Preparation of SWAN runs
When you are preparing a set of Swan computations for a project be sure to devote
considerable time to the preliminary phase. If you don′t you will have to pay
later because you may have to repeat a whole series of production runs.
Essential data for a Swan run are:
If the incoming waves are computed by another wave model such as Wavewatch III or WAM we are in fact dealing with nesting; there is a separate section on nesting that also explains how nesting within the Swan environment can be done.
A more difficult situation occurs when (buoy) observations have to be used to determine the incoming wave field. Incoming waves have to be applied on the whole open boundary of the computational grid while the waves are known at only one location. We distinguish the seaward boundary which is ususally more or less parallel to the coast and the two lateral boundaries which are perpendicular to the seaward boundary.
On the seaward boundary a constant incoming wave field is chosen. Therefore it is important that the seaward boundary is chosen such that the differences between wave parameters along this boundary are small. A seaward boundary is to be in sufficiently deep water (say 20 m or more) and preferably roughly parallel to a depth contour. If the buoy is not in suffiently deep water it is better to choose the seaward boundary in deeper water and estimate the wave conditions on this boundary. After the computation it can be decided by comparing the results at the buoy location with the observations whether the estimation was accurate enough.
Wave conditions on the seaward boundary are often difficult to determine, on the lateral boundaries they are completely unknown. Therefore these boundaries must be sufficiently far away from the region of interest so that their influence there is negligible. This is easily verified: apply two different incoming wave fields; a. no incoming waves on the lateral boundary (i.e. no BOUNDARY command for this side), b. the same incoming wave field as on the seaward boundary. The second option is physically unrealistic, the waves will be too high near the coast. Swan will reduce the wave height near to the boundary by means of the dissipation in the model. Click to see an example.
Note that in a nesting situation there is no need to choose a seaward boundary parallel to the coast. Exercise: explain why this is so.
The boundaries determine the size and the location of the computational grid. The resolution of the grid depends on the variations in the wave field. See the section on numerical modelling for more information. It is discussed here also whether the computation has to be stationary or non-stationary. A section on nesting explains the practicalities of nesting Swan models.
Depending on the situation other input fields have to be applied, e.g. current field,
wind field, or variable bottom friction.
A wind field is essential in the case of a lake or closed sea because then this
is the only mechanism that will generate wave energy.
In a coastal region where waves enter from sea or ocean the influence of wind is
often minor. The dominant wave frequency is low and with such waves the wind
needs a large distance to cause significant changes in the wave height and frequency.
A constant wind field often suffices.
This section discusses the choice of the various physical effects that can be included in a Swan model. Many users tend to include all physics that could possibly have any effect on the results. However, it is better to start with the most simple model, retaining only the physics that is essential to obtain a realistic result. The reason is that analysing and explaining the result is simpler. Further physical effects can be added one by one to find out whether they are relevant. In the case of diffraction this procedure is strongly recommended because diffraction can make the model unstable.
Obstacles in Swan are narrow artificial objects such as breakwaters
that partly or wholly block wave propagation.
Swan is often used to optimize breakwater design (location, cross-section).
The location has to be given by means of coordinates of points along the axis of the structure;
unfortunately errors easily occur when determining coordinates.
Swan will not produce any output, apart from error messages, unless it is instructed to do so. Two categories of output commands are needed in order to get output:
The following output is recommended, in addition to the output that is required specifically for the project:
It is possible to check the input fields (bathymetry, current field etc.) without making a wave field computation; you can do this by setting the number of iterations in SWAN to zero, i.e.:
NUM ACCUR STAT 0Also the boundary conditions will be read by SWAN, so it is possible to check these as well, e.g. by making spectral plots for output locations on the boundary.