Effects of slope lengths on rill erosion under different hydrodynamic conditions in black soil sloping farmland of Northeast China

Abstract: Rill erosion under concentrated water flow is important in studying the mechanism of rill erosion and predicting soil erosion and also provides a database to estimate other parameters of erosion model, such as detachment rate and erodibility. This research aims to study the rill erosion of black soil over cultivated slope to reveal that the sediment concentration varies with slope length. Laboratory simulation was conducted to measure the distribution of sediment concentration along with slope length. This experiment involved four slopes (5(, 10(, 15(, and 20() and three flow rates (1, 2, and 4 L/min). Sediment concentrations at different slope lengths (0.5, 1, 2, 3, 4, 5, 6, 7, and 8 m) which determined by the distance between inlet and outlet were measured under different hydraulic conditions. Concentrated water flow was pumped to scour the soil slope from the designed inlet location to the outlet of the soil flume. Sediment-laden water samples were collected from the sediment collector when water flow run from inlet to outlet and became steady. The samples of different sediment concentrations were dried by an oven and calculated to determine the distribution of sediment along the slope length. As the experiment repeated under different hydraulic conditions, the data series of sediment concentration changed with slope length were attained. The results showed that sediment concentration increased with slope length under different hydraulic conditions; however, this increase rate gradually decreased. The data series of sediment concentration were fitted to slope length with an exponential function. The coefficients of determinations (R2) ranging between 0.85 and 0.99 showed that the exponential function could well fit the experimental data. With the exponential function and the experiment data, the maximum sediment concentration (A) and decay coefficient (B) were regressed. With the increased in slope gradient and flow rate, the water flow needs a shorter slope length to get to a higher maximum sediment concentration compared with the circumstance of low slope gradient and flow rate. Parameter sensitivity analysis was adopted to research the influential degree of each parameter on the maximum sediment concentration. The sensitivity of flow rate is higher under low slope gradient and flow rate compared with the condition of higher slope gradient and flow rate. Slope gradient exerted higher impact on the maximum sediment concentration than flow rate. The slope gradient had a greater influence on parameter A with that under lower slope gradient which means that the effect of slope gradient on A decreased with the increase of slope gradient under the same flow rate. The parameter B generally increased with the increases of slope gradient and flow rate which means sediment in water flow increased faster and need shorter slope length to achieve the maximum value. This study does not quantify the rill erosion process over black soil slope but also provides a foundation for estimating some model parameters such as detachment rate and erodibility.