Dynamics, Mixing, and Sediment Transport in Freshwater Plumes

Freshwater plumes from small rivers entering the ocean drive complex near-field dynamics characterized by sharp density gradients, stratified turbulence, and coherent vortical structures. Despite their importance in coastal transport and environmental processes, many fundamental physical mechanisms as the plume evolves remain insufficienty characterized. We perform direct-numerical simulations (DNS) of freshwater plumes under subcritical and supercritical conditions, replicating laboratory experiments by Yuan & Horner-Devine (2017) [Phys. Fluids 29, 106603]. The simulations solve the incompressible Navier-Stokes equations with the Boussinesq approximation using a second-order accurate finite-volume model.

Our results reveal different plume evolution regimes based on the Reynolds number and the densimetric Froude number. For supercritical cases (Fr=2.14), mixing is enhanced by the development of Kelvin-Helmholtz instabilities and the emergence of secondary structures. A global energy analysis quantifies turbulent dissipation and the evolution of background potential energy, providing new insights into entrainment and mixing efficiency. We further examine the influence of suspended sediments on turbulent dynamics, demonstrating how sediment concentration changes the near-field plume evolution, affecting the dynamics of vortical structures, vertical fluxes, and mixing rates. These findings provide a detailed characterization of coherent turbulence, entrainment, and sediment-driven feedbacks in these buoyancy-driven flows, with implications for predictive modeling of river-ocean interactions and sediment-laden stratified turbulence.