Debris flow overflowing flexible barrier: physical process and drag load characteristics

The multiple-barrier mitigation strategy in the debris-flow source area is an effective approach to inhibit debris-flow entrainment and scale amplification along the flow path. However, less is known as debris flow overflows a filled barrier and cascades downstream. The present study investigates the physical processes and load characteristics of debris-flow overflowing a model flexible barrier, using a well-instrumented medium-scale flume. Volumetric solid concentration is varied ranging from 0.4 to 0.6 so that the control factors (e.g., state of liquefaction and turbulent drag) in the overflow process can be identified. The results demonstrate that effective stress is not observed in the incoming debris flows for solid concentration up to 0.6, denoting a contribution of turbulent drag to the overflow drag. The formed hydraulic jump substantially elevates the overflow depth above the flexible barrier, which has not been considered in the current design guideline. Decomposition of the total overflow load reveals that the increase in total load is actually dominated by the static load due to the elevated flow depth, rather than the turbulent drag. Downward momentum transfer to the lower section of barrier is observed and re-liquefaction of the deposited debris is a prerequisite for the downward momentum transfer. Different from the high solid-concentration cases, the deposit of low solid concentration (0.4) behind the barrier is partially drained with nontrivial effective stress, which further hinders the downward momentum transfer to the base of the barrier. The findings of present study indicate that the debris-flow properties are the key control factors for the complicated debris-barrier interaction.