Laboratory experiments and numerical modeling of wave transformations on a complex reef platform

Abstract

        Low-lying and small-scale coral reef islands are considered extremely vulnerable to the impacts of strong storms. Wave erosion is expected to destabilize reef islands and reduce them uninhabitable on the coast. Reef island morphodynamics have found to be dominated by nearshore wave hydrodynamics on reef platforms. Due to interactions between incident waves and reef morphology, wave characteristics on reefs are highly dependent on reef size, shape and island position, especially for the scale between O(100m) and O(1000m). In this study, laboratory experiments and numerical modeling are used to investigate wave transformation on a reef platform with a complex morphology. A prototype 1:100 three-dimensional reef island and platform were constructed on a wave basin of 50-m in length, 50-m in width and 1.0-m in depth, and nine wave gauges installed on the reef platform to observe water surface elevation under high-energy wave events from three incident directions. A numerical model XBeach is employed to compute short-period waves, long-period infragravity waves, wave setup and currents. Parameters in the wave-breaking dissipation formulation are calibrated against experimental data. Due to a varying of reef slopes surround the reef platform, the optimal breaking parametric values would be disparity between one- and two-dimensional simulations. The ideal values are decided by the model performance in two-dimensional simulation for wave and current fields. Long-period infragravity waves interlaced through refraction and diffraction on the complex localized reef morphology are further examined if the multiple mechanisms can drive shoreline amplification of long waves. The wave characteristics would provide critical indicators for accurate assessments of island shoreline change and better resolve the coast vulnerability.