West Africa Swell Spectral Shapes

Wave spectra measured at sites off West Africa are dominated by the constant presence of one or several swell wave systems. The West Africa Swell Project (WASP JIP) was carried out to propose and assess parametric models for the shapes of the swell components. Bias, variability, and dispersion of estimates are affected by the length/stationarity compromise of the record lengths and the window-tapering used to reduce their variability. In particular, shapes with sharp angles are strongly smoothed, for instance a triangular peak would appear round and reduced by 15 to 25% with rectangular or Tuckey windowing. Models that consider each wave system individually, and an arbitrary number of those, were preferred to global ones. Partitioning of directional spectra is thus a prerequisite, and needs to be tuned taking account some prior knowledge of the swell characteristics. Triangular, log-normal, Gaussian and Glenn-Jonswap shapes were considered. Sampling variability makes it difficult to distinguish between those shapes as far as swells are concerned. The models also indicate that the width of the spectrum in frequency should be inversely proportional to the peak frequency. Directional spreading width shows a similar trend. Fits to the measurements established proportionality factors for each location.