Landslides After Wildfires Expected Yearly in Southern California

After wildfires have devastated a landscape, stripping trees, roots, and vegetation from the ground, hillsides become less stable. Then when it rains, the land can shift and slide with little warning, wiping out homes and damaging everything in its path.

Post-wildfire landslides are now likely to happen almost every year in Southern California and the area can expect major landslides every 10 to 13 years, a new study finds.

Climate change causes changes in the state’s wet and dry seasons which increases rainfall, according to the results, published by researchers from the U.S. Geological Survey (USGS) in the journal Advancing Earth and Space Science.

Study lead author Jason Kean, a hydrologist at the USGS in Denver, conducts rapid debris-flow hazard assessments after wildfires across the West. A debris flow is a fast-moving type of landslide. These analyses are used to assess risk and to develop emergency response plans.

“Over the years we've seen how challenging it can be to develop these plans in the short time window between the fire and first rainstorm. In the worst case, the rainstorm that puts out the fire is the rainstorm that triggers the debris flow,” Kean tells Treehugger. “This time squeeze has pushed us to think about evaluating these hazards before a fire even breaks out. We do this using what-if wildfire and rainstorm scenarios.”

He explains that it’s the same idea that earthquake scientists use. They don’t know exactly when one will occur, but they’ve mapped out where, how big, and how frequent they will be, and those maps are critical when creating emergency response plans.

“Here, we're trying to do the same thing for debris flows after wildfire. It represents a shift in thinking from being purely reactive to wildfires to being proactive in planning for their inevitability.”

Predicting Landslides

For the study, researchers combined fire, rainfall, and landslide data with computer simulations to forecast where landslides are likely to occur after wildfires in Southern California. They predicted how big those landslides might be and how frequently they might happen.

The results showed that small landslides can now be expected to occur nearly every year in Southern California. Major landslides capable of damaging 40 structures or more can be expected every 10 to 13 years. That’s about as often as magnitude 6.7 earthquakes occur in California, according to the study.

“It doesn't take a particularly intense rainstorm to trigger a debris flow in a burned watershed with steep slopes. It's the kind of rainfall you experience when you're driving in a storm and you have to put your windshield wipers on high,” Kean says. “That's heavy rain, but that level of rain that happens at least every year, if not more than once a year in Southern California.”

As more intense rainfall is expected in the coming years, landslides could be even more frequent.

Wildfires make steep slopes and hillsides more susceptible to landslides for two reasons. The soil is easier to erode because the fire removes the vegetation and other organic material than normally protects and stabilizes it, Kean says.

The heat from the fire also can make the soil repel water.

“Water from rainstorms does not get absorbed by the soil in the usual manner. Instead, it beads up on the surface and runs off,” Kean says. “The rapid runoff quickly entrains easily erodible sediment and becomes a slurry that continues to grow downstream, picking up boulders along the way.”

Landslides happen in areas that haven’t been burned too, Kean points out, but it takes much less rain to make one after a fire than it does in an area without a fire.

Time for a Response Plan

There is often not much time between wildfires and the following storms. In Southern California, fall is the busiest time for wildfires, while winter is the rainy season. That can leave a few months to prepare or even less.

“For example, late-season fires in December in Southern California can sometimes be extinguished by the onset of winter rains. Fortunately, federal, state, and local emergency response teams start evaluating post-fire risk as soon as they can, even before the fire is out,” Kean says. 

“But there is a lot to do, and there are often multiple fires burning at the same time, which stretch resources thin.  If we start planning for inevitable fires now, we can get a jump start on developing post-fire emergency response plans.”