Effectiveness of grass strips in trapping suspended sediments from runoff

摘要

Little information is available concerning the performance of grass strips for erosion control from steep cropland. An experiment was conducted on 5-m-long grass strips with slopes of 3°~15° that were subjected to silt laden runoff and simulated rainfall, to investigate the sediment trapping processes. The grass strips had three treatments including intact grass control (C), no litter (dead grass material covering the soil surface was removed) (NL), and no litter or leaves (only 2~3 cm grass stems and roots were reserved) (NLL). Generally the grass strips had a high effectiveness in trapping sediment from steep cropland runoff. Sediment trapping efficiency (STE) decreased with increasing slope gradient, and even for a 15° slope, STE was still more than 40%. Most sediment deposited in the backwater region before each grass strips. The removal of grass litter or/and leaves had no significant influence on STE. The sediment median size (D_(50)) in inflow was greater than that in outflow, and the difference (ΔD_(50)) decreased with increasing slope. A positive power relationship between STE and ΔD_(50) can be obtained. Grass strips were more effective in trapping sediments coarser than 10 or 25 μm, but sediments finer than 1 μm were more readily removed from runoff than particles in the range of 2 to approximately 10 μm. Grass litter had less influence on flow velocity than leaves because the deposited sediment partially covered the litter layer. Mean flow velocity and its standard deviation were negatively correlated with STE, and they can help make good estimation of STE. Results from this study should be useful in planting and managing forage grass to effectively conserve soil loss by runoff from steep slopes on the Loess Plateau of China.