Debris flows and El Niño in southern California

The effects of past wildfires on debris-flows

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.

Fires also destroy vegetal cover, which increases exposure of the soil surface to raindrop impact. The looser soil is more susceptible to erosion. 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.

The fire-flood history of southern California wildfires shows that recently burned areas have many more debris flows. 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.

Example of debris flows following fires: Homes on the north side of San Bernardino, during the winter of 1980, are buried nearly to the roofs by debris flows that come from an area above the homes that burned in 1980. Debris flows filled and overran the flood control basin located in the foreground, which has been partly cleaned out at the time the photo was taken.

Debris flow hazards and landscape: what locations are dangerous?

Debris flow hazards and rainfall: what does heavy rainfall do?

How to prepare for debris flows

Geologic mapping in southern California

For further information, contact David Miller

http://geology.wr.usgs.gov/wgmt/elnino/scampen/fire.html, 29 December 1997, Contact: El Niño Web Team