MAPPING POST-FIRE GEOMORPHIC PROCESSES ON STEEPLANDS IN SOUTHERN CALIFORNIA FROM TERRESTRIAL LIDAR DEMS

Volumes of post-fire debris flows are greatly enhanced by valley network aggradation providing readily transportable sediment during subsequent storms. Sediment transported by dry ravel and overland flow in response to wind and rain on recently burned steeplands creates temporal and spatial patterns of local sources and sinks. To quantify these post-fire changes in southern California, we mapped geomorphic process signatures on high-resolution topography derived from repeat ground-based terrestrial lidar surveys (TLS) of two steep, zero-order catchments.

Through repeat TLS, generation of bare-earth model DEMs, and field mapping we documented how patterns of exhumed bedrock, sediment redistribution/volume changes on hillslopes, and sediment infilling of low-order valleys rapidly altered both the variable source area for overland runoff and the sediment available for transport by post-fire debris flows. Hillshade, gradient, and curvature bases with 1-m contours were most useful to distinguish geomorphic processes whereas elevation difference maps between repeat surveys were most useful in estimating volumes of sediment redistribution. We derived ten distinct map units through field observations of topographic character, sedimentary structures, and depositional or erosional relationships. Associated descriptions of map units include surface morphology, sediment texture and character, consistence, vegetation and root types, and relative age categories. Map unit boundaries varied over time in response to changing rainfall/runoff relationships, storm magnitudes, and local conversion to sediment supply limited conditions where the underlying bedrock or competent A-horizon was exhumed. Observations indicated that macro-scale rills (~5-20 cm wide) primarily formed where loose, granular material and post-fire ash and charcoal deposits remained. Hence, the absence of rilling does not necessarily indicate a lack of overland flow, but rather may denote the exposure of more resistant materials with lower infiltration capacities. An understanding of how such surfaces may promote runoff generation and potential mobilization of sediment aggraded in the valley axes, generally <1m³ volume per meter of channel length, during runoff events improves predictions of debris flow initiation.