General description and instructions for use 11
make an in p u t grid s o lar ge th at it com p l et el y covers the computationa l gri d .
In the region outside the input grid SWAN assumes th at the bottom level, the water level,
bottom friction, vegetation, mud and ice are identical to those at the nearest boundary of
the input grid (lateral shift of that boundary). In the regions not covered by this lateral
shift (i. e. in the outside quad r ants of the corners o f the input grid), a constant field equal
to the value at the nearest corner point of the input grid is ta ken. For the current and
wind velocity, SWAN takes 0 m/s for points outsi d e th e inp u t gr id.
In SWAN, the bathymetry, current, water level, wind, bottom fric t io n , vegetation, mud
and sea ice may be time varying. In that case they need to be provided to SWAN in
so-called in p u t time windows (they n eed not be identical with the co m p u t a ti o n al , output
or other inp u t windows). It is best to make an input window larger than the computa-
tional time window. SWAN assumes zero values at times before the earlie st begin time
of th e input parameters (which may be the begin time of any input parameter such as
wind). SWAN assumes constant values (the last values) at times after the end time of each
input parameter . The inpu t windows should start early en ou g h so that the initial state of
SWAN has propagated through the computational area before reliab l e out p u t of SWAN is
expected.
One should choose the spatial resolution for the input grid s such that relevant spatial
details in the bathymetry, currents, bottom friction, vegetation, mud, ice and wind are
properly resolved. Special care is required in cases with sharp and shallow ridges (sand
bars, shoals) in the sea bottom and extremely steep bottom slopes. Very inaccurate ba-
thymetry can result in very inac cu r at e refra ct io n comp utations the results of which can
propagate into areas where refraction as such is not significant (the results may appear to
be unstable). For instance, waves skirting an island that is poorly resolved may propagate
beyond the i sla n d with entirely wrong directions. In such a case it may even be better to
deactivate the refraction computations (if refraction is irrelevant for the problem at hand
e.g. because the refracted waves will run into the coast anyway and one is not interested
in that pa r t of the coast). In such cases the ridges are vitally important to obtain good
SWAN results (at sea the waves are ’clipped’ by depth-induced breaking over the ridges
which must
therefore represented in SWAN comput a t io n ) . This requir e s not only that
these rid ges should be well represented on the input grid but also after interpolation on
the computational grid. This can be achieved by choosing the grid lines of the inpu t grid
along the ridges (even if this may require some slight ”shifting” of the ridges) and
choosing
the computational grid to be ident i cal to the input grid (otherwise the ridge may be ”lost”
in the int er polation from the bottom input grid to the comp u t at i o n al gr i d ) .
Finally, one should use a time step that is small enou g h th a t t im e variations in the bathy-
metry, current, water level, wind and bottom friction are well resolved.