Debris flows are mixture of grains and water that surge downslope rapidly. These natural hazards claim lives and destroy infrastructure every year across the globe. Conventionally, mitigation of debris flow is often done by installing single rigid barrier along the predicted flow path. However, for large volume debris flow, the size of single rigid barrier can become unrealistically large, making it costly and difficult to construct. Instead of single rigid barrier, installing multiple rigid barriers is a viable option for the mitigation of large volume debris flow. Multiple barriers progressively reduce the kinetic energy of the debris flow and retain debris, thus reducing the size of individual barrier. However, the current design guidelines for these barriers are based on volume rete...[ Read more ]

Debris flows are mixture of grains and water that surge downslope rapidly. These natural hazards claim lives and destroy infrastructure every year across the globe. Conventionally, mitigation of debris flow is often done by installing single rigid barrier along the predicted flow path. However, for large volume debris flow, the size of single rigid barrier can become unrealistically large, making it costly and difficult to construct. Instead of single rigid barrier, installing multiple rigid barriers is a viable option for the mitigation of large volume debris flow. Multiple barriers progressively reduce the kinetic energy of the debris flow and retain debris, thus reducing the size of individual barrier. However, the current design guidelines for these barriers are based on volume retention and overlook the debris-barrier interaction. Consequently, the existing design is not able to capture the influence of overflow dynamics resulting from flow-barrier interaction. Depending on the flow composition and first barrier height, flow barrier-interaction varies and consequently affects the design of the second barrier. Up till now, the influence of these factors on the existing design is not well understood. A systematic investigation is thus needed to provide scientific basis for efficient design of multiple barriers considering parameters such as solid fraction, barrier height and barrier spacing.

In this study, physical and numerical modelling using Lattice Boltzmann Method (LBM) were adopted. The influence of solid fraction on the overflow and landing mechanisms of single rigid barrier were investigated using 5 m-long flume. A similar setup was used to study the impact mechanism of dual rigid barriers spaced at a distance of 1 m for two extreme flow types, namely water and dry sand. Furthermore, to minimise the scaling effects pertaining to two-phase flows, debris flow impacting dual rigid barriers was modelled in 28 m-long flume. Additionally, the LBM based numerical model was calibrated against the experiments conducted in 28 m-long flume. The calibrated model was then used to conduct a parametric study on the combined influence of first barrier height and spacing between barriers on the impact dynamics of the second barrier.

It is found that flows with larger solid fraction v_s result in shorter landing distances x_i due to enhanced grain contact friction. The landing distance x_i also increases with Fr_b (ratio of kinetic energy to the potential energy due to barrier height) because of lower energy dissipation due to low barrier potential. Existing guidelines that recommend barrier spacing based on the geometry of the retained material are insufficient and only valid for Fr_b ~1. For robust and conservative design, minimum barrier spacing should use the maximum of the landing distance x_i and retained volume-based spacing L_min.

Measured results show that for granular flows (v_s = 0.6, with air as pore fulid) when overflow occurs, the impact force at first barrier increase by up to 40% due to the surface drag induced by overflowing debris over retained material. Moreover, a simplified impact equation with dynamic impact coefficient α = 1.5 and static pressure coefficient k = 1 is recommended to estimate design load conservatively for a wide range of flow type on the first rigid barrier of a dual rigid barrier system. In addition, the impact force on the second barrier spaced at a constant distance increases linearly with an increasing Fr_b of first barrier. Furthermore, for the same first barrier height (or Fr_b), with increasing spacing between barriers, the impact force at the second barrier reduces.