Experimental Study On Wave Set-Up Over Coral Reef

Most islands in the South China Sea are coral reefs.For the deep-sea coral reef,the reef-top is generally shallow,and the slope of the reef-face is steep.The depth over the reef-face changes dramatically from several kilometers to several meters.When the wave in the deep sea propagates the coral reef,wave shoaling happens over the steep reef-face and wave breaking can be observed on the reef-edge or the forepart of the reef-top.Although most incident wave energy is dissipated by wave breaking,the wave set-up,wave-generated flow or the low-frequency wave due to the wave transformation and wave breaking in the extreme weather including typhoon and storm surge can threaten the man-made structures on the backend of reef-top.Considering the specificity of the coral reef,the comprehensive and knowledgeable study to the deep-sea waves propagating steep reef-face to the shallow reef-top is scarce.Meanwhile,wave set-up over the reef-top is one of the hot topics in the academic circles and engineering sector.So the researches to the hydrodynamics over coral reefs have profound scientific and engineering significance.Based on some previous classic scientific works,several two-dimensional section experiments for wave transformations over the coral reef are carried out.In our experiments,the reef model includes a steeply sloping reef-face with a plane slope of 1:1 jointing a horizontal reef-top,and the lagoon locates behind the reef-top.The datum for wave elevations are acquired in the condition of the monochromatic wave and spectral wave propagating the steep coral reef at four various depths over the reef-top.The wave transformation,wave height attenuation,wave set-up and wave energy transfer are analyzed contrastively in the condition of various submerged depths and incident waves.For the monochromatic waves,wave shoaling can be observed at reef-face,wave breaks violently near the reef-edge,and the criterion for the locations of the wave breaking points are demonstrated.Wave height attenuates obviously after wave breaking,which can be steady out of the surf-zone,and is controlled by the submerged depths over the reef-top.Wave set-down is generated by wave shoaling,and wave breaking can lead to wave set-up.Wave set-up increases gradually along the surf-zone over the reef-top,and achieves the maximum value out of the surf-zone on the end of the reef-top.The maximum wave set-up on the reef-top increases with the increasing incident wave heights and decreases with the increasing submerged depths,but has a complex relationship with the wave periods.The maximum wave set-up is analyzed nondimensionally,and the reef profile shape factor K_p is suggested.For the spectral waves,wave breaking,wave heights distribution,wave set-up and wave set-down are similar to that at the condition of the monochromatic waves.Wave energy transfers from the peak frequency to the low and high frequencies when wave shoals,and wave energy at the peak frequency dissipates dramatically with wave breaking.The wave energy of low-frequency wave is dominant gradually with the wave propagating the reef-top.With the submerged depths increasing,the wave breaking point moves forward and the width of the surf-zone is shortened,and the wave energy of low-frequency wave decreases.The infragravity waves can be acquired by filtering the original wave elevations by setting some frequency ranges(0.02Hz,0.2Hz).The infragravity wave elevations and relative wave energy along the reef model are considered,and the influence of the incident waves and submerged depths on them are also analyzed,and are compared with the original wave elevations and relative total wave energy,respectively.In this paper,the monochromatic waves and spectral waves transformation characteristics over the steep coral reef at various submerged depths are comprehensively contrastively studied.The relevant results can provide experimental datum and references for engineering project on the reef-top and the numerical experiments of the wave transformations on the sloping coral reefs.