Intense rain triggered mudslides in Glenwood Canyon, Colorado, during June and July last year — one year after a fire scorched the area. Credit: AP/Shutterstock

The devastating mudslides that follow forest fires

Jan 12, 2022 at 9:40am

Jane Palmer

The summer of 2021 brought ideal fire weather to southern British Columbia in Canada. A dome of hot, high-pressure air settled over the area, sending temperatures soaring into record territory after months of drought.

In early July, an average of 40 fires sprang up each day in the province — in what would become one of the region’s worst wildfire seasons ever recorded. Flames scorched the steep, tree-covered slopes above the valleys that are home to the Trans-Canada Highway and national railway lines, as well as oil and gas pipelines. The blaze raised concern about another imminent threat: landslides from the destabilized hills.

In mid-November, a massive storm known as an atmospheric river dumped a month’s worth of rain on the region in just two days. When the downpour hit the burnt, scarred slopes, it set off giant surges of mud and debris that swept across the highway and railway lines. “They severed several key transportation corridors in western Canada, which then meant critical supply-chain interruptions,” says Matthias Jakob, a geoscientist at BGC Engineering in Vancouver, Canada, who worked on assessing the potential for landslides in the region.

Although they might not be as deadly as hurricanes or heatwaves, landslides such as this can cost hundreds of millions of dollars in economic damage, Jakob says. And more are expected this winter as atmospheric rivers pummel the northwest of the United States and British Columbia, following a year of intense and widespread fires.

This one-two punch of fire and flood, is just a taste of what’s to come there, and in many other regions. Climate change is increasing the frequency and intensity of fires around the world, and it is also leading to more extreme bouts of precipitation1. When rain hits recently burnt hillsides it can trigger a particularly deadly type of landslide called a debris flow — a water-laden mass of soil, rocks and other matter that can surge downslope with devastating force.

In regions prone to these types of hazard, scientists have developed models that can feed into warning systems and save lives. But wildfires are now claiming hill slopes that have rarely been burnt before. This is happening around the world, from the boreal forests of Alaska and Canada to the mountains of Austria, where fires are burning more frequently and more intensely than ever before.

“There will be an increased likelihood of both fire and post-fire debris flows in areas where they are currently only a theoretical possibility,” says Bruce Malamud, a geophysicist at King’s College London.

The central region of British Columbia has always had wildfires but now the province is even seeing blazes in coastal areas. The models used to forecast inland debris flows simply wouldn’t work for these regions, where the soils and vegetation differ, Jakob says. It’s a similar scenario in the United States, where fires in the past few years have scorched areas of northern California, Oregon and Washington that rarely burn.

Evidence suggests that the debris-flow behaviour is going to be different in these wetter, more vegetated regions than in arid southwestern states, where fires and landslides are more common, says Jason Kean, a hydrologist at the United States Geological Survey (USGS) National Landslide Hazards Program in Golden, Colorado.

“While it’s kind of clear what’s happening in the drier areas, the picture gets murkier as we move north,” Kean says. “So right now, we are scrambling to collect data to figure out how well our current model works and how to make a better one.”

Continue reading the article from Nature here