Quantifying Turbulent Mixing in Plunging River Inflows: Insights from Field Measurements in Lake Geneva

Abstract

Hyperpycnal river inflows into lakes or reservoirs will plunge and generate gravity-driven underflows, called turbidity currents when driven by high sediment concentrations. These underflows either reach the lake bottom or detach from the bed to form interflows at layers of equal density. Since underflows transport sediment, contaminants, nutrients, and oxygen, their trajectory significantly impacts lake and reservoir water quality. Mixing in the plunging region, where inflowing water entrains ambient water, critically influences the dilution of inflowing river water and therefore the final depth of intrusion, making its quantification essential. This study focuses on quantifying plunging mixing from flow velocity measurements in a laterally unconfined river inflow. Field data were collected from the plunging Rhône River entering Lake Geneva using a boat-towed ADCP across six inflow conditions characterized by the densimetric Froude number (Frd). The plunging mixing coefficient (Ep), comparing the underflow discharge post-plunging to the initial river inflow discharge, was used to quantify the mixing. Results reveal that for larger Frd values (Frd > 3), Ep aligns with estimates from laterally confined laboratory experiments (Ep = O(0.1)). In contrast, for smaller Frd values, Ep correlates with field tracer measurements in unconfined inflows (Ep = O(1)). Ep decreases with increasing Frd, challenging existing numerical simulations that predict the opposite trend. This study provides new insights into turbulent mixing associated with hyperpycnal river inflows and emphasizes the importance of realistic field conditions in modeling plunging river inflows and predicting intrusion depths.