Simulating Flow Dynamics in Debris Flows: Transition from Rigid to Erodible Beds in Granular Dam Break Experiments

Debris flow, a prevalent natural hazard in mountainous terrains, exhibits distinct flow dynamics depending on its occurrence over a bedrock (rigid bed) or atop a substantial deposition (erodible bed). The investigation of this flow transition is imperative for the comprehension and mitigation of debris flow dangers. This study introduces a unsteady, and non-uniform model, conceptualized to simulate the transition between rigid and erodible beds in debris flows. The model's foundation lies in the principles of mass, momentum, and kinetic energy conservation. It integrates a linearized mu(I) rheology to articulate granular flow deformation, thereby capturing the intricate interplay among particles during flow. Additionally, the model considers the impact of Coulomb friction along the sidewalls. To derive numerical solutions, the governing equations undergo depth integration, employing the HLL scheme (Harten, Lax, and Van Leer) in synergy with a finite volume numerical method. Furthermore, to corroborate the model's predictions, a novel granular dam break experiment was conducted. These experiments utilized a narrow glass channel (3.5 meters in length and 0.04 meters in width), with variations in the initial deposit depth downstream to establish diverse basal boundary conditions. High-speed camera footage facilitated the application of the Particle Tracking Velocimetry (PTV) method for capturing granular motion and generating a velocity field. A thorough analysis of the measured velocity field enabled the validation of the model's predictions, affirming its efficacy in accurately simulating the flow transition between rigid and erodible beds in debris flows.