Field characterization of the negatively buoyant inflow of the Rhône River into Lake Geneva

River inflows have a major influence on lake and reservoir water quality through their input of momentum, heat, oxygen, sediment, nutrients and contaminants. The fate of these components is controlled by the hydrodynamic processes at the interface between the inflowing river and the receiving lake or reservoir. The inflow can be positively buoyant, leading to a near surface inflow current (overflow), or negatively buoyant, causing it to plunge and form a gravity-driven current near the bed (density current) and/or intermediate current (interflow). In the case of a plunging inflow, the plunging process provides upstream boundary conditions for density currents, which can continue for significant distances along the lakebed. It is therefore important to understand the mixing processes involving entrainment of ambient water into the plunging flow. The hydrodynamics of the plunging process are still poorly understood, especially in laterally unconfined configurations.

Field measurements of a laterally unconfined plunging flow of the Rhône River into Lake Geneva are presented. A vessel-mounted ADCP was used to measure the three-dimensional velocity field of the plunge region. Remote sensing images of the lake surface in the plunge region were captured with a static camera system set up on a nearby mountain overlooking the inflow. Additionally, a mobile camera system attached to a balloon was operated above the inflow to capture high-resolution videos of the inflow. Both camera systems were equipped with RGB and IR cameras. The ADCP measurements and remote sensing images were combined to detect mixing processes in three dimensions.

The remote sensing images show that the incoming river flow forms a distinct plume of sediment-rich water with a triangular shape leading away from the river mouth in downstream direction towards a sharp tip. Horizontal vortical structures visible at the surface, range from larger gyres, over vortex shedding and dipole formation downstream of the plume, to smaller scale structures such as Kelvin-Helmholtz instabilities at the plume edges. The ADCP measurements show the presence of vertical secondary circulation cells in transects perpendicular and parallel to the plume centerline. In addition, intermittent ‘boils’ of sediment rich water up to more than 200 meter downstream of the plume were observed in the images.