Sediment Retention by a Stiff Grass Hedge under Subcritical Flow Conditions

Abstract

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.

Annualized Agricultural Non-Point Source model application for Mississippi Delta Beasley Lake watershed conservation practices assessment

“…This includes various conservation tillage (e.g., no-tillage, mulch tillage, and reduced tillage) options (Andraski et al 2003;Daverede et al 2003;Dabney et al 2004;Locke et al 2005), Conservation Reserve Program (CRP) practices such as grass filter strips and riparian buffers (Dillaha et al 1989;Line 1991;Cooper and Lipe 1992;Robinson et al 1996;Hussein et al 2007), and in-field structures such as impoundments that retard flow and allow suspended sediment sufficient time to settle out (Laflen et al 1978;Lindley et al 1998). Data on how these programs and practices are affecting water quality is needed to help decision makers determine a cost/ benefit ratio of BMP or conservation program implementation.…”

Modelling Sediment Trapping by Non-Submerged Grass Buffer Strips Using Nonparametric Supervised Learning Technique

“…As in many case studies the length of hedge strips is short the length of the decay curve, which is the place that water surface profile tends to decrease to the level of the exit point of the buffer, can be close to the strips length. Therefore the friction slope in narrow strips is higher than the bed slope, which produces more pronounced stream power in this region (Ghadiri et al, 2001;Hussein et al, 2007a;Raffaelle et al, 1997;Rankins and Shaw, 2001). Erosion has been observed in narrow dense grass strips by Ghadiri et al (2001), this being due to a high energy slope within the narrow strip.…”

“…In some studies (Ghadiri et al, 2001;Hussein et al, 2007a, b;Loch et al, 1999;Meyer et al, 1995;Munoz-Carpena et al, 1999), the effectiveness of grass strips in reducing the sediment concentration has been introduced as the index of efficiency of vegetative buffer strips. Most of these studies are carried out in controlled conditions on impermeable surfaces.…”

“…Whilst the efficiency of vegetative buffer strips in removing sediment can be considerable, they are much less effective in removing particulate or sediment-associated nutrients as they are mostly attached to fine particles (McKergow et al, 2004). Dillaha et al (1989) found that the effectivness of vegetated buffer strips reduces with time, due to sediment deposition, while Hussein et al (2007a) observerd that the outflow sediment concentration is reasonably constant during runoff events. This difference is likely a result of whether the main area of deposition is upstream the grass strips or within them.…”

“…Controlled condition or flume studies have been conducted mostly to provide an undertanding of the effectiveness of different factors in vegetated buffer strips performance in controlled environments (Ghadiri et al, 2001;Hussein et al, 2007a, b;Meyer et al, 1995). Field experiments have been carried out to assess the effectiveness of vegetated buffer strips under natural conditions but with less control over interfering factors (Daniels and Gilliam, 1996;Ghadiri et al, 2011;Magette et al, 1989;Parsons et al, 1994;Robinson et al, 1996).…”