Improving the Performance of SWAN Modelling to Simulate Diffraction of Waves Behind Structures

Abstract

Phase-decoupled wave models such as SWAN model cannot in general deal with the diffraction of waves behind coastal structures or islands. Improvements have been made recently to the SWAN model by including the diffraction effect that allow energy transfer between the directional spectral components. For a successful simulation of the diffraction problem the grid size should be smaller than 1/10 of the wave length. However, this limitation on the grid size seems to be unrealistic to cover a large computational domain, and a practical way to deal with the diffraction problem for the case of large grid size is indispensable. The tests of the performance of the SWAN model for the diffraction of monochromatic waves with various grid sizes show that the diffraction effect disappears behind a breakwater when the grid size increases. To improve the model performance the directional turning rate given by the SWAN model is artificially increased by increasing the diffraction parameter. To test the performance of the modified SWAN model a simple case of monochromatic wave incident on a semi-infinite breakwater in a water of constant depth is considered. As a result, the diffraction effect appears in a shadow zone even for the case of large grid size. The appropriate amplification factor for the diffraction parameter is found by trial and error for a given grid resolution, and an empirical formula for amplification factor is obtained. A practical way to improve wave diffraction effect for the case of large grid size in SWAN is proposed.