Progressive channel erosional processes of the 2020 Heixiluo debris flow in Dadu River, southwestern China

Debris flows are the common gravity-driven mass flows in mountain areas that have high sediment concentration, wide grain sizes, and strong impact force.  Erosion or entrainment is the main mechanism by which the flows significantly increase their volume and destructive potential when they progressively move down over colluvial or alluvial beds. However, the scale and mechanism of erosion are poorly understood due to scarcity of field data. We present on-site data of a rare event in which three consolidated landslide dams were incised deeply by debris flows in a small catchment, southwestern China. The highest erosion rate was up to 1.3 m/min or 568 m³ per unit channel length. The channel topographical condition controls transition from erosion to deposition and the locations of local erosion maxima. An outburst-flood erosion model incorporating flow discharge, channel slope and erodibility is adopted to simulate the progressive erosion of these dams. The infrequent case confirms the key role of debris flows in alpine landscape evolution and provides field data (case data) for developing advanced erosion models. This work provides new insights into the role and scale of debris-flow erosion in catchment evolution.