Los Angeles – Of the many dangers posed by the torrential rains socking southern California this week, mudslides and their accompanying debris flows probably pose the biggest threat – and people living in the region's many hillsides and canyons need to be alert, officials here warn.

Short bursts of dense rain, those approaching one inch an hour, are the conditions that tend to wreak the most havoc on hillsides, say officials ranging from Caltrans to the US Geological Survey (USGS). The threat of a slide is particularly acute where land has been denuded by wildfire, they add.

Those are the very conditions that confront the greater Los Angeles area. Los Angeles got three-quarters of an inch per hour overnight – coming on top of four straight days of rain that broke an 89-year record for precipitation.

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“People don’t really understand how truly destructive and dangerous debris flows can be,” says Susan Cannon, a research geologist with the US Geological Survey (USGS) stationed in Los Angeles, now home for the holidays. “If you’re in a burned-out area and get an evacuation notice, take it very, very seriously.” In southern California's San Bernardino, she notes,16 people died in 2003 when they couldn't get out of the way of a muddy debris flow in time.

“All last winter we had stories of people running for their lives,” she says, noting one incident in which a football-field-size catch basin in Dunsmore Canyon, near the northernmost section of Glendale, was covered 60 feet deep with mud. “The stories usually happen at night in the rain, with a wall of mud barreling down on someone with the speed of a locomotive.”

One problem area officials are watching is a 250-square-mile swath in the of foothills near La Canada Flintridge and La Crescenta, north of Los Angeles, which were stripped of most vegetation during the huge Station Fire 16 months ago. In February, rain and mud damaged at least 50 homes there, and people had to evacuate in a hurry, Cannon says.

Much has been learned in the past decade, as researchers have studied mudslide threats in the area. The USGS has coordinated with the National Weather Service to build models of how much saturation different kinds of soil can take, estimate the amount of water expected from coming storms, and then make their evacuation warnings accordingly. Variables include what areas have had vegetation denuded by fire, and how much leaves and other woody debris have been removed by residents. Researchers have also learned that when plants are burning, they release a gas into the soil that produces a waxy layer inches beneath the turf. That waxy layer halts water infiltration – and creates a slippery slide for soil sitting above it, paving the way for a landslide.