Debris flows are powered by sediment supplied from steep hillslopes where soils are often patchy and interrupted by bare‐bedrock cliffs. The role of patchy soils and cliffs in supplying sediment to channels remains unclear, particularly surrounding wildfire disturbances that heighten debris‐flow hazards by increasing sediment supply to channels. Here, we examine how variation in soil cover on hillslopes affects sediment sizes in channels surrounding the 2020 El Dorado wildfire, which burned debris‐flow prone slopes in the San Bernardino Mountains, California. We focus on six headwater catchments (<0.1 km2) where hillslope sources ranged from a continuous soil mantle to 95% bare‐bedrock cliffs. At each site, we measured sediment grain size distributions at the same channel locations before and immediately following the wildfire. We compared results to a mixing model that accounts for three distinct hillslope sediment sources distinguished by local slope thresholds. We find that channel sediment in fully soil‐mantled catchments reflects hillslope soils (
Landscapes after wildfire commonly experience accelerated hillslope erosion, which often contributes to the mobilization and volume of debris flows. However, quantitative studies of the erosion and its relationship to rainfall, runoff, and landscape characteristics have been limited to a narrow range of physiographic conditions. We estimated the volume and delivery rate of slurry (a water‐sediment mixture) supplied to stream channels during a post‐wildfire rainstorm that generated large debris flows in six catchments above Montecito, CA, in 2018. We mapped the distribution of rills and measured their cross‐sectional geometries to quantify the influences of runoff, lithology, and hillslope characteristics on the sediment volumes released by rill erosion, and we scaled the results up to the 19.5 km2of burned hillslopes in the source catchments. We computed the likely rate of surface runoff during the rainstorm and developed an empirical model for the evolution of a representative hillslope‐spanning rill to illustrate the magnitude and speed of the erosion process. Rilling was the dominant form of erosion across the hillslopes of the source catchments, and the rapid evacuation and mixing of water and sediment during rill formation supplied a slurry with high solids concentrations to stream channels. Colluvium on shale formations was more continuous, finer‐grained, and probably less permeable than colluvium on sandstones, and these differences affected the extent and dimensions of rills. As a result, shale hillslopes were the dominant source of slurry to the debris flows and supplied over twice as much slurry per unit burn area as sandstones.
more » « less- NSF-PAR ID:
- 10362970
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Earth Surface
- Volume:
- 126
- Issue:
- 11
- ISSN:
- 2169-9003
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Landscapes following wildfire commonly have significant increases in sediment yield and debris flows that pose major hazards and are difficult to predict. Ultimately, post-wildfire sediment yield is governed by processes that deliver sediment from hillslopes to channels, but it is commonly unclear the degree to which hillslope sediment delivery is driven by wet versus dry processes, which limits the ability to predict debris-flow occurrence and response to climate change. Here we use repeat airborne lidar topography to track sediment movement following the 2009 CE Station Fire in southern California, USA, and show that post-wildfire debris flows initiated in channels filled by dry sediment transport, rather than on hillsides during rainfall as typically assumed. We found widespread patterns of 1–3 m of dry sediment loading in headwater channels immediately following wildfire and before rainfall, followed by sediment excavation during subsequent storms. In catchments where post-wildfire dry sediment loading was absent, possibly due to differences in lithology, channel scour during storms did not occur. Our results support a fire-flood model in bedrock landscapes whereby debris-flow occurrence depends on dry sediment loading rather than hillslope-runoff erosion, shallow landslides, or burn severity, indicating that sediment supply can limit debris-flow occurrence in bedrock landscapes with more-frequent fires.more » « less
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