The U.S. Geological Survey on Tuesday issued a grim forecast for foothill communities hit by the Station fire, saying major mudslides are highly likely during the winter rain season.
Scientists identified Pacoima Canyon, Big Tujunga Canyon, the Arroyo Seco, the West Fork of the San Gabriel River and Devils Canyon as being at particular risk. In those areas, the report said there was an 80% likelihood of flows. Under certain conditions, some flows could contain up to 100,000 cubic yards of debris -- enough to cover a football field with mud and rock about 60 feet deep.
Under the worst-case scenario, in which there would be 12 hours of gentle, sustained rain, the report said thick flows of soil, rocks and vegetation could stream downhill into neighborhoods as far south as Foothill Boulevard in such communities as La Canada Flintridge and La Crescenta.
"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.
The Station fire burned 250 square miles in August and September, leaving hillsides barren. There is little vegetation left to prevent water, sediment, rocks and branches from rushing down toward thousands of homes when it rains.
The much-anticipated report, which includes maps depicting the potential paths of destruction, gave communities along the fire-ravaged areas of the San Gabriel Mountains an early and frightening look at what might happen when a heavy rainstorm pounds the area.
"We are very seriously worried," said La Canada Flintridge Mayor Laura Olhasso. "It's highly possible that some of the homes that were saved from fire will be lost to mud."
Federal geologists used computer models to estimate the likelihood of debris flows 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 slopes, 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 Gabriels.
If drainage basins in the mountains fill up, Cannon said, debris could stream into neighborhoods.
Triggered by rainfall, debris flows can travel faster than a grown person can run. The rushing water, soil and rocks can destroy bridges, roads and buildings, and seriously injure or kill people in the way.
The goal of the assessment, officials said, is to help guide state and local planners as they work to protect lives and property in the storm season. Foothill communities are beginning to set up sandbags and concrete barriers to divert any mud flows into the streets and away from homes.
The Los Angeles County Department of Public Works has been preparing for the winter storm season for the last month, said spokesman Bob Spencer.
"A lot of this is part of our normal preparation, but because of the devastation caused by the fire, that effort has been increased this year, particularly on the public outreach side," Spencer said.
The county oversees an elaborate flood-control system, which includes 14 major dams, about 500 miles of open channels and 3,000 miles of storm drains. Maintenance crews are clearing out 28 basins, which trap debris in the burn areas before storm flows reach neighborhoods, and are expanding eight of them, Spencer said.
The Public Works Department has organized meetings in many of the 23 communities bordering the Angeles National Forest to advise residents about the danger of debris flows and has set up a website to provide news, advice and forecasts from all the agencies involved: www.dpwcare.org. The department is also dispatching engineers to assess the risk to individual homes and draw up plans to protect them. About 350 of these plans have already been presented to property owners, Spencer said.
But in some cases, where homes directly front blackened hillsides, there is little that can be done.
"We have a couple homes where the county told the property owner, 'Put plywood over your windows and just leave,' " Olhasso said. "The one thing that residents need to understand is that if they have this plan, they need to put it in place now; it can't wait until the 24-hour forecast for rain. By then it could be too late."
The USGS and the National Weather Service have collaborated on a debris flow early warning system, and city officials are preparing evacuation routes and route closures for when the rains come. La Canada Flintridge is also revamping its reverse 911 alert system to call residents' cellphones and personal digital assistants, in addition to their home numbers.