Abstract
River confluences are characterized by a complex mixing zone with three-dimensional turbulent structures, which can be affected by the presence of an ice cover during the winter. The objective of this study is to characterize the flow structure in the mixing zone at a medium-size (~ 40 m) river confluence with and without an ice cover. Detailed velocity profiles were collected under the ice along the mixing plane with an Acoustic Doppler Velocimeter. For the ice-free conditions, drone imagery was used to characterize the mixing layer structures for various flow stages. Results indicate that during the ice-free conditions, very large Kelvin-Helmholtz (KH) coherent structures are visible due to turbidity differences, and occupy up to 50% of the width of the parent channel. During winter, the ice cover affects velocity profiles by moving the highest velocities towards the center of the profiles. Large turbulent structures are visible in both the streamwise and lateral velocity components. The strong correlation between these velocity components indicates that KH vortices are the dominating coherent structures in the mixing zone. A spatio-temporal conceptual model is presented to illustrate the main differences on the three-dimensional flow structure at the river confluence with and without the ice cover.
Highlights
River confluences are characterized by a complex mixing zone with three-dimensional turbulent structures
The position of the mixing interface and the type of flow structures are influenced by the junction angle, the discharge ratio (Qr), the momentum flux ratio (Mr) and the bed morphology, in particular bed discordance [1,2,3,4]
Some have highlighted the importance of streamwise-oriented structures (SOV) [3,5] whereas others have focused on the distortion of vertical Kelvin-Helmholtz (KH) vortices induced by bed discordance [4,6]
Summary
River confluences are characterized by a complex mixing zone with three-dimensional turbulent structures. The position of the mixing interface and the type of flow structures are influenced by the junction angle, the discharge ratio (Qr), the momentum flux ratio (Mr) and the bed morphology, in particular bed discordance [1,2,3,4]. Whilst there is still no clear consensus on the role of these various structures, there is an agreement that vortices in the mixing zone can play a significant role in sediment transport [4,5,7] and on the morphological evolution of the confluence. There remains an overall paucity of studies on medium-sized confluences (30-50 m in width) to help develop general models of mixing processes that are not scale dependent
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