How does shrub stem coverage affect the hydraulic properties of concentrated flow and sediment yield during gully bed erosion?

Abstract

Vegetation plays a critical role in influencing runoff processes and soil loss during gully bed erosion. However, it is still unclear how the stem coverage affects gully bed erosion processes by altering the runoff hydraulics and soil sedimentation. A series of in situ scouring experiments were conducted to investigate the influence of shrub stem coverage on the concentrated flow pathway characteristics, hydrodynamic parameters, and sediment concentration during gully bed erosion processes. The Flow pathway characteristics expressed by the Number of flow pathways (FN), total Flow path Width (FW), Tortuosity Ratio (TR), and Fractal Dimension (FD) were quantified by analyzing photographs of the gully bed surface taken during experimental periods. Structural equation model was used to analyze the comprehensive effect of stem coverage on hydraulic erosion of gully beds. The results showed that FN, FW, and TR increased linearly, and FD increased exponentially as stem coverage increased. Compared with the bare gully bed, the flow velocity and shear stress of gully beds with shrub stem covers decreased by 17.47%–25.19% and 4.75%–11.42%, respectively, while the Darcy-Weisbach friction factor increased by 35.94%–68.71%. The sediment concentration of stem-covered gully beds decreased by 11.82%–26.93%. The increasing stem coverage promoted concentrated flow branching and significantly increased FW, which in turn altered hydraulic parameters, particularly reducing flow velocity, and ultimately reducing sediment concentrations indirectly. These results contribute to partially explaining the differences in flow hydraulics and soil loss of vegetated gully beds in previous studies that failed to account for changes in flow pathways.