Dynamics of run-off hydraulic characteristics during gully headcut erosion driven by head height

In the granitic red soil hills region of South China, gully headcut erosion is considered one of the main processes in the development of gullies. However, the dynamics of run-off hydraulic characteristics such as hydrodynamics and energy consumption during erosion have not been clarified. We investigated the effects of five headcut heights (25 to 125 cm) on hydrodynamic and energy consumption characteristics using field in situ water discharge scouring experiments. The results demonstrated that the flow velocity (V), Froude number (Fr), Reynolds number (Re), run-off shear (tau) and run-off power (omega) all indicate the upstream area (UA) > gully bed (GB). Except for V and Fr, several other hydraulic parameters exhibited a trend of sandy soil layer > red soil layer, and a fluctuating increase with increasing headcut height was observed in the GB. The jet velocity at the edge of the headcut (V-b), jet shear stress (tau(j)), potential energy conversion (triangle E-P) and bottoming instantaneous kinetic energy (EKbottom) all increased exponentially with increasing headcut height by factors of 1.10 to 1.37, 1.16 to 2.00, 1.07 to 2.06 and 1.22 to 1.86, respectively. Except for triangle E-P, all others performed a trend of sandy soil layer > red soil layer. The energy consumption at the gully head (triangle E-H) and its contribution tended to significantly increase with increasing headcut height, and the triangle E-H of sandy soil layer was 13.20% to 49.89% higher than that of red soil layer. When the headcut height was greater than 100 cm, the contribution of energy consumption exceeded 50%. In addition, the sediment deposition process throughout the system was significantly influenced by the hydraulic and jet characteristics, and energy consumption was positively correlated with soil loss. The study results contribute to a profound understanding of gully headcut erosion and its hydrodynamic mechanisms.