Terrestrial Laser Scanning and Post-Wildfire Geomorphic Transport Processes (Invited)

Landscapes denuded by wildfire typically display accelerated transport rates delivering sediment to valleys as potential source material for entrainment during subsequent rainfall. We generated high-resolution topography by surveying steep, low-order drainage basins using repeat terrestrial laser scanning (TLS), lidar, to i) understand the sediment transport processes characteristic of post-wildfire erosion and debris-flows, ii) map geomorphic process signatures, and iii) portray point measurements of flux derived from sediment traps watershed-wide. At the 2009 Station fire burn area in the San Gabriel Mountains, CA, we surveyed topography using TLS five times spanning seven months and multiple debris-flow producing storms. Using lidar-generated bare-earth model DEMs and field mapping using differential GPS, we documented how patterns of rain splash, overland flow scour, and rilling contributed to debris-flow generation. For field mapping, derivative products from TLS, including hillshade, gradient, and curvature bases with 1-m contours, were most useful to distinguish geomorphic processes at watershed scale, whereas elevation difference maps (0.02 m resolution) between repeat surveys were most useful in estimating volumes of sediment redistribution in response to given storms. During our campaigning, we fortuitously surveyed topography within hours both preceding and following the first appreciable storm of the winter season following wildfire. This 28-mm, 1-hr storm induced widespread erosion along the valley axis exceeding 1.5 m depth, in addition to meter-scale debris flows with levees and lobate terminal snouts, some depositing locally where local hillslope gradient decreased. Numerous additional storms throughout the winter produced rills (~5-20 cm wide) forming primarily where loose, granular material derived from weathered granitic bedrock and post-fire char deposits remained, and extended to the drainage divide at small drainage areas. Field observations and time-series elevation difference grids derived from TLS revealed that drainage networks were cut and filled as unconsolidated cohesionless sediment was redistributed by ravel and eolian processes within days to weeks following storms as sediment dried. Although the low-order drainage network was spatially variable, consistent patterns of ridgetop lowering, downslope transport, and widening and deepening within the valley axis continued through the winter. Measuring post-fire sediment transport rates indicative of varying gradient hillslopes using traps, we infer that non-linear exponential relations effectively represent hillslope sediment flux. Such relations are used to spatially extrapolate point-measurement flux rates derived from sediment traps to the landscape scale using topographic gradient maps derived from TLS. This approach allows for approximating the volume of sediment accumulated within the valley network for potential entrainment during rainfall.