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 effi ciency (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 signifi cant infl uence on STE. The sediment median size (D 50 ) in infl ow was greater than that in outfl ow, 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 fi ner than 1 μm were more readily removed from runoff than particles in the range of 2 to approximately 10 μm.Grass litter had less infl uence on fl ow velocity than leaves because the deposited sediment partially covered the litter layer. Mean fl ow 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.
It is still unclear how slope steepness (S) and revegetation affect resistance (f) to overland flow. A series of experiments on runoff hydraulics was conducted on granular surfaces (bare soil and sandpaper) and grassed surfaces, including grass plots (GP), GP with litter (GL), and GP without leaves (GS) under simulated rainfall and inflow (30
Abstract:It is important to evaluate the impacts of grasses on soil erosion process so as to use them effectively to control soil and water losses on the Loess Plateau. Laboratory-simulated rainfall experiments were conducted to investigate the runoff and sediment processes on sloped loess surfaces with and without the aboveground parts of grasses and moss (GAM: grass and moss; NGAM: no grass and moss) under slope gradients of 5°, 10°, 15°, 20°, 25°and 30°. The results show that runoff from GAM and NGAM plots increased up to a slope gradient of 10°and decreased thereafter, whereas the runoff coefficients increased with gradient. The average runoff rates and runoff coefficients of NGAM plots were less than those of GAM plots except for the 5°slope. This behaviour may be due to the reduction in water infiltration under moss. The difference between GAM and NGAM plots in average runoff rates varied from 1Ð4 to 8%. At the same gradients, NGAM plots yielded significantly (˛D 0Ð05) more sediment than GAM plots. Average sediment deliveries for different slopes varied from 0Ð119 to 3Ð794 g m 2 min 1 from GAM plots, and from 0Ð765 to 16Ð128 g m 2 min 1 from NGAM plots. Sediment yields from GAM plots were reduced by 45 to 85%, compared with those from the NGAM plots. Plots at 30°yielded significantly higher sediments than at the other gradients. Total sediments S increased with slope gradients G in a linear form, i.e. S D 9Ð25G 39Ð6 with R 2 D 0Ð77 Ł , for the GAM plots, and in an exponential model, i.e. S D 40Ð4 exp(0Ð1042G) with R 2 D 0Ð93 ŁŁ , for the NGAM plots. In all cases, sediment deliveries decreased with time, and reached a relative steady state at a rainfall duration of 14 min. Compared with NGAM plots, the final percentage reductions in sediment delivery from GAM plots were higher than those at the initial time of rainfall at all slopes.
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