Experts say the ground is so saturated now that the danger of flooding and debris flow will remain heightened through the rest of the rainy season.The wettest December since 1889 has left hillside areas across Southern California dangerously saturated, bringing a heightened risk of landslides and further flooding in the next few months.
More than 14 inches of rain has fallen in some hillside areas in just the last two weeks, and officials said the saturation levels could intensify in January and February, when Southern California typically gets most of its rain for the year. Engineers are using helicopters to fly over some hillside areas hit by recent fires, looking for signs of fissures or earth movement.
"It gets to the point where the water that's falling is no longer even going into the ground — it's just skipping off the ground," said Bob Spencer, spokesman for the Los Angeles County Department of Public Works. "A lot of residents are under the false impression that once the sun comes out, everything is fine. That's not the case. The soil beneath the surface can take months to completely dry out."
So far this winter, sections of Glendale, La Cañada Flintridge and La Crescenta threatened by mud cascading off the burned slopes of the San Gabriel Mountains have seen no major flooding, in large part because officials have managed to keep debris basins in the hills clear. But Spencer and others said the major danger through the rest of the winter would be the ground itself giving way amid more rain.
"With every storm that comes in, it increases the risk of potential mudslides and debris flows," he said. "The risk is there now, and it is going to remain there throughout the winter season."
In areas burned by recent fires, as little as a quarter-inch of rain can begin to cause slopes to slide. In areas with more vegetation, debris can begin to flow after about 10 inches of rain, said Douglas Morton, landslide expert with the U.S. Geological Survey.
In the San Bernardino National Forest, two weeks of pounding rain saturated the earth and washed away several sections of Highway 330, a key route to Big Bear.
Geologists were sent into the mountains this week to determine how much mountain roads have been compromised. Although these roads can withstand big accumulations of snow, last week's warm front instead brought large amounts of rain, which undermined the roads.
"We've had some storm damage in the last several years, but that was nothing compared to the damage we have now," said Darin Cooke, spokesman for the California Department of Transportation.
View entire article here: http://www.latimes.com/news/local/la-me-12-30-rain-20101230,0,2384258.story
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