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Record rains leave hillsides perilously soaked

December 29, 2010 |  4:15 pm

The wettest December since 1889 has left hillside areas across Southern California dangerously saturated with rain, bringing a heightened risk of landslides and more 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 its most rain of the year.

MudslideThumb 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, sections of Glendale, La Canada Flintridge and La Crescenta threatened by mud cascading off the burned slopes of the San Gabriel Mountains have seen no major flooding this winter, 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 is the ground itself giving way amid more rains.

“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 of the mountain roads have been compromised. While 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 California Department of Transportation spokesman Darin Cooke.

The rains also threatened a section of Highway 18, another key route, forcing motorists to take a small detour in one section between Kuffel Canyon and Arrowhead Villa roads, where guardrails were beginning to sink and the hill slope was slipping. Caltrans reopened the road on Monday night.

“Normally … it doesn’t rain that much in a short period of time. The mountain could not hold it,” Cooke said. “The side slopes fell and slid down and our roadways happened to be in the middle of them, and it took the roads with them.”

The latest storm moved through the region Wednesday, dumping more than 0.8 inches of rain in downtown L.A., where more than 10 inches of rain has fallen so far this month. The storm caused some traffic problems, including a mudslide that blocked lanes of the 91 Freeway near Green River Parkway.

The rains gave way to gusty winds and dropping slow levels, which could cause problems in the Grapevine and other key mountain passes. Another storm is expected to hit Saturday night.

-- Sam Allen

Photo: (Don Bartletti / Los Angeles Times / December 29, 2010) Loosened by the rain, a car-size boulder sits on the shoulder of Highway 79 just outside of Julian, Calif. County workers were kept busy clearing rocks and mud from roads in rural San Diego County. At higher elevations, they were clearing snow.