A major crack discovered on the Rattlesnake Ridge in Yakima County, Washington in October 2017 is spreading some 6.3 cm (2.5 inches) per day, officials said. This slow-moving landslide is now threatening to dump as much as 765 500 m3 (1 million cubic yards) of rock down the ridge, but most of it is expected to be trapped in a quarry and not reach Interstate 82 or the Yakima River. Roughly 50 residents living near the base of the ridge have been warned to evacuate.
The area of concern is on the east side of the Union Gap, east of Thorp Road near the Columbia aka Anderson Quarry, officials said.
"With this type of incident, there is no definitive answer as to if or when significant movement of the slide will occur or how far the slide will travel. The geologic experts that have been monitoring this slide believe that since the slide is slow moving and on a gentle slope that the landslide event will be small in nature and hopefully stabilize itself," the Yakima Valley Office of Emergency Management said in a press release.
Rattlesnake Ridge crack on December 31, 2017. Credit: Steven Mack
Video courtesy KING5
It's unclear when the crack first formed but officials are monitoring it since October and have started creating what will eventually be a 180 meters (600 feet) of concrete-filled shipping containers between the base of the ridge at the Thorp Road and nearby Interstate 82. The containers are intended to defend the interstate against moderate rockfall, officials said, but won't protect from a major landslide.
According to George Machan, a senior associate geotechnical engineer with Cornforth Consultants, the landslide is projected to occur sometime between now and the end of February 2018.
Video courtesy KING5
Machan said there is an extremely remote possibility that some of the estimated 380 000 – 765 000 m3 (500 000 – 1 million cubic yards) of material is going to get into the river. Instead, most of it is expected to fall into the quarry pit and stabilize.
However, as the slow-moving slide, which is moving at an average of 6.3 cm (2.5 inches) per day, the surrounding areas could see increased rockfall, he said.
Emergency Management is preparing for a variety of different case scenarios, the Yakima Valley OEM said.
Thorp Road between Birchfield Road and Gangl Road will remain closed until further notice, it said. "The homes along Thorp Road south of the Quarry have been put on a Level 3 – Recommended evacuation. I-82 is still open but WSDOT is monitoring the area for rockfall but so far has not seen any. If WSDOT feels that the Intestate has become unsafe they will close it. In the meantime, they have placed signs along the highway alerting drivers of the potential for rockfall."
Featured image: Rattlesnake Ridge crack on December 31, 2017. Credit: Steven Mack
If you value what we do here, create your ad-free account and support our journalism.
Your support makes a difference
Dear valued reader,
We hope that our website has been a valuable resource for you.
The reality is that it takes a lot of time, effort, and resources to maintain and grow this website. We rely on the support of readers like you to keep providing high-quality content.
If you have found our website to be helpful, please consider making a contribution to help us continue to bring you the information you need. Your support means the world to us and helps us to keep doing what we love.
Support us by choosing your support level – Silver, Gold or Platinum.
Other support options include Patreon pledges, one-off payments using PayPal and purchasing products from our webshop.
Thank you for your consideration. Your support is greatly appreciated.