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Federal scientists say risk of mudslides from Station fire extremely high

October 6, 2009 | 12:29 pm

The U.S. Geological Survey today issued a grim forecast for foothill communities hit by the Station fire, saying huge mudslides and debris flows are highly likely during the winter rainy season.

Scientists have been spending the last few weeks studying terrain destroyed by the largest fire in Los Angeles County history to determine which areas have the greatest risk of mudslides.

They identified Pacoima Canyon, Big Tujunga Canyon, the Arroyo Seco, the West Fork of the San Gabriel River and Devils Canyon as being 80% likely to experience flows of up to 100,000 cubic yards of debris, enough cover a football field with mud and rock to about 60 feet deep.

“Some of the areas burned by the Station fire show the highest likelihood for big debris flows that I’ve ever seen,” said Susan Cannon, a USGS research geologist and one of the authors of the emergency assessment. Cannon has been studying debris flows after fires for 11 years.

“We don’t have the science to model where it will travel, but there’s a really good chance of a big debris flow happening within that drainage system," she added.

USGS geologists used computer models to estimate the likelihood of debris flow in 678 drainage basins in the burned area, as well as how voluminous the material might be and where it might go.

They based their projections on the steepness of the slope, the extent and severity of the fire, soil characteristics and possible rainfall. The assessment posed two scenarios -- a three-hour, high-intensity thunderstorm, and a 12-hour gentle rainstorm -- and found high probabilities that each would cause large debris flows in neighborhoods that front the San Gabriel Mountains.

If drainage basins in the mountains fill up, Cannon said, debris flows could stream into neighborhoods in 12 urban areas.

Triggered by rainfall, debris flows can travel faster than a grown person can run, and the rushing water, soil, and rocks can destroy bridges, roads and buildings, and seriously injure or kill people in the way. The Station fire burned through a 250-square-mile expanse in August and September.

The goal of the assessment, officials said, is to help guide state and local planners as they work to protect lives and homes when rains come to the burn area. Cannon, the geologist, said the situation in the San Gabriel Mountains is reminiscent of conditions in the San Bernardino Mountains in 2003, when on Christmas Day a flash flood hit a campsite near a burn area, killing 14 members of a church group.

--Tony Barboza


Photo: A home sits high above Vogel Flats, surrounded by charred hillsides now at risk of winter mudslides that threaten homes that survived the Station fire along Stonyvale Road below. Credit: Robert Gauthier / Los Angeles Times

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