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USGS - science for a changing world

Post-Fire Debris Flow



This debris flow video was captured by resident Howard Davis. It shows an example of the power of debris-flow events as mud and boulders rolled down the street in front of his house in Devore, CA, on Christmas Day 2003. USGS streamgage data help emergency managers prepare for hazards associated with debris flow and flooding events. Video courtesy of Mr. Davis.

Post-fire landslide hazards include fast-moving, highly destructive debris flows that can occur in the years immediately after wildfires in response to high intensity rainfall events, and flows that are generated over longer time periods that are accompanied by root decay and loss of soil strength. Post-fire debris flows are particularly hazardous because they can occur with little warning, can exert great impulsive loads on objects in their paths, can strip vegetation, block drainage ways, damage structures, and endanger human life.

Post-fire debris flows are most common in the 2 years after a fire; they are usually triggered by heavy rainfall. Flooding and increased runoff may continue for several years, but it is unusual for post-fire debris flows to be produced beyond the second rainy season. Some of the largest debris-flow events have been triggered by the first intense rainstorm of the storm season. It takes much less rainfall to trigger debris flows from burned basins than from unburned areas. In southern California, as little as 7 millimeters (0.3 inches) of rainfall in 30 minutes has triggered debris flows, and any storm that has intensities greater than about 10 millimeters/hour (0.4 inches/hour) is at risk of producing debris flows.

While multiple factors can affect debris-flow occurrence, post-fire debris flows generally are triggered by one of two processes: surface erosion caused by rainfall runoff, and landsliding caused by infiltration of rainfall into the ground. Surface erosion runoff processes are by far the most prevalent contributor to debris flows. This is because fires commonly reduce the rate at which water can seep into the soil, which increases runoff and erosion. Landsliding processes are much less common causes of fire-related debris flow, but prolonged heavy rains may increase soil moisture even after a wildfire. The wetted soil then may fail, producing infiltration-triggered landslides. Wildfires can also result in the destabilization of pre-existing deep-seated landslides over long time periods

Emergency Assessment of Post-Fire Debris-Flow Hazards

Wildfire can significantly alter the hydrologic response of a watershed to the extent that even modest rainstorms can produce dangerous flash floods and debris flows. The USGS conducts post-fire debris-flow hazard assessments for select fires in the Western U.S. We use geospatial data to understand the size and shape of the watershed, and burn severity, soil properties, and rainfall characteristics to estimate the probability and volume of debris flows that may occur in response to a design storm.