As cracks widen in Rattlesnake Ridge, here's what to know about landslide risk

Two hours southeast of Seattle along Interstate 82, drivers will find the Rattlesnake Hills – with a fissure in the ridge so big it can be seen from the road on a clear day.

With the daunting crevice comes a slow-moving, 20-acre landslide that has gradually inched downward on Rattlesnake Ridge near Yakima for months.

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Geologists and engineers don't know exactly when it will happen or what the scale of the slide will be if it hits the base of the ridge. But they’ve conceived several scenarios of what could happen – with some alarming residents, drivers and emergency crews.

Here’s what experts know now about the fissure, the slide, and how leaders are preparing.

Where exactly is this going on?

Not to be confused with the popular hiking trail sharing the same name in North Bend, Rattlesnake Ridge is 12 miles south of Yakima on private land.

Rattlesnake Ridge stands beside Ahtanum Ridge, and between both ridges is the gap -- where nearly 54,000 vehicles a day travel on Interstate 82 and U.S. Highway 97. The massive chunk of land moves on the ridge above them.

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Those roads have not been closed yet, but traffic leaders shut down Thorp Road along the base of Rattlesnake Ridge when rocks started to fall onto it.

When will the slow-moving slide really give way?

No one knows for sure when a so-called "failure event" will happen. The initial timeframe was between January and March, but as data evolves the timeline changes.

According to DNR spokesman Joe Smillie, while the slide is still slow-moving, experts have stopped giving precise predictions of when an event may happen. Because the slide is no longer accelerating like it was earlier this month—it’s too hard for experts to give a concise timeline of when it may occur.

In the latest findings, an independent group of engineers picked by the state found that the landslide may creep for years rather than giving way all at once.

Experts are discussing whether to keep monitoring, but they have not come to a solution.




Where will the slide go?

Whenever this happens, there are many possibilities.

The most likely scenario:

The landslide will continue to slowly move to the south, where the landslide mass will fall into a quarry pit and accumulate. Most of the slide will stay in the pit, but some rocks are expected to fall around it -- possibly reaching Thorp Road. In this scenario the landslide likely doesn't hit I-82 or the Yakima River.

Unlikely scenario:

The landslide may move beyond the pit and reach I-883.

Very unlikely scenario:

The slide will run out beyond I-82 and reach the Yakima River.

Extremely unlikely scenario:

The landslide moves west and blocks I-82 and the Yakima River.




What caused these cracks and slow-moving slide?

An investigation has not been opened into what caused the cracks, because the focus right now is on preparing emergency responses.

What's being discussed about the cause currently speculative. Reports question the Anderson Quarry that's been operating since 1995 and collecting materials to make asphalt. Some wonder whether removing part of the ridge destabilized it, though Washington Department of Natural Resource can't confirm that with KIRO 7 News.

As far as how the crack even got there, state geologists say that the crack shows up in aerial photos as far back as the 1970s, and that it's enlarged over time. Although still speculative, the slide could be a re-activation of a much older slide; the cause of that older slide is also unknown.

As far as the geology-related factors? According to Washington DNR, the landslide consists of Columbia River Basalt flows sliding over a weaker sedimentary interbed, which is slopping toward the quarry at about 15 degrees.

When did the landslide start happening in this event?

A local pilot discovered cracks on the ridge in October. Mining operations stopped, and geologists and engineers installed monitoring instruments after learning about the 200-foot-deep fissure.




The cracks have been widening and new ones have developed since the October report —with a landslide mass with rocks and debris moving down the ridge.

The landslide is moving south at the rate of 1.6 feet a week. Since the slide is moving each day, it’s gaining momentum.

“The slide will increase in movement to the point where there will be a large movement of that mass,” said Horace Ward, senior emergency planner with the OEM.

What could change the scenarios?

Rainfall isn't driving the landslide, but a large snowfall and quick melt could speed it up – possibly leading up to a worst-case scenario.  
According to KIRO 7 News PinPoint Meteorologist Morgan Palmer, there's no large, heavy precipitation events are rather unlikely in the area late in the winter season.




"There’s a lot unknown about the slide, but the expectation is that only a rather large precipitation event would have any impact on the behavior of the slide,” Palmer said.

“This is very different from the Oso landslide, which consisted of waterlogged soil after a long period of above-normal precipitation. The area around Rattlesnake Ridge doesn’t receive but a fraction of precipitation of the wet areas on the west side of the Cascades, thus any impact from rain and snow is believed to only come from significant or extreme events.”

