If enough rain falls, some flows could contain enough debris to cover a football field with about 60 feet of mud and rock, and could reach far into communities along the San Gabriel Mountains. And with an El Nino expected???
Soil, rocks and branches like these in the fire-scarred Angeles National Forest could be washed far into foothill communities by winter rains, according to the U.S. Geological Survey.
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 Cañada 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 Cañada 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.
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."
In Big Tujunga Canyon, residents still struggling to clear piles of debris and ash from homes destroyed in the Station fire said they fear there is worse to come.
"There’s nothing to hold that back," said Bronwen Aker, pointing to the charred slope behind her red cabin, inherited from her grandmother, in the canyon community of Vogel Flats. "It’s going to come down; it’s not a maybe."
Adi Ell-Ad is hopeful the sandbags will work, "if we don’t get that much debris flow."
"If it’s overwhelming," he said, "then nothing will stop it."
It’s going to be a long winter for those folks. The forecast doesn’t look good.
Tomorrow I’ll tell you about my new little ‘friend’ living in my garage and my plan to get rid of him!!! Here is today’s song. Don’t ask but yeah!