Tidal modulation of incident wave heights: Fact or Fiction?

This contribution investigates the hypothesis that incident wave power is modulated by the tide. Eulerian measurements of wave height recorded by three wave buoys in intermediate water depths (8-45 m A.C.D.), over a seven year period were analysed in a search for evidence of this semi-diurnal variability in incident wave heights. The study site (Perranporth, UK) was a highly macrotidal environment with a maximum spring tidal range of approximately 7.5 m. Autospectra of wave height time-series displayed a significant peak at semi-diurnal frequencies that was coherently coupled to the tidal displacement. At this site maximum wave power was seen to occur on the rising tide, on average 1 hour 6 minutes before high water. The observed semi-diurnal variability in wave height increases in magnitude towards the shoreline. This contribution presents field evidence for tidal modulation of incident wave power by the tide and suggests a possible explanation for the observations in terms of an analytical model for attenuation of wave power by contra tidal flows. ADDITIONAL INDEX WORDS: Wave-current interaction, viscous effects, tidal push, wave damping, wave dissipation INTRODUCTION It has long been suggested by surfers (possibly the most avid group of wave-watchers) that in macrotidal areas breaker heights commonly increase during the certain phases of the tide. Frequently, (but not exclusively) it is observed that incident wave height increases during the rising tide, a phenomenon is often referred to by surfers as the ‘tidal push’, The phrase ‘tidal push’ implies that the incoming tide somehow eases the passage of shoreward propagating waves, even providing them with extra energy, whilst the outgoing tide opposes the passage of waves, somehow dissipating wave energy. This pattern of wave height modification is at variance with conventional wave current interaction studies which show convincing evidence that wave heights will grow in adverse flows. Although observations of increasing wave height during the rising tide currently lack any solid scientific support they are extremely common and globally wide-spread amongst the surfing community. In the last two years the ‘tidal push’ phenomenon which is generally reported to follow a slack tide lull at low-tide has also received interest from the scientific community. This is evidenced by numerous communications documented on the coastal scientific communities’ e-mail circular, the ‘coastal list’. However, to the authors’ knowledge there has currently been no data presented or formal publication to substantiate or reject these subjective observations. This contribution investigates the hypothesis that nearshore wave power can be modulated by the tide.