Study Of The Effects Of Tidal Curent On Wave Breaking And Wave-Induced Setup Around Reef Islands

In recent years,with the development of engineering constructions on those reef islands in South China Sea,facilities such as lighthouses,wharfs,airports,oil and gas exploitation platforms around the coral reefs have been increasingly.But such facilities are often located in the remote oceans,where the sea state is bad and the surrounding seabed topography changes dramatically.How to maintain their safety in extreme wave events such as storm surges is an urgent problem.According to,a recent global survey of the wave and tidal conditions experienced by coral reefs has found that roughly one-third of reefs worldwide could be considered as tide-dominated,which means not only the tidal level,but also the tidal current could possibly control the reef-flat hydrodynamics at some reef sites.Therefore,a study of the influence of tidal current on the wave processes over coral reefs could not only improve the theory of coral reef hydrodynamics but also provide some theoretical guidance for the engineering projects around the reef islandsBased on an idealized reef model,two wave flume experiments were carried out under a variety of combined wave-current conditions.A pipe was constructed to connect both the bottom of lagoon and the bottom near wavemaker was installed to generate both shoreward currents(flood tides)and seaward currents(ebb tides)across the reef profile.In the first experiment,effects of tidal currents on the breaking-wave characteristics were studied and the breaking waves were measured by a video camera.In the second experiment,effects of tidal currents on the maximum wave setup on the reef flat were investigated and the mean water level across the reef was measured by wave gauges.Subsequently,a one-dimensional theoretical model based on the cross-shore momentum balance was proposed to predict wave setup on the reef flat under different tidal currents,and a sensitivity analysis of the model parameters was conducted.Result analysis shows that the relative reef-flat submergence(the ratio of reef-flat submergence to deep-water wave height)is a key parameter to characterize such wave transformation and breaking characteristics as transmission coefficient,breaker type,breaking location,surf-zone width,and breaker depth index.The influence of current flow rate on the breaking-wave properties appears to be significant within our tested range.The findings in this study highlight the necessity to include the current effect in existing analytical or numerical models developed for cross-shore wave transformation,wave-induced setup and wave-driven flow over fringing reefs.Result analysis shows that the mean water level on the reef flat was consistently lower during the shoreward current(flood tide)than that during the seaward current(ebb tide).Maximum wave setup on the reef flat decreased significantly as the current flowrate changed from its seaward maximum to its shoreward maximum.A theoretical model,which was derived based on the cross-shore balances of momentum and energy,was proposed to reproduce the wave setup on the reef flat in the presence of current.The model was then validated by the laboratory data.It shows that the theoretical model can predict the cross-shore variation of wave height,and mean water level as well as the maximum wave setup on reef flat.The key model parameter was found to decrease rapidly with the increasing current flowrate,and it suggested that the effect of current on the wave setup could be modeled in a way by changing the incipient breaking wave height and location.