Study On Wave Propagation And Evolution On Coral Reef

The topographic features of coral reef have clear difference with gentle slope coast.The violent wave breaking around reef edge and rapid change in water depth from reef front to reef flat,lead to different hydrodynamic processes of wave set-up,wave-induced flow and infragravity wave than traditional coast.Serious damages have frequently observed at reeffringed coast during strom and extreme sea state.Furthermore,with the future rise of the sea level,increase of the frequency of extreme wave events,and the continuous degradation of coral reef,the ability of the reef to defend the shoreline from coastal flooding decreases.The risks faced by reef-fringed shoreline and reef-top structure are getting higher in the future.Therefore,the investigations on the hydrodynamic characteristics of the coral reef and corresponding hazard mitigation measure are of realistic importance.Laboratory experiments are conducted to study wave propagation and evolution on coral reef under different incident wave conditions and water depths.The reef models in the experiment are designed and simplified according to common reef terrain.Reef model consists of reef face with slope of 1:1 and 8.6 m horizontal reef flat.10 cm thickness porous media overlay is placed on the reef model to conduct the permeability feature of coral community and reef rock.The influences of porous layer on the wave height,wave set-up and wave-induced flow are discussed.The relationships among infragravity wave height,reef surface and wave breaking are revealed.The results show that the porous media leads to the decrease of the gravity,infragravity wave and wave-induced flow.The wave set-up on the reef flat decrease with the increase of the median diameter,porosity and thichness of porous layer.The influences of reef-top structure on the reef hydrodynamics are investigated by laboratory experiments.The variations of wave height and wave set-up after building vertical structure are carefully discussed.The test results show that the wave set-up and wave height increases significantly after building vertical structure,which lead to the risk increase of overtopping.The rise of the water level on the reef flat leads to strong back flow and violent wave breaking around the reef crest.Based on the experiment results and the conservation of wave energy,the formula for the wave set-up on the smooth reef flat is improved.By fitting into an experiment formula on the wave-induced flow,the wave set-up on the platform reef is analyzed.The depth-integrated,time-averaged momentum equation and the radiation stress are deduced to predict the wave setup on porous fringing reef.Based on this theoretical model,the influences of porous media on the wave setup are discussed.Based on the analysis of the reef hydrodynamics,a new permeable wave-dissipation structure with V-type baffle is proposed.The wave decay effects of this structure are tested through laboratory experiments.The experiment results show that this permeable structure decreases the wave height of gravity and infragavity wave effectively without lifting the water level.This permeable structure can provide effective protection for the shoreline and reef-top structure.