Study Of The Effects Of Reef Roughness On Wave Transformation And Wave Runup Around Reef Islands

In recent years,with the development of coral reefs at domestic and international,facilities such as lighthouses,wharfs,airports,oil and gas exploitation platforms around coral reefs were increasing.Such engineering facilities are often located in open sea areas,where the wave conditions are harsh and the seabed topography changes dramatically.How to construct and maintain them in extreme climates such as storm surges is an urgent problem.Among the numerous marine dynamic factors around coral reefs,wave action is considered to be the most important.Besides of wave breaking,due to the existence of coral communities,the roughness of coral reef surface is 1-2 orders of magnitude higher than beach.Thus the roughness of reef surface is also a major factor affecting the wave transformation on coral reef coast.Therefore,the study of tidal current influence on the hydrodynamic process of coral reefs near shore will help to further improve and develop the theory of coral reef hydrodynamics.Moveover,it can also provide some theoretical reference for the construction and maintenance of reef projects and be used to verify breaking wave numerical models.Based on a generalized coral reef model,this paper uses a cylindrical array to simulate the coral reef surface with antler corals through wave flume experiments.The characteristics of wave breaking are studied.The experiment mainly uses video capure system combined with wave height acquisition system to test the combination of a series of regular wave conditions under the smooth and rough reef.Firstly,the influence of reef surface roughness on wave breaking characteristics is studied.Furthermore,the influence of roughness reef on wave transformation and wave runup is analyzed.Finally,based on CFD(Computational Fluid Dynamics)method,a reef numerical model is established,and used porous model theory to simulate roughness reef bottom.The feasibility of the numerical model is verified by experimental data.Furthermore,cross-shore variations of wave driven current and vorticity is also discussed using the model.The analysis results show that the existence of reef roughness can significantly change the wave breaking process.Compared with smooth reef,the wave breaking points under rough reef model are shoreward,the relative wave surfzone width is shorter,the wave breaking height is smaller,and the water breaking depth is smaller.Then,based on the experimental results,the empirical relations of the breaking point location,the relative surfzone width,the breaking index(wave breaking height index,water breaking depth index)under the two reef conditions are proposed.Compared with the smooth reef surface,the wave amplitudes at each wave gauge location point under the rough reef model are all reduced,the wave skewness outside the surfzone is significantly reduced,and the wave asymmetry and Ursell number are increased.The 2nd harmonic waves are important components of the transmitted waves on the reef flat.The rough reef surface damps the 1st and the 2nd harmonics,and its impact on the higher harmonics is insignificant.The relative reef-flat submergence is a key parameter to describe the wave transmission.Wave transmission coefficient drops significantly but energy dissipation coefficient increases by an average of 8%for all tested scenarios when the reef surface becomes rough.However,wave reflection from the reef seems to be independent of the surface roughness.Wave run-up on the back-reef beach increases with the increasing transmitted wave height.Finally,an empirical formula to predict regular wave run-up on the back-reef beach is proposed to account for the large surface roughness.Based on the open-source CFD package OpenFOAM,a coral reef numerical model is established.The large roughness reef bottom is simulated by porous model.The wave transformation and wave runup of regular waves under smooth and rough reef surfaces is studied.The numerical calculation results are in good agreement with the results of experiments.It is shown that the numerical model can better predict the surface elevation and the wave runup under the rough reef surface.Finally,the wave driven current and vorticity along the reef is also shown.