A new model for coupling surface and subsurface water flows: With an application to a lagoon

SummaryBuilt upon the work of Yuan et al. (Yuan, D., Lin, B., Falconer, R.A., 2008. Simulating moving boundary using a linked groundwater and surface water flow model. Journal of Hydrology 349, 524–535), we present in this note an integrated, vertically averaged numerical model based on an unstructured finite volume/finite difference method for simulating coupled surface and subsurface water flows in a coastal hydrological system. The modelled domain was divided into a surface water flow layer and a groundwater flow layer, with the former overlying the latter across the interface. In contrast to the work of Yuan et al. (2008), the governing equations for describing these two different flows were discretised using an unstructured triangular mesh and then assembled into a single set of linear algebraic equations for computing the solutions. A semi-implicit Eulerian–Lagrangian method was used to deal with the momentum convection term of the surface flow equation. In this method, the constraint on the model’s time step imposed by the Courant–Friedrichs–Lewy condition for stable solutions is relaxed. The model was validated against the results of a previous experiment on the interaction of surface and subsurface flows in a coastal lagoon. This showed that the new model retained the same degree of accuracy in its predictions even with time steps increased by two orders of magnitude from those allowed in the model of Yuan et al. (2008). The resulting increase of computational efficiency will enable model applications to large scale hydrological systems commonly encountered in the coastal zone.