Debris flow initiation by runoff in a recently burned basin: Is grain‐by‐grain sediment bulking or en masse failure to blame?

Postwildfire debris flows are frequently triggered by runoff following high‐intensity rainfall, but the physical mechanisms by which water‐dominated flows transition to debris flows are poorly understood relative to debris flow initiation from shallow landslides. In this study, we combined a numerical model with high‐resolution hydrologic and geomorphic data sets to test two different hypotheses for debris flow initiation during a rainfall event that produced numerous debris flows within a recently burned drainage basin. Based on simulations, large volumes of sediment eroded from the hillslopes were redeposited within the channel network throughout the storm, leading to the initiation of numerous debris flows as a result of the mass failure of sediment dams that built up within the channel. More generally, results provide a quantitative framework for assessing the potential of runoff‐generated debris flows based on sediment supply and hydrologic conditions. Key Points Debris flow initiation at our study area resulted from the mass failure of channel bed sediment Sediment eroded from hillslopes during the rainstorm made substantial contributions to debris flow volume Slope stability models applied to channel bed sediment can provide estimates for runoff‐generated debris flow timing and magnitude