Joint influences of topographical and flow conditions on debris-flow erosion and deposition: insights from the 2020 Heixiluo event in the Dadu River Basin

Debris flows have a high capacity for transporting sediments from hillslopes or steep channels into rivers via erosion and deposition. However, the lack of well-defined critical conditions for the two processes hampers the development of a fully physical model incorporating them. In this paper, we explore the topographical and flow conditions of erosion and deposition through a case study of the Heixiluo catchment in southwestern China, where a high-magnitude debris flow has deeply incised three consolidated landslide dams. The highest erosion rate was up to 1.3 m/min, or 568 m 3 per unit channel length. The channel topographical conditions controlled the local maximal erosion and the transition from erosion to deposition. We examine the relationship of erosion depth with unit discharge, channel slope, and channel bankfull width, and identify the critical values below which the deposition occurred. The channel slope and unit discharge have a higher correlation with the erosion depth than with the bankfull width. An outburst-flood erosion equation incorporating flow discharge, channel slope, and erodibility is integrated to compute the progressive channel erosion at two cross-sections. The back-calculated erodibility is two orders of magnitude higher than in other cases of water floods. This study provides new insights into the role and scale of channel topography and flow discharge on debris-flow erosion and deposition.