happen next – which proved to be difficult based on the varying scientific reports that came out that winter. Erring on the side of caution, these agencies informed the public that they were preparing for a worst-case scenario. 

They covered the ridge with monitoring instruments and found that the slide reached its peak speed in January, when it moved an average of 1.6 to 1.7 feet per week. As months rolled by, this rate of movement continued, says Ward. 

In February, evacuations were lifted and residents moved back into their homes at the base of the ridge. By May, the landslide began a trend of deceleration that has continued through today. 

Nonetheless, the Rattlesnake Ridge landslide is constantly being monitored with seismometers, a GPS system, LIDAR and other laser-based technology.

Ward said the event has proven to be a good lesson about getting more information out to the public when it’s available. He says the public will certainly be informed of significant developments with the slide as he becomes aware of them. 

Click here for the latest updates from the Yakima Valley Office of Emergency Management. 

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