DISASTERSPerfect Storm: Megafires Set the Stage for Debris Flows
Storms now pounding Southern California are raising the specter of more danger for residents — debris flows in areas already reeling from devastating fires. Flows pose bigger threat than typical mudslides.
Storms now pounding Southern California are raising the specter of more danger for residents — debris flows in areas already reeling from devastating fires. Debris flows are like mudslides, except much faster and more destructive, as they contain a mix of rock, plants and even boulders or trees in addition to mud.
Andrew Gray, an associate professor of watershed hydrology at UC Riverside, is an expert on the way water and sediment move through landscapes after wildfires. He weighs in here on the threats to life and infrastructure posed by these fast-moving flows, and how residents can best respond.
Q: In your mind, what is the “perfect storm” for a debris flow — the perfect combination of conditions that would precipitate one?
A: First and foremost, if you are living near any of the recent burn scars in Southern California, please pay attention to local hazard warnings during the rainstorms this week ¾ especially evacuation orders.
The best (worst) recipe for post-fire debris flows in Southern California is high-intensity rainstorms interacting with steep hillslopes that were very recently burned. The atmospheric rivers that deliver rain to Southern California in the winter commonly meet or even exceed the rainfall intensities required to trigger post-fire debris flows.
Q: What can we expect this week and in the coming months in the burn scars from the recent Southern California fires?
It looks like the storm this week will result in moderate to high debris flow risks across much of the Palisades Fire and Eaton Fire burn areas. The USGS conducts emergency assessments of post-fire debris flow hazards. Their approach is the current standard in the Western U.S., and many hazard managers rely on their maps of debris flow risks.
The kind of debris flows that we mostly see during the first year after fires in mountainous shrublands are runoff-driven. They are the result of large supplies of water and sediment to the channels draining burn scars during high-intensity rainfall. The soil surface of recently burned shrublands tends to have much lower infiltration rates than unburned areas, especially if the burn severity is moderate to high, which is common in these fires.
When rain falls on these surfaces, more of it reaches the group because of the lack of vegetation to intercept it, and then a larger proportion of this becomes runoff. The burned soil surfaces also tend to be very easy to erode.
