USER MANUAL SWAN Cycle III version 40.72A

14 Chapter 2
• each wet computational grid point that is linked to only one other wet computational
grid point, is removed and
• each wet computational grid point that has no wet links is removed.
The effect of this filter is that if exception values are used for the depth grid or the
curvi-linear computational grid, one-dimensional water features are ignored in the SWAN
computations (results at these locations with a width of about one grid step may be
unrealistic). If no exception values are used, the above described filter will not be applied.
As a consequence, one-dimensional features may appear or disappear due to changing
water levels (flooding may create them, drying may reduce two-dimensional features to
one-dimensional features).
The computational time window must be defined by the user in case of nonstationary
runs. The computational window in time must start at a time that is early enough that
the initial state of SWAN has propagated through the computational area before reliable
output of SWAN is expected. Before this time the output may not be reliable since usually
the initial state is not known and only either no waves or some very young sea state is
assumed for the initial state. This is very often erroneous and this erroneous initial state
is propagated into the computational area.
The computational time step must be given by the user in case of nonstationary runs. Since,
SWAN is based on implicit numerical schemes, it is not limited by the Courant stability
criterion (which couples time and space steps). In this sense, the time step in SWAN is
not restricted. However, the accuracy of the results of SWAN are obviously affected by
the time step. Generally, the time step in SWAN should be small enough to resolve the
time variations of computed wave field itself. Usually, it is enough to consider the time
variations of the driving fields (wind, currents, water depth, wave boundary conditions).
But be careful: relatively(!) small time variations in depth (e.g. by tide) can result in
relatively(!) large variations in the wave field.
As default, the first guess conditions of a stationary run of SWAN are determined with the
2 nd generation mode of SWAN. The initial condition of a nonstationary run of SWAN is
by default a JONSWAP spectrum with a cos 2 (θ) directional distribution centred around
the local wind direction.
A quasi-stationary approach can be employed with stationary SWAN computations in a
time-varying sequence of stationary conditions.
The computational spectral grid needs to be provided by the user. In frequency space,
it is simply defined by a minimum and a maximum frequency and the frequency resolution
which is proportional to the frequency itself (i.e. logarithmic, e.g., ∆f = 0.1 f). The
frequency domain may be specified as follows (see command CGRID):
• The lowest frequency, the highest frequency and the number of frequencies can be
chosen.