The allocation and erosive effects of on-wall flow on loess gully heads

On-wall flow and jet flow converted from upstream flow at the gully brink play important but differential roles in gully head erosion. However, on-wall flow allocation at gully brinks and its erosive effects on vegetation-covered loess gully heads remain unclear. Simulated flow scouring experiments were conducted on grass-covered gully heads under different inflow rates (3.0–7.2 m³/h) and upstream slope gradients (1–7°) to investigate the on-wall flow proportion and relationships with the hydraulic parameters of upstream flow, as well as erosion and sediment yield characteristics. The results showed that the on-wall flow rates linearly increased with increasing inflow rate (P < 0.01) but decreased as the slope gradient increased, whereas the jet flow rate increased as the inflow rate or slope gradient increased. The on-wall flow proportions ranged from 24.6 % to 58.7 % and decreased with increasing inflow rate or slope gradient, whereas the jet flow proportions exhibited the reverse trend. The on-wall and jet flow proportions had the most significant correlations with the Froude number, following a negative power function and positive logarithmic function (P < 0.01), respectively. The breadth and depth of the scour hole that developed on the gully headwall increased linearly with the on-wall flow rate (P < 0.01). Jet flow had a limited effect of accumulation on scour hole development, with average increases of 13.5 % and 8.6 % in the breadth and depth, respectively. On-wall flow generally contributed 17.6 %–48.4 % of the sediment driven by on-wall and jet flows but played a dominant role under 1° upstream slope gradient and 3.6 m³/h inflow rate in which the contribution reached 64.9 %. On-wall flow might have dominated the runoff volume and played a dominant role in the sediment yield under low upstream slope gradients and inflow rates and was essentially responsible for scour hole development, which could destabilize and retreat the gully head.