Modelling of obstacles

SWAN can estimate wave transmission through a (line-)structure such as a breakwater (dam). It is assumed that the obstacle is narrow compared to the grid size, i.e. a subgrid approach is applied. If in reality the width is large compared with grid size, the feature preferably is to be modeled as a bathymetric feature. The following text refers to narrow obstacles.


Such an obstacle will affect the wave field in three ways:

• it will reduce the wave height of waves propagating through or over the obstacle all along its length,
• it will cause waves to be reflected, and
• it will cause diffraction around its end(s).

In irregular, short-crested wave fields, however, it seems that the effect of diffraction is small, except in a region less than one or two wavelengths away from the tip of the obstacle (Booij et al., 1993). Therefore the model can reasonably account for waves around an obstacle if the directional spectrum of incoming waves is not too narrow, unless one is interested in the wave field deep into the shadow zone.


Since obstacles usually have a transversal area that is too small to be resolved by the bottom grid in SWAN, an obstacle is modelled as a line in the computational area. See Section 3.12 for the numerical implementation of obstacles in SWAN.