The downside of Southern California's rain: Debris flows

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Southern California's upcoming rains could cause trouble for those living near hillsides, says Caltech geology professor Michael Lamb. That's because sediment gathered in riverbeds has been building up due to a lack of major storms.

“The gun is loaded so to speak," said Lamb.

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He notes a big downpour could sweep up that debris and cause a flow of mud, logs and even boulders into populated areas. And Lamb adds, L.A. is built on a basin where rivers empty their water.

Debris flow video

Communities living near recent fires could face a similar problem, Lamb notes, since fires bake the soil dry. That turns the normally absorbent ground into a water-resistant surface, almost like concrete.

Large rainstorms send water down those hills with minimal absorption, meaning debris on the hill can get washed into communities below.

Eric Boldt with the National Weather Service says this storm is expected to drop a lot of rain in little time — the perfect conditions for this kind of problem.

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