Physical modelling investigation of reef island morphodynamics under rising sea levels

Abstract

Anticipated sea level rise throughout the 21st century is expected to pose a significant threat to the future stability and persistence of low-lying reef islands, which, as sea-levels rise, are expected to experience substantial erosion, increased flooding and inundation. However, existing studies attempting to project the future of reef islands are constrained by the currently poor understanding of how islands will physically respond to changing environmental conditions. This thesis develops a physical model of a reef island and platform to explore the geomorphic response of islands to changes in sea level, wave conditions and sediment supply. The scaled physical model replicated the morphology, sediment characteristics and reef platform wave processes at Fatato Island, Funafuti Atoll, Tuvalu. Simulations of island response to increasing sea levels indicate that islands not only move laterally on reef platforms, but overwash processes provide a mechanism to build and maintain the freeboard of islands above sea level. Implications of island building are profound, as it enables islands to offset existing scenarios of dramatic increases in island flooding. Furthermore, modelling results demonstrate that a sediment supply has a substantial effect on island adjustments, promoting the increase of the elevation of the island while dampening island migration and subaerial volume reduction. Model results were used to develop a conceptual model of future island trajectories under various environmental conditions, demonstrating that some islands may be less vulnerable to physical destabilization and inundation than existing flood risk models suggest. The results of this thesis provide valuable insights into island morphodynamics and new opportunities to improve existing flood risk models. Incorporating the morphodynamic behaviour of islands in response to both physical and ecological processes improves capability to forecast island change and resolve future island susceptibility to SLR.