Debris Slide Risk After Wildfires: New Study Reveals
Columbia River Gorge Debris Slide Risk Not Always Higher Post-Fire, Study Finds
PORTLAND, Ore. – Intense rainfall often triggers debris flows in the Columbia River Gorge, but a new study suggests fire’s impact on these events may be less important than previously thought. Researchers from Portland State university, utilizing advanced remote sensing technology, have uncovered evidence that the region’s steep, rocky terrain is predisposed to frequent debris flows irrespective of recent wildfires.
The 2017 Eagle Creek fire brought renewed attention to landslide hazards in the Gorge, prompting concerns that burned areas would experience increased debris flow activity. However, the research team, led by PSU geologist Amanda Sanders, found that many of the most active watersheds were already prone to sediment movement long before the fire.
“These watersheds are highly active and inherently hazardous, irrespective of fire,” Sanders explained. “We wont our research to help agencies like ODOT better understand this geologically-complex landscape.”
To investigate, Sanders and her team employed airborne lidar – a remote-sensing technology that uses laser pulses to create detailed 3D maps of the terrain, even beneath dense tree cover. This allowed them to analyze physical changes on the ground,pinpointing areas of erosion and mapping debris flow paths. The data revealed a concentration of activity in watersheds near Dodson, Oregon, east of Multnomah Falls, characterized by exceptionally steep and rapidly eroding slopes.
The team discovered massive sediment accumulations in fan-shaped formations at the base of rocky catchments. While appearing stable due to vegetation cover, lidar imaging revealed a history of frequent debris flows spanning thousands of years. The slopes also accumulate sediment at a faster rate than more stable terrain, likely due to temperature fluctuations causing rockfall. This process contributes to debris flows occurring roughly every few decades.
“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.
Crucially, the researchers found erosion rates were similar before and after the 2017 fire, indicating the rocky habitat wasn’t particularly sensitive to the burn. This suggests fire plays a relatively small role in triggering these events,and that understanding the area’s geological history is paramount.
The findings have significant implications for hazard mitigation. Sanders’ team is currently finalizing a tool to help the Oregon Department of Transportation (ODOT) and other stakeholders predict debris flows. This predictive capability will enable more effective use of safety measures, such as roadside warning signs and temporary road closures, during periods of heavy rain.
While the study offers valuable insights, the inherent hazards of the Columbia River gorge remain a serious concern. The ongoing research aims to provide a more thorough understanding of this dynamic landscape and improve safety for residents and travelers alike.