Impact of Density Gradients on the Secondary Flow Structure of a River Confluence

Duguay, Jason; Biron, Pascale M.; Lacey, Jay

doi:10.1029/2022wr032720

Cited by 6 publications

(20 citation statements)

References 44 publications

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“…The Froude number of the left channel (F r = U/ √ gD, where U l is taken as U) in the slow experiments was 0.048, similar in magnitude to the F r of 0.04 of the Massawippi during the density SOV observations of Duguay et al (2022b). The Froude number of the right channel (F r = 0.086) is also of similar magnitude to that of the Coaticook (0.12) in Duguay et al (2022b). The values of ρ straddle that of 0.50 kg m −3 estimated between the Coaticook and Massawippi rivers during the coherent density SOV observations of July 9th, 2020, discussed by Duguay et al (2022a).…”

Section: Tested Flow Conditionssupporting

confidence: 55%

“…a discharge-weighted average of the tributaries' bulk velocities, the bulk velocity of the tributary or the bulk velocity of the main channel). Also, Duguay et al (2022b) showed that F D is incapable of accounting for the effects that the sign of the density gradient has on secondary flow structure (i.e. a fast denser channel encountering a slow light channel produces different structures than a fast-light channel encountering a slow-dense channel).…”

Section: The Densimetric Froude Numbermentioning

confidence: 99%

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Density effects on streamwise-orientated vorticity at river confluences: a laboratory investigation

Duguay,

Biron,

Lacey

2023

J. Fluid Mech.

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Small differences in the densities of a river confluence's tributaries (i.e. 0.5 kg m$^{-3}$) have been proposed to cause coherent streamwise-oriented vortices (SOVs) in its mixing interface. These secondary flow structures are thought to result from density-driven gravity currents being laterally confined between the converging flows. However, empirical evidence for density SOVs and the confined gravity current mechanism is lacking. To this end, experiments are carried out in a laboratory confluence permitting a spectrum of thermal density differences between its tributaries. Particle image velocimetry and laser-induced fluorescence are used simultaneously to study the mixing interface's dynamics. The sensitivity of the mixing interface's secondary flow structure to the confluence's momentum ratio and the magnitude of the density difference is evaluated. Density SOVs are confirmed in the mixing interface and are caused by the gravity currents being confined laterally as the opposing flows merge and accelerate downstream. The SOVs are largest and most coherent when the momentum of the dense channel is greater than that of the light channel. The dynamics of these secondary flow structures is strongly coupled to periodic vertically orientated Kelvin–Helmholtz instabilities. The striking similarities between the empirically reproduced SOVs herein and those recently observed at the Coaticook-Massawippi confluence (Quebec, Canada), despite a two-order magnitude difference in physical scale, suggest density SOVs are a scale-independent flow structure at confluences when specific, yet relatively common, hydraulic and density conditions align.

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Section: Tested Flow Conditionssupporting

confidence: 55%

Section: The Densimetric Froude Numbermentioning

confidence: 99%

Density effects on streamwise-orientated vorticity at river confluences: a laboratory investigation

Duguay,

Biron,

Lacey

2023

J. Fluid Mech.

Self Cite

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“…Table 1 also lists the water chemistry characteristics, and the relative density difference Δ ρ observed during the field surveys. A notation is adopted to convey both the magnitude and direction of ∆ ρ (Duguay et al., 2022b). In Survey 1, the Yangtze River was estimated to be 0.9 kg/m 3 denser than the Poyang Lake and because the Yangtze River is the left tributary, density decreased across the mixing interface from left to right.…”

Section: Resultsmentioning

confidence: 99%

“…On the one hand, the density effects might have been weakened by the larger relative bed roughness associated with the lower water depth in Survey 3 (Best & Roy, 1991;Constantinescu et al, 2016). On the other hand, the density effect of Survey 3 may not be as large as the density Froude number indicated (Duguay et al, 2022b;Pouchoulin et al, 2020).…”

Section: Density Differencementioning

confidence: 98%

Mixing Dynamics at the Large Confluence Between the Yangtze River and Poyang Lake

Yuan

Gualtieri

et al. 2022

Water Resources Research

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Confluences are locations where flows from different tributaries converge, resulting in mixing in the post-confluence channel. Complete transverse mixing between the two streams can occur over the mixing interface over a distance when two streams that have significantly different sediment loads, temperatures, or dissolved chemical and nutrient loads confluence (Gaudet & Roy, 1995;Lewis et al., 2020;Lewis & Rhoads, 2015). The mixing distance scales with the product of the post-confluence channel width by its aspect ratio (Rutherford, 1994). Thus, mixing at river confluences may occur over very long distances, especially for large rivers. This requirement is confirmed in many cases by aerial field measurements and satellite observations (Bouchez et al., 2010;Rathbun & Rostad, 2004;Stallard, 1987;Umar et al., 2018). However, in some other cases mixing could be strongly influenced by the complex flow structure within the confluence hydrodynamic zone (CHZ), and consequently occur over very different length scales Lane et al., 2008;Pouchoulin et al., 2020). This study focuses on the mixing dynamics within the CHZ of river confluences.

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“…The extent to which even weak vertical stratification can modify the generalized flow patterns of confluences has been investigated computationally (Ramon et al, 2014, 2016; Horna‐Munoz et al, 2020; van Rooijen et al, 2020) and has been confirmed in the field at confluences where density effects are relatively strong (Herrero et al, 2018; Ramon et al, 2013), but extensive field investigations are lacking. Recent work suggests that even small density effects can produce local effects on flow structure in the vicinity of the mixing interface (Duguay et al, 2022a, 2022b).…”

Section: Introductionmentioning

confidence: 99%

Spatial patterns of transport‐effective flow at three small confluences: Relation to channel morphology

Shukla

Lewis

Rhoads

2023

Earth Surf Processes Landf

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The spatial patterns of transport‐effective flows at confluences and the relation of these patterns to channel morphology remain poorly understood. This field study uses acoustic Doppler current profiler measurements to explore the spatial structure of different transport‐effective flows at three small stream confluences where measurements of flow structure have been obtained previously using electromagnetic current meters or acoustic Doppler velocimeters for events incapable of mobilizing bed material or for transport‐effective events with much smaller discharges. Results show that flow accelerates from upstream to downstream for all six measured flows and that in each case a distinct region of velocity deficit exists within the confluence. Accelerating flow within this velocity‐deficit region eventually transforms into the high‐velocity core of flow downstream of the confluence. Patterns of secondary flow are indicative of helical motion, with some helical cells exhibiting senses of rotation inconsistent with patterns of streamline curvature defined by patterns of depth‐averaged velocity vectors—a type of fluid motion that contrasts with flow structure documented at lower stages. Channel form, especially bed morphology, generally reflects acceleration of flow at the confluences; all three sites exhibit zones of scour. Despite mobility of bed material during measured flows, channel morphology does not change substantially at any of the sites, but systematic change in channel form for two different transport‐effective events with different momentum flux ratios is observed at one of the three confluences. At this confluence, the type of change documented during the two events is consistent with previous work on changes in morphology before and after events with different momentum flux ratios. The results of this study improve understanding of the connections between flow structure and channel form at small lowland stream confluences.

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Impact of Density Gradients on the Secondary Flow Structure of a River Confluence

Cited by 6 publications

References 44 publications

Density effects on streamwise-orientated vorticity at river confluences: a laboratory investigation

Density effects on streamwise-orientated vorticity at river confluences: a laboratory investigation

Mixing Dynamics at the Large Confluence Between the Yangtze River and Poyang Lake

Spatial patterns of transport‐effective flow at three small confluences: Relation to channel morphology

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