Direct observations of density-driven streamwise oriented vortices at a
river confluence
Abstract
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.