Influence of interannual tidal modulation on coastal flooding along the Western Australian coast

Diurnal and semidiurnal tides are modulated over a range of time scales, including systematic annual and interannual variations. Although identified for other parts of the world, the effects of interannual tidal modulations have had limited attention on the Western Australian coast. Research described here identified that tidal modulations are a significant and regular factor in the frequency with which high water level thresholds are exceeded. Hence, tidal modulations provide a predictable contribution to the coastal management effort required on a year-to-year basis and allow prediction of periods where there is enhanced risk of flooding to coastal infrastructure. As has been demonstrated elsewhere, these cycles are obscured within conventional harmonic and extreme analysis, and their identification requires dedicated techniques. In this study, annual standard deviations and exceedance frequency have been used to examine both hourly and high-pass-filtered water levels to establish the influence of tidal modulations. The relative contribution of the two principal cycles and their subharmonics varies along the Western Australian coast from north to south and hence is strongly linked to the tidal form. High-tide levels for Western Australian locations with diurnal tidal dominance are dominated by the lunar nodal cycle, with a clear 18.6 year signal in the Fremantle-Bunbury region. The cycle most recently peaked in 2007 with declining tidal peaks expected until 2017. High-tide levels for locations with semidiurnal tidal dominance are mainly affected by the lunar perigean subharmonic, causing a 4.4 year cycle along the Northwest Shelf. The last peak occurred in 2006 with the next peak due in 2011.