Mechanisms of Volume-Dependent Debris Flow Impact on Multiple Barriers Using an Efficient Two-Phase MPM

As powerful and destructive geological processes, debris flows are rapidly moving slopes of rock, mud, and water that present a serious hazard to people and the built environment. Multiple barrier system is an effective, economical and sustainable approach for progressively mitigating large-volume flows. Existing multiple-barrier design recommendations are mainly based on experiments and simulations of flows with volumes far less than those in real-world scenarios. However, the two-phase nature of debris flows determine that their behaviour is volume dependent. This may lead to unconservative design of multiple barriers. Therefore, it is crucial to investigate how the flow volume affect the flow-interaction. Additionally, much attention has been focused on the flow-dual barrier interaction, hence existing recommendations should be improved for multiple barrier design. In this study, an improved, efficient, two-phase material point method was proposed to investigate the impact, overflow, and landing of flows interacting with barriers. The proposed method was validated against existing experimental data on flow-barrier interactions, including dry granular flows, water, and two-phase flows. Then the method was used to conduct three series of parametric studies. The first series specifically examined the pore fluid pressure and the impact force on the second barrier. The second series varied flow volume to investigate its influence on overflow and landing. The flow volume ranges four orders of magnitude—from experimental volumes (1 m3) to extreme scenarios in Hong Kong (103 m3). Finally, the multiple barriers with different barrier heights were simulated to mitigate debris flows. Key findings reveal that the normalised landing distance for 8000 m3 flows can be 4.5 times for that 1 m3 flows, because the associated energy dissipation is reduced to one-tenth. Pore fluid pressure and shear rate-dependency determine the variation in solid shear stress across different volumes, which in turn governs the resulting energy dissipation. This is because, in larger volumes (e.g., 8000 m3): (i) the fluidisation ratio 𝜆 often exceeds 0.8, and (ii) the reduced shear rate weakens dilatancy and results in a lower friction coefficient. The volume-dependency can be neglected for volumes beyond 8000 m3, since the energy dissipation shows negligible further decrease. A parameter 𝜇eq is proposed to quantify the shear stress under different volumes. An optimised barrier spacing and height are identified. Spacings smaller than the optimum increase flow depth at subsequent barriers by up to 30%, amplifying impact forces. However, if the barrier spacing larger than the value, flow accelerates under gravity. Therefore, it is suggested the barrier spacing ranging from 2 to 4 times the landing distance. The barrier height is recommended to be 2.5 times the flow depth, to enhance the energy dissipation when 𝜇eq ≥ 0.18 and avoid turbulence after landing when 𝜇eq ≤0.10. Design recommendations on multiple barriers are improved and validated. The landing distance is calculated based on the 𝜇eq, to consider the effects of flow volume. The landing factor is suggested to be 1.5 for the first barrier, accounting for the acceleration during landing. After landing, the energy dissipated by basal friction is reduced through a 30% increase of 𝜆. The results show that the impact force on the multiple barriers is conservatively estimated.