Study: Slides Could Cover Football Field, 60 Feet Deep

A geologist studying the wildfire-denuded San Gabriel  Mountains confirmed worries that rainstorms will likely trigger debris  flows that could affect foothill neighborhoods.

Other threatened areas include Big Tujunga Canyon, Pacoima Canyon,  Arroyo Seco, the San Gabriel River's West Fork and Devils Canyon, according to  the U.S. Geological Survey.

Susan Cannon of the USGS, who has been studying debris flows for more  than a decade, said Tuesday that recent studies show some local watersheds could produce  debris flows up to about 100,000 cubic yards, or enough material to cover a  football field 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," she said.

The mammoth Station Fire -- the biggest in county history -- burned  about 250 square miles, or about a quarter of the Angeles National Forest just  north of Los Angeles.

"Our assessment used a set of computer models to estimate the  probability of debris flow, how large the event might be, and were it might go,  based on the steepness of the area, the extent and severity of the fire, soil  characteristics and possible rainfall," Cannon said.

The USGS looked at the effects of three-hour downpour, a typical  thunderstorm and 12 hours of light, but steady rain.

Cannon warned that debris flows typically move faster than a person can  run.

"People may remember that 16 people were killed by debris flows during  the Christmas Day storm in 2003, but few realize that those were only two  debris flows out of the hundreds that were triggered from the burn area," she  said, adding that nearly every burned watershed produced a debris flow.

Gail Farber, Los Angeles County's director of  public works, said county  crews were preparing for problems.

"We are hoping for the best but preparing for a difficult storm season  by inspecting all debris basins throughout the county and cleaning out those  that need it to ensure full capacity," she said.

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