Physical modelling of reef island topographic response to rising sea levels

Abstract

Low-lying coral reef islands are considered extremely vulnerable to the impacts of climate change, particularly sea-level rise and increasing storm magnitude. Inundation and erosion are expected to destabilise reef islands and render them uninhabitable within decades. However, such assertions are founded upon the assumption that reef islands are geomorphologically inert landforms. Whereas, recent planform and cross-sectional analysis of reef island morphodynamics demonstrates that many islands are highly dynamic landforms changing in size, shape and position on the reef platform. This paper describes the first physical modelling experiments to model the whole-of-island morphodynamic response of reef islands to changes in sea level and wave regime. A wave basin is utilised to construct a 1:50 scale three-dimensional model of Fatato Island (Tuvalu) and document the whole-of-island response across a broad range of wave and water-level conditions. Whole-of-island results corroborate flume studies and provide new insights into the modes and styles of island change. Spit rotation, as well as lagoonward island recession and vertical oceanward crest accretion, are identified as the core island responses to increasing water level and wave conditions. However, results show that the rate and magnitude of physical adjustment is strongly dependent on the rate and magnitude of sea-level rise and wave conditions. Results challenge existing models of future island susceptibility to wave driven flooding, demonstrating that washover processes can provide a mechanism to build and potentially maintain island freeboard above sea level. These insights highlight an urgent need to incorporate island morphodynamics into flood risk models in order to produce accurate assessments of future wave-driven flood risks and better resolve island vulnerability.