CFD-DEM-Based Simulation Study on Lateral Sudden Sediment Supply and Riverbed Evolution in a Mountainous Stream Channel Induced by Multi-Stage Slope Slumps

Under dynamic loading (e.g., earthquakes, extreme rainfall), multi-stage slope slumps occur as downstream slopes lose anti-sliding stability, triggering intensive lateral sediment supply that governs mountainous channel evolution. This study uses a coupled CFD-DEM model to simulate how water–sediment conditions regulate sediment transport and riverbed deformation. Results show that during the first sediment supply event, particle motion is initially slower under wet than dry conditions but accelerates due to buoyancy, with the peak average particle velocity along the gully axis decreasing by 11.5% and exhibiting negligible flow rate dependence. In the channel, higher flow rates raise particle velocity and downstream sediment flux, while a prolonged supply interval elevates peak velocity and delays its occurrence. For subsequent events, peak gully axis and vertical velocities increase with sediment supply mass, with weak dependence on flow rate or interval. Post-peak particle motion accelerates with these three factors, enhancing sediment entrainment effects. Increasing flow rate from 1.7 to 2.2 L/s, supply mass from 0.75 to 1.50 kg, and interval from 4 to 6 s significantly strengthens substrate dynamic response, with the peak average velocity rising by 78.3%, 33.3%, 67.0% and maximum displacement by 80.7%, 51.2%, 67.6%, respectively. Channel particle velocity is more sensitive to flow rate but suppressed by greater sediment mass and shorter intervals. The deposited riverbed has three zones: first-supply-dominated, mixed, and subsequent-supply-dominated. Higher flow rates restrict depositional area expansion but increase thickness, whereas greater subsequent sediment expands its dominant zone while reducing thickness, with minimal influence from supply intervals. This study offers theoretical insights for preventing water–sediment disasters in mountainous areas.