Geologic and aspect-driven controls on post-fire sediment export from steep headwater catchments

Abstract:

Post-fire hydro-geomorphic responses and their drivers are typically transient and non-uniform in space, which makes them difficult to study. This highlights the continued need for monitoring of recently burned environments subject to rainfall over various spatial and temporal scales to further our mechanistic understanding of post-fire erosional processes. In this study we seek to understand the role of aspect-driven differences in vegetation and differing geologic units on post-fire erosion magnitudes in burned first and second order catchments. We performed point cloud-based change detection on repeat airborne lidar scans on ~90 km² of steep terrain burned by the 2018 Holy Fire in southern California bracketing runoff-producing storms during the first winter following fire and analyzed additional remote sensing products including imagery and normalized burn area reflectance. Maps of change over the full season show that most headwater catchments experienced gully erosion and channel scour into both longer-term accumulations of colluvium and post-fire dry ravel. Preliminary results showed that the highest erosion magnitudes (>2 cm) occurred in catchments with slopes greater than 25 degrees, likely driven by a greater occurrence of runoff-generated debris flows. However, channel erosion magnitudes did not display monotonic relationships with burn severity and slope. This could be partially explained by the fact that more polar-facing watersheds generally had greater soil burn severity values from denser fuel loads but actually had similar but slightly lower net erosion values (1.0 cm ± 0.3 cm) compared to equatorial-facing catchments (1.3 cm ± 0.3 cm). It is possible that this may be driven by the greater prevalence of skeletal biomass and root networks of remaining vegetation on polar-facing slopes, though more work will be needed to verify this. When catchments were subdivided by dominant geologic units it was found that those underlain by deep-seated quaternary landslides had somewhat greater erosion values (1.7 ± 0.4 cm) than those without mapped landslides (1.2 ± 0.3 cm). These preliminary results highlight the potential importance of geologically recent landslide activity and vegetation differences as controls on post-fire erodibility in steep mountainous settings.