Assessment of subgrid method in a finite-volume model

We present an implementation of a subgrid finite-volume model based on a nonlinear semi-implicit solver originally proposed by Casulli (2009), but with different approaches for constructing the tangential velocity as required by all Arakawa C-grid models. The model is carefully benchmarked through a few revealing test cases that consist of a theoretical nonlinear inundation case and a field application to San Francisco Bay. Through these tests, we seek to answer a critical science question, i.e. whether a structured grid with the help from a subgrid representation of underlying bathymetry can produce comparable results to an unstructured grid. Results suggest that comparable results for surface elevation can be achieved provided that the resolution of the base cell is sufficient fine; on the other hand, use of subgrid does not significantly improve the velocity results. The latter is attributed to the assumption that the surface elevation is assumed to be constant at the base cell. Future research priorities for this type of models should focus on improving the subgrid representations of the elevation and velocity. (C) 2020 Elsevier Ltd. All rights reserved.