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

我们发展了一种数值方法来预报岛屿周围海面上任意点的水波频谱。该方法是基于对海岛周围波场的数值预报。利用已知的波浪绕射理论，在等水深的海面上对任意截面的圆柱形结构物进行了波浪绕射，得到了波浪场。该方法的特点在于，与通过现场观测得到的预测结果相比，该方法对任意点的波谱预测简单。在本研究中，首次使用边界单元法进行了波场的计算。首先对数值计算中需要考虑的问题进行了研究。然后，利用所获得的海岛周围的波浪绕射，计算了海浪谱(方向谱和频谱)。在数值预报的同时，于2003年10月至2004年2月对日本福冈北九州白岛县储油岛周围的海浪进行了现场观测，并根据得到的海浪资料计算了海浪谱。用该方法进行了数值预报，并与现场观测结果进行了比较。结果表明，散射波的影响距离岛屿较远，因此在通过现场观测估算入射波谱时应考虑这一点，该方法对海岛周围任意点的波谱都能给出较好的综合预报。

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.