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

Lewis, Quinn W.; Rhoads, Bruce L.; Sukhodolov, Alexander N.; Constantinescu, George

doi:10.1029/2019wr026817

Cited by 29 publications

(47 citation statements)

References 63 publications

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“…These different hydrodynamic regions generally correspond to those identified in previous laboratory and field research on confluence hydrodynamics based on visual observations of dye tracing or the movement of neutrally buoyant particles within the CHZ (Best, 1987) and on velocity measurements at cross sections (Rhoads & Kenworthy, 1995, 1998). The spatial pattern of these regions conforms to those documented in other work that has examined time‐averaged flow structure at the Saline confluence using acoustic Doppler velocimeter (ADV) measurements (Lewis et al., 2020; Rhoads & Sukhodolov, 2001) and drone‐based LSPIV methods (Lewis & Rhoads, 2018a, 2018b). The ADV measurements provide information on patterns of fluid motion throughout the water column, but only at slices through the flow that happen to intersect different hydrodynamic regions.…”

Section: Discussionsupporting

confidence: 81%

Large‐Scale Particle Image Velocimetry Reveals Pulsing of Incoming Flow at a Stream Confluence

Sabrina

Lewis

Rhoads

2021

Water Resources Research

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Despite widespread recognition that confluences are characterized by complex hydrodynamic conditions, few studies have mapped in detail spatial patterns of flow at confluences and variation in these patterns over time. Recent developments in large‐scale particle image velocimetry (LSPIV) have created novel opportunities to explore the spatial and temporal dynamics of flow patterns at confluences. This study uses LSPIV to map two‐dimensional flow structure at the water surface at a confluence and to examine variation in this structure over time. Results show that flow within the confluence is characterized by a large region of flow stagnation at the junction apex, a region of low velocities at the downstream junction corner, and a region of merging of the two flows along a mixing interface within the center of the confluence. Interaction between the incoming flows varies over time in the form of episodic pulsing in which one of the two tributary flows first decelerates and then subsequently accelerates into the confluence. The cause of this pulsing remains uncertain, but it may reflect unsteadiness in the water‐surface pressure‐gradient field as the two flows compete for space within the confluence. No large‐scale vortices are evident within the mixing interface for the particular flow conditions documented in this study, but such vortices do occur along the margins of the stagnation zone where shearing action between fast‐moving and slow‐moving fluid is strong. The results of the study provide insight into the time‐dependent dynamics of the spatial structure of flow at stream confluences.

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Section: Discussionsupporting

confidence: 81%

Large‐Scale Particle Image Velocimetry Reveals Pulsing of Incoming Flow at a Stream Confluence

Sabrina

Lewis

Rhoads

2021

Water Resources Research

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“…Previous studies have reported that transverse mixing is related to the momentum flux ratio (M r ). Lewis et al [16] and Lewis and Rhoads [41] reported that transverse mixing increases with M r . Additionally, the results presented by Pouchoulin et al [30] support the positive correlation between transverse mixing and M r based on field measurements, where D T /hu * was 1.34, M r = 1.15 and 2.21, and M r = 1.54.…”

Section: Solute Mixing Downstream Of Junctionmentioning

confidence: 96%

“…The discharge of the main and tributary channels was set to Fr d = 0.37 for a discharge ratio (Q r ) of 0.25, such that the numerical simulations would have a separation zone downstream of the junction. Based on these hydraulic conditions, the momentum flux ratio (M r ) in this study was 1.62, which is included in the range for previous studies (0.29-2.52) [16,30]. The free-slip condition was employed for the wall boundary.…”

Section: Model Settings and Validationsmentioning

confidence: 99%

“…A circular motion is formed as a result of discharge from the tributary channel at the outer side of the confluence. The complex characteristics of the flow in the confluence change with the junction angle, discharge ratio between the main and tributary flows, discordance of the channel bed at the entrance to the confluence, geometric symmetry, density contrasts, and potentially, the width-depth ratio [13][14][15][16].…”

Section: Introductionmentioning

confidence: 99%

“…The effect of density contrast on mixing is more dominant farther downstream from the confluence than differences in the velocity of the main and tributary channel [33]. Lewis et al [16] demonstrated that a tributary flow with strong momentum flux induces helical motion, which moves the mixing interface toward the outer bank, causing transverse mixing to increase. The strong momentum flux affects not only the helical motion but also the flow separation, which causes solute storage.…”

Section: Introductionmentioning

confidence: 99%

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Analysis of Storage Effects in the Recirculation Zone Based on the Junction Angle of Channel Confluence

2021

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In this study, numerical simulations using the Environmental Fluid Dynamics Code model were conducted to elucidate the effects of flow structures in the recirculation zone on solute storage based on the junction angle. Numerical simulations were performed at a junction angle of 30° to 90° with a momentum flux ratio of 1.62. The simulation results revealed that an increase in the junction angle caused the recirculation zone length and width to increase and strengthened the development of helical motion. The helical motion increased the vertical gradient of the mixing layer and the mixing metric of the dosage curves. The recirculation zone accumulated the solute as a storage zone, which formed a long tail in the concentration curves. The interaction between the helical motion and recirculation zone affected the transverse mixing, such that the transverse dispersion had a positive relationship with the helical motion intensity and a negative relationship with the recirculation zone size. Transverse mixing exhibited an inverse relationship with the mass exchange rate of the recirculation zone. These results indicate that the transverse dispersion is replaced by mixing due to strongly developed storage zones.

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

Cited by 29 publications

References 63 publications

Large‐Scale Particle Image Velocimetry Reveals Pulsing of Incoming Flow at a Stream Confluence

Large‐Scale Particle Image Velocimetry Reveals Pulsing of Incoming Flow at a Stream Confluence

Analysis of Storage Effects in the Recirculation Zone Based on the Junction Angle of Channel Confluence

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

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