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Increasing California Landslide Risk Prompts USGS Study

Experts suggest that as rainfall is more concentrated in California, more landslides will occur. A USGS study sheds light on dangerous debris flows and what could be done to reduce potential damage via mitigation.

The Montecito city sign along the 101 freeway on Jan. 15, 2018 in Montecito, Calif. (Katie Falkenberg/Los Angeles Times/TNS)
It’s been well documented that wildfires in California and elsewhere are becoming more frequent and more intense, and climatologists say the warming atmosphere assures that this is the new normal. That trend also means different patterns of precipitation — perhaps the same annual rainfall totals but more intense patterns of precipitation during certain periods.  

What this all adds up to is more wildfires followed by more landslides like the one that struck the city of Montecito in Santa Barbara County in January 2018. That event produced debris flows so robust they killed 23 people despite warnings.  

A recent study by the United States Geological Survey was aimed at shedding light on these debris flows and what could be done to reduce potential damage via mitigation efforts. Experts suggest that as rainfall is more concentrated, more landslides will occur than before and may be an annual event in California. 

“Climatologists are telling us that we might be expecting more intense precipitation and a longer burning season,” said Jason Kean, a hydrologist for USGS and part of a team that creates hazard maps of debris flow after a wildfire to give responders some direction. He is the lead author of the study.  

“That’s like a double bad combination for post-fire debris: You’re going to have the dry time period which provides time for the fire to ignite and then when it does rain it’s going to rain harder so all these things are signs in the wrong direction." 

Already in 2020, a landslide washed out a portion of Highway 1 near Big Sur on California's Central Coast after a severe rainstorm. The landslide closed 23 miles of highway, which will take weeks to months to repair.  

Gary Griggs, a coastal erosion expert at the University of California, Santa Cruz, said to expect more of the same. “As the climate continues to change, the weather will get warmer with more drought and more fires as we have experienced in recent years,” he wrote in an email. “Precipitation is expected to be more concentrated in the winter months, and when combined with the condition of the hillsides after major fires, debris flows and landslides are more than likely to increase in frequency and extent.”

Kean said even with competent response to the Montecito landslide, there was just not enough time to react as the fire was literally still burning when the storm hit. 

That, in part, is what the study attempts to address, getting ahead of the issue of landslides before they strike with intelligence about where they are likely to strike and mitigation efforts, including mapping for evacuation.  

Kean said the study suggests doing some of the mapping ahead of time and doing simulations to get an even better idea of how and where a mudslide will occur.  

“That [the Montecito slide] is the motivation behind this,” he said. “Why don’t we start looking ahead of wildfire scenarios because we know, at least in Southern California and a lot of other places in the West, it’s going to burn. Let’s do a synthetic fire, a what-if scenario, and start planning for what could happen.” 

Kean said residents of California, especially Southern California, are well versed in how to respond to an earthquake because of the communication efforts that have taken place over the years, and that something similar could be done for landslides.  

“People are pretty earthquake aware but these debris flows after a fire are not on people’s radar,” Kean said. “People got the evacuation notice in Montecito but, A, they were fatigued because they had just evacuated from the fire, and B, they just didn’t think anything like this was possible, that you could have car-sized boulders rolling through the neighborhood.”  

Creating the debris flow maps ahead of time and sharing those with the public could help raise awareness and aid evacuation, Kean said. Other mitigation strategies include creating ponds or basins to catch the flow before it runs into neighborhoods, and those are present in some Southern California areas already.