SWASH is not a Boussinesq-type wave model. In fact, SWASH may either be run in depth-averaged mode or
multi-layered mode in which the computational domain is divided into a fixed number of vertical
terrain-following layers. SWASH improves its frequency dispersion by increasing this number of layers
rather than increasing the order of derivatives of the dependent variables like Boussinesq-type wave models.
Yet, it contains at most second order spatial derivatives, whereas the applied finite difference approximations
are at most second order accurate in both time and space.
In addition, SWASH does not have any numerical filter nor dedicated dissipation mechanism to eliminate short wave instabilities. Neither does SWASH include other ad-hoc measures like the surface roller model for wave breaking, the slot technique for moving shoreline, the source functions for internal wave generation, and the alteration of the governing equations for modelling wave-current interaction. As such, SWASH is very likely to be competitive with the extended Boussinesq-type wave models in terms of robustness and the computational resource required to provide reliable model outcomes in challenging wave and flow conditions. Therefore, it can be seen as an attractive alternative to the Boussinesq-type wave models.