Role of Future Reef Growth on Morphological Response of Coral Reef
Islands to Sea-Level Rise
Abstract
Coral reefs are widely recognised 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 through overwash deposition and island accretion, but the
impact of different future reef accretion trajectories on the
morphological response of islands remain 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 across the island crest plays a key role in the ability of
islands to keep up with SLR and maintain freeboard, with a value of (10
l m s) 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.