title: Revisiting a 50-year-old conceptual model of plunging riverine inflows
Creators: name: Thorez, Stan Albert
name: Lemmin, Ulrich
name: Barry, D. Andrew
name: Blanckaert, Koen
Publisher: name: TU Wien
Year: 2023
Subjects: Plunging
Entrainment
Turbidity current
Gravity current
Lakes
Reservoir
Mixing processes

description: 1. Introduction Lake, reservoir and ocean water quality is impacted greatly by riverine inflows discharging into them, as these inflows form an important input of oxygen, sediment, pollutants, nutrients, momentum and heat. When the density of these inflows is higher than that of the receiving water body, the river water plunges upon contact and forms a gravity-driven density current near the bed. These currents can transport the aforementioned constituents to the deep hypolimnion. The final destination of these constituents is largely controlled by hydrodynamic mixing processes along its pathway, of which the mixing in the plunging area can form a significant proportion due to the large amounts of turbulence related to plunging. Therefore, a deepened understanding of the plunging process is key. Several attempts were made in the 1970s and 1980s to model plunging inflows, with a focus on laterally-confined, dammed-river reservoirs. Recently, several hypotheses were made regarding laterally-unconfined plunging inflows, which are typical for lakes and oceans. There exists, however, a general lack of direct measurements that could help to arrive at an updated, all-encompassing conceptual model for such inflows. 2. Objectives In this work, an effort was made to test existing hypotheses regarding the hydrodynamics of laterally-unconfined, plunging inflows by direct field measurements, and to extend and generalize the existing conceptual model of plunging. 3. Methods The study site for this work is the Rhône River inflow into Lake Geneva in Switzerland. A combination of boat-towed ADCP measurements and remote time-lapse imagery was used to elucidate hydrodynamic processes related to plunging over a large range of spatial and temporal scales. 4. Results The ADCP velocity data reveals that the Rhône River plunges at contact with Lake Geneva. While plunging along the river axis, the inflow undergoes a lateral slumping motion, caused by its density excess. Just outside the main flow body, this slumping drives two counter-rotating, secondary currents transporting river water back to the surface. The remote time-lapse imagery shows that, at the surface, the inflowing, sediment-rich river water forms a distinct plume with a triangular shape leading away from the river mouth towards a sharp tip. Furthermore, a myriad of vortical structures is visible at the surface in the plunging zone. The lateral slumping, as seen in the ADCP data, explains the triangular shape of the inflow plume at the surface, while the remote imagery provides evidence for the transport of sediment-rich water to the surface by secondary currents. 5. Conclusion Outside of the along-river-axis plunging, all other processes described are additions to the existing conceptual models for laterally-confined plunging, which therefore can be extended and generalized for laterally-unconfined plunging. The vortical structures in particular are suspected to increase the mixing taking place in the plunging region.