Study: Risk of debris flows in Calif. fire area
PASADENA, Calif. — 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 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.
“If it starts raining hard and you know you are in a hazardous area, then just take your own initiative and leave,” she said.
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.
The foothill communities along the San Gabriels are now protected by a county system of basins to catch debris while allowing water to flow down flood-control channels to the ocean.
The emergency assessment of the fire’s aftermath used modeling to determine what kind of flows could be triggered by a three-hour storm and a 12-hour storm during the next two winters. The three-hour storm is considered to have a 100 percent chance of occurring each year; the 12-hour event has a 50 percent chance.
“We chose very commonly occurring storms,” Cannon said. “These storms could actually happen in any winter. It doesn’t depend on if it’s an El Nino winter. They have a good chance of occurring.”
Cannon said the sharp angle of the terrain is a critical factor.
“In our model, the parameter that matters the most is the slopes that are burned that are greater than 30 percent, and most of the slopes in the area are steeper than 30 percent,” she said.
Cannon noted that while the so-called urban interface between foothill communities and the wilderness is protected by flood-control basins, there are significant hazards within mountain canyons.
In 2003, two blazes known as the Old and Grand Prix fires scorched more than 150,000 acres of the San Bernardino Mountains, which lie to the east of the San Gabriels. A Christmas Day storm triggered torrents of debris and floodwaters, killing 16 people gathered at a church facility in a canyon.
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