Wildfires, Debris Flows, and El Niño in Southern California
Large debris flows are common after wildfires. However, the effects of burning the mixed chaparral vegetation that grows on the hillsides of southern California are complex and varied. The vegetation above ground level and extending below the surface to depths of 1 or 2 inches can be burned to ash or merely charred, depending on how fast the fire moves and its temperature. The upper part of the soil becomes dry and loose and, even in dry weather, tends to slide down steep slopes to accumulate in gulleys and ravines. Fire can also cause a water-repellant layer to form at shallow depth in the soil, preventing or slowing infiltration of water and resulting in increased surface run-off during rainfall.
Loss of vegetal cover also increases exposure of the soil surface to raindrop impact. The looser soil is more susceptible to erosion. The fire-flood history of southern California wildfires shows that recently burned areas produce more debris flows than unburned areas. Compared to vegetated hills, debris flows in burned areas start earlier — no prior rainfall is necessary — and after less intense, briefer storms because virtually no rainfall is absorbed in the soil.
U.S. Geological Survey scientists are currently working to better characterize the changes that result from wildfires and to identify the relative importance of various topographic, vegetative, and rainfall conditions that result in debris-flow activity after fires.
USGS scientists have compiled a map showing perimeters of 1997 southern California wildfires that burned more than 300 acres. The wildfire perimeters enclose areas that can undergo extreme surface run-off from hillslopes. Areas downslope from the burned areas could potentially experience flooding and debris flows in the event of significant rainfall. Large debris flows might overwhelm existing flood control facilities. Data for the wildfire perimeters were obtained from the California Division of Forestry, California Office of Emergency Services, USDA Forest Service, and Orange, Los Angeles and Ventura Counties.
