Debris flow initiation from ravel-filled channel bed failure following wildfire in a bedrock landscape with limited sediment supply

Abstract

Steep, rocky landscapes often produce large sediment yields and debris flows following wildfire. Debris flows can initiate from landsliding or rilling in soil-mantled portions of the landscape, but there have been few direct observations of debris flow initiation in steep, rocky portions of the landscape that lack a thick, continuous soil mantle. We monitored a steep, first-order catchment that burned in the San Gabriel Mountains, California, USA. Following fire, but prior to rainfall, much of the hillslope soil mantle was removed by dry ravel, exposing bedrock and depositing ∼0.5 m of sandy sediment in the channel network. During a one-year recurrence rainstorm, debris flows initiated in the channel network, evacuating the accumulated dry ravel and underlying cobble bed, and scouring the channel to bedrock. The channel abuts a plowed terrace, which allowed a complete sediment budget, confirming that ∼95% of sediment deposited in a debris flow fan matched that evacuated from the channel, with a minor rainfall-driven hillslope contribution. Subsequent larger storms produced debris flows in higher-order channels but not in the first-order channel because of a sediment supply limitation. These observations are consistent with a model for post-fire ravel routing in steep, rocky landscapes where sediment was sourced by incineration of vegetation dams—following ∼30 years of hillslope soil production since the last fire—and transported downslope by dry processes, leading to a hillslope sediment-supply limitation and infilling of low-order channels with relatively fine sediment. Our observations of debris flow initiation are consistent with failure of the channel bed alluvium due to grain size reduction from dry ravel deposits that allowed high Shields numbers and mass failure even for moderate intensity rainstorms.

Hydrogeomorphic Recovery and Temporal Changes in Rainfall Thresholds for Debris Flows Following Wildfire

“…The Fish Fire was part of the San Gabriel Complex, a combination of the Fish and Reservoir Fires. Postfire debris flows have been extensively studied in the SGM (Cannon et al., 2011; DiBiase & Lamb, 2019; Gartner et al., 2014; Kean et al., 2011; McGuire et al., 2017; Palucis et al., 2021; Staley et al., 2014; Tang et al., 2019a). Whereas our focus here is on the Fish Fire, we also include data and model results from past studies that focused on the nearby 2009 Station Fire in order to more effectively parameterize our hydrologic model.…”

Movement of Sediment Through a Burned Landscape: Sediment Volume Observations and Model Comparisons in the San Gabriel Mountains, California, USA

“…Post‐wildfire erosion has even been found to generate the majority of long‐term erosion in some mountain systems (Kirchner et al., 2001; Ellett et al., 2019; Orem & Pelletier, 2015). Consequently, understanding how fire influences both short‐term hazards, such as debris flows (Cannon, 2001; Cannon & DeGraff, 2009; Cannon et al., 2003; Kean et al., 2011, 2012; Nyman et al., 2011; Palucis et al., 2021), and longer‐term geomorphic changes to the landscape (Meyer et al., 1992; Pierce & Meyer, 2008), requires datasets that quantify the spatial redistribution of sediment through hillslope‐channel systems following fire.…”

Multi‐Stage Soil‐Hydraulic Recovery and Limited Ravel Accumulations Following the 2017 Nuns and Tubbs Wildfires in Northern California

“…This may be due solely to the dearth of high‐intensity rainfall necessary to trigger post‐fire debris flows in the region (e.g., Figure S1 in Supporting Information S1). Considering how dry ravel sedimentation may act to substantially reduce critical discharge for in‐channel failure, however (Palucis et al., 2021), our observations of limited post‐fire dry ravel transport suggest that burned Northern California watersheds may be comparatively less prone to post‐fire debris flow hazards even while other sedimentary hazards such as increased reservoir sedimentation may still persist (East et al., 2021).…”