Quantifying post-wildfire hillslope erosion with lidar

Following a wildfire, flooding and debris- flow hazards are common and pose a threat to human life and infrastructure in steep burned terrain. Wildfire enhances both water runoff and soil erosion, which ultimately shape the debris flow potential. The erosional processes that route excess sediment from hillslopes to debris-flow channels in recently burned areas, however, are poorly constrained. In this study we examined erosional processes through repeat terrestrial lidar surveys in a steep mountainous watershed that experienced a high-severity burn in the 2016 San Gabriel complex fire. Three lidar surveys were conducted during a wet winter (2016-2017) on a hillslope plot. We used geomorphometric techniques to better contextualize erosion observations in areas with rills and between rills (interrill areas). A challenge was effectively differentiating DEM pixels that were in the constantly evolving rill network as well as those outside the rill network. By applying a series of DEM filtering processes we found that it was possible to efficiently identify the small-scale rill networks. Our results challenge previously held beliefs about sediment erosion on burned hillslopes, suggesting that prior estimates made without access to high resolution topography likely underestimated the role of interrill erosion.