Dynamics, Mixing, and Sediment Transport in the Near -Field of Freshwater Plumes

Freshwater plumes generated by small rivers play a signficant role in coastal processes. In glacially fed systems, such as those found in Patagonia, strong buoyancy forcing and turbulence produce sharp density interfaces and complex flow structures that regulate plume spreading and vertical exchange. Understanding the physical mechanisms controlling mixing and sediment transport in these environments is essential for linking small-scale turbulence to larger-scale coastal processes.We present results from direct numerical simulations (DNS) of freshwater plumes discharging into denser ambient fluid under subcritical and supercritical conditions. The simulations resolve the 3D coherent structures, capturing the development of interfacial instabilities and vortical motions that control entrainment and mixing efficiency. We show that plume dynamics transition between regimes dominated by shear-driven instabilities and large-scale overturning, with distinct implications for vertical density fluxes and plume thickness.We also explore the influence of suspended sediment on plume dynamics, focusing on how particle settling modifies turbulence, alters effective vertical transport, and feeds back on interfacial structure. The interactions of sediment transport with stratified turbulence significantly affect near-field plume evolution. These results provide new physical insights into mixing and transport in buoyancy-driven flows and help bridge idealized turbulence studies with the behavior of natural glacial river plumes in coastal environments.This work has been supported by ONR-Global grant N62909-23-1-2004.