Postfire debris flows occur quickly after rainfall starts



Areas damaged by forest fires can be vulnerable to debris flows because vegetation is no longer holding dirt and rocks in place, and debris flows in burned areas can be triggered by much less rainfall than would be needed to trigger a debris flow in an unburned area. Such debris flows are common in burned steep terrain in Southern California and sometimes cause significant damage to property and even loss of life—for instance, 16 people died in debris flows above San Bernardino on 25 December 2003. To better understand the conditions that lead to debris flows, Kean et al. measured properties including rainfall, channel bed pore fluid pressure, and hillslope soil water content for 24 debris flow events that occurred in five different watersheds that burned in the 2009 Station and Jesusita fires in the San Gabriel and Santa Ynez mountains. They looked at the timing of rainfall and debris flow events.