POST-STATION FIRE DEBRIS FLOW ANALYSIS IN THE SAN GABRIEL MOUNTAINS

Debris flows are a source of substantial erosion in mountainous areas; consequently, their occurrence, spatial density, and characteristics provide essential data for understanding erosion rates and volumes. The 2009 Station Fire in the San Gabriel Mountains burned an extensive area (649.75 square kilometers), and thus destabilized the slopes setting them up for subsequent debris flows. GIS mapping of post-fire debris flows within the burn area, combined with field mapping, allowed the calculation of area and volume calculations and the spatial density of these flows. Most debris flows initiated from burned, previously undisturbed, upper channel hill slopes averaging 28° and have a spatial density of one flow per every two square kilometers. Total flow material deposited is 715,071 cubic meters and affected 2.5% of the total burn area. The relationship between the area affected by each debris flow and the volume deposited in each flow reveals a positive linear correlation and suggests that as the area affected by a debris flow increases, so does the depositional volume of the flow. Assuming a 30 year recurrence of fires and subsequent debris flows, these flows account for 0.12 ± 0.03 mm/yr of erosion within the burn area. This erosion rate accounts for 7.5 -13.3% of the total erosion rate in the San Gabriel Mountains, according to the erosion rate of 0.9 -1.6 mm/yr (Lavé and Burbank, 2004), despite flows covering only 2.5% of the area. This new data demonstrates that debris flows are more efficient in causing significant erosion compared to typical erosion processes. This data provides an empirical dataset for future debris flow hazard analysis, as well as providing a quantitative assessment of post-fire erosion rates in the San Gabriel Mountains.