Conservation and restoration of coral reef may be one of effective Nature based Solutions (NbS) for adaptive and sustainable coastal disaster prevention and mitigation measures against intensifying coastal hazard under the threat of global warming. Following the sea level rise, the bed level of the coral reef may also grow up, and the elevated reef induces more wave dissipation through wave breaking and bed frictions. On the other hand, the increased breaking wave dissipation may yield the higher wave setup on the reef, which may amplify coastal inundation. Appropriate and accurate projection of the future change of the balance of wave dissipation and setup, and resulting wave runup and overtopping, is therefore of importance for evaluation of the effectiveness of coral conservation and restoration as one of NbS for sustainable coastal disaster mitigation and prevention measures. In line with these motivations, this study investigates how coral reefs with different conditions affect the extent of coastal inundation.
Majuro atoll was selected as a case study site. We obtained the cross-section profiles of the bed level over the fringing reef around Majuro atoll. Characteristics of offshore wave conditions were analyzed based on the reanalysis data sets of ERA 5. Using the obtained cross-section profile, wave deformation, setup, runup, and wave overtopping induced by breaking and broken waves were computed based on phase-averaged energy balance equations and non-linear shallow water equations with forcing terms based on wave radiation stress. The model was validated through the comparison with the laboratory data presented by Demirbilek et al.(2007). The model was applied for various tide and incident wave conditions. As a coral-based NbS adapting to the future climate change, we account for coral restoration, beach formation as a result of increased sediment supply from restored coral, and elevation of the reef bed bed. The same computation of coastal hazard was performed on the modified reef profiles.
Through the comparison of computed results for various wave and tide conditions on the reef bed profile with and without coral-based NbS, it was found that elevated reef bed contributed to wave dissipation through bed friction and depth-limited wave breaking. While the elevated reef also slightly increased the wave setup due to increase in wave dissipation, it reduced the wave runup and overtopping as a result of additional wave dissipation. The presentation discusses further details in adaptive features of coral-based NbS along the coast with fringing reef.