In the Land of Landslides

Officials hope to avoid another Niles-type Landslide 45 minutes south at Rattlesnake Ridge.

Niles Wa., 2009

In 2005, the State Department of Natural Resources (DNR) inspected a quarry in the southeast foothills of the Cascade Mountains in Washington State and told them that they were digging at the toe of a landslide. What’s a toe of a landslide? To borrow a metaphor from the History Link Files who tells this story: imagine you dump gravel down a flight of stairs. You start digging where the pile has stopped–the toe–a step or two before the ground. If you dig enough, more of the pile will slide down from above.

Essentially, geologically ages ago, the south side of Cleman mountain had slid into the Naches River Valley in a cataclysmic landslide 6 miles long creating the Sanford Pasteur Formation. Much later, still thousands of years ago, the Nachez river had undercut an edge of this formation and another massive landslide covered the river in hundreds of feet of debris. Over the many  years, the debris had gradually eroded away creating a hillside which present day homeowners had developed. (Click through the slide show to see it.)

This slideshow requires JavaScript.

Until in 2005, the DNR asked a quarry operating there to submit a plan for monitoring the slope as it was on landslide territory. In 2007, the quarry did it’s own investigation of the slope which concluded that the quarry operations were too small to be exasperating slope instability.

At 6 am, October 11, 2009, the slope gave way for a third time to the largest landslide in recorded Washington State history. As History Link describes:

It occurs near the small Yakima County community of Nile located in the eastern foothills of the Cascades about 10 miles northwest of Naches, near Cleman Mountain. State Route 410 travels west from Yakima through the Nile Valley and across the Cascades at Chinook Pass. The landslide lifts this road and breaks it into huge slabs of asphalt scattered every which way. It lifts the river, leaving rainbow trout dying among high rocks that used to be the riverbed.  Five homes are damaged or destroyed in the landslide and another 20 are damaged by flooding as the river finds its new way around rock and debris some 40 feet thick. The slide covers 80 acres, taking several power poles; as a precaution Pacific Power cuts service to about 800 customers. Residents of the sparsely populated area are evacuated. Factors causing the landslide are speculated to be the action of the Naches River undercutting the steep slope, the slippery geological situation of a layer of basalt sliding over a deeper layer of sand, and the activities of a gravel quarry engaged in undercutting the slope.

Thankfully, no one was injured, but the landslide permanently altered the course of the Naches river disrupting fisheries, flooding 20 homes, changing bridges and roads, and nearly destroying Yakima’s water treatment plant. ( Read more about the massive project to build a new road and a new river channel here. Cool diagrams and pictures.)

 

Rattlesnake Ridge, Wa., 2018

01042018_yakima_0914042-780x990

Almost exactly 8 years later, October 2017, and only 45 minutes south,  a crack was discovered on the top corner of a hill called Rattlesnake Ridge above a quarry near I 82. The quarry moved operations away from the slope and hired a geologist to monitor it.

Presently, the fissure is about 250 feet deep, though geologists believe that the basalt bedrock has not cracked (which is good news; only surface dirt will sluff off if a landslide occurs). The main fissure is growing at about 2.5 inches a day or about 1.5 feet per week, and gaining momentum. The Red Cross is calling it a slow landslide.

This drone footage courtesy of geologist Steven Mack (for the Yakima Herald) is from about a week ago. It gives very good establishing and close up shots of the slope.

gfx-2_1515538942989_10438003_ver1-0_640_360
courtesy of Kiro 7 with an excellent report here

Geologists believe that the landslide will continue to fall slowly south into the quarry and stabilize, though it is possible that a million cubic feet of dirt will fall southwest onto Thorpe road and parallel I-82–farther if the bedrock is indeed broken. The 70 or so residents nearby have been evacuated and the quarry owners are offering to pay for hotel accommodations. No one wants another Nile-type slide which trapped Nile residents for ten days while an emergency access road was being built.

Meanwhile, the most likely threat is anticipated to be rock falling on the road. To that end, the Department of Transportation (DOT) has placed a wall of shipping containers filled with cement barriers along the shoulder of I-82. It won’t save the road from a landslide, but it will help with falling rocks.

01042018_yakima_091404-780x520
courtesy of the Yakima herald

So this won’t be another Oso slide?

How did we get this far without discussing the Oso, Washington landslide in 2014 which killed 43 people? Because it’s a very different scenario from both the Nile landslide and the Rattlesnake Ridge (potential) landslide. From local news investigative report, Kiro 7:

After speaking with experts, Washington state leaders are confident that the Rattlesnake Ridge landslide is very different from the deadly Oso landslide that took 43 lives nearly three years ago.

Geologists explained to KIRO 7 that Oso was mud while Rattlesnake Ridge is consolidated rock on the move.

Also, the OSO slide was affected by rainfall. Water does not appear to be a factor in the Rattlesnake Ridge landslide.

“And (with) this one we have more time to prepare,” Public Lands Commissioner Hilary Franz told reporters. “And understand what’s going on and respond to it.”

Interestingly (as the report continues), the Rattlesnake Ridge fissure is visible in aerial photos as far back as the 1970s. Though it’s too soon to tell, geologists are speculating that the cause of the cracks may be similar to the Nile’s landslide situation: a “reactivation” of a much older landslide caused by gravel sitting on basalt sitting on sand being pushed by the Cascade faults.

We’re keeping an eye on this slow moving landslide for you. Stay tuned for developments.

Further Reading

Advertisements

What do YOU think?

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s