Particle selectivity of sediment deposited over grass barriers and the effect of rainfall.

Abstract

Particle selectivity of the sediment deposited over vegetative barriers is of importance to predict sediment transport and particulate pollutant load into surface waters. Grassed barriers with 20–90% covers at 15° slope were subjected to silt-laden inflows in the presence and absence of simulated rainfalls to investigate the sediment deposition processes. The results show that regrass of steep croplands can effectively trap eroded sediment from upslope, and the rowed grass barriers can strengthen sediment deposition. The deposition order of sediment particle sizes (μm) follows (>50) > (25–50) > (10–25) = (<2) > (2–10), and the particle selectivity weakens with increasing grass covers. Clay particles had a similar deposition efficiency to overall sediment, implying the effectiveness of regrass in controlling soil nutrient loss. The contribution of grass to total overland flow resistance is almost equivalent to the percentage of grass cover. For steep grassed slopes, raindrop impact significantly decreases sediment deposition, but limitedly affects particle selectivity of deposited sediment and overland flow hydraulics. Both raindrop kinetic energy and stream power available for surface soil contribute to sediment deposition in net deposition areas of grass barriers. These imply that rainfall effect on sediment delivery over vegetated barriers derives from the additional raindrop energy, rather than the variation in runoff hydraulics. These results can help to clarify the effect of raindrop impact on sediment transport and to evaluate the benefit of revegetation in decreasing sediment yield and its particulate nutrient load into surface waters.