Can reef islands keep up with sea level? Exploring the interplay between sea-level rise, sediment supply, and overwash processes

Coral reef islands are accumulations of carbonate sediment deposited subaerially atop coral reef platforms. We hypothesize that the long-term evolution of reef islands is primarily controlled by the interplay between sea-level rise, sediment supply, and sediment overwash. Reef islands are supplied with sediment from offshore, in the form of reworked coral skeletons that originate at the reef edge and are carried onto the reef platform by waves, as well as in situ production on the reef flat itself. However, the primary mechanism that allows reef islands to keep pace with sea level is storm overwash, which enables the vertical transport of sediment from the periphery to the top of the island. Given the current lack of understanding on how production and overwash processes interact, we have constructed a morphodynamic model to elucidate and quantify how reef islands may respond to sea-level rise and changes in sediment production. Model results demonstrate that even if reef islands can remain subaerial over the coming century, this will require significant deposition of sediment atop the island and, in many cases, the island is expected to roll considerably over itself; both of these morphologic changes will negatively affect homes and infrastructure atop these islands. The model also suggests that as reef islands approach the lagoon edge of the reef platform, shoreline erosion and island drowning can be enhanced as sediment overwashes into the lagoon. Interestingly, this situation can only be avoided if either a high offshore sediment supply bulwarks the island in place or the system undergoes similar rates of overwash sedimentation from both the ocean and the lagoon sides. The model also allows us to explore the potential for increased overwash with increased storminess, increases in sediment supply due to bleaching or disturbance, or reduction of sediment supply as a result of reduced calcification rates due to ocean acidification.