Numerical estimation of wave spectra around an island by using wave diffraction theory

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We have developed a numerical method to predict water wave spectra at arbitrary points on the sea around an island. The method is based on numerical prediction of the wave field around the island. The wave field is obtained by using known wave diffraction theory on a cylindrical structure of arbitrary cross section in the sea of constant depth. The feature of the method lies in its simplicity in the prediction of wave spectra at any points in comparison with the prediction obtained through field observation.In this research, the calculation of wave field was first conducted using a boundary element method (BEM). The problems to be taken into consideration in numerical calculations were first investigated. Then, using obtained wave diffractions around the island, wave spectra (directional spectra and frequency spectra) were calculated. Along with the numerical prediction, field observation of waves around an specific island (Shirashima oil storage island, Kitakyushu, Fukuoka, Japan) was carried out from Oct. 2003 to Feb. 2004, and wave spectra were calculated from the obtained wave data. The numerically predicted spectra using the present method were compared with those obtained from field observation. It was confirmed that the effect of scattered waves reaches far from the islands and thus this should be considered in the estimation of incident wave spectra through field observation, and that the present method can give good overall prediction for the wave spectra at arbitrary points around islands.