Three‐dimensional turbulent structures at a medium‐sized confluence with and without an ice cover

Confluences are important sites for mixing within river networks. Past work has shown that mixing within confluences is highly variable; in some cases flows mix rapidly and in other cases flows remain unmixed far downstream of the confluence. The fluvial processes that govern mixing within confluences remain poorly understood. This study relates patterns and amounts of mixing to three-dimensional flow structure at three small confluences. It focuses on lateral fluxes of streamwise momentum, which theoretical considerations suggest should influence lateral mixing. Patterns and amounts of mixing differ at the three sites. Considerable mixing occurs at an asymmetrical confluence with strong helical motion within flow from the lateral tributary, which produces substantial differences in advective lateral transport of streamwise momentum over depth. Minor mixing occurs at a comparatively symmetrical confluence where incoming flows have relatively equal momentum fluxes; however, helical motion within one of the flows locally increases mixing. At a symmetrical confluence where one incoming flow has much greater momentum flux than the other, mixing occurs largely through progressive lateral shifting of the mixing interface toward the minor tributary because of the strong lateral flux of streamwise momentum by the dominant tributary. At all three confluences, lateral turbulent transport of streamwise momentum is an order of magnitude less than advective lateral transport of streamwise momentum. The study indicates that generalization of mixing at confluences remains challenging but that advective lateral fluxes of streamwise momentum related to secondary currents (helical motion) or primary flow (cross currents) greatly enhance mixing at confluences.

Section: Discussionmentioning

confidence: 99%

Advective Lateral Transport of Streamwise Momentum Governs Mixing at Small River Confluences

Lewis

Rhoads

Sukhodolov

et al. 2020

“…However, based on drone imagery, Biron et al. (2019) observed that only KH vortices dominated within the shear layer for discharge ratios ranging from 0.09 to 1.02 in a medium‐sized confluence. Twin surface‐convergent helical cells were observed on either side of the shear layer in a vertical plane, which rapidly evolve into a single, channel‐scale circulation cell due to the differences in the curvature of the two combining flows (Bradbrook et al., 2000; Rhoads & Kenworthy, 1995, 1998).…”

Section: Introductionmentioning

confidence: 99%

“…The shear layer characterized by von Kármán vortex street for the momentum ratios close to unity, that is, wake mode (Constantinescu et al, 2011(Constantinescu et al, , 2012 or vortex pairing for those far from unity, that is, KH vortices dominating mode (Rhoads & Sukhodolov, 2004) were reported. However, based on drone imagery, Biron et al (2019) observed that only KH vortices dominated within the shear layer for discharge ratios ranging from 0.09 to 1.02 in a medium-sized confluence. Twin surface-convergent helical cells were observed on either side of the shear layer in a vertical plane, which rapidly evolve into a single, channel-scale circulation cell due to the differences in the curvature of the two combining flows (Bradbrook et al, 2000;Rhoads & Kenworthy, 1995.…”

mentioning

confidence: 99%

Hydrodynamics, Sediment Transport and Morphological Features at the Confluence Between the Yangtze River and the Poyang Lake

Yuan

Tang

et al. 2021

100

Confluences are critical nodes in a river network and affect flow, sediment transport, water quality, and ecological patterns. Significant changes in hydrodynamics, bed morphology as well as environmental and ecological features occur at the confluence, where the flows from two tributaries combine and adjust to the postconfluence planform geometry. Confluence of two flows with different momentum and velocity ratios (the ratio of the variable of one channel to that of the other) often results in a downstream shear layer driven by the Kelvin-Helmholtz (KH) instability. It enhances turbulent mixing and exchange of momentum and other matter (e.g., sediment and pollutants;

“…Transverse mixing due to molecular diffusion is usually negligible compared to shear dispersion and advective effects over a range of spatial scales. Mixing is due firstly to the shear between the two incoming flows, characterized by turbulent eddies that develop at the mixing layer scale (Biron et al, 2019) and substantially enhance transverse mixing in the near-field downstream of the confluence. In addition to shear dispersion, mixing may be enhanced by convective effects due to large-scale, persistent flow structures, notably, helical motion and upwelling motion.…”

Section: Introductionmentioning

confidence: 99%

Predicting Transverse Mixing Efficiency Downstream of a River Confluence

Pouchoulin

Coz

Mignot

et al. 2020

Predicting mixing processes, especially transverse mixing, downstream of river confluences, is necessary for assessing and modeling the fate of pollutants transported in river networks, but it is still challenging. Typically, there is a lack of transverse mixing solutions implemented in 1-D hydrodynamical models widely used in river engineering applications. To investigate the mixing processes developing downstream of a medium-sized river confluence, three high-resolution in situ surveys are conducted at the Rhône-Saône confluence in France, based on geolocated specific conductivity and hydroacoustic measurements. Contrasting mixing situations are observed depending on hydrological conditions. In some cases, the two flows mix slowly due to turbulent shear at their vertical interface. This can be modeled by an analytical solution of the advection-diffusion equation. In other cases, the waters from one of the two tributaries move under the waters of the other tributary. The induced local circulation enhances transverse mixing but not vertical mixing and the flow remains stratified vertically, which may be missed when surface or satellite images are analyzed qualitatively. Stratification may be predicted by comparing the time scales for shear and density-driven adjustment. Shear-dominated transverse mixing of depth-averaged concentrations can be predicted analytically and implemented in 1-D hydrodynamical models. However, the initiation of apparently rapid transverse mixing due to density-driven circulation remains to be better understood and quantified. Plain Language Summary Predicting how waters mix downstream of river confluences is necessary for assessing and modeling the fate of pollutants transported in river networks, but it is still challenging. Typically, there is a lack of transverse mixing solutions implemented in models widely used in river engineering applications. To investigate the mixing processes developing downstream of a medium-sized river confluence, three high-resolution in situ surveys are conducted at the Rhône-Saône confluence in France. Contrasting slow or rapid mixing situations are observed depending on hydrological conditions. The transverse mixing of depth-averaged concentrations can be predicted analytically and implemented in 1-D hydrodynamical models. However, the initiation of rapid transverse mixing due to difference in fluid density remains to be better understood and quantified.