BOTTOM ROUGHNESS DEFINITION IN CORAL REEF CONTEXT (LA REUNION ISLAND, FRANCE)

Tropical coastlines and small islands are particularly vulnerable because of their physical exposure to climate change hazards and their limited adaptative capacity. Located in oceanic regions, low-latitude low-lying islands are subject to tropical cyclones, storms, flooding and drought, leading the UNESCO (2014) initiative to carry strategies for building Small Islands resilience in the context of global change. Many of these tropical islands and coastlines are bordered by coral reefs that can provide substantial defense by reducing as much as 98percent of incident wave energy. Coral reefs reduce the risk by inducing the breaking at the reef crest and by dissipating the remaining energy by bed friction linked to the architectural complexity of the reef system. The predictions regarding future climates over the global ocean point towards a rise in sea levels, an increase of storm intensities, and the alteration of coral reefs induced by the climate change in combination to anthropogenic stresses at global and local scales, contributing to more severe wave-driven flooding events in tropical islands and atolls. In order to predict submersion risks in such conditions, numerical modelling is used, such as BEWARE dataset (Pearson et al. 2020), knowing water depth over the reef flat, incident wave conditions, reef flat width, beach slope and bed friction. The focus of this study is La Saline coral reef, located in the West of La Reunion Island (France) in the Indian Ocean.