Climate Adaptation for a Natural Atoll Island in the Maldives - Predicting the Long-Term Morphological Response of Coral Islands to Sea Level Rise and the Effect of Hazard Mitigation Strategies

Coral atoll islands, common in (sub)tropical oceans, consist of low-lying accumulations of carbonate sediment produced by fringing coral reef systems and are of great socio-economic and ecological importance. Previous studies have predicted that many atoll islands will become uninhabitable before the end of this century due to sea level rise exacerbating wave-driven flooding. However, the assumption that such islands are morphologically static has been challenged by observations and modeling that show the potential for overwashing and sediment deposition to maintain island freeboard. Reliable long-term predictions of island change and future flood risk, essential for adaptation planning, are, however, lacking. Here, we adopt a novel, efficient approach for modeling the long-term island response and illustrate the morphological response of an atoll island to future sea level rise and the effect of various adaptation measures. We demonstrate that wave-driven sediment deposition increases island (beach) crest freeboard. We find that the assumption of static island morphology leads to a significant increase in the predicted frequency of future island flooding compared to morphodynamically active islands. Reef adaptation measures were shown to modify the inshore wave energy, influencing the equilibrium island crest height and therefore the long-term morphological response of the island, while beach restoration mainly delays the island's response. Accounting for long-term natural island dynamics, including the morphodynamic feedback from adaptation measures, offers more realistic projections of future flood risk compared to current static island model predictions. These local projections of island response can serve as decision support tools for climate adaptation.