Partitioning the Wave Spectrum into Wind Sea and Swell Components

Wave models or two-dimensional measurements of wave spectrum, determine the distribution of wave energy over frequency and different directions. However most of spectrum measurements are not directional. In this study, we are planning to determine how much energy in waves comes from storms taken place in distant areas and old waves have reached the study areas (swells), and how much energy is from the transfer of energy from the local wind to the waves near the measuring device (wind sea). For separating wave spectrum into wind sea and swell components, several methods have been proposed. In this article, first we reviewed different methods of separation, and then all methods were evaluated for a set of measured data. Based on comparisons done in this study, we concluded that the Earle and Hwang methods underestimate separation frequency. As a result, swell wave height is underestimated and mean period of swell components is over-estimated by these methods. Although the method of Portilla intrinsically chooses one one of local minimum of spectrum as separation frequency, it seems that the method is highly dependent on spectrum noise.