Quantifying mixing processes at a river-lake interface: The case of the plunging negatively buoyant inflow of the Rhône River into Lake Geneva

Plunging river inflows affect lake and reservoir water quality, reservoir storage capacity and hazards. After plunging, they form gravity-driven density currents near the bed. The plunging process at the river-lake/reservoir interface provides upstream boundary conditions for density currents and controls the pathway and fate of all constituents they carry. It is therefore important to identify and quantify the turbulent flow and mixing processes diluting the plunging inflow by entraining ambient water. In the present study, for the first time, the three-dimensional flow field of a laterally unconfined plunging inflow was measured in the field using a vesselmounted ADCP at the Rhône River inflow at Lake Geneva (Switzerland/France). The ADCP data provide insight into the related hydrodynamic processes and a direct quantification of the plunging mixing coefficient Ep. The value found for Ep is an order of magnitude larger than previously observed in laterally confined configurations.