The interaction between basal and earth-surface flows such as dam-break flows, hyper-concentration flows and debris flows, for example, entrainment and erosion, have crucial influences on their dynamic characteristics and rheological behaviour. Among these mass flows, debris flows that are able to entrain a great number of masses tend to be the most catastrophic. A fully coupled CFD-DEM technique is utilised here to evaluate the erosion effect of debris flow on the sand basal, in which basal is modelled via a non-cohesive particle assembly using DEM and debris flows are recognised as a non-Newtonian fluid. Herschel-Bulkley-Papanastasiou (HBP) rheological model is implemented to capture the viscous behaviour of debris flows. Besides, considering the gap-graded feature of basal, the traditional coarse-grain method that accounts for porosity based on particle centres may result in inaccuracy in void fraction determination. Thus, the semi-resolved method is used to determine physical fields. Morphology of the basal is updated each time step following DEM calculation cycles.
Debris flows have been widely recognised as one of the catastrophic geological hazards. The initiation of development of debris flows tends to be unpredictable and speedy. Massive mixtures can be entrained during the motion of debris flows, which fairly exaggerates the damage induced by debris flows. Among composites of debris flows, larger solid particles like boulders take a crucial role in resultant destruction. Regards of the high flowing speed and damage of debris flows, experiment studies were mostly conducted with the centrifuge model and large-scale model box [1-3]. In real cases, debris flows triggered by rainfall are frequently confronted. As such, it is of great importance to understand its flowing behaviour and the interaction between solids and fluids bearing inside debris flows. Coupling methods provide an avenue to unveil the multiscale behaviour of debris flows, among which the CFD-DEM method proposed by Tsuji et al. [4, 5] might be the most favoured one. Primarily, Li et al. [6, 7] revealed the interaction between saturated debris flows and flexible structures via the divided void fraction scheme as stated in [8, 9]. Following these, Kong et al. [10] further applied the CFD-DEM method in a real case. A thorough perspective about the generation of the dead zone and the impact of debris flows on flexible barriers is also given in [11] and [12], respectively. Zhao et al. also studied the moving transition process of submerged debris flows with the unresolved CFD-DEM method. In addition to the impact on engineering structures and flowing behaviour itself, debris flows also entrain a great number of masses that exaggerate damage during moving. Zheng et al. [13] discussed the effect of viscous shear flows on the entrainment of debris flows using the unresolved CFD-DEM method. Up to date, most of the numerical studies treated the basal as rigid or uniformly graded. However, the fabric of basal particles is known to be gap-graded under some circumstances, in which boulders might be entrained along the path debris flows passed by. Conventional CFD-DEM coupling schemes like the coarse grid method might not be suitable for gap-graded issues as the ratio of fluid cells to particle diameter shall be smaller than at least 1/3 [14]. To tackle the situation where particle diameter might be equal or larger than fluid cells, Jing et al. [15] put forward the big-particle model and compared it with the previous divided model.
