Large-scale turbulent mixing at a mesoscale confluence assessed through drone imagery and eddy-resolved modelling

Confluences are sites of intense turbulent mixing in fluvial systems. The large-scale turbulent structures largely responsible for this mixing have been proposed to fall into three main classes: vertically orientated (Kelvin-Helmholtz) vortices, secondary flow helical cells and smaller, strongly coherent streamwise-orientated vortices. Little is known concerning the prevalence and causal mechanisms of each class, their interactions with one another and their respective contributions to mixing. Historically, mixing processes have largely been interpreted through statistical moments derived from sparse pointwise flow field and passive scalar transport measurements, causing the contribution of the instantaneous flow field to be largely overlooked. To overcome the limited spatiotemporal resolution of traditional methods, herein we analyse aerial video of large-scale turbulent structures made visible by turbidity gradients present along the mixing interface of a mesoscale confluence and complement our findings with eddy-resolved numerical modelling. The fast, shallow main channel (Mitis) separates over the crest of the scour hole's avalanche face prior to colliding with the slow, deep tributary (Neigette), resulting in a streamwise-orientated separation cell in the lee of the avalanche face. Nascent large-scale Kelvin-Helmholtz instabilities form along the collision zone and expand as the high-momentum, separated near-surface flow of the Mitis pushes into them. Simultaneously, the strong downwelling of the Mitis is accompanied by strong upwelling of the Neigette. The upwelling Neigette results in similar to 50% of the Neigette's discharge crossing the mixing interface over the short collision zone. Helical cells were not observed at the confluence. However, the downwelling Mitis, upwelling Neigette and separation cell interact to generate considerable streamwise vorticity on the Mitis side of the mixing interface. This streamwise vorticity is strongly coupled to the large-scale Kelvin-Helmholtz instabilities, which greatly enhances mixing. Comparably complex interactions between large-scale Kelvin-Helmholtz instabilities and coherent streamwise vortices are expected at other typical asymmetric confluences exhibiting a pronounced scour hole.