Reef island response to sea-level rise explored through a simple morphodynamic model

Abstract:

Coral reef islands are coherent accumulations of carbonate sediment deposited on top of coral reef platforms, typically reaching little more than a few meters above modern sea level. In many cases, these islands represent the entirety of inhabitable land for island nations. Most of these islands formed during falling or stable sea level, which means that their past behavior may not be indicative of their response to current and future rapid sea-level rise. Recent results from physical models and coupled hydro-dynamic sediment transport models reinforce the concept that island inundation should lead to increased island overtopping, leading to sediment overwash and landward transgression. Here we present results from a simple, spatially explicit model of reef island morphodynamic evolution based upon the interplay between sea-level rise, offshore sediment supply, and sediment overwash from both the seaward and lagoonward shore. Particularly important is the morphodynamic model’s ability to simulate a change in reef island shape as a response to sea-level rise. For different constant rates of sea-level rise, modeled reef islands exhibit a range of behaviors, including seaward expansion if sediment input is high, and in all cases a tendency towards drowning if overwash fluxes are too small (even if supply of sediment from offshore is high). A particularly unstable condition arises if the lagoon shoreline of a transgressing island reaches the far edge of the reef flat, in which case the island drowns substantially sooner. Because projected sea-level rise is not expected to be constant, we also simulate reef island response to different projected sea-level-rise scenarios over the coming century and beyond. Overall, the model demonstrates that understanding the future of reef islands requires increased quantification of sediment fluxes from both offshore and onto the top of these dynamic landforms.