Researchers warn foothills below burned hills face mudflow danger

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A charred landscape is left in the wake of the 250-square-mile Station Fire as firefighters work to complete the final defensible fireline to contain the massive fire in the San Gabriel Mountains on September 16, 2009 in the Angeles National Forest, northeast of Los Angeles.

U.S. Geological Survey researchers warn foothill areas below hillsides burned by the Station Fire could face mudflow danger this winter.

The Station Fire is nearly out, but the danger is far from over. Scientists at the U.S. Geological Survey say hillsides stripped bare by the flames could bring trouble from mud and debris flows this winter, even with normal rainfall.

The fire burned away vegetation from more 160,000 acres of the Angeles National Forest. In an interview with KPCC’s Larry Mantle, Lucy Jones with the USGS said signs of the potential danger from runoff are obvious.

“Go look up at the mountains right now, and you can see what’s called ‘dry ravel’ has already begun,” said Jones.

“You can see little rivulets. The hillsides are starting to slough off the top cover, and it’s getting deposited on the streams just on gravity and wind.”

The USGS has been working on computer models to see how bad the rainstorm runoff will be in the foothills below the Station Fire.

USGS researcher Sue Cannon told KPCC’s Susan Valot that the models show there’s a high probability of large mud and debris flows. Some basins could produce 100,000 cubic meters of debris in a rainstorm.

“That’s enough material to cover a football field 60 feet deep in mud and rocks,” said Cannon.

After 2003’s Old and Grand Prix Fires in the San Bernardino National Forest, the scorched landscape produced some of the largest debris flows ever seen, said Cannon. Fourteen people celebrating Christmas Day in Waterman Canyon were killed when a flash flood sent a river of mud, boulders, and trees pouring through their campground.

“They can travel faster than a man can run,” said Cannon.

“Certainly when they’re traveling through a steep, tightly confined drainage, they can really zip through there. So they’re hard to stay out of the way of.”

Geologists with the U.S. Geological Survey have assessed the burned area. They’ve identified and mapped areas where normal winter rainstorms could trigger debris flows.

“I think the important thing is that you can’t prevent the risk but you can manage it,” said Jones. “We’ve got probably the best system in the world. But this is a higher risk then we have faced before.”

Jones said work crews can dig basins to help minimize debris flow, but those can easily overflow when it rains. She said anyone living below the Station Fire area should obey evacuation orders during rainstorms.

“People think they know debris flows," said Jones. "But there’s a mistake in thinking that it’s gonna be dirty water down your street carrying some boulders. In the ‘78 flow, it’s described as a 20 foot black wall. So it can be pretty thick. And the big thing we gotta do is divert it so it can get down the road and they can try and pick it up later and try and keep it out of our residences.”

Sue Cannon with the USGS said the biggest risk after the Station Fire will come where rivers and tributaries run. Those areas include Pacoima Canyon, Big Tujunga Canyon, the Arroyo Seco, and the west fork of the San Gabriel River.

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