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Rainstorms could send huge flows of water laden with mud, rocks and other debris toward cities below steep slopes of the San Gabriel Mountains stripped bare by a wildfire near Los Angeles this summer, federal scientists said Tuesday.
The U.S. Geological Survey used two very probable types of storm scenarios to map where inundation by debris flows could occur below drainages along the highly populated base of the mountain front.
“Because of the fire, there’s a significant hazard posed by debris flows and this hazard will occur even in response to a wimpy little storm,” USGS research geologist Susan H. Cannon said.
In the emergency assessment, the USGS mapped where inundation would occur if flows struck when flood-control catch basins were empty and where the flows would go into neighborhoods if the basins were already full.
The study found that some of the burned watersheds could release up to 100,000 cubic yards of material, enough mud and rock to cover a football field 60 feet deep.
The Los Angeles County Department of Public Works has been cleaning out the several dozen catch basins in the areas affected by the 250-square-mile (400-kilometer) fire, and officials said the process would be complete by Oct. 15.
The USGS said its assessment provides critical information for designing mitigation measures, planning evacuations and issuing warnings.
Cannon cautioned that predicting storms and debris flows is difficult so residents need to stay informed and take responsibility for protecting property and themselves.
The fire was ignited by arson on Aug. 26 near a ranger station in the Angeles National Forest and threatened foothill suburbs and canyon homes as it grew into the largest wildland blaze in county history. Eighty-nine homes were destroyed and two firefighters died when their truck plunged off a road.
The connection between wildfires and debris flows from the San Gabriel Mountains has been documented since the early 20th century, including a storm in 1934 that unleashed runoff so intense that 30 people were killed, more than 480 homes were destroyed and a nearly 60-ton boulder was pushed out of a canyon, the report said.