POST-WILDFIRE GEOMORPHIC TRANSPORT PROCESS MAPPING USING TERRESTRIAL LASER SURVEYS

Hillslopes denuded by wildfire typically display accelerated transport rates delivering sediment to valley bottoms as potential source material for debris flows during subsequent rainfall. To understand the sediment transport processes characteristic of post-wildfire erosion and debris-flows, and to map geomorphic process signatures on high-resolution topographic base maps, we surveyed steep, low-order drainage basins using repeat terrestrial laser surveys (TLS). Since 2005, surveys have been conducted within 6 burn areas in southern California. Here we present the results of 5 repeat surveys spanning 7 months within the 2009 Station burn area in the San Gabriel Mountains, CA. Through generation of bare-earth model DEMs, and field mapping using GPS of a field site, we documented how patterns of rain splash, overland flow scour, and rilling contributed to debris-flow generation. We fortuitously surveyed topography within hours both preceding and following the first appreciable storm of the winter season following wildfire. On November 12^th, 2009, 28 mm of rainfall over approximately one hour generated widespread erosion of the valley axis exceeding 1.5 m, in addition to meter-scale debris flows with levees and lobate terminal snouts, some locally depositing where local hillslope gradient decreased (<35˚). Numerous storms throughout the winter produced widespread macro-scale 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 within 0.5-2.0 m of the watershed drainage divide. Field observations and the TLS time series 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, presumably lowering apparent cohesion from matric suction. 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. The associated geomorphic process boundaries varied over time in response to changing rainfall/runoff relationships, storm magnitudes, and local conversion to sediment supply limited conditions.