Hydrodynamics of Flow in a Channel with Two Lateral Submerged Groynes

Installing groynes in a river has important implications for controlling the scouring process along river banks, maintaining channels for navigation, restoring fish habitat and on the mass transport processes. In many rivers flooding occurs several times a year causing most groynes to become submerged. The momentum convected into the embayment between successive groynes by the flow overtopping the groyne crest can significantly change the whole circulation pattern inside and around the embayments. In the submerged case the embayment can exchange momentum and mass both horizontally via the lateral channel-embayment interface and vertically via the roof interface. Thus, studying the effect of submergence on the momentum and mass transfer processes at groyne fields is important. Large Eddy Simulation is used to investigate the dynamics of the main coherent structures present in the flow around two vertical submerged groynes situated in a long flat-bed open channel. The Reynolds number defined with the height of the groyne D and the bulk channel velocity, U, is Re=13,600. The total channel depth is h=1.4D corresponding to flood conditions and the incoming channel flow is fully turbulent. The statistically averaged three dimensional (3D) vortical structures located in the upstream recirculation zone, embayment, detached shear layers, and downstream of the second groyne are visualized and the flow dynamics in these regions is described. The differences between the instantaneous coherent structures in the turbulent flow and the vortical structures present in the mean flow field are also highlighted and explained. The role of these large scale structures in the mass exchange between the embayment region and the main channel is investigated.