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

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 similar to 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 similar to 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 postfire ravel routing in steep, rocky landscapes where sediment was sourced by incineration of vegetation dams-following similar to 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.

Automated summary

Steep, rocky landscapes often produce large sediment yields and debris flows following wildfire. Observations in the San Gabriel Mountains, California, showed that debris flows were initiated from dry ravel after a fire, evacuating accumulated material in the channel network. Sediment budgets confirmed that most sediment deposited in a debris flow matched that evacuated from the channel, with additional contribution from hillslope erosion during rainfall. Overall, the study found that failure of channel bed alluvium due to dry ravel deposits could lead to debris flow initiation even during moderate intensity rainstorms.