SWASH (an acronym of Simulating WAves till SHore) is a nonhydrostatic waveflow model and is intended to be used for predicting transformation of dispersive surface waves from offshore to the beach for studying the surf zone and swash zone dynamics, wave propagation and agitation in ports and harbours, rapidly varied shallow water flows typically found in coastal flooding resulting from e.g. dike breaks, tsunamis and flood waves, density driven flows in coastal waters, and largescale ocean circulation, tides and storm surges.
The basic philosophy of the SWASH code is to provide an efficient and robust model that allows a wide range of time and space scales of surface waves and shallow water flows in complex environments to be applied. The governing equations are the nonlinear shallow water equations including nonhydrostatic pressure, and optionally the equations for conservative transport of salinity, temperature and suspended sediment. In addition, the vertical turbulent dispersion of momentum and diffusion of salt, heat and sediment load are modelled by means of the standard kε turbulence model. The transport equations are coupled with the momentum equations through the baroclinic forcing term, while the equation of state is employed that relates density to salinity, temperature and sediment.
The need to accurately predict smallscale coastal flows and transport of contaminants encountered in environmental issues is becoming more and more recognized. In principle, SWASH has no limitations and can capture flow phenomena with spatial scales from centimeters to kilometers and temporal scales from seconds to hours. Yet, this model can be employed to resolve the dynamics of wave transformation, buoyancy flow and turbulent exchange of momentum, salinity, heat and suspended sediment in shallow seas, coastal waters, estuaries, reefs, rivers and lakes. Examples are smallscale coastal applications, like waves approaching a beach, wave penetration in a harbour, flood waves in a river, oscillatory flow through canopies, salt intrusion in an estuary, internal waves, and largescale ocean, shelf and coastal systems driven by Coriolis and meteorological forces to simulate tidal waves and storm surge floods.
It should be emphasized that SWASH is not a Boussinesqtype wave model. Conceptually, the vertical structure of the flow is a part of the solution. In fact, SWASH may either be run in depthaveraged mode or multilayered mode in which the computational domain is divided into a fixed number of vertical terrainfollowing layers. SWASH improves its frequency dispersion by increasing this number of layers rather than increasing the order of derivatives of the dependent variables like Boussinesqtype wave models do. Yet, SWASH contains at most second order spatial derivatives, whereas the applied finite difference approximations are at most second order accurate in both time and space. This is probably the main reason why SWASH is much more robust and faster than any other Boussinesqtype wave model. This approach receives good linear frequency dispersion up to kh ≤ 7 with two equidistant layers at 1% error in phase velocity (k and h are the wave number and water depth, respectively). In addition, SWASH does not have any numerical filter nor dedicated dissipation mechanism to eliminate short wave instabilities. Neither does SWASH include other adhoc measures like the surface roller model for wave breaking, the slot technique for moving shoreline, and the alteration of the governing equations for modelling wavecurrent interaction. See also an interesting paper on this subject.
SWASH is close in spirit to SWAN with respect to the pragmatism employed in the development of the code in the sense that comprises are sometimes necessary for reasons of efficiency and robustness. Furthermore, like SWAN, the software package of SWASH includes userfriendly pre and postprocessing and does not need any special libraries. In addition, SWASH is highly flexible, accessible and easily extendible concerning several functionalities of the model.
Applications drawn from the work of the Fluid Mechanics research group at Delft University convey an impression of the capabilities of SWASH. Also, many scientific papers, reports and other documents on SWASH have been published.
