Characteristics of Wave Dissipation and Setup on Coral Reefs Around Tropical Islands

Conservation and restoration of corals can be a potential nature-based solution for sustainable disaster prevention and mitigation under the threat of sea level rise due to global warming because it may adaptively elevate the coral reefs catching up with the sea level rise. While well-developed coral reefs have functions to dissipate waves, the wave dissipation induces the wave setup on the reef. Hence, evaluation of the hazard reduction effect of coral reefs should account for such trade-off relationship between wave dissipation and the wave-induced current and setup, which should be significantly dependent on the reef topography. Ultimate goal of this study is to understand and evaluate overall hazard reduction effects of coral reefs accounting for the influence of climate change. As a part of this goal, this paper first investigated the overall topographic characteristics of coral reefs of Pacific Tropical Islands using Satellite Derived Bathymetry (SDB) data based on LANDSAT imagery. Second, wave deformation and the setup induced by breaking and broken waves were computed based on phase-averaged energy balance equations and non-linear shallow water equations with forcing terms of wave radiation stress. Based on the overview of topographic characteristics, several case study sites were selected, and the numerical simulation was conducted both for one-dimensional cross-shore profiles and for the two-dimensional reef topography. Through the comparisons of these simulations, it was found that two-dimensional reef topography significantly lowers the wave setup by a factor ranging from 0.1 to 1.0 depending on the topographic characteristics.