Methods to obtain representative surface wave spectra, illustrated for two ports of north-western Australia

Abstract

Simple automatic methods of classifying surface wave spectra in terms of spectral shape are outlined and are used to examine the monthly wave climatology for the port approaches to Dampier and Port Hedland on the North West Shelf of Australia. Waves and swell at these shallow sites occur independently in three frequency bands. These correspond to low-energy long-period swell from distant sources (periods of 12 to 20 s), cyclones or energetic local storms (periods of 8 to 12 s), and local winds (periods of 4 to 8 s). Summer cyclones generate highest waves, but sea breezes are the dominant mode of wave generation. Waves and swell tend to occur episodically and independently in any month, and the peak frequency shifts during the wave generation process, so that a representative spectrum formed by simple averaging of spectra would be grossly oversmoothed. To overcome this, the monthly climatology is presented in terms of reference spectral shapes (the Ochi and Hubble 1976 North Atlantic formulations) that are most likely to occur for particular frequency bands and ranges of significant wave height. Measured spectra may then be selected as representative spectra from groupings associated with the most likely reference shapes, with further criteria such as spectral width being used to define what is typical. In some cases, the reference spectra provide good enough fits to enable them to be used directly as representative spectra.