So this won’t be like, Oso?

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."

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An adjunct geology professor at Washington State University told The Yakima Herald that the movement seems similar to that which caused a massive landslide onto Highway 410 near Naches in 2009.

Five homes were damaged in the landslide and 20 were flooded as the Naches River went around rock and debris. No one was injured.

How much is expected to fall in the Rattlesnake slide?

It's estimated that the landslide volume measures at 1 million cubic yards of rock and soil. That covers an area of about 20 acres. Though, it's unclear how much of the rocks and debris will hit the base of the ridge and beyond.

Why are people evacuating?

Rocks – around the size of baseballs and basketballs – are already falling onto the road below. Nearly 50 people at the base of the hill, which is right in the landslide's path, are urged to leave immediately.  
Local firefighters and members of the Washington State Emergency Division knocked on doors in early January. At first some people in the community didn't believe the risk, but now most people urged to evacuate have done so.

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Emergency management leaders set up a Red Cross shelter in Yakima, and most people who have evacuated were given hotel rooms.

There's nothing crews can do to stop the slide, not even detonating the land

. So, their focus is minimizing damage and planning the response for after the landslide happens.

Why can't you blast it?

Simply, blasting an unstable hillside isn't safe.

A controlled blast requires crews drilling 20 to 40 feet into the hillside for live charges, but a slow-moving slide would not allow that to be done accurately.

Even if crews proceeded with blasting despite that risk, the explosion could make the hillside more unstable, according to Washington State Department of Transportation.

WSDOT gave this experience on its blog for context:

"For some perspective, we've spent the past several years blasting rock from the hillsides along I-90 as part of the project to stabilize rock slopes and add more lanes. When we blast, it's in very small sections and then we excavate any loose material after the blast. Over the past five years we've removed a little more than one million cubic yards of material – or about 200,000 cubic yards every construction season (April through October). The Rattlesnake Ridge slide is made up of about four million cubic yards of material."

How is Washington monitoring?

Nearly a dozen agencies have collaborated in deploying GPS monitoring stations and seismometers. The site is under 24-hour observation because if something changes, crews need to be able to respond on a minute-by-minute basis.




In addition to state and contracted scientists, Gov. Jay Inslee and Public Lands Commissioner Hillary Franz want to find third-party resources to further look into the slide.

“We want to get an independent review by an independent third-party geologist and we will do that to make sure that we have the ultimate degree of confidence in assessing the nature of this risk,” Inslee said.

"This is to try to give the public more confidence and bringing in other eyes," Inslee said. "But I will say, to me at least, we have very impressive monitoring through these multiple systems."

How is Washington preparing? 

Crews lined 600 feet of Thorp Road closest to I-82 with about large shipping containers filled with concrete, some brought to Yakima County from Seattle and Tacoma. Bulldozers filled each freight car with up to 14-tons tons of cement barriers to weigh the cars down.
The purpose is to stop debris falling off the hill from spilling onto the busy highway.

Representative Dan Newhouse (R-WA) urged Gov. Inslee to declare a state of emergency for Yakima County in response to the slide. The declaration is vital for potential in the event a federal response is needed.

The Yakima City Council on Jan. 17 approved a resolution for city officials to request state or federal assistance should they require help in the event of a disaster.

Will I-82 shutdown?

The Washington Department of Transportation could close I-82, forcing about 30,000 drivers who travel that stretch to use a 15-mile detour.




WSDOT says if the rocks coming off the hill get bigger – boulder or small vehicle sized – they will shut down the highway. It also says if geologists indicate the landslide is picking up significant speed, it will shut down the highway.

Will there be flooding?

Even though flooding is a worst-case scenario, and not likely, there is an evaluation for a potential risk if debris reaches the Yakima River and waterflows must find new ways to get around it.
"It is not predicted for this to happen, but if there was a flooding event in Union Gap, there are already emergency preparations to deal with any flooding event," Inslee said in a news conference.

Federal river managers met over the weekend to talk about how to mitigate flooding if landslide debris were to dam the river.
Chad Stuart, manager of the Yakima River for the U.S. Bureau of Reclamation, told National Public Radio that no one wants to see this situation come into the spring.

"The river is traditionally much higher that time of year," he said. "And we have a lot more unregulated flows due to melt-off and rain."

As scientists and emergency leaders monitor this situation, that could change at a moments notice, Washington Department of Natural resources is tweeting the latest information. Find their thread here.

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