Recruitment and Dispersal of Post-Wildfire Debris Flows

Abstract

Quantifying sequences of events and materials involved in the growth and dispersal of post-wildfire debris flows across entire mountain catchments and piedmont fans is rarely possible. However, understanding these processes facilitates assessing future flow magnitudes and recurrence risk. This study analyzed the evolution of debris flows generated by a rainstorm following near-complete burning of vegetation in six mountain watersheds. The flows transported large volumes of boulders to downstream fans, devastating Montecito, California. With rainfall-runoff modeling, lidar, photogrammetry, and field surveys, we quantified the hydrological and sedimentological components of the debris flows as they evolved from hillslope runoff to boulder-rich fan deposits and ocean discharge. Runoff from burned soils drove larger amounts of rill erosion and slurry generation on shale hillslopes than on sandstones. Hillslope slurry mobilized ravel deposits and fine sediments stored in the channel network, mainly on shales. Channels draining sandstones mainly supplied the flows' boulder loads. One-quarter of the mountain-shed sediment escaped to the ocean, while all boulders settled on the fans. Flows confined to primary channels remained erosive across the fans except where channel gradients and dimensions decreased in response to fault-related topography and where bridges trapped boulders, intensifying in-channel and overbank deposition. Although the results derive from a single event, they illustrate how a sequence of processes and landscape conditions determine debris-flow evolution across catchments and fans. Identifying debris-flow components highlights useful measurement and modeling methods to improve prediction while highlighting current limits on understanding critical processes and transient antecedent conditions.