Role of Future Reef Growth on Morphological Response of Coral Reef Islands to Sea‐Level Rise.

Coral reefs are widely recognized for providing a natural breakwater effect that modulates erosion and flooding hazards on low‐lying sedimentary reef islands. Increased water depth across reef platforms due sea‐level rise (SLR) can compromise this breakwater effect and enhance island exposure to these hazards, but reef accretion in response to SLR may positively contribute to island resilience. Morphodynamic studies suggest that reef islands can adjust to SLR by maintaining freeboard (island crest elevation above still water level) through overwash deposition and island accretion, but the impact of different future reef accretion trajectories on the morphological response of islands remains unknown. Here we show, using a process‐based morphodynamic model, that, although reef growth significantly affects wave transformation processes and island morphology, it does not lead to decreased coastal flooding and island inundation. According to the model, reef islands evolve during SLR by attuning their elevation to the maximum wave runup and islands fronted by a growing reef platform attain lower elevations than those without reef growth, but have similar overwash regimes. The mean overwash discharge Qover across the island crest plays a key role in the ability of islands to keep up with SLR and maintain freeboard, with a Qover value of O (10 l m−1 s−1) separating island construction from destruction. Islands, therefore, can grow vertically to keep up with SLR via flooding and overwash if specific forcing and sediment supply conditions are met, offering hope for uninhabited and sparely populated islands. However, this physical island response will negatively impact infrastructure and assets on developed islands. Plain Language Summary: Coral reef islands are particularly exposed to the impacts of sea‐level rise. They are usually fronted by "living" coral reef platforms that protect the island shoreline from energetic wave action. Healthy reef platforms grow vertically and can keep up with rising sea level, maintaining a constant water depth in front of the island. It is therefore suggested that future reef growth may be a critical factor in reducing the vulnerability of coral reef islands to sea‐level rise. We use a computer model to simulate the response of coral reef islands to sea‐level rise with and without future reef growth. We find that as sea level rises, the islands evolve by retreating, while at the same time building up vertically. Island build‐up is accomplished by waves overwashing the island and depositing sediment on the top of the island. According to our model results, vulnerability of the reef islands to sea‐level rise is not dependent on whether the reef platform grows or not. In both cases, islands are regularly overwashed, but this is necessary for islands to grow vertically. Island accretion by overwash offers hope for uninhabited and sparely populated islands, but will negatively impact infrastructure and assets on urbanized islands. Key Points: Reef islands evolve during SLR by attuning their elevation to the maximum wave runup; thus, gravel islands build up higher than sand islandsAs long as mean overwash discharge across the island crest is < O (10 l m−1 s−1) coral reef islands accrete vertically during sea‐level riseFuture reef growth does not increase the ability of islands to adjust to sea‐level rise on the medium‐term (<50 years) [ABSTRACT FROM AUTHOR]