Debris Flow Surges Amplification Controlled by Topography and Rheology

Field observations in the Jiangjia Ravine show that surge characteristics evolve systematically along the flow path. As the flows transition from steep upstream slopes to gentler downstream reaches, surge forms shift from high-frequency low-amplitude surges to low-frequency high-amplitude surges. To explain the spatial evolution of surges, we develop a mechanistic model governed by slope geometry and yield stress. The shear stress can fall below the yield stress at slope breaks, temporarily blocking the flow. Subsequent surges with higher shear stress exceed the yield stress and remobilize the stored material. Experimental results show that on the steep slope, the mixture generates unsteady roll waves. Once these waves reach the gentle slope, they further amplify and evolve into distinct surge fronts, confirming the proposed model. These findings establish a conceptual framework for understanding the accelerated evolution of debris-flow surges.