"That's something we've known since the early to mid-20th century," said Josh West, an associate professor of earth sciences at the University of Southern California. "It's this one-two punch of fire and debris flows."
Not only does an intense wildfire consume the vegetation and shallow root system that holds soil in place, it can also turn slopes into a raincoat that repels rather than absorbs water.
Scientists aren't precisely sure how that happens, said Jonathan Godt, a natural hazards program coordinator for the U.S. Geological Survey.
But two processes probably play a role: Volatilization of organic compounds in the soil creates waxy substances that coat sediments. And fire dries the soil to such a degree that it loses the ability to soak up rainfall, like a shriveled sponge.
On top of that, Southern California's mountain ranges are steep and highly erodible. And because of tectonic plate movement, they are slowly rising. "What goes up must come down," West said.
Based on past data, the USGS has found that debris flows are likely to occur in Santa Barbara burn areas if rainfall rates hit half an inch per hour, said Francis Rengers, an agency research geologist.
They far exceeded that threshold in Tuesday's predawn hours, effectively turning a fire hose on the barren, fire-baked front country of the Santa Ynez Mountains.
The steepness of the slopes above Montecito added to the speed of the flows, which killed at least 17 people and destroyed at least 100 homes.
The sloping nature of the local coastal plain also helped carry walls of mud and rubble for miles, all the way to coastal Highway 101.
"How far the really big boulders went from the mountain front was definitely eye-opening," said USGS research scientist Dennis Staley, who flew over the burn area Wednesday.
Before the storm, local emergency personnel issued mandatory evacuation orders for areas at higher elevation, below the burn scar. Voluntary evacuation warnings went out to areas closer to the shore.
"It's so unpredictable you don't know where it's going to jump out or where it's going to go," Fayram said. "That's what led to the decision for a very broad evacuation call."
As to why some areas got an evacuation warning and others an order, Fayram said either way, they were messages to get out.
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