Impact of density gradients on secondary flow structure at a river confluence

Abstract

A small gradient in the densities (Δρ) of two rivers was recently shown to develop coherent streamwise orientated vortices (SOVs) in the mixing interface of their confluence. We further investigate this phenomenon at the Coaticook and Massawippi confluence (Quebec, Canada) using eddy-resolved numerical modelling to examine how the magnitude and direction of Δρ· affect this secondary flow feature. Results show that a front from the denser channel always slides underneath the lighter channel independent of the direction of Δρ. When the fast tributary (Coaticook) is denser, coherent clockwise rotating density SOVs tend to form on the slow (Massawippi) side. However, when the slow Massawippi is denser by the same magnitude, anticlockwise secondary flow caused principally by shear induced interfacial instabilities develop on the fast Coaticook side. This shows the inertia of the tributary opposing the lateral propagation of the dense front shapes the secondary flow characteristics of the mixing interface. Moreover, in the absence of a density difference, anticlockwise SOVs are predicted by the model which correspond well to new aerial observations of anticlockwise SOVs at the site. A densimetric Froude number (Fd) convention accounting for the direction of Δρ is proposed to accurately convey the local inertial forces that oppose the lateral propagation of the dense front. Finally, a conceptual model of the mixing interface's secondary flow structure over a spectrum of plausible Fd values is proposed. The Fd convention provides a flexible and consistent metric for use in future studies examining the effects of Δρ on river confluence hydrodynamics.

Mixing Dynamics at the Large Confluence Between the Yangtze River and Poyang Lake

“…On the one hand, the density effects might have been weakened by the larger relative bed roughness associated with the lower water depth in Survey 3 (Best & Roy, 1991; Constantinescu et al., 2016). On the other hand, the density effect of Survey 3 may not be as large as the density Froude number indicated (Duguay et al., 2022b; Pouchoulin et al., 2020).…”

“…Table 1 also lists the water chemistry characteristics, and the relative density difference Δ ρ observed during the field surveys. A notation is adopted to convey both the magnitude and direction of ∆ ρ (Duguay et al., 2022b). In Survey 1, the Yangtze River was estimated to be 0.9 kg/m 3 denser than the Poyang Lake and because the Yangtze River is the left tributary, density decreased across the mixing interface from left to right.…”