Swell wave island interaction and wave piloting in the southern pacific ocean

Gerbrant van Vledder

Friday 3 july 2015

11:30 - 11:45h at Europe 2 (level 0)

Themes: (T) Extreme events, natural variability and climate change, (ST) Interaction society and water systems

Parallel session: 15I. Extreme events - Society

For many centuries the seafarers of Oceania have used wave piloting to navigate between remote island groups in the Southern Pacific. Wave piloting is the art to ‘read’ the waves to keep course and to sense the presence of nearby islands. This almost lost art relies on a thorough knowledge of swell wave climatology and experience to sense effects of island-wave interaction. According to Genz et al. (2009), much needs to be learned to understand their traditional ways of wave piloting and reconcile them with our scientific knowledge. This reconciliation is a multi-disciplinary approach in which anthropology, oceanography and wave modelling are key components. Results of a global implementation of the third-generation wave prediction model WAVEWATCH III_ have been used to reconstruct time series of 30-years of 2-D wave spectra. A spectral partitioning scheme has been applied to split these time series in wind seas and multiple swell systems. The resulting swell climatology has been compared with traditional knowledge. In general, the wave environment is dominated by swells. An intriguing aspect of wave piloting is the way the seafarers of Oceania could sense the presence of nearby islands which lie behind the horizon. Wave reflection, diffraction, refraction, focussing and shadowing effects all play a role to assess the relative position of the seafarer w.r.t. the island (Hunt, 2013). However, there are some traditional claims of special wave effects (e.g. the dilep) that still have no proper scientific explanation. Therefore, using results of the swell climatology, numerical computations have been carried our using the third-generation SWAN model and the non-hydrostatic wave model SWASH to study this interaction for academic and real island geometries. A key problem was to estimate the reflection characteristics of swell waves against islands. In this way a better understanding has been achieved of the wave-island interaction and the art of wave piloting.