Abstract: Increased flooding due to sea level rise (SLR) is expected to render reef islands, defined as sandy or gravel islands on top of coral reef platforms, uninhabitable within decades. Such projections generally assume that reef islands are geologically inert landforms unable to adjust morphologically. We present numerical modeling results that show reef islands composed of gravel material are morphodynamically resilient landforms that evolve under SLR by accreting to maintain positive freeboard while retreating lagoonward. Such island adjustment is driven by wave overtopping processes transferring sediment from the beachface to the island surface. Our results indicate that such natural adaptation of reef islands may provide an alternative future trajectory that can potentially support near-term habitability on some islands, albeit with additional management challenges. Full characterization of SLR vulnerability at a given reef island should combine morphodynamic models with assessments of climate-related impacts on freshwater supplies, carbonate sediment supply, and future wave regimes.
DISCUSSION
Our numerical model simulations, validated against small-scale physical model tests, indicate that reef islands will undergo physical transformations in response to SLR and can maintain island surfaces above sea level. Notably, we present the first process-based numerical model simulations of future island change that highlight lateral displacement of shorelines and vertical building of the island crest and island surface. These adjustments are mediated through island rollover, driven by overtopping and overwash processes, and a tentative threshold separating these two regimes for our modeled gravel barrier subjected to wave conditions of Hs = 2–4 m is a mean overwash discharge across the island crest of 0.01 m3 m−1 s−1 associated with maximum overwash depths of 0.2 to 0.4 m. Our results further indicate that the magnitude and pace of change will be dependent on both the rate of SLR and changing wave regimes. These modeled trajectories of island dynamics are consistent with modes of island change observed in recent studies throughout the Indo-Pacific (17, 19, 20–22).
The morphological modeling approach adopted here considers coral reef island response to climate change only as a result of rising sea level. However, increased ocean water temperature is expected to increase the intensity of tropical storms, resulting in enhanced coastal flooding (31), thereby accelerating the rollover process identified in this study, and also has substantial adverse effects on the health of coral reef systems that may modify carbonate sediment production regimes that contribute to island building and maintenance (32). In addition, island habitability is not only a function of island freeboard it also depends on the island planform area, which, without sediment input from the reef structure, may reduce as a result of rollover. Storlazzi et al.(4) have demonstrated that enhanced coastal flooding due to SLR is expected to lead to increased contamination of the freshwater aquifer, where they occur, a process not accounted for in the present numerical modeling approach. It is also important to emphasize that the reef island modeled here is made of gravel, and because of the reduced mobility and increased hydraulic conductivity of gravel compared with that of sand, it could be argued that gravel islands may be particularly responsive and able to keep up with rising sea level. Notwithstanding a modeling approach that only considers the morphodynamic impacts of SLR and the complex set of factors that influence island habitability (33–35), our results confirm recent assertions that the physical foundations for island communities may persist (21). Compared with a static reef island model, the vertical buildup of island elevation by overwash processes modeled here can also offset the increase in future flood risk due to SLR. However, our results also indicate that communities are likely to be confronted with ongoing and escalating rates of island physical change that will stress populations and require careful consideration of the full spectrum of adaptation strategies.
Our analysis provides an empirical basis to help inform appropriate adaptation pathways in island nations, with continued habitation of islands underpinning the majority of these approaches (Fig. 4). The simulated morphodynamic trajectories suggest a cascade of responses is likely, beginning with island keep-up and marginal island narrowing under slower rates of SLR and dominant overtopping regimes. Faster-paced lateral migration of islands and increased reduction in freeboard are projected under faster rates (and greater magnitudes) of SLR and higher wave regimes, producing dominant overwash regimes (Fig. 4). In the most extreme cases, dominant overwashing forces loss of freeboard and rapid rollover of island sediment reservoirs. This cascade of morphological changes supports recent studies (21, 36) that indicate that physical responses are likely to vary between islands, reflecting differences in antecedent condition (e.g., sedimentary fabric and abundance, island size, and presence/absence of conglomerate platform) and environmental boundary conditions (storm wave climate and rate of SLR). Such differences in morphodynamic behavior present the opportunity to develop nuanced adaptation solutions in different island settings, rather than adopt a one-solution approach that ultimately results in island abandonment and relocation (10). Islands with artificial shoreline defenses compromise the ability of shorelines to undergo natural adjustment to changes in the process regime and lock communities into hard structural solutions and a maladaptive dependency. Under extreme scenarios of change, islands may become uninhabitable, and community relocation and structural solutions may become the only alternatives. However, between the binary outcomes—hold the line and community migration—exist a suite of alternate solutions that reflect the dynamic nature of island change and allow planning and soft engineering strategies. Furthermore, given the progressive nature of island transformations, the suite of options provide opportunities for adaptive planning pathways to be developed at the island scale (37), which allows resources to be deployed in a more efficient manner and avoid maladaptive interventions (38).
[Fig. 4 Conceptual diagram of reef island morphological adjustment to future SLR under different environmental and management scenarios.]
Island response is driven by extrinsic factors (rate of SLR, storm characteristics, and overtopping/overwashing balance) and controlled by intrinsic factors (presence/absence of conglomerate platform beneath the island, reef growth, size of the island, and sediment supply). The most appropriate adaptation strategy (managed realignment, nourishment, coastal defense, and relocation) to deal with island change is strongly determined by the type of island response to SLR. For example, an island that is narrowing, but maintaining freeboard, could benefit more from nourishment than coastal defense. If an island is already completely defended, preventing overtopping and overwashing, the only way to maintain habitation is upgrading the coastal defenses (or relocation). The width of the black bars represents the magnitude/importance/relevance of the factor in question.
