The effect of swell on wave spectra of extreme sea states offshore Sarawak, Malaysia

Wave spectra specified for the design of offshore structures are often expressed in terms of the Pierson Moskowitz (PM) and JONSWAP formulations, which describe the frequency spectrum of fully-developed and fetch-limited wind-seas, respectively. At locations offshore Sarawak, Malaysia, measurements of extreme sea states have spectral shapes with characteristics of a wind-sea, but the frequencies of the spectral peaks are substantially too low to be supported by the measured local winds. As extreme sea states offshore Sarawak are predominantly produced by northeast monsoon surges in the South China Sea, we dissect the wind and wave fields of such an event, by examining the winds and the wave spectra from a hindcast of the event, together with measured wave spectra at a location offshore Sarawak, Malaysia, with the specific aim of identifying the nature of the underlying wave field that produces the spectral shape of the measured extreme sea states. It is found that while the wave spectra of extreme sea states offshore Sarawak have unimodal shapes like those of wind-seas, the wave field is dominated by swell energy located at and around the spectral peak frequency, with a comparatively small wind-sea component at higher frequency.