Three-dimensional morphodynamic modeling of reef islands in response to climate change

Coral reef islands have been viewed significantly vulnerable to the effects of climate change, especially due to their low elevations when facing sea-level rise (SLR) and intensifying storms. Wave-driven flooding and erosion are anticipated to undermining these islands and forcing them to be uninhabitable in a few decades. Although, several existing investigations have tried to tackle the horizontally one-dimensional (1DH) morphological change of reef islands subjected to waves, experimental and numerical investigations on the three-dimensional (3D) island evolution are rare in the literature. Therefore, this study employs a non-hydrostatic phase-resolving model originated from the XBeach tool (XBeach-NH), combined with its sediment transport module. The model was applied to reproduce the published 3D wave basin experiments which used the Fatato Island located at Funafuti Atoll, Tuvalu as the prototype. After model validation, the morphological responses of a more generalized 3D reef island to various climate change indicators, including SLR, increased storm activity and coral growth/degradation were investigated. Simulation results showed that vertical crest accretion, spit rotation, lagoonward island recession, island subaerial area and volume reduction were identified as the island's core responses. Reef islands were also found to have the capacity to adjust vertically to SLR, increased storminess and reef degradation below a threshold. However, their evolution processes can be accelerated as the values of these indicators further increased, eventually resulting in the destruction of the island. The above findings emphasize the pressing necessity to have the island's morphodynamic response integrated into the coastal flood models, thus the island's future persistence and stability could be better resolved.