Sediment dynamics in a sand bed stream with riparian vegetation

The characteristic bed morphology of streams results from the action of various sediment fluxes. Sediment fluxes are difficult to quantify, and as a result, few studies have been able to characterize the relative importance of different transport mechanisms in shaping a river's bed at the reach scale. Even fewer studies have been able to address the impacts of introduced riparian vegetation on those fluxes. This study provides the first comprehensive set of flow and sediment measurements required to compute the fluxes in a sand bed stream with riparian vegetation. Data were collected in a reduced-scale laboratory model of a reach of a sand bed stream in southeastern Australia that is undergoing rehabilitation using in-stream riparian vegetation. The reach has a large-radius meander and a variable width. Laboratory measurements included streamwise and transverse velocities, bed shear stresses, bed load transport, suspended sediment concentrations, and bed topography with and without riparian vegetation placed along the outer bank of the bend. The results unveiled the importance of secondary circulation as well as converging and diverging flow patterns in shaping the bed topography. Modifications due to the vegetation included a shift of the main flow away from the vegetated outer bank and an overall straightening of the flow in the reach, resulting in an increased deposition near the vegetated bank and a reduced deposition near the inner bank. This highlights the need for overall reach assessment of flow and sediment dynamics before revegetation, as its effects go beyond local bank protection.