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A team of federal geologists is helping state and local officials assess the impact of mudslides that devastated a community in Southern California last month. More than 20 people were killed as a massive debris flow rushed through the town of Montecito. Public safety officials now say people who live in areas prone to mudslides need a better understanding of the dangers they pose. Stephanie O'Neill reports.
STEPHANIE O'NEILL, BYLINE: About 12 hours before torrential rains hit the Santa Barbara County town of Montecito, Anne Wilder got a knock on her front door. It was a sheriff's deputy. He'd come to tell her that a forecast of heavy rains had triggered mandatory evacuations in her neighborhood.
ANNE WILDER: He took our names, wanted to know the headcount of the household and if we were going to stay or go. I told him I'm waiting for my husband to get home. We were going to make that decision when he got home.
O'NEILL: When he arrived, the couple, who had just unpacked from a nearly two-week-long evacuation from the massive Thomas Fire, weighed the risks of riding out a storm that threatened flash flooding.
WILDER: I was thinking about that, oh, the creek bed is far enough away from our house that if the creek rises and it, you know, goes over the banks I think we're far enough away that we're OK.
O'NEILL: Wilder also lives far enough from the base of the town's mountains that she didn't feel threatened by the common landslide. Those involve a defined chunk of rain-saturated earth that slips off a hillside. So she and her husband decided to stay. But what hit Montecito wasn't something they'd anticipated. And they weren't alone. The powerful rock and boulder-filled slurry that wiped out neighborhoods shocked even veteran first responders, among them Ventura County Fire Division Chief John McNeil.
JOHN MCNEIL: When you see, you know, cars up in trees and houses just completely removed from their foundations and flattened out, it's just - it's amazing to think about the force.
O'NEILL: These flows of mud, rocks and burned wood are called debris flows. And they move fast. Jonathan Godt is with the U.S. Geological Survey in Boulder, Colo.
JONATHAN GODT: Typically they're moving at speeds way beyond that you could outrun. So it might be 30, 40 miles an hour. They can travel well beyond the mountain range from - like we saw in Montecito, miles in some cases.
O'NEILL: About 30 square miles in the case of Montecito's deadly flow that killed at least 21 people, destroyed more than a hundred homes and damaged more than 300 others. Debris flows of this magnitude are not common in Southern California, but the thick ash, exposed soil and tangled piles of burned vegetation left by wildfires like the Thomas Fire make them more likely and more destructive.
GODT: Because they have so much momentum, they're very effective at eroding additional material. And they can grow in size and volume and velocity as they move down slope.
O'NEILL: Godt says pretty much anyone who lives near burned mountains from Montecito to Montana needs to better understand the destructive power of these flows and then stay vigilant during every intense rain.
GODT: Right now in Southern California and in other parts of the western U.S. the concern is, when will the next big storm come and what area will it impact?
O'NEILL: The USGS offers online hazard maps that color-code the probability of debris flows in wildfire areas. But Godt says because these flows are so powerful, predicting their path is extremely difficult. In Montecito, the debris flow spared Anne Wilder's home. But about two blocks away, in an area deemed less likely to get hit, the deadly slurry gobbled up houses.
WILDER: Knowing what we know now, we wouldn't stay. Our thinking now is that whenever it rains we're going to leave.
O'NEILL: And that's a sentiment local emergency officials in Montecito hope everyone who lives near the burned mountains will adopt during storms this winter. For NPR News, I'm Stephanie O'Neill. Transcript provided by NPR, Copyright NPR.