Keywords: wind waves, action balance equation, hybrid grid, finite difference, finite volume.

Computational efficiency and reliability (in terms of accuracy and robustness) are among the most important factors in the deployment of numerical models. Those requirements need to be balanced with the ones of spatial and temporal resolution dictated by the application area. For example, numerical modelling of waves in coastal waters typically requires a high level of spatial resolution to incorporate complex coastal boundaries and bathymetry resulting in high computational costs. The need for high spatial resolution has led to the introduction of finite element and finite volume schemes using unstructured grid techniques. While those have the advantage of being spatially flexible, leading to accurate coastline specification, their complexity can lead to high computational costs. A possible solution to that is the use of a hybrid grid system. This technique introduces the concept of multi-zone grid generation and local variation in the number of unstructured grids. The aim of the technique is to allow different layers of grid geometry and resolution with an increase in computational efficiency of the associated numerical modelling scheme. In this paper a hybrid grid system is introduced into a spectral wave model described by the action balance equation. The hybrid grid system consists of a coupled finite difference and finite volume method which can accommodate unstructured meshes with a variability in geographical resolution suitable for representing irregular shorelines and complex bottom topography.

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