The mechanism of flow retardation and erosion control by vegetated buffer strips on sloping lands

Abstract

Buffer strips are commonly used to reduce runoff sediment transport on sloping lands and in the riparian zones of rivers and streams. One of the important aspects of buffer strip which is not well understood is the flow hydrology around the strips. The work reported here extends the current understanding of the physical processes involved in sediment and contaminant reduction by a vetiver grass buffer strip. Experiments were carried out in the Griffith University Tilting-Flume Simulated Rainfall facility using three different soils on three slopes. A dense vetiver strip was inserted in the path of surface runoff in the flume. The soils were made into slurry and introduced to the surface flow. The inflow and outflow of sediment were measured, together with the runoff rate. The rate of deposition in front of the buffer was measured at different distances and times in front of the buffer. Results indicated that the Vetiver grass strip caused a region of enhanced flow depth, upstream of the buffer. The region increased in depth and decreased in length with increasing slope. Buffering action resulted in the deposition of up to 95 % of the added sediment in the backwater region. Suspended sediment loads in the outflow increased with slope but remained primarily in the finer particle size range compared to the input sediment. With most fine particles remaining is suspension in the emerging water downstream of the strips, the value of Vetiver buffer strip in controlling pollutant transport on the slopes and into water bodies is not certain. Water depths, sediment concentrations and rate of deposition were simulated using both hydraulic and erosion/deposition models and predictions compared with data from the flume experiments.