Pacific Northwest
Photo / Courtesy of Ben Leshchinsky

Coast weather may increase bluff erosion, landslides

This is the first in a series of stories about the environment brought by the News Guard from Lincoln City, Oregon.

The News Guard is providing a series of reports about the Oregon Coast environment.

Our first report, illustrated state officials concerns about increasing beach erosion.

In this second report, Steve Lundeberg, public information representatives at Oregon State University, outlines the erosion impact along the coast from extreme storms.

Unstable slopes on Oregon’s coastline could see a 30 percent jump in landslide movements if extreme storms become frequent enough to increase seacliff erosion by 10 percent, a new study by Oregon State University shows.

For many slope failures traversing Highway 101, these cliffs form the base of active slides that already move a little every year, said the study’s corresponding author, Ben Leshchinsky, a forest engineering and civil engineering researcher at OSU.

The findings are especially impactful for slides along the 360-mile-long coastline that are highly susceptible to “toe erosion” – the removal of buttressing material at the base via waves or river scour.

“The really big slides, the monstrous landslides that span hundreds of acres, are so large and driven by water that their instability is less dominated by erosion,” Leshchinsky said. “But the ones that are highly exposed to erosion from the ocean, if we see increased extreme events like storm surge, then we’ll see increased bluff erosion. These creeping landslides that are already active and moving every year, they’ll move a lot more. If erosion increases 10 percent, slide movements might increase by 20 or 30 percent.”

Also known as slow earthflows, the slides are prone to gradual movement, the result of being marginally stable to begin with, coupled with seasonal changes in the water pressure within their soil.

Fluctuations in groundwater levels are primarily what drive changes in pore pressure and in turn landslide movement, but undercutting and “toe retreat” also play key roles, Leshchinsky said. As a bluff’s toe erodes, the soil in the bluff shifts gradually – sometimes ending in collapses that can topple houses and bury roads such as Highway 101, which runs the length of the Oregon coast.

“What’s going on may often be imperceptible until a landslide begins to compromise infrastructure like underground utilities or roads,” he said.

Leshchinsky and collaborators developed a model to determine the relationship between progressive landslide movement and slope geometry, undercutting processes and hydrological changes. The model showed agreement with data gathered at three monitored slide sites along Oregon’s shoreline. Read full article.