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Too much rain? Next few months will tell

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.

December 30, 2010|By Sam Allen, Rong-Gong Lin II and Hector Becerra, Los Angeles Times
  • Dog walker Don Allen walks Tramp, left, and Owen while crossing Broadway in the rain in downtown Los Angeles.
Dog walker Don Allen walks Tramp, left, and Owen while crossing Broadway… (Mel Melcon / Los Angeles…)

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.

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 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."

In the Angeles National Forest, the saturated ground has increased the likelihood of falling rocks and toppling trees, especially in combination with the high winds forecast for the region during the next two days.

The latest rainstorm moved through the region Wednesday, dumping more than 0.8 inches in downtown Los Angeles, where more than 10 inches of rain has fallen so far this month. The rain 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.

The series of storms last week caused major flooding in Laguna Beach, caused a mudslide that damaged dozens of homes in the San Bernardino County city of Highland and prompted the closures of major roads including Pacific Coast Highway in Malibu, which was shut again Wednesday.

The rains caused significant damage to commuter rail service in L.A. County, where power outages, tangled overhead electric lines and lightning storms forced lengthy delays on Metro's rail service. Officials advised transit patrons to give themselves plenty of time if they plan to take the system to the Rose Parade.

The weather has also turned the grassy fields surrounding the Rose Bowl into a muddy swamp, meaning parking near the football game is likely to be more scarce this year.

In Glendale, engineers were traveling by foot and vehicle to the canyons and to Deukmejian Wilderness Park to take photos and monitor hillsides for signs of collapse or accumulations of debris that could sweep into neighborhoods below, said Glendale Public Works Director Steve Zurn.

The severity of the slide danger will depend greatly on how much rain Southern California gets in the next two months.

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