Post-fire debris flow erosion in the San Gabriel Mountains, California: evidence from the Station Fire, 2009

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 (650 square km), and thus destabilized the slopes setting them up for subsequent debris flows. GIS mapping of 58 post-fire debris flows within the burn area from the first year after the fire, allowed the quantification of debris flow density within the burn area. Most debris flows were initiated from burned, previously undisturbed, upper channel hill slopes averaging 28°. The spatial density was one flow per every two square kilometers. Nine flows were chosen for field investigation and debris volume determination. Debris flow volumes from the field measured flows were correlated with drainage basin area above each flow to reveal a positive linear correlation of 0.053. The correlation coefficient was then used to calculate total material from each drainage area affected. Total flow material deposited was 717,300 ± 179,310 cubic meters and covered 6% of the total burn area. The volume of deposition can be quantitatively determined based on the regression equation of V = 0.053A, (V = volume in cubic meters, A = area in square meters). Based on a fire recurrence interval of every 30 years in the San Gabriel Mountains, post-fire debris flows account for 0.13 ± 0.02 mm/yr of erosion within the burn area. This erosion rate accounts for ~10% of the total erosion rate of ~1 mm/yr determined for this area in the San Gabriel Mountains, despite flows covering only 6% of the area. These data provide 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.