Shedding effects of sediment composition and bed morphology on debris flow dynamics and entrainment mechanism: Insights from laboratory experiments

Sediment composition and bed morphology are two crucial factors that can significantly affect the dynamics of debris mixtures and attendant entrainment characteristics. In this study, three types of bed sediments with varying contents of coarse and fine grains and different bed longitudinal transition angles were designed for our small-scale flume experiments. The profound deceleration, redirection, and regime transition of debris flows when moving over the transition sites were detected based on experimental observations and measurements. For the coarse-grained bed sediment (CBS) and fine-grained bed sediment (FBS), high flow noses developed with an exploded or wedge-shaped front. For the wide-grading bed sediments (WBS), the debris mixture can be regarded as a tabular flow. These effects on debris flow dynamics became more pronounced as the bed transition angle increased. The measured pore pressures decreased successively for the CBS, WBS, and FBS, which may be attributed to the varying hydraulic permeability of different bed sediments. A possible mechanism of pore pressure generation in non-uniform erodible beds is proposed, resulting from local pore contractions within both the debris flow and bed sediments. The spatial distribution of the entrained depths for different bed sediment compositions is similar when the initial bed morphology is plain. As the transition angle increased, the entrained depths varied significantly as obvious erosion holes emerged, which may be ascribed to the intense frontal interactions between the flowing mixture and static bed material and the propagation of shock waves. A comparison between the calculated erosion rates of five mechanical models and the measured global time average of erosion depth proved that the two-phase erosion model (Pudasaini and Fisher, 2020), which comprises both the erosion of solid and fluid phases, is appropriate for describing the eroding forces of debris flows in erodible beds with slight bed longitudinal transitions.