Erosion Mechanisms of Debris Flow on the Sediment Bed

Debris flows are common geological hazards in mountainous regions worldwide. The scale of debris flows can be significantly enhanced by basal erosion and bank collapse in the transportation process, resulting in an increase in casualties and property losses. However, the mechanisms of this growth are largely unclear. Here, we conduct a series of experiments to investigate the erosion of two different bed sediments (coarse‐grained and widely graded) by released flows with three different densities and two different volumes. The erosion mechanisms of bed sediments are revealed by comparing detailed sensor data for flow level, pore pressure and total normal stress. A flow nose develops on the coarse‐grained bed sediment, resulting in a high flow depth and low velocity, while a tabular flow develops on the widely graded bed sediment, leading to a low flow depth and high velocity. The mean erosion rates of the coarse‐grained bed sediment are generally higher than those of the widely graded bed sediment due to significant pore pressure developed in coarse‐grained bed sediment. The feedback effect of bed sediment on the erosion process strongly influences the flow depth and velocity, which in turn affects the mean erosion rate of bed sediment. The interaction between the overlying flow and sediment bed controls the erosion pattern: coarse‐grained bed sediment is eroded by a layer of mass movement whereas widely graded bed sediment is progressively scoured. The interaction between debris flow and bed sediment during erosion is principally attributed to pore‐pressure transmission. Key Points Flow depth and velocity over a coarse‐grained sediment (d50 = 2.4 mm) and a widely graded sediment (d50 = 0.9 mm) differed substantially The coarse‐grained bed sediment was eroded by mass movement while the widely graded bed sediment was progressively scoured The interaction between the overlying flow and sediment bed controlled the erosion pattern