Predicting vegetation buffer efficiency in reducing runoff transport of sediments and nutrients.

Abstract

Vegetative buffer strips are widely used as a conservation measure to reduce erosion and transport of sediments and associated pollutants across landscapes. Buffers generally reduce sediment and pollutant loads through a combination of deposition and infiltration processes. The physical processes involved in sediment deposition by a stiff Vetiver grass buffer strip at low flow rates were examined in a series of experiments carried out in a 1x6m flume of a rainfall simulator. Experiments were carried out using sub-critical flows on three different soils introduced to flow path as slurry upstream of the Vetiver strip at 1, 3 and 5% slopes. Water and sediment profiles were measured at different time intervals while experiments were in progress. The strip caused a region of increased flow depth (backwater), upstream of the buffer which increased in depth and decreased in length with increasing slope. As slope increased, sediment was deposited closer to the grass strip, moving into the grass strip at 5% slope. The buffer strip was less effective in reducing sediment transport as slope increased and differences between slopes were significant. These experiments quantified the reduction in sediment and particulate-sorbed nutrients from overland flow and data were used to test the newly developed model of GUSED-VBS (the Griffith University Soil Erosion & Deposition model-Vegetated Buffer Strip) for assessing and predicting buffer efficiency for sediment and sorbed nutrients. This model couples the hydraulics, sediment deposition and topography in order to predict water and sediment profiles upstream of a buffer strip with time. Unlike other models, GUSED-VBS simulates the evolution of the deposited layer by dynamically adjusting the bed elevation, the water profile and the flow velocity as a result of sediment accumulation. The model successfully predicts water and sediment profiles while masses of deposited sediment and sorbed nutrients (P, N) were generally simulated within 20 % of measured values. Further model development is in progress which will incorporate infiltration to provide a coupled overland/vadose approach to simulating flow through vegetative buffers.