After the fire comes the mud

Delayed Disasters

This belated disaster came as no surprise to University of Oregon-Eugene geologist Joshua Roering. He has been studying landslides in the Pacific Northwest for nearly two decades. Among other things, Roering’s group investigated the extent to which the soil here behaves differently after a fire than in drier areas. As their work shows, the water-repellent layer on the ground after a forest fire in the Pacific Northwest is not as continuous as it is further south.

“It doesn’t matter how you kill a tree”(Joshua Roering, University of Oregon)

In extreme rainfall after a fire, for example, water can penetrate the ground. “The fact that several severe storms in the region have not triggered debris flows suggests that the hydrological system behaves differently here than in southern California,” says Roering. But that does not mean that there is no danger after a forest fire in the northwest.

Rather, the risk of mudslides, which researchers also observe after the timber harvest, is delayed. When a tree is felled, its roots decompose over a period of years. The more the trees lose their footing, the higher the risk that the slope will slide down when it rains. “It doesn’t matter how you kill a tree,” says Roering. “In three, four or five years, the root systems in the upper meters of the soil lose about 90 percent of their strength.”

If this description of the landslide danger in the forested Pacific Northwest is confirmed, researchers and emergency managers would have to include in their forecasts those storms that occur several years after a forest fire. “Then we’re sitting on a ticking time bomb, so to speak, in terms of badly burned areas with steep slopes over major transport corridors and highways,” says Roering.

Houses cannot run away

In contrast, researchers do not expect the same delay for the drier central region of British Columbia, which has a different soil structure than the coastal regions of the Pacific Northwest. “We have to develop our own model for British Columbia,” says Jakob. “But we need a lot more data for that.”

In Australia, too, researchers are studying the changing patterns of fires and debris flows. In the southeast of the continent where Gary Sheridan works, wildfires are commonplace. However, they are now also found in the humid, alpine areas of Tasmania. Instead of developing a completely new landslide model, Sheridan’s group is trying to refine their models with the data from these fires and then assess the predictive power of the debris flows that occur. According to Sheridan, a difficult and lengthy process.

“Forest fires can have a wide range of consequences, from almost no change in the landscape to catastrophic mudslides,” says Sheridan. The international landslide research community is currently discussing a number of common factors that could be incorporated into a universal prediction model. However, we are still a long way from such a model. Landscape, climate and the behavior of the soil are simply too different from place to place: »The risk is different in every place. Still don’t know why.”

Models of such landslides help save lives. It becomes more difficult if you also want to protect buildings or transport networks. “British Columbia simply doesn’t have the money to protect roads, railway lines or pipelines from every single slope that could trigger a debris flow,” says Jakob. “Although early warning systems can prevent people from being harmed, infrastructure is still damaged or destroyed.”

Data is still missing

Improved models could be used in the future to identify roads or pipelines that are at risk. Jakob suspects that this would allow authorities to use their protective measures in a more targeted manner. Homeowners could also limit the risks to their property and life with the help of landslide models. “In view of the dramatic and rapid climate change, many people may not even know that they are at risk from landslides after fires,” says Jakob. However, such efforts are not without controversy. Politicians and contractors have been known to oppose the creation of landslide hazard maps, which they see as an impediment to growth and income.

So landslide researchers need time and more storms to optimize their post-fire landslide predictions. For this winter storm season, many have already installed instruments on slopes burned in California’s 2021 fires, including in the region of the Caldor and Dixie fires — the largest single fires in state history. Kean hopes that the network of measurements will provide insight into which precipitation intensities trigger debris flows and which do not, how much water seeps into the ground, how much runs off, and many other things that will help improve current models or help develop region-specific models .

“The biggest challenge is the lack of resources relative to the extent of the devastation caused by the wildfires,” says Kean. The Dixie Fire alone burned almost 400,000 hectares. But you only have a limited number of instruments. This limits the ability to find out something about the behavior of debris flows in new regions.

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