Vegetated buffer strips are widely used to reduce fluxes of eroding soil and associated chemicals, from hillslopes into waterways. Sediment retention by buffers is time-dependent, with its effectiveness changing with the deposition process. Our research focuses on settling of sediment upslope of stiff grass buffers at three slopes, under subcritical flow conditions. A new model is developed which couples the hydraulics, sediment deposition and subsequent adjustment to topography in order to predict water and sediment profiles upslope of a buffer with time. Experiments to test the model were carried out in the Griffith University Tilting-Flume Simulated Rainfall facility using subcritical flows at 1%, 3% and 5% slopes. Water and sediment profiles were measured at different times as Vertisol sediment was introduced upslope of a vetiver grass strip. A region of increased flow depth (backwater) was produced upslope of the strip which increased in depth and decreased in length with increasing slope. Backwater height could be predicted from flow rates and thus could be used as an input for the model in the absence of experimental data. As slope increased, sediment was deposited closer to the grass strip, moving into the grass strip itself at 5% slope. The grass strip was less effective in reducing sediment in the outflow as slope increased and differences between slopes were significant. Model prediction of water and sediment profiles compared reasonably well with measured data, giving low root mean square errors and high coefficients of model efficiency. Masses of deposited sediment were generally simulated within 20% of measured values. However, simulated particle size distributions of deposited sediment were less accurate. ª

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There is general agreement among researchers that at least for some time during erosion events sediment leaving an area is finer than that of the soil under surface erosion. But in some cases, it has been observed that coarser particles are transported at greater rates than fines. This paper reports on the results of an investigation into the processes that control the size distribution of the sediment during runoff erosion events at low flow rates and streampowers. Experiments were carried out on three contrasting soil types in the 1 · 6 m flume of Griffith University's large rainfall-runoff simulation facility at a slope of 2%, with overland flow confined to uniform rectangular rills pre-formed in the soil bed. The time variation in size distribution of the exiting sediment was measured for all experiments. The results supported a selective bimodal particle size class pattern for transported sediment with peaks for the finest size class of <0.001 mm and also for the larger class of 1-2 mm. Particles between 0.1 and 0.5 mm appeared to resist transportation. It seems that this selectivity in transport may indicate different transport mechanisms, such as suspension, saltation and rolling, can dominate in different sediment size classes, perhaps reflecting differences in resistance to transportation (or transportability) at the low streampowers investigated.

Reduction of diffuse fluxes of sediments from catchments is frequently achieved through use of vegetative buffers, but the dynamics of this reduction are not fully understood. The physical processes involved in sediment deposition by a stiff grass buffer (hedge) at a low, subcritical, flow rate were therefore examined. Flow experiments were carried out in the Griffith University Tilting-Flume Simulated Rainfall facility using a 0.3 m width, vetiver hedge (Vetiveria zizaniodes L., sterile cultivar Monto) at 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 bedload movement in addition to the deposition/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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