The debris flow can carry boulders and trees along with mud, making it more dangerous than a regular flood or a dry landslide. “When you mobilize them—keep them mobile with this muddy matrix in between them—they can go 3 kilometers,” says Kean. The video below shows just how easily the flow tosses boulders around.
Boulders can also act like a moving dam, retarding the flow. “So you get this surging behavior, where these things are holding back a whole bunch of flow behind it,” adds Kean. “It really makes the flow depth increase several times what you'd get in a water-only flood.” The Montecito debris flow got to be 30 feet deep at points, lodging boulders in trees. Humans are powerless to stop it—sandbags just become more debris for the flow.
But it can be predicted, giving residents a chance to evacuate. The work begins as soon as a wildfire’s smoke clears enough for satellites to get a good look at how badly areas have burned. This information is relayed to scientists on the ground, who enter those burned regions as soon as it’s safe.
The greatest risk of debris flow is in the zones that burned the most intensely, and scientists spend two or three days wandering the fire perimeter taking soil samples. “That involves testing the hydrophobicity of the soil, the water repellency,” says Eric Huff, staff chief of Cal Fire’s Forest Practice Program, “and actually getting down on hands and knees and looking at what that surface soil layer is showing, whether or not organic matter has been completely burned off.”
With all this data in mind, they can calculate how much rain you would need to potentially let loose a debris flow for different regions of the map. Often half an inch of rain in an hour is enough to kick one off, or as little as five minutes of heavy rain. “It's kind of a run-of-the-mill storm,” says Kean. “It's heavy rain, but it's not particularly exceptional.”