Landslide risk doesn’t always rise after a wildfire, Columbia River Gorge study finds

Following a forest fire, there is often the hypothesis that the burned landscapes will be more sensitive to landslides. But new research from the University of Oregon suggest that it is not always as simple.

An analysis of the Gorges de la Rivière Columbia, which runs along the border between Oregon and Washington, shows that the rocky rocky watersheds in this area have been subject to debris and rock falls for thousands of years. These events did not increase measurablely after the Fire of Eagle Creek, which burned 47,000 acres of throat in three months in 2017.

The geologist Uo Josh Roering and the members of his laboratory published their conclusions in Scientific advancesemphasizing the importance of the context and geological history in the assessments of the risks of landslide. The study could also help to shed light on safety and risk awareness projects in gorges, in burned and unruly areas.

After the fire of Eagle Creek, the land managers of Oregon were concerned with the landslides, in particular near the interstate transport corridor 84 which crosses the gorge. Their fears were largely informed by what happened in places like southern California, where the slides after the fire caused devastating losses and millions of dollars of damage.

This phenomenon can occur because, while forest fires destroy vegetation and soil cover, the slopes become more prone to the movement of debris, erosion and falling rocks, said Roering, which can be more easily triggered by rain and storm events.

“When Eagle Creek burned such a massive area of the Gorges de la Rivière Columbia, the natural question was: will that happen here?” Said Roering. “The gorges provided an excellent laboratory to examine how the fire affects steep and rocky landscapes.”

In the latest laboratory project, Roering and doctoral student Maryn Sanders analyzed recent debris in his throat to better understand the probability of a slope movement after a fire and to explore how to predict when the debris flows occur. Debris flows occur when loose sediments – such as mud, rocks and other debris – move on a slope, often powered by a storm or a high rain.

Sanders and his team turned to a distant detection technology known as Lidar Aéroporté, or detection and ranging from light, which allows them to see through the cover of the tree so that they can analyze physical changes on the ground below, like where erosion has occurred.

This tool, alongside the field observations, helped them to draw the flow of debris so that they can assess the movements in the study area.

While Sanders mapped the data, she found that many flow of debris was concentrated in the watersheds near Dodson, a few kilometers east of Multnomah falls on the side of the throat Oregon. These are among the steepest and fastest state watersheds in the state.

The flow of debris in this region were particularly frequent and destructive. They caused deaths and threatened from human lives, houses and additional infrastructure, which makes them even more vital for state agencies.

Sanders noticed some interesting characteristics of the landscape when she studied data, which suggests that fire may not be the most important cause of the movement of the slope in this area. He also suggested that the rocky rocky terrain behaves differently from the slopes in a place like southern California.

The researchers found massive quantities of accumulation of sediment in fan -type formations at the base of rocky watersheds in the watersheds. At first glance, these characteristics seemed unpretentious because they were covered with vegetation, but with Lidar imagery, it was clear that something more notable was going under the surface.

“The size and composition of the fans suggest that frequent debris flows occur in these watersheds for a very long time, in the magnitude of thousands of years,” said Sanders.

She also observed that the slopes collected the sediments much faster than the more stable terrain, probably by temperature fluctuations which cause the fall of rock. This prepares them to produce debris more frequently, generally every few decades.

Sanders examined more closely and analyzed erosion rates in the region. It found frequent debris flows throughout its geological history and saw that the landscape had behaved in a coherent manner over thousands of years, which remained relatively unchanged after the fire of 2017.

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

However, the frequency, size and nature of debris flow in the gorges remain a continuous concern. Researchers are in the latest stages of the development of a tool that could help the Ministry of Transport of Oregon and other stakeholders predicting debris in their throat.

This would help them better use the safety features such as road warning panels and closings, alerting travelers of the increased risk of landslides during intense storms.

“These watersheds are very active and intrinsically dangerous, whatever the fire,” said Sanders. “We want our research to help agencies like Odot to better understand this geologically complex landscape.”