After nearly three weeks of battling fires in the Los Angeles area, light rain has initiated concerns about potential flooding and debris flows. These flows are especially dangerous on burned hillslopes where intense fires have left hydrophobic soils unable to absorb water. As rain falls, a hazardous slurry of ash, sediment, and water forms and rushes downhill. The danger was highlighted by severe cases in areas like Montecito, underscoring the ongoing threat to communities even after the fire is extinguished, making it crucial for officials to monitor conditions closely.
When rain does fall on a burned area like this, water mixes with the ash, rocks and sediment to form a slurry. This slurry of debris then pours downhill in small gullies called rills, which then converge to form bigger and bigger rills, creating a torrent of sediment, water and debris rushing downhill.
Debris flows can move with the speed of a freight train, picking up or destroying anything in their path. They can move tons of sediment during a single storm.
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