Transverse Mixing in a River with Complicated Channel Geometry

Abstract

This paper investigates the flow behavior and the associated mixing in a river system with complicated geometry by means of two-dimensional numerical analysis. The basic mathematical models for flows and mixing in the generalized curvilinear coordinate system are derived from the usual equations for shallow water in the Cartesian coordinate system, and the numerical models with use of FDM is developed. The models are applied to the Yodo River System which has three main tributaries; flow behavior and concentration distributions at/below the confluence of the Yodo River System are obtained, and they are successfully compared with the observed data. The characteristics of flow behavior and the transverse mixing at/below the confluence of the Yodo River System are also investigated by both numerical and simplified physical experiments under various hydraulic conditions. The characteristics may be summarized as follows: 1) The distribution of the river bed elevation is non-uniform in the longitudinal and the transverse directions. Therefore, the transverse distribution of the momentum flux vector varies in the longitudinal direction, 2) This variation makes the convective transport of concentration in the transverse direction, with the convection having more dominant roles in transverse mixing than the dispersion transport, 3) Therefore, the concentration distribution is much influenced by the discharge condition and the river bed topography. These features are also demonstrated by our simplified physical model study.