Coastal flooding and mean sea-level rise allowances in atoll island

Angel Amores,Marta Marcos,Jochen Hinkel,Gonéri Le Cozannet

doi:10.1038/s41598-022-05329-1

Abstract

Atoll islands are among the places most vulnerable to climate change due to their low elevation above mean sea level. Even today, some of these islands suffer from severe flooding generated by wind-waves, that will be exacerbated with mean sea-level rise. Wave-induced flooding is a complex physical process that requires computationally-expensive numerical models to be reliably estimated, thus limiting its application to single island case studies. Here we present a new model-based parameterisation for wave setup and a set of numerical simulations for the wave-induced flooding in coral reef islands as a function of their morphology, the Manning friction coefficient, wave characteristics and projected mean sea level that can be used for rapid, broad scale (e.g. entire atoll island nations) flood risk assessments. We apply this new approach to the Maldives to compute the increase in wave hazard due to mean sea-level rise, as well as the change in island elevation or coastal protection required to keep wave-induced flooding constant. While future flooding in the Maldives is projected to increase drastically due to sea-level rise, we show that similar impacts in nearby islands can occur decades apart depending on the exposure to waves and the topobathymetry of each island. Such assessment can be useful to determine on which islands adaptation is most urgently needed.

Highlights

Atoll islands are among the places most vulnerable to climate change due to their low elevation above mean sea level
From a policy coastal-management perspective the crucial questions that need to be addressed are: (1) for how long a given atoll island is safe from mean sea-level rise and associated wave-induced flooding if no adaptation action is taken, and (2) by how much do flood defenses or island elevations need to be raised in order to keep islands safe until a given moment in time?
By how much do defenses or islands have to be raised? We address the second policy question by applying our parameterisation of wave setup and the computation of the wave-induced flooding to estimate by how much the island elevation or its defenses need to be raised in order to cope with mean sea-level rise and waves

Summary

Introduction

Atoll islands are among the places most vulnerable to climate change due to their low elevation above mean sea level. We present a large set of numerical simulations that provide the wave-induced flooding over an atoll island as a function of the incoming wave parameters (namely significant wave height, Hs , and peak period, Tp ), the Manning’s friction coefficient, the length of the reef flat and the island height, under different mean sea level conditions.

Results

Conclusion