Accounting for density-driven secondary flows at river confluences with a 2-D depth-averaged hydro-morphodynamic model

At river confluences, transverse density gradients induce secondary currents that interact with those generated by streamline curvature, affecting flow patterns and sediment dynamics. Here, a hydro- and morphodynamic two-dimensional numerical model is enhanced to account for density-driven secondary flows. The model solves the Shallow Water Equations coupled with transport equations for water temperature and streamwise angular momentum, driven by both streamline curvature and spanwise density gradients. A morphodynamic module computes bedload, suspended sediment transport, and the bed evolution. The model is tested against three-dimensional CFD results and applied to the Yangtze River and Poyang Lake confluence in both fixed and mobile bed modes. The results, which favorably compare to measured data, highlight the role of temperature dynamics in the pattern and intensity of secondary currents and their contribution in shaping the riverbed. The model allows for long-term morphodynamic simulations at low computational effort. • Secondary currents at confluences arise from curvature and transversal gradients of density. • A 2-D model including curvature effects is enhanced to include density-induced secondary flow. • The model is applied to schematic cases and to the Yangtze River-Poyang Lake confluence. • The model enables accurate long-term simulations with affordable computational cost.