ABSTRACT: A 2D depth-integrated subgrid hydrodynamic model (FrehdC) is designed to simulate effects of subgrid-scale topography on flow and scalar transport in shallow coastal marshes using computationally-efficient grid cells that are coarser than many of the channelized paths through the marsh. The subgrid-scale topography is parameterized into four depth-dependent variables (subgrid cell volume and three subgrid face areas) that characterize the high- resolution features of coarse grid cells. These variables are pre-stored in a table and embedded into the governing equations as model inputs to scale cell storage, mass and momentum fluxes across cell faces. A block-checking procedure is designed to automatically preserve high-resolution surface connectivity during grid-coarsening. By testing on both synthetic domain and real marshes, this new model is able to approximate fine-grid simulation results of surface elevation, inundation area, flow rate and salinity with less computational cost.