Newly released federal and state wildfire reports offer a grim assessment on threats to life, homes and drinking water supplies if hard rains hit steep slopes charred in last month's wildfires.
Those most at risk appear to be residents of rural eastern Orange County, portions of Bouquet Canyon and Val Verde in Los Angeles County, and vacation homes in the Angeles and San Bernardino national forests, according to the reports and government officials.
More than $6 million in federal emergency funds have been approved in the last week to stabilize denuded forest lands, many of which sit above threatened communities.
In many cases though, residents will have to rely on their own judgment to determine if they should evacuate, as applications of sandbags or straw bales may do little to help.
The warnings come amid predictions of a dry, La Nina winter throughout most of drought-stricken Southern California. However, officials said that even a single day of pounding rains could trigger a disaster because there are no plants to hold soil, rocks and debris in place.
"La Nina? You can't speak in those terms," said Randy Westmoreland, a U.S. Forest Service soils scientist who led a multi-agency team that evaluated risks in the wake of the Santiago fire.
"You could get all your storm in one event, and it could be 80% above normal . . . in five minutes," Westmoreland said. "If you get one big storm with the right intensity, it could trigger flooding and mudslides."
Modjeska Canyon in Orange County, which sees runoff from the Cleveland National Forest, could be hardest hit. Soil and storm experts said that because of last month's wildfires, mudslides in the canyon could measure "19 times greater" than mudslides occurring before the fires.
Also, flash floods and other runoff threaten hundreds of homes that exist between two canyon creeks, experts said.
The flooding could be four to eight times greater than in pre-fire conditions, experts said.
Westmoreland, who has done such assessments for nearly 20 years, said, "It's some of the worst risk I've seen because so much vegetation burned."
Westmoreland spent days hiking through and flying over the burn areas with other team members.
"You basically went from fully vegetated to no coverage at all, just ash and bare dirt, so now you have nothing to hold anything back," Westmoreland said. "Worse than a flood is a debris flow through that canyon. That is my worst fear, the mud and water and ash and everything [the water] picks up along the way."
