Role of Future Reef Growth on Morphological Response of Coral Reef Islands to Sea‐Level Rise

Abstract

Coral reefs are widely recognized 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 (island crest elevation above still water level) through overwash deposition and island accretion, but the impact of different future reef accretion trajectories on the morphological response of islands remains 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 Qover across the island crest plays a key role in the ability of islands to keep up with SLR and maintain freeboard, with a Qover value of O (10 l m−1 s−1) 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.

Evolución de la utilización de los recursos y el cambio climático en Oceanía

“…The coral reefs developed a dynamic equilibrium with the environment being subject to readjustment, erosion and sedimentation in response to varying sea levels, wind patterns and storms (Neumann & Macintyre, 1985). Reefs accrete to maintain positive freeboard while retreating lagoon ward (Masselink et al, 2021). Mangrove forests thrive if the SLR remains under 5 mm/yr (NTU, 2020).…”

Cap. 30. Hawái y las Islas del Pacífico Afiliadas a los Estados Unidos. En: La Quinta Evaluación Nacional del Clima

“…Los atolones muy urbanizados podrían no ser capaces de adaptarse al SLR futuro, lo que provocaría la pérdida de tierras y la posible inhabitabilidad 98,106,107,108 .…”

“…Se proyecta que aproximadamente 3 pies de SLR afecte al menos el 58 % del entorno construido de Guam 90 y la mayoría de los atolones de los FSM y la RMI 98 . Es posible que los atolones muy urbanizados no puedan adaptarse al SLR futuro (KM 9.2, 12.2) 106,107 , provocando la pérdida de tierras y la posible inhabitabilidad 98,108 . Las SST elevadas pueden aumentar el blanqueamiento y la degradación de los arrecifes de coral, que son las principales defensas naturales de algunas islas (Figura 30.11; KM 30.4) contra las inundaciones costeras 272 .…”

Impacts of Particle Shape, Skeletal Porosity and Density on the Settling Velocity of Gravel‐Size Coral Debris

“…Coral islands in tropical and subtropical regions are formed from coral reef debris and associated marine organism materials, which provide diverse ecosystems, enhance coastal protection, and bring economic benefits to islands (Green et al., 2018; Harris et al., 2018; Hughes et al., 2003; Masselink et al., 2021; Perry et al., 2011). However, due to sea level rise and wave climate changes, coral islands with low‐lying accumulations of carbonate bioclasts, also known as skeletal fragments, are suffering significant erosion and flooding threats (Butler et al., 2013; Kench et al., 2014; Masselink et al., 2020; Pearson et al., 2017; Webb & Kench, 2010).…”