EGU25-17770, updated on 15 Mar 2025
https://doi.org/10.5194/egusphere-egu25-17770
EGU General Assembly 2025
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
Near-field plunging dynamics of laterally unconfined hyperpycnal plumes over inclined beds
Georgios Giamagas1, Cyrille Bonamy1, Koen Blanckaert2, and Julien Chauchat1
Georgios Giamagas et al.
  • 1CNRS, Laboratoire des Écoulements Géophysiques et Industriels, Gières, France (george.giamagas@tuwien.ac.at)
  • 2Research Unit Hydraulic Engineering, Institute of Hydraulic Engineering and Water Resources Management, TU Wien, Vienna, Austria

The flow of a hyperpycnal river plume into a lake with an inclined bed and without any lateral confinement is a common occurrence in nature.  A characteristic example of such geophysical fluid flow system is the one of  sediment-laden dense river inflows in freshwater lakes (e.g. river Rhone - lake Geneva). In this case, the plunging process close to the river mouth, as well as its dependence on the properties of the inflow, affect critically the subsequent hydrodynamics in the lake. A better understanding of these flow processes is therefore significant for the effective management of the lake water quality and overall ecology. In this talk, we focus on the complex near-field plume dynamics and establish novel criteria for plunging, based on Large Eddy Simulations (LES) of an unconfined hyperpycnal saline plume over an idealized bed. More specifically, we focus on the effect of the variation of the inflow densimetric Froude number, Frd, which is the non-dimensional parameter describing the ratio between inertial and buoyancy forces acting on the plume. In particular, three simulations were performed at different values of Frd, within the range of values encountered in the plunging of river Rhone in lake Geneva. It is found that the near-field dynamics of the hyperpycnal plume is different in the unconfined plunging scenario compared to the case where the flow is confined by lateral walls and that it critically depends on Frd. Indeed, it is a well-established result that in the confined case the plunging of the hyperpycnal plume occurs at the location downstream where a balance between dynamic pressure forces (inertia) and the resisting hydrostatic pressure forces (gravity) is obtained. However, in absence of any lateral confinement the plunging begins immediately upon the entrance of the dense river water in the lake, due to lateral slumping. The slumping takes place at both lateral sides of the plume in the form of a collapse of the dense water column followed by a spread across the lake bottom that is very similar to a lock-release flow configuration. This results in an earlier departure of the plume from the lake surface in the unconfined case compared to the confined case under similar inflow conditions. We are able to determine the effect of Frd on the plunge curve, as well as the extend of the plunging zone on the lake bed, before the plume turns into a gravity current and continues its propagation down the slope. In addition, an explanation is provided for the field observations of surface leakage, where sediment-rich water is detected at the lake surface even downstream of the plunge curve. This explanation focuses on the effect of Frd on the dynamics of the turbulent mixing layers that develop at the interface between the incoming river water and the surrounding lake water. These mixing layers facilitate a substantial transfer of mass and momentum from the inflowing river water to the ambient lake water.

How to cite: Giamagas, G., Bonamy, C., Blanckaert, K., and Chauchat, J.: Near-field plunging dynamics of laterally unconfined hyperpycnal plumes over inclined beds, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17770, https://doi.org/10.5194/egusphere-egu25-17770, 2025.