Sediment retention by a stiff grass hedge under subcritical flow conditions

Reduction of diffuse fluxes of sediments from catchments is frequently achieved through the use of vegetative buffers, but the dynamics of this reduction are not fully understood. The physical processes involved in sediment deposition by a stiffgrass buffer (hedge) at a low, subcritical flow rate were therefore examined. Flow experiments were performed in the Griffith University tilting-flume simulated rainfall facility using a 0.3-m-width, vetiver {Vetiveria zizaniodes L., sterile cultivar Monto) hedge at a 5% slope. Sediments comprising a sandy soil (Podzol) and red clay (Ferralsol) were introduced into the flow upstream of the buffer and the resultant hydrology, sediment deposition, and outflow characteristics were measured. Flow retardation produced a backwater upstream of the vetiver hedge and sediment deposition varied with soil type in this backwater. The backwater region was greatly extended by the deposition process, increasing overall sediment trapping efficiency. Buffering action reduced mean sediment loads in the outflow to 3.2 and 6.0% of the inflow concentration for the Podzol and Ferralsol, respectively, with a significant difference (P < 0.01) between the soils. In contrast to other buffer research where deposited sediments were coarsest upstream of the backwater, we found the coarsest particles at the downstream end for the Podzol and Ferralsol, indicating possible bed load movement in addition to the deposition and entrainment processes that dominate supercritical flow. The type of flow therefore affects the size distribution as well as the amount and efficiency of sediment deposition in front of vetiver hedges.

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