Limitations

The DIA approximation for the quadruplet wave-wave interactions depends on the width of the directional distribution of the wave spectrum. It seems to work reasonably in many cases but it is a poor approximation for long-crested waves (narrow directional distribution). It also depends on the frequency resolution. It seems to work reasonably in many cases but it is a poor approximation for frequency resolutions with ratios very different from 10% (see command CGRID). This is a fundamental problem that SWAN shares with other third-generation wave models such as WAM and WAVEWATCH III.


The LTA approximation for the triad wave-wave interactions depends on the width of the directional distribution of the wave spectrum. The present tuning in SWAN (the default settings, see command TRIAD) seems to work reasonably in many cases but it has been obtained from observations in a narrow wave flume (long-crested waves).


As an option SWAN computes wave-induced set-up. In 1D cases the computations are based on exact equations. In 2D cases, the computations are based on approximate equations. This approximation in SWAN can only be applied to open coast (unlimited supply of water from outside the domain, e.g. nearshore coasts and estuaries) in contrast to closed basin, e.g. lakes, where this option should not be used. The effects of wave-induced currents are always ignored.


SWAN does not calculate wave-induced currents. If relevant, such currents should be provided as input to SWAN, e.g. from a circulation model which can be driven by waves from SWAN in an iteration procedure.


In areas where variations in wave height are large within a horizontal scale of a few wave lengths, diffraction should be used. However, the computation of diffraction in arbitrary geophysical conditions is rather complicated and requires considerable computing effort. To avoid this, a phase-decoupled approach is employed in SWAN so that same qualitative behaviour of spatial redistribution and changes in wave direction is obtained. This approach, however, does not properly handle diffraction in harbours or in front of reflecting obstacles.


SWAN can be used on any scale relevant for wind generated surface gravity waves. However, SWAN is specifically designed for coastal applications that should actually not require such flexibility in scale. The reasons for providing SWAN with such flexibility are:

• to allow SWAN to be used from laboratory conditions to shelf seas and
• to nest SWAN in the WAM model or the WAVEWATCH III model which are formulated in terms of spherical coordinates.

Nevertheless, these facilities are not meant to support the use of SWAN on oceanic scales because SWAN is less efficient on oceanic scales than WAVEWATCH III and probably also less efficient than WAM.