Evaluating critical hydrodynamic characteristics of rill formation on saturated soil slopes

As a specific near surface hydrological condition, soil saturation can significantly affect the critical hydrodynamic characteristic and soil erosion rate of rill formation, leading to severe rill erosion. Nevertheless, few studies have investigated the characteristics of critical hydrodynamic parameters and their relationships with rill erosion rate under critical hydrodynamic conditions of rill formation on saturated soil slopes. Consequently, the quantification of critical hydrodynamic parameters and their effects on rill erosion rate under critical hydrodynamic conditions of rill formation on saturated soil slopes is of great significance for understanding the dynamic mechanism of rill formation and evolution and for predicting and controlling soil loss. In this study, indoor simulated rainfall experiments were performed and a new analytical model (Vc(NAM)) was applied to calculate the aforementioned critical parameters under a wide range of hydraulic conditions comprising five slope gradients (SG) (2°, 5°, 10°, 15°, and 20°) and three rainfall intensities (RI) (30, 60, and 90 mm/h). The results indicated that the new analytical model (Vc(NAM)) was suitable for estimating critical hydrodynamic parameters on saturated soil slopes. The critical flow velocity (Vc), the critical shear stress (τc), and the critical stream power (ωc) apparently increased, whereas the critical rill length (Lc) decreased with the increase of slope gradients and rainfall intensities. Moreover, the erosion rate at the critical condition increased with decreasing Lc and increasing Vc, τc, and ωc. Pearson correlation analysis indicated that τc and ωc were significantly positively correlated, whereas Lc was negatively correlated with erosion rate under the critical conditions. Stepwise regression analysis revealed that the erosion rate under critical hydrodynamic conditions of rill formation could be well predicted by τc (R2 = 0.83) with the linear model. The results provide an accurate model for evaluating critical conditions of rill formation and a basis for further understanding the intrinsic dynamic mechanism of rill formation on saturated soil slopes.