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 lacdk 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.

中文翻译：

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在沉积物供应有限的基岩景观中，野火后填塞的河道床破坏引发的碎屑流

陡峭的岩石景观通常会在野火后产生大量沉积物和泥石流。泥石流可以从景观的土壤覆盖部分的滑坡或钻孔开始，但很少有直接观察到泥石流在缺乏厚实连续土壤地幔的陡峭岩石部分的景观。我们监测了美国加利福尼亚州圣盖博山脉燃烧的陡峭一级集水区。火灾发生后，但在降雨之前，大部分山坡土壤地幔被干裂移除，暴露基岩并在通道网络中沉积约 0.5 m 的沙质沉积物。在一年的重复暴雨期间，河道网络中开始发生泥石流，疏散堆积的干碎石和下面的鹅卵石床，并将河道冲刷成基岩。水道紧靠犁过的台地，这允许完整的沉积物收支，证实约 95% 沉积在泥石流扇中的沉积物与从通道疏散的沉积物相匹配，并有少量降雨驱动的山坡贡献。随后更大的风暴在高阶通道中产生了泥石流，但由于沉积物供应限制，在一级通道中没有产生泥石流。这些观察结果与在陡峭的岩石景观中的火灾后散落路线模型一致，其中沉积物是通过焚烧植被大坝（自上次火灾以来大约 30 年的山坡土壤生产之后）并通过干燥过程向下坡输送，导致山坡沉积物供给限制和沉积物较细的低阶河道充填。