Experimental investigation on the infragravity wave on different reef systems under irregular wave action

Abstract

This paper presents the results of a laboratory investigation into the role of infragravity motions in reef hydrodynamics over different reef systems. Laboratory experiments are performed with reef profiles of a platform reef and a fringing reef under irregular wave conditions. The propagation and the distribution of the infragravity wave are different over these two reef systems. Analysis of the measured time histories of water surface elevation shows that shorter sea-swell (SS) waves, longer infragravity (IG) wave and mean water level on the reef flat are significantly larger in the fringing reef system. The IG wave height is much larger on the fringing reef than that on the platform reef. This marked increase of the IG wave is considered to be due to: (1) the superposition of incoming IG wave and reflected IG wave from shoreline; (2) more violent wave breaking in the surf zone that may enhance the transfer of wave energy from the frequency band of the shorter wave to IG wave. In this experiment, the wave breaking mechanism controls the relationship between IG wave height and water depth.