Journal of Geophysical Research: Earth Surface
© American Geophysical Union
Impact Factor: 3.426
ISI Journal Citation Reports © Ranking: 2014: 19/175 (Geosciences Multidisciplinary)
Online ISSN: 2169-9011
Associated Title(s): Journal of Geophysical Research
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 burnt 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 a debris flow above San Bernardino on 25 December 2003. To better understand the conditions that lead to debris flows, Kean et al. (2011) 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. Although there were differences between the sites and between different events at the same site, the researchers consistently found that debris flow events began very quickly-within 30 min-after the onset of a rainstorm. Debris flow was not found to be correlated with soil moisture, and debris flows were primarily initiated by surface water runoff rather than by landslides. This was true across sites of different size and different geologic material. The data could be useful in constraining future models of debris flow following fires, but more important, they show that to give enough practical lead time, early warning of debris flow must rely on weather forecasts-once a heavy rain starts, it is too late to warn people of an impending debris flow.