Where the Coastal Landslides Meet the Road


With “world-class landslides,” Oregon is fertile ground for learning how to handle this geologic phenomenon

STATEWIDE – Landslides are a way of life in Oregon. Unfortunately, the chances of them occurring have risen after the devastating wildfires we experienced this year. Aside from disasters, however, landslides are always front and center for some people.

Curran Mohney is a senior engineering geologist in ODOT’s Statewide Project Delivery group, where he analyzes thousands – or more – pieces of data regularly to try to get a handle on the complex situation.

ODOT Senior Engineering Geologist Curran Mohney stands among trees being split by a slow-moving landslide. Photo courtesy of ODOT

“Primarily what I want to know is how much time we have left for our highways in certain areas,” explained Mohney. “What’s the life span of our highways on the coast and in our stressed areas? How fast are landslides accelerating, especially with climate change drivers? How long until we lose that battle?”

Because, Mohney said, Mother Nature will ultimately win.

The value of monitoring

One project Mohney directs is in year 4 of its 7-year life and involves monitoring five landslide sites along our coastal highways. In January 2017, Mohney, along with students and professors from Portland State and Oregon State universities, installed sensors and monitors so data collection could begin.

“It’s coming along really, really well – almost better than I thought it would,” Mohney said. “We have some really smart kids, young and enthusiastic, and several very experienced professors; I couldn’t have asked for better.”

One of those students presented an update at last March’s Northwest Transportation Conference. Andrew Senogles from OSU said they’re gathering valuable information every day.

“We can use the data for deciding where and when we do our repairs, and it will help us prioritize,” he said. “We can also look forward. For example, if we’re thinking of moving a roadway, we’ll know more about the risks.”

Learning quickly in geologic time

Two of the slides Mohney is monitoring have had serious movement in the past several years: Arizona and Hooskanaden (see up-close photos on ODOT’s Flickr site). In both, the team lost instruments, but that is not unexpected.

“The deep monitors are generally viewed as sacrificial anyway; it just happened sooner than we thought because the landslides moved so fast,” Mohney said. Because these instruments are installed in active landslides, they will always be sheared off by the slide movement at some point.

“We still have surface change detection using UAV (unmanned aerial vehicles, or drones) and ground-based Lidar.” This newer technology allows Mohney and team to pick up where the in-ground instruments leave off when they are sheared.

A buried tree and a buried animal (pictured here) helped ODOT Senior Engineering Geologist Curran Mohney date the Hooskanaden Slide to around the mid-1700’s – near the time of the last Cascadia Subduction Zone earthquake. Photo courtesy of ODOT

Because things usually move slowly from a geologic perspective – like at the millions-of-years pace – Mohney said they actually “got lucky” with Arizona and Hooskanaden in that they caught them having episodes of rapid movement.

“That in itself is pretty cool; we were able to monitor the movement in real time,” he said.

Predicting slides is one of the potential benefits of this project, even though it’s extremely difficult to do with any degree of reliability. Senogles, the OSU student, said it’s still worth trying because the more data you collect, the more accurate you can be.

“We’re learning things like how a slide moves and at what rate,” he said.

Using the knowledge

Mohney said the project is increasing knowledge that will benefit the state in many ways.

“It’s telling us things about how and why landslides happen in certain places,” he said. “Just imagining what our issues are going to be with climate change and Cascadia (the Cascadia Subduction Zone Earthquake) – it seems insurmountable. So if we can figure out anything about where, why, how, then we can be prepared. Maybe we can go out ahead of time and make smart decisions.”

Senogles admitted that even with the monitoring, they couldn’t have predicted the recent pulse of movement on the Hooskanaden slide. The learning opportunities continue, however: the team is currently monitoring movement in the Arch Cape and Silver Point landslides using their high resolution instruments. Why is this important?

Because we can use the knowledge we’re gaining for practical purposes, according to Mohney. Geologists agree that climate change is going to make conditions more likely for devastating events, but how much worse and where are critical unknown factors.

“We can clearly see, however, that in some places, it doesn’t make sense to spend money. It might be better, for example, to change the alignment of a road instead of battling the inevitable.”

Another way to use the information is to age-date a slide and learn from it. Every landslide has elements that indicate its approximate age: its shape and radiocarbon dating of buried animal bones and plant matter, for example. Depending on what you find, you can learn whether a landslide occurred because of seismic events or, for example, just from heavy rains.

“Learning about the age and the causes of slides can help us make better decisions about our seismic lifelines or things we need to do to adapt to climate change impacts,” Mohney said.

And there’s no shortage of work to be done. In the last 16 years, ODOT has collected information about more than 4,000 landslides on the transportation system – and that’s with only about 50% of highways surveyed.

 “We have world-class landslides in Oregon, and OSU, for one, is attracting top-notch people in their programs. It’s real plum stuff for graduate students to work on.”

How old is Hooskanaden?

Mohney and team used buried material, including a tree trunk, to radiocarbon date the Hooskanaden Slide on the south coast and found that it occurred about 270 years ago – around 30 years younger than the last Cascadia Subduction Zone earthquake. He said that means if the slide wasn’t directly initiated by the Cascadia event, it was certainly affected by it. The fun part? They’re not sure if that buried tree was from the bottom, bottom of the slide or perhaps just the bottom of a younger pulse from the slide.

About Author

Shelley Snow is the Strategic Communications Coordinator with Oregon Department of Transportation

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