Wave Attenuation by Restored Coral Reef Canopies: Implications for Coastal Protection

Abstract

The significant bottom roughness of coral reefs plays a vital role in mitigating wave-driven hazards along many tropical coastlines. As reef-building corals decline globally, reef restoration has been increasingly used to re-establish reef structure that influences wave dissipation. This study investigated the hydrodynamic processes that govern wave attenuation by restored reef canopies created using a common approach involving modular structures populated with coral colonies, based on the Mars Assisted Reef Restoration System. Three different coral covers were tested in a wave flume under various wave conditions and water depths typical of coral reef flats. We found that wave attenuation increased significantly with coral cover, driven by how specific attributes of the coral geometry controlled in-canopy flows and hydrodynamic forces. Two independent measures of wave energy dissipation generated by the reef were compared: (a) the rate of work done by hydrodynamic forces (calculated from force-velocity timeseries measurements), and (b) the observed wave height attenuation across the reef. These measures agreed well over the different coral covers and wave conditions studied, demonstrating that accurate quantification of in-canopy drag forces and flows enables reliable prediction of wave dissipation. The validated predictive framework was then used to evaluate the effectiveness of coral reef restoration under varying wave conditions and coral covers, showing that well-designed reef canopies can dissipate more than 50% of the incoming wave energy under typical reef flat conditions. These findings offer new insights into the hydrodynamic processes that govern wave attenuation by restored coral reef canopies, highlighting the potential of restoration to mitigate coastal hazard risk.