1 EVALUATION OF DIFFRACTION BEHIND A SEMI-INFINITE BREAKWATER IN THE SWAN WAVE MODEL François Enet, Alphonse Nahon 1 , Gerbrant van Vledder, David Hurdle Alkyon Hydraulic Consultancy & Research, PO Box 248, 8300AE Emmeloord, The Netherlands Email: enet@alkyon.nl , nahon@alkyon.nl , vledder@alkyon.nl , hurdle@alkyon.nl 1 INTRODUCTION Information on the penetration of waves behind breakwaters is important for the design of harbours and to assess the safety of mooring systems. Such information is usually obtained with numerical modelling. The numerical modelling has to take care of all processes affecting the waves as they approach and enter a harbour. In most cases harbours are located along a shallow coast where refraction and diffraction affect the penetration of waves into a harbour. For large harbour areas also local wave generation may play role. The effects of refraction and local wave generation are easily accounted for with phase-averaged models. Examples of such models are the discrete spectral models WAM and SWAN (Booij et al., 1999). These models are not normally used to account for diffraction. Diffraction is usually computed with phase-resolving models, like Boussinesq (e.g., Borsboom et al., 2001) or mild- slope models (Berkhoff, 1972). Traditionally, phase-averaged models have been used to compute the wave conditions in the coastal zone and phase-resolving models have been used to determine the wave conditions in the harbour, if necessary coupling the models. A recent example of such a coupling is given in Groeneweg et al. (2004). Phase-averaged models are more efficient than phase-resolving models. Therefore, several attempts have been made to include the effects of diffraction in spectral wave models (Rivero et al., 1997) and Holthuijsen et al., (2003). Recently, version 40.51 of the SWAN model was released (Holthuijsen et al., 2004). This version includes an approximation to diffraction according to the equations given in Holthuijsen et al. (2003). Since the SWAN model is widely used in coastal engineering practice, we were interested in the applicability of diffraction in the SWAN model. The purposes of this paper are therefore: - Test the implementation of diffraction in SWAN with an emphasis on numerical stability and guidelines for proper usages; - Verify the diffraction approximation against an analytical solution; - Assess the importance of diffraction for wind waves and for swell waves; - Formulate guidelines for modelling diffraction with SWAN. As a first step, we have investigated the performance of diffraction in SWAN for a simple situation; the semi-infinite breakwater in water of constant depth and omitting the growth by wind and dissipation by depth effects. The input wave conditions consisted of a narrow banded frequency spectrum with various amount of directional spreading. 1 At Alkyon on internship from Matmeca, University of Bordeaux I, France, alphonse.nahon@etu.u- bordeaux1.fr