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Direct observations of density-driven streamwise oriented vortices at a river confluence
  • Duguay Duguay,
  • Pascale Biron,
  • Jay Lacey
Duguay Duguay
Concordia University, Concordia University

Corresponding Author:duguay.jason@gmail.com

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Pascale Biron
Concordia University, Concordia University
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Jay Lacey
Université de Sherbrooke, Université de Sherbrooke
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When rivers collide, complex three-dimensional large-scale coherent turbulent structures are generated along the confluence’s mixing interface. These structures play important roles in mixing streamborne pollutants and suspended sediment, and have considerable bearing on the morphology and habitat quality of the postconfluent reach. A particular structure of great interest, streamwise orientated vortices (SOVs), were first detected in numerical simulations to form in pairs, one flanking each side of the mixing interface rotating in the opposite sense of the other. Since, it has proved difficult to detect SOVs with conventional pointwise velocimetry instrumentation. Despite the lack of empirical or observational evidence to confirm their existence and understand their dynamic behaviour, SOVs are nevertheless considered important drivers of mixing and sediment transport processes at confluences. Their causal mechanisms are also not fully understood, hindering progress towards a robust conceptual model of confluence turbulent mixing. To address these gaps, we present and analyse direct observations of highly dynamic and coherent SOVs captured in aerial drone video at a mesoscale confluence presenting a stark turbidity contrast between its tributaries. Eddy-resolved modelling demonstrates the dynamics of the SOVs can only be reproduced when a small density difference (Δρ) is imposed between the tributaries ( Δρ = 0.5 kg/m3). Our results conclusively demonstrate that SOVs do exist and that a small difference in density between the tributaries inverts the sense of rotation of the SOVs and their vertical position within the water column, causing important effects on the confluence’s turbulent mixing regime.