Effects of vegetation stem coverage on the hydraulic characteristics and on-wall and jet flow proportions at the gully head

Abstract

Vegetation in the upstream area played a critical role in influencing runoff processes and subsequent headcut erosion at the gully head. However, it remains unclear how the vegetation stem coverage affects the hydraulic parameters and the on-wall and jet flow characteristics at the gully head. A simulation flow scouring experiment was performed to explore the influences of the vegetation stem coverage (5 levels) on the hydraulic parameters and on-wall and jet flow proportions at the gully head under inflow discharges ranging from 3.6–9.6 m3 h−1. The results indicated that the runoff in the upstream area occurred in the supercritical turbulent flow regime, and its velocity decreased while the Manning roughness coefficient and Darcy–Weisbach coefficient increased with increasing vegetation stem diameter. The runoff velocity at the gully brink decreased with increasing vegetation stem diameter and increased as a linear function of the inflow discharge (R2 = 0.849–0.997). With increasing vegetation stem diameter, the proportion of on-wall flow increased, and the proportion of jet flow decreased. The on-wall and jet flow proportions ranged from 7% to 92% and 8% to 93%, respectively, under inflow discharges from 3.6–9.6 m3 h−1, thereby attaining the highest correlation with the runoff velocity, Reynolds number, and runoff kinetic energy and following a significant logarithmic function. The vegetation stem influences the hydraulic parameters of the upstream area and gully head and thus influences the on-wall/jet flow proportion. This study could help elucidate the effect of vegetation on the hydraulic mechanism of headcut erosion and provide a theoretical foundation for improving gully headcut erosion models.