Effects of grass coverage and distribution patterns on erosion and overland flow hydraulic characteristics

Grass coverage and its spatial distribution patterns have crucial influences on erosion. The laboratory scouring experiments were conducted to research the influence of grass cover on runoff, erosion rates, and overland flow hydraulic characteristics in the plots with differing grass coverage rates (30, 50, 70, and 90 %), grass distribution patterns (where US, MS, and DS stand for the grass laid on upslope, middle-slope and down-slope, respectively) and with a bare soil plot (CK) at a slope gradient of 20. The results illustrate that the grassplots had a 2.06-10.94 % runoff reduction and 28.57-75.4 % sediment decreases, respectively, as compared with CK plot. There was no significant difference in the runoff rate among the three grass distribution patterns for the same grass coverage, while DS had the lowest sediment yield rate and greatest sediment yield reduction in comparison with US and MS. The sediment yield rates were found to have a significantly negative exponential relationship with the grass coverage (p<0.01), while the sediment concentration had a significantly negative linear relationship with the grass coverage (p<0.01). The overland flow velocity (V) increased with increasing inflow discharges and deceased with increasing grass cover, and it was negatively correlated with the grass coverage following a linear trend (p<0.01). ThemeanFroude number (Fr) holds to a similar variation law with the changes in the V. There was no significant relationship found to exist between the grass coverage and Reynolds number (Re). The average Darcy-Weisbach resistance coefficient (f) of the whole slope for grass plots was 2.2-25.6 times of that for CK plot, and f was found to be an exponent correlated with the coverage rate (p<0.01). In addition, f was negatively correlated with the erosion rate following a power function (p<0.01); however V, Fr, and Re were positively correlated with the erosion rate (p<0.01). The sediment yield rate itself was a function of the runoff rate for each treatment, and their relationships could be well described by the linear equation (p<0.01). These results indicate that both grass coverage rates and distribution patterns have significant effects on hydrological characteristics of overland flow.