A Numerical Study of the Influence of Channel-Scale Secondary Circulation on Mixing Processes Downstream of River Junctions

Abstract

A rapid downstream weakening of the processes that drive the intensity of transverse mixing at the confluence of large rivers has been identified in the literature and attributed to the progressive reduction in channel scale secondary circulation and shear-driven mixing with distance downstream from the junction. These processes are investigated in this paper using a three-dimensional computation of the Reynolds averaged Navier Stokes equations combined with a Reynolds stress turbulence model for the confluence of the Kama and Vishera rivers in the Russian Urals. Simulations were carried out for three different configurations: an idealized planform with a rectangular cross-section (R), the natural planform with a rectangular cross-section (P), and the natural planform with the measured bathymetry (N), each one for three different discharge ratios. Results show that in the idealized configuration (R), the initial vortices that form due to channel-scale pressure gradients decline rapidly with distance downstream. Mixing is slow and incomplete at more than 10 multiples of channel width downstream from the junction corner. However, when the natural planform and bathymetry are introduced (N), rates of mixing increase dramatically at the junction corner and are maintained with distance downstream. Comparison with the P case suggests that it is the bathymetry that drives the most rapid mixing and notably when the discharge ratio is such that a single channel-scale vortex develops aided by curvature in the post junction channel. This effect is strongest when the discharge of the tributary that has the same direction of curvature as the post junction channel is greatest. A comprehensive set of field data are required to test this conclusion. If it holds, theoretical models of mixing processes in rivers will need to take into account the effects of bathymetry upon the interaction between river discharge ratio, secondary circulation development, and mixing rates.

Hydrodynamics, Sediment Transport and Morphological Features at the Confluence Between the Yangtze River and the Poyang Lake

“…As mentioned before, we infer that the flow with a large velocity near the surface rushed to the right bank and abruptly changed its direction to the left (Figure 5a), creating such anti‐clockwise cell. The topographic steering near the bed, for example, the swell in the middle of CYP6, may also play a role in generating this cell (Lyubimova et al., 2020). At CYP4 and CYP5 a deep scour hole, 6 m deeper than that at CYP3 and 3 m deeper than that at CYP6, was observed.…”

Shallow mixing interfaces between parallel streams of unequal densities

“…The dynamics and structure of the MI become even more complex once buoyancy starts playing an important role (Laraque, Guyot & Filizola 2009;Prats et al 2010;Lyubimova et al 2014Lyubimova et al , 2020Ramón et al 2014;Lewis & Rhoads 2015;Gualtieri, Ianniruberto & Filizola 2019;Gualtieri et al 2019;Horna-Munoz et al 2020). For sufficiently large density contrast between the incoming streams, the rates of mixing between the two streams were found to be quite different from the ones observed for cases with negligible density contrast (Horna-Munoz et al 2020).…”

“…2010; Lyubimova et al. 2014, 2020; Ramón et al. 2014; Lewis & Rhoads 2015; Cheng & Constantinescu 2018; Gualtieri, Ianniruberto & Filizola 2019; Gualtieri et al.…”

Characteristics of Flow Movement in Complex Canal System and Its Influence on Sudden Pollution Accidents

“…In addition to the existence of secondary flow in bends, branching or confluence rivers also exist. erefore, Chen et al [13] and Lyubimova et al [14] found that while twin secondary circulations are found at channel confluence, their contribution to the mixing depends on their local positions with respect to the river streams, and the water depth drives the most rapid mixing and notably. omas et al [15] evaluated the influence of secondary flow disturbance on bifurcation evolution.…”