This study develops a semi-3D model based on a vertical-horizontal splitting (VHS) method to analyze the flow in open-channel bends. In horizontal, the surface elevation and depth-averaged velocity components are computed by 2D depth-averaged model. In vertical, assume the 3D velocity profile of Navier-Stokes equations is equal to the depth-averaged velocity plus the deviation of velocity profile, and then the vertical
governing equations can be derived by subtracting the 2D depth-averaged equations to the 3D Navier-Stokes equations. In order to fit the complex geometry in both side wall and bed slope of channel, the orthogonal curvilinear coordinate system is used in horizontal gird, and sigma coordinate system is used in vertical grid. As for the numerical solution procedure, the two-step split-operator approach, which includes dispersion process (advection and diffusion terms) and propagation process (bed shear stress and pressure terms), is adopted to solve the 2D depth-averaged flow equations to improve the application flexibility. Implicit difference methods are adopted to relax the time step restriction allowing large time steps. Finally, three sets of experimental data including mildly curved and sharply curved channel are used to demonstrate the capability and accuracy of the semi-3D model, and the results of 2D depth-averaged model are also compared. The simulation results of semi-3D model show well agreement with experimental data considering different curved channels, bend lengths, secondary current and transverse mixing conditions.