A computationally efficient subgrid model for coupled surface and groundwater flows.

We present the development of a high-efficiency and high-resolution subgrid model for simulating surface water and groundwater interaction in coastal regions. The governing equations are derived based on an assumption of the fully-saturated subsurface flow and a phase-averaging method, in which two phase functions are defined respectively for the surface phase and subsurface phase at the subgrid level. Both the surface water and groundwater processes are modeled by solving the phase-averaged equations on a coarse grid, taking into account the subgrid effects. Model validation with laboratory data indicates that the subgrid model is capable of simulating flow processes within permeable structures. The model was applied to simulating flooding and draining processes and the interaction of surface flows and groundwater flows in a meso-tidal salt marsh. The model results demonstrate that the subgrid model can be used in a complex salt marsh system with numerous narrow channels, creeks and artificial ditches, with accuracy similar to that of the full model running directly on the high-resolution grid, but computational costs can be two orders of magnitude smaller than for the full grid model. Tidal influence on the phreatic surface of the groundwater in the marsh system was discussed. • A subgrid model is developed for simulating surface wave and groundwater interaction. • High-resolution subgrid effects are accounted for in the coarse grid computations. • The model is validated against laboratory experiment. • Simulate the interaction of surface and groundwater flows in a meso-tidal marsh. [ABSTRACT FROM AUTHOR]