Dry sediment loading of headwater channels fuels post-wildfire debris flows in bedrock landscapes

Roman A Dibiase,Michael P Lamb

doi:10.1130/g46847.1

Roman A Dibiase, Michael P Lamb

Open Access

https://doi.org/10.1130/g46847.1

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Abstract

AbstractLandscapes 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.

Highlights

Sediment yields following wildfire commonly greatly exceed background erosion rates (Moody et al, 2013), threatening life and property at the wildland-urban interface in mountainous terrain (Cannon and DeGraff, 2009)
Rather than being driven by severe storms and soil hydrophobicity that act on hillslope soils, post-wildfire sediment yield in this model is determined by dry sediment supply, which in turn is a function of the storage capacity of sediment stored behind vegetation dams (DiBiase and Lamb, 2013; Lamb et al, 2013) and the connectivity between steep hillslopes and headwater channels (DiBiase et al, 2017)
Overall, our data highlight key differences in the fire-flood cycle between soil-mantled and bedrock landscapes that are important for understanding post-wildfire debris-flow hazards and longer-term landscape evolution

Summary

Introduction

Sediment yields following wildfire commonly greatly exceed background erosion rates (Moody et al, 2013), threatening life and property at the wildland-urban interface in mountainous terrain (Cannon and DeGraff, 2009). Predicting the magnitude of this increase in sediment yield and the consequences of wildfire for longer-term landscape evolution requires a mechanistic understanding of how sediment is delivered from hillslopes to channels and the degree to which post-wildfire erosion is limited by hillslope sediment supply (Roering and Gerber, 2005; Lamb et al, 2011). Rather than being driven by severe storms and soil hydrophobicity that act on hillslope soils, post-wildfire sediment yield in this model is determined by dry sediment supply, which in turn is a function of the storage capacity of sediment stored behind vegetation dams (DiBiase and Lamb, 2013; Lamb et al, 2013) and the connectivity between steep hillslopes and headwater channels (DiBiase et al, 2017). Steep landscapes commonly exhibit a patchwork of soil-mantled and bare-bedrock hillslopes (DiBiase et al, 2012), making it challenging to determine the relative importance of wet versus dry transport processes

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