The effects of fire on the generation of debris flows in southern California
Debris flows following Are are a common, but poorly understood, problem in southern California. Research to date suggests that they result from greatly accelerated rates of surface erosion by both wet and dry processes during the days and weeks following a fire. Significant amounts of hillslope debris are delivered to stream channels during the fire by a process called dry ravel. An important feature of postfire erosion is the rapid development of extensive rill networks on hillslopes. These rill networks are linked to a layer of water-repellent soil that forms a few millimeters below the ground surface during the fire. These rill networks result from numerous, tiny debris flows that occur on the hillslopes during the early storms. The rill networks form rapidly, often in a matter of minutes, and provide an efficient means for transporting surface runoff to stream channels. This helps explain why postfire debris flows often occur during very small storms and after short periods of rainfall.