Current confluence research emphasizes three broad controls on flow
structure generation: (1) planform curvature; (2) topographic steering;
and (3) anisotropic turbulence associated with flow separation and
shear layer dynamics. The relative importance of these processes in
explaining observed flow structures is controversial, a situation that
may be related to the fact that different investigators have examined
different confluence configurations. This paper uses a
three-dimensional numerical model, with a fully elliptic solution, a
free surface treatment, and a turbulence model based on a renormalized
group (RNG) to help to provide a physically based explanation of the
controls upon flow structure generation for both a laboratory
(rectangular) and a field confluence (the confluence of the Kaskaskia
River and Copper Slough) and to identify the particular conditions
under which particular flow structures are observed. Results suggest
that an analogy with back-to-back meanders is possible for symmetrical
configurations but that there will be progressive divergence from this
state as confluence asymmetry increases. In asymmetric situations a
dual-cell structure may be limited to the immediate vicinity of the
junction because of the effects of streamline curvature and topographic
steering. These differences can be explained by consideration of the
dynamic pressure field, which may be specific to each confluence
configuration. As such, this study partially reconciles differing views
over what controls time-averaged flow structures in river channel
confluences, although further research into the interaction of these
processes with instantaneous velocity fluctuations is required.