16 October 2009

The Naches / Nile River landslide and the role of the quarry

Posted by Dave Petley

Spectacular image of the Naches / Nile River landslide, from the Washington State DNR Flickr site
Hat tip to Andrew Giles for pointing this out to me.

The Seattle Times have run an article today about the role of the quarry in the triggering of the Naches landslide. They quite rightly point out that there is no firm evidence either way as to whether the quarry played a role or not, but the do make some interesting comments about the possible role:

“Washington’s Department of Natural Resources warned a Naches, Yakima County, gravel pit four years ago that its operations might be destabilizing a portion of the slope that collapsed onto Highway 410 this week. Records from 2005 show a department geologist noticed a 10-foot-wide fissure between the towering basalt cliffs and a broad talus slope below. The gravel mine appeared to have removed deposits that were buttressing the slope, documents say. “Your surface mining activity may be exacerbating slope instability and, therefore, may be creating a potential hazard to adjacent property and danger to the public health, safety (and) welfare,” says a notice issued to the mine owners in September 2005.”

The article quotes Prof. David Montgomery from the University of Washington, who is undeniably a world class academic geologist:

“It’s definitely premature to rule out the gravel pit as a contributing factor…They were definitely digging at the toe of an active landslide, and that’s a recipe for a slide.”

The article then goes on to note that the Department of Natural Resources (DNR) wrote a letter dated July 2008 that noted that the agency still had not received a monitoring plan.

The image above clearly shows that this is primarily a slump/rotational failure as outlined in my previous post. This is rather beautifully illustrated by the image below, also from Washington State DNR Flickr site. The back-tilting of the trees are a sure-fire sign that the slope has rotated:

There is undoubtedly some translational movement too, as would be expected in such a large failure.