Swell propagation in a natural coastal channel in the SWAN model

Compare the 1D spectrum computed in SWAN (Svasek, 2007) with it measured by buoy OS4 in the Oosterschelde Estuary, serious underestimation of wave energy can be found on low-frequency band. Investigation has been performed to find the possible reasons for the missing energy on low frequency part. In this study, diffraction is hypothesized as the reason owing to 1) the model constructed by Svasek has no diffraction; 2) diffraction does re-distribute the wave energy from areas with rapid spatial variation in amplitude to areas with low amplitudes. Before the diffraction is added to the Oosterschelde Estuary in the SWAN model, two sub-tests have been studied beforehand. 1. A ray tracing model (REFRAC model) is applied to the Oosterschelde Estuary for two purposes: 1) to find the existence of diffraction in the Oosterschelde Estuary; 2) to validate the refraction effects in the SWAN model. A parallel case called Canyon case is applied to the REFRAC model first to provide a reference. The results show that areas with rapid variations in amplitudes exist in the Oosterschelde Estuary. It is necessary to take diffraction into consideration. The effects of refraction in the SWAN model work well. 2. Three academic cases have been performed in advance to validate the diffraction implementation in the SWAN model. They are the Semi-infinite Breakwater Case, the Gap in Infinite Breakwater Case and the Ridge Case, respectively. The results show that diffraction has apparent effects in the SWAN model. The underestimation of wave energy in the lee of the breakwaters has been improved. Diffraction in SWAN compensates the refraction over irregular bottom albeit it is not accurate enough. Model in SWAN with diffraction, under certain spatial resolution and with smaller number of smoothing steps is suggested. By performing the Oosterschelde Estuary in the SWAN model with diffraction, the results show that diffraction has apparent effects when the incoming waves at the boundaries are unidirectional. However, when the incoming waves have broader directional spreading, diffraction cannot solve the underestimation which can be resulted from the fact that diffraction effects of the wave components may cancel each other. Therefore, further investigations with respect to local wind sea and the ambient currents are suggested as future work since these two processes induce the wave energy on low-frequency band flow. In addition, the effects of the directional spreading of the incoming waves at the boundaries are suggested to be further studied since the value of the directional spreading is critical to the diffraction implementation in the SWAN model.Hydraulic EngineeringCivil Engineering and Geoscience