Modeling Motu: Using hydrodynamic modeling to parameterize morphodynamic interactions between reef flats and reef islands

Between the deep open ocean and backing lagoon, atolls and barrier reefs are comprised of a series of linked environments, from the coral-dominated and carbonate-producing fore-reef, the shallow reef flat typically consisting of sediment and/or concreted carbonate, to, in some cases, detrital reef islands. Using the hydrodynamic components of the XBeach model, we have developed a consistent conceptual model that can be used to explain the development of a constant-depth reef flat that subsequently extends lagoonward as sediment is produced at the reef edge. As reef flats widen, shear stresses are minimized mid-flat, which could lead to the development of incipient islands that form at a distance from the ocean. Emergent islands block across-reef flow, and subsequently are expected to prograde seawards until reaching a steady-state distance from the shore. To quantify this conceptual model, and to better understand how interconnected reef systems may respond to climate change effects, including sea-level rise, change in storminess, and different carbonate production rates, we use XBeach model results spanning a variety of forcing conditions (offshore wave height) and reef geometries (reef flat width and depth, presence of reef island) to parameterize a numerical model of coupled reef flat and reef island dynamics. Model results demonstrate that during rising sea levels, the reef flat can serve as a sediment trap, starving reef islands of detrital sediment that would otherwise fortify the shore against sea-level-rise-driven erosion. On the other hand, if reef flats are currently shallow (likely due to geologic inheritance), such that sea-level rise does not result in sediment accumulation on the flat, sea-level rise will have less of an effect of reef island shorelines. Overall, the model allows a parameterized framework that can be applied to understand the interconnected response of coupled reef environments to climate change, with potential application to atolls in different environmental settings.