Coastal landslides vs. our roads: Who wins?

With “world-class landslides,” Oregon is fertile ground for learning

Animal bones help date the Hooskanaden Slide
A buried tree and a buried animal helped Geologist Curran Mohney date the Hooskanaden Slide to the mid 1700s.

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 one of those folks. He’s 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.

“Primarily what I want to know is how much time we have left for our highways in certain areas,” he explained. “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.

Curran Mohney stands among trees
Geologist Curran Mohney stands among trees being split by a slow-moving landslide.

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, 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.”

Two of the slides Mohney and his team are 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’s not unexpected.

“The deep monitors are generally viewed as sacrificial anyway; it just happened sooner than we thought because the landslides moved so fast,” he said. “We still have surface change detection using UAV (unmanned aerial vehicles, or drones) and ground-based Lidar.”

This newer technology allows the team to pick up where the in-ground instruments leave off when they are sheared.

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.

In March 2019, the slide moved across U.S. 1010, disrupting traffic for several months. The ancient slide is about 270 years old, according to radiocarbon dating.

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.”

Another way to use the information is to age-date a slide and learn from it (see below for information on Hooskanaden). 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 the students have a really good time with it. 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 grad 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.