Laboratory Investigation of the Impact of Lateral Spreading on Buoyancy Flux in a River Plume

Abstract

he relationship between lateral spreading and mixing in stratified gravity currents is investigated by comparing laterally confined and unconfined currents in a series of laboratory experiments. The vertical turbulent buoyancy flux is determined using a control volume approach with velocity and density fields derived from combined particle image velocimetry (PIV) and planar-laser-induced fluorescence (PLIF). Lateral spreading is determined in the unconfined experiments based on plan-view imaging using the optical thickness method (OTM). The authors find that lateral spreading dramatically modifies the plume structure; the spreading plume layer consists of approximately linear density and velocity profiles that extend to the surface, whereas the channelized plume profiles are uniform near the surface. Lateral spreading decreases the average plume density relative to laterally confined currents with similar inflow conditions. However, the local turbulent buoyancy flux in the spreading experiments is approximately equal to that in the confined experiments. This apparent paradox is resolved when the plume areas are taken into account. The total mixing integrated over the horizontal plume area is significantly higher in the spreading experiments. Thus, the experiments suggest that spreading does not appreciably alter the turbulent mixing processes at the base of the plume. However, it significantly increases the area over which this mixing occurs and, through this mechanism, increases the net dilution of river water at a fixed distance from the river mouth. Finally, the authors hypothesize that the spreading does not significantly increase the local turbulent buoyancy flux because spreading occurs preferentially near the surface, whereas buoyancy flux is greatest in the core of the current.