|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 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 large-scale 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 non-hydrostatic 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 k-ε 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 small-scale 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 small-scale 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 large-scale 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 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, the source functions for internal wave generation, and the alteration of the governing
equations for modelling wave-current interaction. See also an interesting
paper on this subject.
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.
Applications drawn from the work of the Fluid Mechanics research
group at Delft University convey an impression of the capabilities of SWASH.