Los Angeles

Rainfall Accumulation Increases Risk of Mudflows

Slopes in burn zones have already begun shedding mud, and they may not be the only ones at risk if this rainy season continues at its above average pace.

With two more stormfronts moving in Tuesday over Southern California, rainfall to date for downtown Los Angeles stood at 7.24 inches, which is 2.18 inches -- 30 percent -- above the historical average, according to the National Weather Service.

Previous storms this season have dislodged tons of mud, rocks and debris from recently burned and destabilized slopes in the San Gabriel Mountains. Much of the mud and debris in those flows has been captured by debris basins maintained for that purpose by Los Angeles County Public Works. Concrete k-rail barriers have been installed to channel the mud that bypasses basins.

Workers at the Las Lomas Debris Basin above Duarte have faced the Sisyphean task of repeatedly removing debris to unclog the basin to make room for the next round of muck unleashed by the next storm.

Outside the burn zone Tuesday, a mudflow in the Santa Monica Mountains partially blocked traffic lanes on Kanan Dume Road before it could be cleared.

In the Hollywood Hills on Laurel Canyon Blvd, motorists encountered rocks and mud that have sloughed off the steep hillsides, over the curb, and onto the edge of the roadway.

Flows of mud and debris become more likely as soil approaches saturation with water, and at saturation point the soil cannot absorb any more, increasing chances of flooding.

Knowing where slopes have failed previously is a significant indicator they will fail again. But given variation between soils and slopes and triggers, determining when remains a challenge, said Caltech professor Jose Andrade, the executive officer for mechanical and civil engineering, with a focus on soil mechanics.

"Mudslides are like earthquakes," said Andrade. "They're impossible to predict."

Water accumulation is an indicator, and in the Bay Area, the United States Geological Survey is developing a system that would warn of increased landslide risk in cases where water can be detected building up between soil and bedrock.

More easily predicted is flooding on roadways where storm drains are clogged.

During intense rainstorms, Laurel Canyon Boulevard has periodically served as an unintended flood channel, the runoff at times strong enough to wash cars off the road, as in 1978.

At other times, water washed right through the Canyon Country Store, recalled owner David Shamsa. He said the canyon has fared better in heavy rain since the city upgraded the capacity of the storm sewer system.

After five years of drought, Shamsa and canyon residents are welcoming this season's rain, recurring but so far rarely intense.

"We haven't had torrential downpours like in the past," said Riley Bray, a musician who grew up in the area. "So far, so good."

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