Debris slide risk doesn’t always rise after a wildfire, study finds

(Press-News.org) In the wake of a wildfire, there’s often an assumption that burned landscapes will be more susceptible to landslides. But new research from the University of Oregon suggests it’s not always that simple.  

An analysis of the Columbia River Gorge, which runs along the border between Oregon and Washington, shows that steep, rocky watersheds in that area have been prone to debris flows and rockfall for thousands of years. Those events didn’t measurably increase after the Eagle Creek Fire, which scorched 47,000 acres of the gorge over the course of three months in 2017. 

UO geologist Josh Roering and members of his lab published their findings Aug. 8 in Science Advances, highlighting the importance of context and geological history in landslide risk assessments. The study also could help inform safety and hazards awareness projects in the gorge, in both burned and nonburned areas.  

After the Eagle Creek Fire, Oregon land managers were concerned about landslides, especially in the vicinity of the Interstate 84 transportation corridor that runs through the gorge. Their fears were, in large part, informed by what’s transpired in places like Southern California, where post-fire slides have caused devastating casualties and millions of dollars in damage.  

That phenomenon can happen because as wildfire destroys vegetation and groundcover, slopes become more prone to debris movement, erosion and rock fall, Roering said, which can be more easily triggered by rain and storm events.  

“When Eagle Creek burned up such a massive area of the Columbia River Gorge, the natural question was: Is that going to happen here?” Roering said. “The gorge provided a great laboratory to examine how fire affects steep and rocky landscapes.” 

In his lab’s latest project, Roering and doctoral student Maryn Sanders analyzed recent debris flows in the gorge to better understand the likelihood of slope movement after a fire and to explore how to predict when debris flows will occur. Debris flows occur when loose sediment — like mud, rocks and other debris — rapidly moves down a slope, often fueled by a storm or heavy rain.  

Sanders and her team turned to a remote-sensing technology known as airborne lidar, or light detection and ranging, which allows them to see through the tree cover so they can analyze physical changes on the ground below, like where erosion has occurred. That tool, alongside field observations, helped them map out debris flows so they could assess movement across the study area. 

As Sanders mapped the data, she found that many debris flows were concentrated in the watersheds near Dodson, just a few miles east of Multnomah Falls on the Oregon side of the gorge. Those are some of the steepest and fastest eroding watersheds in the state.  

The debris flows in that region have been especially frequent and destructive. They’ve caused fatalities and threatened additional human lives, homes and infrastructure, which make them even more vital for state agencies to understand. 

Sanders noticed a few interesting characteristics of the landscape as she studied the data, which suggested fire might not be the most significant cause of slope movement in that area. It also hinted that steep, rocky terrain behaves differently than slopes in a place like Southern California.    

The researchers found massive amounts of sediment accumulation in fan-like formations at the base of the rocky catchments in gorge watersheds. At first glance, those features looked unassuming because they were covered in vegetation, but with lidar imaging it was clear something more notable was going on beneath the surface. 

“The size and makeup of the fans suggest that frequent debris flows have been happening in these watersheds for a really long period of time, in the magnitude of thousands of years,” Sanders said. 

She also observed that the slopes were collecting sediment much faster than more stable terrain does, likely through temperature fluctuations that cause rockfall. That sets them up to produce debris flows more frequently, typically every few decades.  

Sanders took a closer look and analyzed the erosion rates in the area. She found frequent debris flows throughout its geological history and saw that the landscape had behaved in a consistent manner over thousands of years, something that remained relatively unchanged after the 2017 fire.  

“Because we found similar rates of erosion before and after the fire, we believe the rocky environment was not as sensitive to fire,” she said. “Our analysis suggests that fire plays a relatively small role in triggering these events and emphasizes how important it is to consider the history of place.” 

Still, the frequency, size and nature of debris flows in the gorge remains an ongoing cause for concern. The researchers are in the final stages of developing a tool that could help the Oregon Department of Transportation and other stakeholders predict debris flows in the gorge. That would help them make better use of safety features like roadside warning signs and closures, alerting travelers about the heightened risk of landslides during intense storms.  

“These watersheds are highly active and inherently hazardous, irrespective of fire,” Sanders said. “We want our research to help agencies like ODOT better understand this geologically-complex landscape."

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