Hydro-Sedimentary Processes of a Plunging Hyperpycnal River Plume Revealed by Synchronized Remote Imagery and Gridded Current Measurements
Résumé
The present knowledge of plunging hyperpycnal river plumes is mainly based on two-dimensional (confined) laboratory experiments. Several hypotheses on three-dimensional (unconfined) flow processes have been made, but not tested in situ. In this field study, the dominant three-dimensional hydro-sedimentary processes related to unconfined plunging were elucidated by synchronizing autonomous time-lapse camera images with boat-towed acoustic Doppler current profiler measurements. It was found that the flow field complies with two-dimensional conceptualizations along the plume centerline. Perpendicular to the centerline, the plume slumped laterally due to its density excess and simultaneously converged laterally due to its vertical divergence. This combination led to a narrowing of the plume near the surface, resulting in a sediment-rich triangle-shaped pattern on the surface near the inflow, and a rather stable width near the bed. The formation of secondary flow cells transporting riverine water and suspended particulate matter (SPM) away from the plume near the bed, up toward the surface and back toward the plume near the surface, was revealed. An increase of the average SPM concentration and the SPM flux in the main flow direction indicates net sediment erosion under the investigated conditions of high discharge and sediment load. This suggests transient storage of sediment during conditions of lower discharge and sediment load, and high morphological activity in the plunging area. These findings allowed extending a classical conceptual plunging model to laterally unconfined sediment-laden plunging inflows for conditions of well-mixed ambient water in the plunging region and inflow densimetric Froude numbers exceeding 1, common in nature. When higher density river water flows into a lower density lake or reservoir, the river water forms a plume that dives down (referred to as plunging) toward the bottom of the lake or reservoir and flows along its bottom as an underwater current. Previously, the processes related to plunging were poorly understood in typical lake settings. In this study, these processes were investigated by combining camera images of the lake surface and measurements along a grid of both the currents and sediment concentration of the Rhone River plume entering Lake Geneva (Switzerland). Previously predicted processes occurring in the river plume, including plunging along its axis and a combination of lateral collapsing and contracting motions, were confirmed to exist. Other processes, including two swirling flows transporting river water and sediment back toward the surface, were revealed. These contribute to the spreading and mixing of river water and sediment in different ways from those measured in laboratory representations of reservoirs. Furthermore, the plunging river water eroded the lake bed near the river mouth during the measurements. The results from this study lead to a better understanding of plunging flows in lakes and allow for improved modeling and better predictions of such flows. Dominant three-dimensional hydro-sedimentary processes related to laterally unconfined river plume plunging were revealed by field measurements These processes include lateral secondary flow cells spreading over the local depth, sediment transport, and flow-bed interactions The new findings allowed extending a classical conceptual plunging model to laterally unconfined sediment-laden inflows
Auteurs, date et publication :
Auteurs Stan Thorez , Ulrich Lemmin , D. Andrew Barry , Koen Blanckaert
Publication : WATER RESOURCES RESEARCH
Date : 2024
Volume : 60
Issue : 3