Unconfined Plunging of a Hyperpycnal River Plume Over a Sloping Bed and Its Lateral Spreading: Laboratory Experiments and Numerical Modeling

Hyperpycnal (negatively buoyant) river inflow into lakes and oceans often develops three-dimensional (3D) plunging flow patterns when laterally unconfined. To determine the 3D flow pattern characteristics, laboratory experiments of laterally unconfined plunging on a sloping bed were carried out using salinity to control the density difference. The experiments were complemented by numerical modeling based on a high-resolution computational fluid dynamics model. As is the case for confined plunging plumes, it was found that in unconfined plunging, the inflow densimetric Froude number at the river mouth and the bed slope of the receiving water body are the dominant control parameters. However, the results documented that the hydrodynamics of laterally unconfined plunging are fundamentally different: The hyperpycnal plume in unconfined configurations forms a triangle on the surface in the plunge zone due to its convergence near the surface and lateral spreading near the bottom. The triangular pattern extends further into the receiving water when the inflow densimetric Froude number increases or the bottom slope decreases. The unconfined entrainment coefficient, which quantifies the amount of ambient water entrained into the plunging plume, also increases with increasing inflow densimetric Froude number. In general, entrainment is much higher in unconfined than in confined plunging. The plunging densimetric Froude number takes a constant value of ∼0.5 in confined plunging, whereas it increases with increasing inflow densimetric Froude number and can be ≫ 1 in unconfined plunging. Complex patterns of secondary currents occur in the plunging plume. A low-velocity zone whose size increases with the inflow densimetric Froude number is observed near the centerline above the bed.