The proportion of jet flow and on-wall flow and its effects on soil loss and plunge pool morphology during gully headcut erosion

Gully headcut erosion is recognized as the primary process of gully erosion and is the main contributor to sediment yield of gully erosion. However, the proportion of jet flow and on-wall flow induced by headcut and its effects on soil loss and plunge pool morphology are still unclear. A simulated flow-scouring experiment was conducted to explore the proportion of jet flow and on-wall flow and their contributions to soil loss and the effect of on-wall flow erosion on plunge pool morphology under different flow discharge (q(0) = 3.0-7.2 m(3) h(-1)) and headwall height (H-0 = 0.3-1.2 m) conditions. Our results showed that jet flow and on-wall flow accounted 15.7% - 22.6% and 77.4% - 84.3% of total flow volume upstream headcut, respectively. Jet flow, on-wall flow and their interaction contributed 53.5%, 34.9% and 11.6% of total soil loss amount, respectively. Furthermore, H-0 exhibited greater effect on soil loss caused by jet flow and its interaction with on-wall flow, but the soil loss caused by on-wall was mainly controlled by q(0). The width and depth of plunge pool logarithmically increased with scouring time, and the q(0) and H-0 significantly affected the development of plunge pool morphology. On-wall flow reduced plunge pool depth by 27.8%-71.4%, and it weakened plunge pool width by 24.3%-57.3% only under H-0 of 0.3-0.9 m, but for H-0 of 1.2 m and q(0) of >4.8 m(3) h(-1), it improved the plunge pool width by 7.5%. The energy consumption of jet flow showed the closest relationship with plunge pool morphology. The concentrated flow upstream gully head exhibited the stronger effect on plunge pool morphology than H-0. This study is helpful to deepen the understanding of gully erosion mechanism and provide scientific reference for the design of gully erosion prevention and control measures.