Turbulent Scales Observed in a River Plume Entering a Fjord

Stevens, Craig L.; O'Callaghan, Joanne

doi:10.1029/2019jc015448

Cited by 5 publications

(16 citation statements)

References 63 publications

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“…A microstructure profiler (VMP 250, Rockland Scientific) was deployed from the side of the vessel, measuring small-scale velocity shear from which estimates of TKE dissipation rates ( ) were directly obtained (McPherson et al, 2019). The VMP was deployed in an upwards-profiling mode which enabled measurements right to the water surface.…”

Section: Vessel-based Survey Of Currents and Density Fieldmentioning

confidence: 99%

“…3were estimated using a control volume method. As the majority of energy was found in the surface layer (O'Callaghan and Stevens, 2015;McPherson et al, 2019), the upper and lower integration limits were defined by the depth 0 ≤ z ≤ 10 m and along-axis distance 0 < x ≤ 3 km. Across-fjord limits (in the y-direction) were defined by the relative plume width (b(x)) determined by the freshwater conservation equation,…”

Section: Plume Momentummentioning

confidence: 99%

“…The turbulence in the near-field region of the river plume system studied in this paper was investigated by McPherson et al (2019), using direct turbulence measurements from microstructure profilers to examine the drivers of turbulent kinetic energy (TKE) dissipation rate ( ) variability within the plume. In the initial 0.5 km downstream of the plume discharge point, measurements of in the surface plume layer were amongst the highest recorded in oceanic shear flows (maximum > 10 −2 Wkg −1 ), with decreasing with distance from the source.…”

Section: Introductionmentioning

confidence: 99%

“…When a riverine inflow is discharged into the head of the fjord (Pickard and Stanton, 1980), this provides an idealised domain in which to isolate specific aspects of near-field plume dynamics. While the deep bathymetry minimises tidal exchange and the inner basin is sheltered from large ocean swell, fjord-river interactions can been directly applied to coastal plumes as the two systems share many common features: freshwater inflow to the fjord produces similar density gradients observed in major rivers such as the Hudson and Columbia Rivers (MacDonald and Geyer, 2004;Hunter et al, 2010); comparable estimates of occur in each of the near-field plume regions (MacDonald et al, 2007;McPherson et al, 2019); and timevariable discharge rates, common to both systems, similarly impact the plume structure and energetics (Yankovsky, 2001;O'Callaghan and Stevens, 2015).…”

Section: Introductionmentioning

confidence: 99%

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

Stevens¹,

O'Callaghan²,

Lucas³

et al. 2019

Preprint

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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 surface layer. An observed transition from a supercritical to sub-critical flow regime in the initial 1 km indicates the presence of an internal hydraulic jump and the subsequent release of internal gravity waves. 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 the observed hydraulic jump and internal wave radiation could account for the discrepancy in the budget.

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Section: Vessel-based Survey Of Currents and Density Fieldmentioning

confidence: 99%

Section: Plume Momentummentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Stevens¹,

O'Callaghan²,

Lucas³

et al. 2019

Preprint

Self Cite

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show abstract

“…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). Quantifying the role of lateral spreading in the presence of enhanced rates of turbulent mixing is therefore necessary to characterise the dynamics which govern near-field plume structure.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2021

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Observations collected from a fast-flowing buoyant river plume entering the head of Doubtful Sound, New Zealand, were analysed to examine the drivers of plume lateral spreading. The near-field plume is characterised by flow speeds of over 2 ms−1, and strong stratification (N2 > 0.1 s−2), resulting in enhanced shear which supports the elevated turbulence dissipation rates (ϵ > 10−3 W kg−1). Estimates of plume lateral spreading rates were derived from the trajectories of Lagrangian GPS surface drifters and from cross-plume hydrographic transects. Lateral spreading rates derived from the latter compared favourably with estimates derived from a control volume technique in a previous study. The lateral spreading of the plume was driven by a baroclinic pressure gradient toward the base of the plume. However, spreading rates were underestimated by the surface drifters. A convergence of near-surface flow from the barotropic pressure gradient concentrated the drifters within the plume core. The combination of enhanced internal turbulence stress and mixing at the base of the surface layer, and the presence of steep fjord sidewalls likely reduced the rate of lateral spreading relative to the theoretical spreading rate. The estimates of plume width from the observations provided evidence of scale-dependent dispersion which followed a 4/3 power law. Two theoretical models of dispersion, turbulence and shear flow dispersion, were examined to assess which was capable of representing the observed spreading. An analytical horizontal shear-flow dispersion model generated estimates of lateral dispersion that were consistent with the observed 4/3 law of dispersion. Therefore, horizontal shear dispersion appeared to be the dominant mechanism of dispersion, thus spreading, in the surface plume layer.

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Experimental properties of continuously forced, shear-driven, stratified turbulence. Part 1. Mean flows, self-organisation, turbulent fractions

Lefauve

Linden

2022

J. Fluid Mech.

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We study the experimental properties of exchange flows in a stratified inclined duct, which are simultaneously turbulent, strongly stratified by a mean vertical density gradient, driven by a mean vertical shear, and continuously forced by gravity. We focus on the ‘core’ shear layer away from the duct walls, where these flows are excellent experimentally realisable approximations of canonical hyperbolic-tangent stratified shear layers, whose forcing allows mean and turbulent properties to reach quasi-steady states. We analyse state-of-the-art data sets of the time-resolved density and velocity in three-dimensional subvolumes of the duct in 16 experiments covering a range of flow regimes (Holmboe waves, intermittent turbulence, full turbulence). In this Part 1 we first reveal the permissible regions in the multidimensional parameter space (Reynolds number, bulk Richardson number, velocity-to-density layer thickness ratio), and their link to experimentally controllable parameters. Reynolds-averaged balances then reveal the subtle momentum forcing and dissipation mechanisms in each layer, the broadening or sharpening of the density interface, and the importance of the streamwise non-periodicity of these flows. Mean flows suggest a tendency towards self-similarity of the velocity and density profiles with increasing turbulence, and gradient Richardson number statistics support prior ‘internal mixing’ theories of ‘equilibrium Richardson number’, ‘marginal stability’ and ‘self-organised criticality’. Turbulent volume fractions based on enstrophy and overturn thresholds quantify the nature of turbulence between different regimes in different regions of parameter space, while highlighting the challenges of obtaining representative statistics in spatiotemporally intermittent flows. These insights may stimulate and assist the development of numerical simulations with a higher degree of experimental realism.

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Turbulent Scales Observed in a River Plume Entering a Fjord

Cited by 5 publications

References 63 publications

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

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

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

Experimental properties of continuously forced, shear-driven, stratified turbulence. Part 1. Mean flows, self-organisation, turbulent fractions

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