The role of turbulence and internal waves in the structure and evolution of a near-field river plume

Abstract

Abstract. An along-channel momentum budget is quantified in the near-field plume region of a controlled river flow entering Doubtful Sound, New Zealand. Observations include highly resolved density, velocity and turbulence, enabling a momentum budget to be constructed over a control volume. Estimates of internal stress (τ) were made from direct measurements of turbulence dissipation rates (ϵ) using vertical microstructure profiles. High flow speeds of the surface plume over 2 m s−1 and strong stratification (N2∼10-1 s−2) resulted in enhanced turbulence dissipation rates (ϵ>10-3 W kg−1) and internal stress (τ>10-2 m2 s−2) at the base of the surface layer. Internal waves were observed propagating along the base of the plume, potentially released subsequent to a hydraulic jump in the initial 1 km downstream of the plume discharge point. The momentum flux divergence of these internal waves suggests that almost 15 % of the total plume momentum can be transported out of the system by wave radiation, therefore playing a crucial role in the redistribution of momentum within the near-field plume. Observations illustrate that the evolution of the momentum budget components vary between the distinct surface plume layer and the turbulent, shear-stratified interfacial layer. Within the surface plume, a momentum balance was achieved. The dynamical balance demonstrates that the deceleration of the plume, driven by along-channel advection, is controlled by turbulence stress from the plume discharge point to as far as 3 km downstream. In the interfacial layer, however, the momentum equation was dominated by the turbulence stress term and the balance was not closed. The redistribution of momentum within the shear-stratified layer by internal wave radiation and other hydraulic features could account for the discrepancy in the budget.

Spatial Structure, Short-temporal Variability, and Dynamical Features of Small River Plumes as Observed by Aerial Drones: Case Study of the Kodor and Bzyp River Plumes

“…Osadchiev [33] described a mechanism of generation of internal waves in small river plumes as a result of rapid deceleration of an inflowing river jet and formation of a hydraulic jump in vicinity of a river mouth. These internal waves propagate offshore and are regularly observed by satellite imagery in many coastal regions in the World [33,90,91]. Using aerial remote sensing we recorded generation and propagation of these internal waves from the mouth of the side-channel of the Bzyp River on 1 July 2019 ( Figure 11a).…”

Mechanisms of Lateral Spreading in a Near-Field Buoyant River Plume Entering a Fjord

“…This choice of W 0 enables W/W 0 for each drifter deployment to be compared, despite the variability in the deployment locations of the drifters in each experiment. For the lateral transects, W 0 = 100 m, which is the measured distance of the width of the tailrace discharge point (McPherson et al, 2020). Here, W is calculated along each drifter track at intervals of 10 m from the reference point.…”

“…The far-field plume region is influenced by buoyancy, wind stress and rotation (Hetland, 2005). An understanding of the near-field processes, which compete to determine the plume structure and ultimate redistribution of energy and momentum in the plume (Hetland, 2005;MacDonald et al, 2007;MacDonald and Chen, 2012;McPherson et al, 2020), is necessary to properly characterise the local plume behaviour and understand the implications for the larger coastal ocean.…”

“…Lateral spreading, on the other hand, accelerates the plume due to a shoaling of the surface layer and enhances stratified-shear turbulence (Hetland, 2005;MacDonald and Chen, 2012). The role of turbulent mixing in the near-field region of the plume system studied in this paper was quantified by McPherson et al (2020) using direct measurements and a control volume method. A momentum budget determined that the deceleration of the plume was controlled by turbulence stress, with enhanced turbulent kinetic energy (TKE) dissipation rates (ǫ) at the base of the plume (maximum ǫ > 10 −3 W kg −1 ) (McPherson et al, 2019).…”

“…Fjord-river interactions can been directly applied to coastal plumes as the two systems share many common features, whereby a freshwater inflow interacts with a coastal ambient (Garvine, 1987;O'Callaghan and Stevens, 2015). The evolution of the plume width and lateral spreading rate in the near-field are obtained directly from GPS surface drifters and lateral hydrographic data, and then compared with the control volume derived estimates from McPherson et al (2020). The role that the fjord setting, with its steep sidewalls, plays in influencing the lateral spreading of the plume is also examined.…”