|SWASH (an acronym of Simulating WAves till SHore) is a non-hydrostatic wave-flow model and is
intended to be used for predicting transformation of surface waves from offshore to the beach for studying
the surf zone and swash zone dynamics, wave propagation and agitation in ports and harbours, and rapidly
varied shallow water flows in coastal waters.
The governing equations are the nonlinear
shallow water equations including non-hydrostatic pressure and provide a general basis for
describing complex changes to rapidly varied flows typically found in coastal flooding resulting
from e.g. dike breaks and tsunamis, and wave
transformations in both surf and swash zones due to nonlinear wave-wave interactions,
interaction of waves with currents, and wave breaking as well as runup at the shoreline.
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. As a result, SWASH allows for the entire modelling process
to be carried out in any area of interest. SWASH is close in spirit to
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
user-friendly pre- and post-processing and does not need any special libraries.
In addition, SWASH is highly flexible, accessible and easily extendible
concerning several functionalities of the model.
It should be emphasized that SWASH is not a Boussinesq-type wave model.
Conceptually, the vertical structure of the flow is a part of the solution.
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 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
Boussinesq-type 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 ad-hoc measures like the surface roller model for wave breaking, the slot
technique for moving shoreline and source functions for internal wave generation. See also an interesting
paper on this subject.
Applications drawn from the work of the Fluid Mechanics research
group at Delft University convey an impression of the capabilities of SWASH.