A 3-D implicit finite-volume model of shallow water flows

•The developed model is the first one in literature to adopt the SIMPLEC algorithm with under-relaxation to couple the velocity and water level in solution of the 3-D shallow water equations.•It uses non-staggered grid to simplify the numerical algorithms and applies a Rhie and Chow-type momentum interpolation method to eliminate potential oscillations that exist when the linear interpolation is used to determine intercell fluxes on non-staggered grid.•The classical 2-D mixing length model is extended to 3-D situations by using different mixing length functions for the horizontal and vertical shear strain rates.•The model can simulate well the wind-driving flow using a newly developed mixing length function.•The quadtree (telescoping) mesh is a relatively new idea to refine the mesh and take advantage of Cartesian grid. A three-dimensional (3-D) model has been developed to simulate shallow water flows in large water bodies, such as coastal and estuarine waters. The eddy viscosity is determined using a newly modified mixing length model that uses different mixing length functions for the horizontal and vertical shear strain rates. The 3-D shallow water flow equations with the hydrostatic pressure assumption are solved using an implicit finite-volume method based on a quadtree (telescoping) rectangular mesh on the horizontal plane and the sigma coordinate in the vertical direction. The quadtree technique can locally refine the mesh around structures or in high-gradient regions by splitting a coarse cell into four child cells. The grid nodes are numbered with a one-dimensional index system that has unstructured grid feature for better grid flexibility. All the primary variables are arranged in a non-staggered grid system. Fluxes at cell faces are determined using a Rhie and Chow-type momentum interpolation, to avoid the possible spurious checkerboard oscillations caused by linear interpolation. Each of the discretized governing equations is solved iteratively using the flexible GMRES method with ILUT preconditioning, and coupling of water level and velocity among these equations is achieved by using the SIMPLEC algorithm with under-relaxation. The model has been tested in four cases, including steady flow near a spur-dyke, tidal flows in San Francisco Bay and Gironde Estuary, and wind-induced current in a flume. The calculated water levels and velocities are in good agreement with the measured values.