California is being battered by the first of what promises to be an onslaught of multiple storms this winter. It's all caused by an El Niño – an ocean temperature fluctuation - that's already tied for the strongest on record, according to the National Weather Service. While it's bad enough to be pelted by pea-sized hail and buffeted by 45-mile-an-hour winds and see low-lying coastal areas threatened by flash flooding, Californians are especially worried about another, even scarier risk - sudden mudslides that descend upon motorists and sweep away houses in their path.
In one Los Angeles neighborhood, a resident told CBS News that he could do little else but wait for the mud to overwhelm his retaining walls. "The likelihood is that the house will be shoved into the street and the county will be coming to pick it up," he predicted.
U.S. Geological Survey studies of past El Nino events suggest the situation may only get worse. Unfortunately, researchers are still trying to figure out how to predict which areas are most at immediate risk.
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According to the USGS website, the most serious immediate risk is from shallow landslides that become so wet that they can move rapidly over long distances. Such "debris flows," as they're called, have the potential to injure or kill people who can't get out of their path in time. They can move at speeds of up to 35 miles per hour.
The areas most susceptible to debris flows are steep hillsides that recently have burned in wildfires. But this winter the risk could extend further, because several years of drought have killed off vegetation that normally might keep hillsides from collapsing.
But predicting precisely where mudslides will occur in a particular season is tricky. While rainfall maps can give some indication of risks, they're not necessarily a reliable indicator, because the amount of moisture in the hillsides is affected by varying degrees of drying and evaporation that occur between storms. USGS researchers, who've studied mudslides in the San Francisco area, have been experimenting with sensor technology that would detect moisture levels.
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Looming ahead, there's also the risk of deeper-seated landslides in the spring, caused by the prolonged rainfall. Deep landslides move more quickly than the shallow mudslides, so there's less risk of fatalities. But they still can cause serious damage to buildings and infrastructure.
The 1982-83 El Niño caused the equivalent of $1 billion in damage in today's dollars, some of it from mud and rock that came rushing down upon canyons and coastal roads, according to the Los Angeles Times.
Officials in the southern California community of Glendale compiled this scary compilation of pictures of historic debris flows dating back to the 1930s.