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Features of SWASH

General
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 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 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.

Physics
SWASH accounts for the following physical phenomena:
  • wave propagation, frequency dispersion, shoaling, refraction and diffraction
  • nonlinear wave-wave interactions (including surf beat and triads)
  • wave breaking
  • wave runup and rundown
  • moving shoreline
  • bottom friction
  • partial reflection and transmission
  • wave interaction with structures
  • wave-current interaction
  • wave-induced currents
  • vertical turbulent mixing
  • subgrid turbulence
  • wave damping induced by aquatic vegetation
  • rapidly varied flows
  • tidal waves
  • bores and flood waves
  • wind driven flows
  • space varying wind and atmospheric pressure
  • density driven flows
  • transport of suspended load for noncohesive sediment
  • turbidity flows
  • transport of tracer
Computations
SWASH computations can be made on a regular and an orthogonal curvilinear grid in a Cartesian or spherical coordinate system.

SWASH runs can be done serial, i.e. one SWASH program on one processor, as well as parallel, i.e. one SWASH program on more than one processor using an MPI protocol.

Boundary conditions
SWASH provides the following specification of boundary conditions:
  • wavemakers:
    • regular waves by means of Fourier series or time series
    • irregular unidirectional waves by means of 1D spectrum. The spectrum may be obtained from observations or by specifying a parametric shape (Pierson-Moskowitz, Jonswap or TMA).
    • irregular multidirectional waves by means of 2D spectrum. The spectrum may be obtained from a SWAN run or by specifying a parametric shape (Pierson-Moskowitz, Jonswap or TMA) while the directional spreading can be expressed with the well-known cosine power or in terms of the directional standard deviation.
  • absorbing-generating boundary conditions
  • velocity or discharge
  • Riemann invariants
  • full reflection at closed boundaries or solid walls
  • Sommerfeld or radiation condition
  • sponge layers
  • periodic boundaries
Output quantities
SWASH provides the following output quantities (ASCII or binary Matlab files containing tables and maps):
  • surface elevation
  • depth-averaged velocity magnitude and direction
  • layer-averaged velocity magnitude and direction
  • vertical distribution of horizontal velocity
  • velocity in z-direction or relative to sigma plane
  • time-averaged velocities (e.g. undertow)
  • discharges
  • friction velocity
  • turbulence quantities (k, ε and νt)
  • time-averaged turbulence quantities
  • transport constituents (salt, heat and suspended sediment)
  • time-averaged transport constituents
  • pressure at bottom
  • layer-averaged pressure
  • vertical distribution of pressure
  • non-hydrostatic pressure
  • significant wave height
  • wave-induced setup
  • maximum horizontal runup or inundation depth
Limitations
At present, SWASH does not account for:
  • wind effects on wave transformation
  • hotstarting (temporary limit)