Oct 6, 2009 6:46 PM by MSNBC

L.A. foothills warned of mud flows after wildfire

Winter rains could release huge flows of mud and debris in the vast area of the Angeles National Forest burned by a wildfire, affecting San Gabriel Mountain foothill communities, the U.S. Geological Survey said Tuesday.

The agency said its assessment indicated "high probabilities" of large debris flows in both scenarios it examined: a three-hour thunderstorm and a 12-hour storm.

Rainwater roaring through canyons in the nearly 251-square-mile burn area could move up to 100,000 cubic yards of mud, rocks and vegetation - enough debris to cover a football field 60 feet deep, the USGS said.

The August and September fire burned from the northeastern corner of Los Angeles eastward above the suburbs of La Crescenta, La Canada Flintridge, Altadena and Pasadena.

"The base of the San Gabriel Mountains as well as areas in Big Tujunga Canyon, Pacoima Canyon, Arroyo Seco, West Fork of the San Gabriel River, and Devils Canyon" could see large mud flows, the USGS said.

The USGS noted that deadly mud flows followed the 2003 fires around San Bernardino, California.

"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 burned area," Susan Cannon, a USGS research geologist, said in a statement.

"Nearly every burned watershed produced destructive debris flows or floods in response to that storm," she added. "Some of the areas burned by the Station Fire show the highest likelihood for big debris flows that I've ever seen."




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