Flow and particle paths at a natural river confluence with coarse bed material

Abstract

An investigation of flow and particle trajectories was conducted at a gravel bed confluence where the junction angle is gradually reduced from 60 to 20° as the tributary joins the main channel. A shallow and elongated scour zone is present at the mouth of the tributary. Three flow stages ranging from 1 7 to 1 2 bankfull were recorded. Flow velocity vectors were measured from five bridges installed across the channels. Sediment transport paths were determined by following the progression of seeded marked particles through the confluence from May to October 1986. The results show that flow structure is responsive to stages. At lower stages (less than 1 3 bankfull) flow velocity vectors are controlled by local bed topography and specifically by the steeper slopes around the scour zone. At higher stages, velocity vectors become aligned with the plan geometry of the confluence. For all stages, surface flow velocities are higher in the scour zone. Bed shear stress computed from the law of the wall for the deepest flow is higher at the bottom of the scour zone compared to its sides, head and exit. The trajectories of particles seeded on the tributary pass through the scour zone. Particles coming from the main channel displayed a dual pattern first moving laterally towards the scour zone early in the season then following paths that are consistent with the plan geometry of the confluence. This shift in pattern reflects an increase in the peak discharges during the survey period. These results highlight the role of stage in controlling the flow dynamics and particle paths at the confluence.