The pursuit of alternate adaptation pathways does not negate the need to pursue ongoing mitigation action to curtail future SLR and climatic changes on small island nations. However, morphodynamic modeling provides a basis to resolve island-specific trajectories of change to underpin the development of adaptation strategies that may extend the duration of habitation of these islands to at least more than several decades. Future morphodynamic modeling of reef island response to SLR must not only explore further SLR and wave conditions but also need to incorporate the different environmental factors, such as island morphology, reef platform adjustment, and sediment supply.
Our numerical model simulations, validated against small-scale physical model tests, indicate that reef islands will undergo physical transformations in response to SLR and can maintain island surfaces above sea level. Notably, we present the first process-based numerical model simulations of future island change that highlight lateral displacement of shorelines and vertical building of the island crest and island surface. These adjustments are mediated through island rollover, driven by overtopping and overwash processes, and a tentative threshold separating these two regimes for our modeled gravel barrier subjected to wave conditions of Hs = 2–4 m is a mean overwash discharge across the island crest of 0.01 m3 m−1 s−1 associated with maximum overwash depths of 0.2 to 0.4 m. Our results further indicate that the magnitude and pace of change will be dependent on both the rate of SLR and changing wave regimes. These modeled trajectories of island dynamics are consistent with modes of island change observed in recent studies throughout the Indo-Pacific (17, 19, 20–22).
The morphological modeling approach adopted here considers coral reef island response to climate change only as a result of rising sea level. However, increased ocean water temperature is expected to increase the intensity of tropical storms, resulting in enhanced coastal flooding (31), thereby accelerating the rollover process identified in this study, and also has substantial adverse effects on the health of coral reef systems that may modify carbonate sediment production regimes that contribute to island building and maintenance (32). In addition, island habitability is not only a function of island freeboard it also depends on the island planform area, which, without sediment input from the reef structure, may reduce as a result of rollover. Storlazzi et al.(4) have demonstrated that enhanced coastal flooding due to SLR is expected to lead to increased contamination of the freshwater aquifer, where they occur, a process not accounted for in the present numerical modeling approach. It is also important to emphasize that the reef island modeled here is made of gravel, and because of the reduced mobility and increased hydraulic conductivity of gravel compared with that of sand, it could be argued that gravel islands may be particularly responsive and able to keep up with rising sea level. Notwithstanding a modeling approach that only considers the morphodynamic impacts of SLR and the complex set of factors that influence island habitability (33–35), our results confirm recent assertions that the physical foundations for island communities may persist (21). Compared with a static reef island model, the vertical buildup of island elevation by overwash processes modeled here can also offset the increase in future flood risk due to SLR. However, our results also indicate that communities are likely to be confronted with ongoing and escalating rates of island physical change that will stress populations and require careful consideration of the full spectrum of adaptation strategies.
Our analysis provides an empirical basis to help inform appropriate adaptation pathways in island nations, with continued habitation of islands underpinning the majority of these approaches (Fig. 4). The simulated morphodynamic trajectories suggest a cascade of responses is likely, beginning with island keep-up and marginal island narrowing under slower rates of SLR and dominant overtopping regimes. Faster-paced lateral migration of islands and increased reduction in freeboard are projected under faster rates (and greater magnitudes) of SLR and higher wave regimes, producing dominant overwash regimes (Fig. 4). In the most extreme cases, dominant overwashing forces loss of freeboard and rapid rollover of island sediment reservoirs. This cascade of morphological changes supports recent studies (21, 36) that indicate that physical responses are likely to vary between islands, reflecting differences in antecedent condition (e.g., sedimentary fabric and abundance, island size, and presence/absence of conglomerate platform) and environmental boundary conditions (storm wave climate and rate of SLR). Such differences in morphodynamic behavior present the opportunity to develop nuanced adaptation solutions in different island settings, rather than adopt a one-solution approach that ultimately results in island abandonment and relocation (10). Islands with artificial shoreline defenses compromise the ability of shorelines to undergo natural adjustment to changes in the process regime and lock communities into hard structural solutions and a maladaptive dependency. Under extreme scenarios of change, islands may become uninhabitable, and community relocation and structural solutions may become the only alternatives. However, between the binary outcomes—hold the line and community migration—exist a suite of alternate solutions that reflect the dynamic nature of island change and allow planning and soft engineering strategies. Furthermore, given the progressive nature of island transformations, the suite of options provide opportunities for adaptive planning pathways to be developed at the island scale (37), which allows resources to be deployed in a more efficient manner and avoid maladaptive interventions (38).
[Fig. 4 Conceptual diagram of reef island morphological adjustment to future SLR under different environmental and management scenarios.]
Island response is driven by extrinsic factors (rate of SLR, storm characteristics, and overtopping/overwashing balance) and controlled by intrinsic factors (presence/absence of conglomerate platform beneath the island, reef growth, size of the island, and sediment supply). The most appropriate adaptation strategy (managed realignment, nourishment, coastal defense, and relocation) to deal with island change is strongly determined by the type of island response to SLR. For example, an island that is narrowing, but maintaining freeboard, could benefit more from nourishment than coastal defense. If an island is already completely defended, preventing overtopping and overwashing, the only way to maintain habitation is upgrading the coastal defenses (or relocation). The width of the black bars represents the magnitude/importance/relevance of the factor in question.
The pursuit of alternate adaptation pathways does not negate the need to pursue ongoing mitigation action to curtail future SLR and climatic changes on small island nations. However, morphodynamic modeling provides a basis to resolve island-specific trajectories of change to underpin the development of adaptation strategies that may extend the duration of habitation of these islands to at least more than several decades. Future morphodynamic modeling of reef island response to SLR must not only explore further SLR and wave conditions but also need to incorporate the different environmental factors, such as island morphology, reef platform adjustment, and sediment supply.
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