Comprehensive Comparison of Flexible, Silt and Rigid Barriers in Mitigating Multiphase Debris Flows: Insights from Large-scale Simulations

Abstract

The steep hilly terrain, intense rainstorms, and dense population render Hong Kong highly vulnerable to debris flow hazards, which can be reduced by placing flexible, silt and rigid barriers in the runout zones. However, quantitative comparisons between different barriers in intercepting debris flows are lacking, due to the difficulties associated with the analysis of multi-body and multi-phase interactions among the barrier, the solid and the fluid in debris flows. Here, we perform the coupled computational fluid dynamics and discrete element method (CFD-DEM) simulations to probe the intricate three-way interactions. Specifically, a debris flow is simulated as a mixture of discrete gap-graded particles and a continuous viscous slurry. A flexible barrier consisting of interlocking ring elements, brakes, and cables is modeled. The numerical results have reasonably captured both field and experimental observations on key flow-structure interactions. We quantitatively compare the results of 24 large-scale simulations of debris flows with pre-impact Froude numbers ranging from 0.9 to 7.5 in terms of the Spatio-temporal impact load distributions, impact durations, overspreading flow dynamics as well as the impact mechanism transitions with different barriers. This allows to clarify the inter-twined relationships among the flow dynamics, barrier types, and impact loads for the first time. After the transition of impact mechanisms from pile-up to runup, flexible barriers can significantly reduce impact loads and overspreading flow dynamics by the extension of impact duration, the passing through of materials, and barrier deformations, as compared to silt and rigid barriers. Therefore, the flexible barrier can be more effective and pragmatic in arresting debris flows with faster flow dynamics. This study helps reveal the fundamental distinctions of flexible, silt and rigid debris-resisting barriers, which may provide a useful reference for their practical engineering analysis.