Influence of initial headcut height on rill headcut erosion mechanisms via runoff hydrodynamics modulation in granite residual soil

Abstract

Headcut erosion constitutes the beginning of rill erosion and is the main process influencing gully expansion. However, there is a lack of detailed quantitative studies on the sediment–producing process and the dynamic mechanism underlying rill headcut erosion at the beginning stage of the development of large–scale incised gullies (Benggang). In this study, we continuously dynamically monitored the hydrodynamic mechanism and sediment transport pattern in the rill headcut erosion process in cases with intense runoff erosion by setting four initial headcut heights (5, 10, 15, and 20 cm) and two scour flow rates (4 L·min⁻¹ and 8 L·min⁻¹) for two typical soil layers (the laterite and sandy soil layers). The results revealed that both the flow velocity (V) and the Reynolds number (Re) gradually increased and that the Froude number (Fr) gradually decreased with increasing initial headcut height. As the flow rate was varied, the runoff process changed from a transitional flow pattern to a turbulent flow pattern, and the runoff shear stress (τ) and stream power (ω) increased. The Darcy–Weisbach coefficient (f) and runoff power (ω) values of the sandy soil layer were greater than those of the laterite soil layer because of the high surface sand content and loose structure. Moreover, the runoff rates and runoff sediment concentrations in the catchment slope–headcut head–gully bed system increased with increasing initial headcut height. The soil loss rate peaked at the beginning of the experiment and then decreased under fluctuations. Notably, the soil loss rate and headcut erosion velocity of the laterite soil layer were greater than those of the sandy soil layer. The headcut head and gully bed runoff energy consumption increased with initial headcut height, and both the headcut head energy consumption and headcut head runoff energy contributions were greater for the sandy soil layer than for the laterite soil layer. Further analysis revealed that the coupled effects of the hydraulic properties of runoff and changes in energy consumption drove the sediment production process of rill headcut erosion. The accuracy of erosion estimates obtained with prediction models based on the random forest (RF) algorithm were great (LCCC = 1.020 and 1.021; R²_adjusted = 0.979 and 0.972). Headcut erosion expansion can be controlled by preventing the formation of primary and secondary headcut heads. This study provides a theoretical basis for preventing slope erosion, reducing soil erosion and coping with extreme rainfall events.