K-rails were seen lined up along several roadways at the bottom of hillsides in the Duarte area.
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Wildfires have obliterated much of the vegetation in the burn areas, so there is nothing to absorb the rain. In addition, the loss of soil strength increases the risk of mud flow off the charred hillsides.
"When it rains, within five minutes of heavy rain, it funnels it all straight into my property right through this stairway right here," described Azusa resident Ed Heinlein, who lives directly under a steep mountainside.
He knows all too well how quickly mud and water can rush toward his home.
"The fact is, nowadays, I don't even watch the weather. I wait until news people call me or show up and they tell me what's going to happen," Heinlein said.
In Glendora, the Public Works Department is handing out free filled and empty sand bags for anyone who may need them.
Public Works officials have also inspected flood channels and cleared tens of thousands of catch basins and storm drains.
Those who live in areas that were impacted by the Blue Cut Fire - including neighborhoods near the San Gabriel Mountains and Lytle Creek - are also bracing for possible debris flows.
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Debris flow off of these burn areas can move very quickly, so fire officials are promoting what they're calling "Ready Set Go," asking residents to pack important items and protect their properties.
A storm system is expected to bring possible rain Thursday night, with heavier rain expected Friday morning.
A flash flood watch has been issued for most of Southern California until noon on Friday.
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