Dynamic coupling of a three‐dimensional hydrodynamic model with a laterally averaged, two‐dimensional hydrodynamic model

An efficient method to dynamically couple a three‐dimensional (3D) hydrodynamic model with a laterally averaged, two‐dimensional (2DV) hydrodynamic model is described. The method is a useful tool to incorporate narrow tributaries that have vertically 2D flow patterns into a three‐dimensional hydrodynamic simulation for a shallow and large water body that has a 3D flow pattern. The coupling of the two models is facilitated with a free‐surface correction (FSC) method that is unconditionally stable with respect to gravity waves, wind and bottom shear stresses, and vertical eddy viscosity terms. The coupled model solves laterally averaged (2DV) Reynolds‐averaged Navier‐Stokes (RANS) equations for the narrow tributaries and three‐dimensional RANS equations for the larger water body. The coupled 3D‐2DV model was used to simulate hydrodynamics and salinity transport processes in an idealized estuary with a large water body and two narrow tributaries. In the 3D‐2DV simulation, the entire simulation domain was split into one 3D subdomain and two 2DV subdomains. To illustrate the efficiency of the coupled 3D‐2DV model, a 3D simulation was also conducted for the entire simulation domain. It was found that the coupled 3D‐2DV model can generate the same model results as those by the 3D model but uses less than half of the computer time of the 3D simulation for this particular case. An application of the coupled 3D‐2DV model to the Lower Peace River‐Upper Charlotte Harbor (LPR‐UCH) system in southwest Florida, USA, shows that the dynamically coupled model retains the efficiency of both the 3D model and the 2DV model.