Investigating the Role of Raindrop Impact on Hydrodynamic Mechanism of Soil Erosion Under Simulated Rainfall Conditions

Few studies have addressed the role of raindrop impact on hydrodynamic mechanism of soil erosion. In this study, rainfall simulation experiments were conducted to evaluate the role of raindrop impact on processes of runoff and sediment yield, flow-hydraulic characteristics, and dynamic mechanism of raindrop impact under three rainfall intensities (50, 75, and 100 mm h(-1)) at three slope gradients (5 degrees, 7.5 degrees, and 10 degrees). These rainfall intensities and slope gradients were tested under two soil surface conditions: one with raindrop impact and the other without raindrop impact by placing nylon net over a soil pan. Result showed raindrop erosion accounted for 72.3% to 96.2% of the total sediment yield. Furthermore, runoff rate and sediment concentration were greatly reduced during rainfall with the elimination of raindrop impact. The corresponding reduction rates were 1.13% to 84.08% and 48.13% to 96.50%, respectively. After eliminating raindrop impact, hydraulic parameters of flow depth, flow velocity, Reynolds number, and Froude number were markedly reduced, whereas Darcy-Weisbach coefficient was increased 1.40 to 5.31 times; flow velocity was the key hydraulic parameter affecting raindrop erosion. Raindrop impact caused the critical hydrodynamic forces of shear stress, unit stream power, and stream power increase more than two-fold relative to the treatment without raindrop impact. Raindrop erosion rate had a significant linear relationship with the relative increment in unit stream power or stream power induced by raindrop impact. These findings present a new approach for predicting raindrop detachment rate from the point of flow hydraulics and dynamic mechanism of raindrop impact.