Jimmy Guilinger is an assistant professor of applied environmental science at California State University, Monterey Bay, and an expert on post-wildfire debris flows. After recent damaging storms in Southern California, we asked him to tell us more about debris flows—what they are, why they happen, and how Californians can protect themselves.

We’ve been hearing more about debris flows recently. What makes a debris flow distinct from ordinary flooding, and what unique challenges does it pose?

While flood flows are dominated by water, debris flows are a slurry of a lot of things, including boulders. They are typically half water and half sediment, and they are very energetic. They move rapidly, they’re more destructive than floods, and they can jump out of a previous waterway and carve a new path. It’s a lot harder to predict where they’ll go.

We are prone to debris flows here in California because many of our mountain ranges are young and really steep. Soils on these slopes are already close to being unstable, so when you really wet that soil, it falls apart. Then, under the right conditions, it can liquify and run downhill as a debris flow.

Intense rainfall triggers debris flows. And when a steep area burns at high enough severity, it can prime the landscape for debris flows in the first few years following the fire. Many variables contribute to debris flows following fire, including soil water repellency, which is common in chaparral shrublands. Chaparral contains a waxy seal in its leaves that can vaporize during fire and condense on top of the soil, leaving it impermeable to water. Rain just trickles off.

In Northern California, recent studies have shown that soil water repellency breaks down in that region really quickly. Even in Southern California, repellency and other impacts don’t stick around in perpetuity. Landscapes recover remarkably fast—typically within two to three years after a fire, debris flow likelihood drops off significantly.

Recent work by the USGS has shown that most post-fire debris flows in the Western US have been triggered by short-duration rainfall. Even if it rains for only five to 15 minutes, if it rains hard enough, the runoff can generate pretty significant debris flows. Thankfully most of these flows did not lead to loss of life, except in Montecito in 2018. In that case, even though it was a drought year and the total amount of rain wasn’t that impressive, it came down in a matter of minutes. The sediment overwhelmed flood control infrastructure and channels downstream, and 23 people died.

How does climate change play into this dynamic?

Droughts are getting more intense with climate change, creating conditions that are ripe for extreme wildfire. Southern California already has one of the most aggressive fire regimes on the planet. You have 20 million people situated in a naturally flammable ecosystem, so it’s less about fuel buildup and more about the timing and location of ignitions. Elsewhere in the state, it’s a mixture of climate change, fuel buildup from fire suppression, and land use.

And when you have a warmer, more energetic atmosphere, it can hold more moisture—which means it can rain harder. With more extreme wildfires and more intense rainfall, there’s an increased likelihood of post-fire debris flows, particularly larger events like what happened in Montecito in 2018.

Does traditional flood insurance cover debris flows?

The National Flood Insurance Program typically does not cover impacts from debris flows to homes. The state is working on it, but it’s a gray area: Debris flows and floods can happen in the same event. Homeowners should check with their insurance to see if they’re covered.

What’s the best approach to managing this hazard?

Honestly, in steep landscapes where things burn at high severity, you can’t feasibly stop debris flows at the source, so you need to get people out of the way. Forecasts are uncertain, but if you’re seeing conditions ripe for producing these events, it’s best to evacuate people in the most hazardous areas downstream. But evacuation can be difficult for many people. That’s an equity problem that needs to be addressed.

Clearing debris basins also reduces hazards by increasing the amount of space available to catch all the material coming down. However, some areas in the state are burning at rates we haven’t seen before, and we need to find out if they even have debris basins or other flood control infrastructure or if the existing infrastructure can handle a large amount of debris.

New long-term predictive models can identify areas that may burn and then experience a debris flow. With this approach, we can ask: Who lives downstream? What infrastructure is downstream? Taking a proactive approach is critical. If we can determine the areas most susceptible to debris flows, then we can plan better and make investments ahead of time to protect people.