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Large-scale landslide dams can induce significant hazards to human lives by blocking the river flows and causing inundation upstream. They may trigger severe outburst flooding that may devastate downstream areas once failed. Thus, the advancement in understanding the formation of landslide dams is highly necessary. This paper presents three dimensional numerical investigations of the formation of landslide dams in open fluid channels via the Discrete Element Method (DEM) coupled with Computational Fluid Dynamics (CFD). By employing this model, the influence of flow velocity on granular depositional morphology has been clarified. As the grains settle downwards in the fluid channel, positive excess water pressures are generated at the bottom region, reducing the total forces acting on the granular mass. In the meantime, the particle sedimentations into the fluid channel with high impacting velocities can generate fluid streams to flow backwards and forwards. The coupled hydraulic effects of excess water pressure and fluid flow would entrain the solid grains to move long distances along the channel. For simulations using different flow velocities, the larger the flow velocity is, the further distance the grains can be transported to.In this process, the solid grains move as a series of surges, with decreasing deposit lengths for the successive surges. The granular flux into the fluid channel has very little influence on the depositional pattern of particles, while it affects the particle-fluid interactions significantly.The results obtained from the DEM-CFD coupled simulations can reasonably explain the mechanisms of granular transportation and deposition in the formation of landslide dams in narrow rivers.

The International Society for Rock Mechanics (ISRM) has suggested a notched semi-circular bend technique in split Hopkinson pressure bar (SHPB) testing to determine the dynamic mode I fracture toughness of rock. Due to the transient nature of dynamic loading and limited experimental techniques, the dynamic fracture process associated with energy partitions remains far from being fully understood. In this study, the dynamic fracturing of the notched semi-circular bend rock specimen in SHPB testing is numerically simulated for the first time by the discrete element method (DEM) and evaluated in both microlevel and energy points of view. The results confirm the validity of this DEM model to reproduce the dynamic fracturing and the feasibility to simultaneously measure key dynamic rock fracture parameters, including initiation fracture toughness, fracture energy, and propagation fracture toughness. In particular, the force equilibrium of the specimen can be effectively achieved by virtue of a ramped incident pulse, and the fracture onset in the vicinity of the crack tip is found to synchronize with the peak force, both of which guarantee the quasistatic data reduction method employed to determine the dynamic fracture toughness. Moreover, the energy partition analysis indicates that simplifications, including friction energy neglect, can cause an overestimation of the propagation fracture toughness, especially under a higher loading rate. Keywords Dynamic fracture toughness Á Discrete element method Á SHPB Á Rate dependent Á Energy partition Abbreviations DEM Discrete element method ISRM International Society for Rock Mechanics NSCB Notched semi-circular bend SHPB Split Hopkinson pressure bar SIF Stress intensity factor a Crack length of the NSCB sample (m) a a Dimensionless crack length of the NSCB sample A b Cross-section area of the pressure bars (m 2 ) A s Area of the fracture surface (m 2 ) B Thickness of the NSCB sample (m) dF s Increment of the shear force (N) ds Increment of the relative displacement (m) E bYoung's modulus of the elastic bars (MPa) E bond Potential energy stored in bonds (J) E contact Potential energy stored in contacts (J) E friction Friction energy (J) E kinetic Kinetic energy (J) f Im Values of the contact force m on the barspecimen incident interface (N) f Tm

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