Elsevier

Geomorphology

Volume 390, 1 October 2021, 107871
Geomorphology

Assessing atoll island physical robustness: Application to Rangiroa Atoll, French Polynesia

https://doi.org/10.1016/j.geomorph.2021.107871Get rights and content

Highlights

  • Geomorphic, ecological and human factors control atoll island physical robustness.

  • A new method for assessing atoll island physical robustness is developed.

  • The physical robustness of atoll islands varies significantly within a given atoll.

  • Within-atoll relocation of human assets appears a relevant adaptation strategy.

Abstract

This article proposes a comprehensive methodology considering geomorphic, ecological and human variables to assess atoll island physical robustness in the face of climate-ocean pressures. Six variables are considered, including island size, elevation, shape, structure, vegetation and the influence of human activities on island natural capacity to adjust to climate-ocean changes. Each of these variables is ranked on a five-level scale and the rankings are summed up to provide a final estimate in the form of an index of the relative physical robustness of each island. This methodology is applied to twelve islands of Rangiroa Atoll, French Polynesia, considered key to the maintenance of the habitability of the atoll by the local community and exhibiting contrasting physical configurations. The findings emphasize highly contrasting levels of island physical robustness, with indices ranging from 0.33 to 0.75. The main contributors to differences in island physical robustness are, in order of importance, island size and the influence of human activities on island capacity to adjust to climate-ocean changes; island elevation; island shape, structure and vegetation. Some peripheral rural islands that are targeted by the public authorities for future development have a much higher physical robustness than the settled islands. Based on these findings, we advocate, first, for the inclusion of ecological and human variables in assessments of atoll island physical robustness and modelling studies; and second, for within-atoll relocation of people and human assets to more robust island areas and islands.

Introduction

Atoll islands are recently-formed (generally <4000 yr BP) and low-lying (generally <3 m) islands composed of biologically derived carbonate sand, gravel and boulders, resting on circular reef structures at or near contemporary sea level and often encircling a central lagoon (Woodroffe, 2008; McLean, 2011; Gischler, 2016). These islands host relatively large populations in some countries and territories of the Indian and Pacific Oceans, including the Maldives (530,953 inhabitants), Kiribati (117,646 inhabitants), the Federated States of Micronesia (113,815 inhabitants), the Marshall Islands (58,791 inhabitants), French Polynesia (15,544 inhabitants) and Tuvalu (11,646 inhabitants). Because of their physical configuration, atoll countries and territories are among the territories that are the most threatened by climate variability and climate change impacts, including especially the combination of gradual sea-level rise (SLR) and increased storm wave heights, and the degradation of coral reefs under both ocean warming and acidification and increased human disturbances (Bindoff et al., 2019; Cornwall et al., 2021; Duvat et al., 2021; Gattuso et al., 2015; Hoeke et al., 2021; Kane and Fletcher, 2020; Mentaschi et al., 2017; Oppenheimer et al., 2019; Perry et al., 2018; Vitousek et al., 2017).

Physical assessments of risks to atoll island habitability under climate change have investigated the risks of island erosion (Beetham et al., 2017; Beetham and Kench, 2018; Shope et al., 2017; Shope and Storlazzi, 2019; Tuck et al., 2019) and temporary or permanent submergence (Giardino et al., 2018; Owen et al., 2016; Storlazzi et al., 2018). These studies have mainly considered two types of parameters, namely island elevation (which influences marine flooding) and island capacity to naturally adjust to climate-ocean changes through sediment reorganization, that is, changes in position, shape, volume and elevation (which allow for an island to persist). Such studies agree that atoll islands will likely experience increased physical destabilization over the second half of the 21st century as a result of increased marine flooding and shoreline instability, with potential decreases in shoreline elevation, island width and volume, whatever the climate scenario. Beyond these key findings, these studies have limitations. First, they are based on idealized islands that do not reflect the high diversity of atoll island configurations, although recognized by previous studies (e.g. Nurse et al., 2014; Richmond, 1992; Stoddart and Steers, 1977; Woodroffe, 2008). Second, they used models that neglect some variables driving island change, including the response of ecosystems (especially the reef ecosystem and island vegetation) and sediment transport, notably cross-shore and overwash-driven. Third, they overlooked the specific situation of inhabited islands exhibiting a decreased capacity to adjust to climate-ocean changes, as a result of the obstruction of sediment transport pathways and reduction of coastal accommodation space by human developments (Duvat, 2019; Duvat and Magnan, 2019; Duvat et al., 2020a; McLean and Kench, 2015; Schuerch et al., 2018).

Because atoll islands exhibit diverse physical (Richmond, 1992; Stoddart and Steers, 1977; Woodroffe, 2008) and human (Duvat and Magnan, 2019; McLean and Kench, 2015) configurations, which together influence their current and future habitability (Duvat et al., 2021; Magnan et al., 2019), the present article proposes a more comprehensive assessment of their ‘physical robustness’ that considers not only geomorphic, but also ecological and human determinants of this robustness. ‘Physical robustness’ refers to the ability of an atoll island to both resist to extreme events (i.e. ‘absorb a disturbance with minimal alteration’; Masselink and Lazarus, 2019, p. 5), and naturally adjust (i.e. its size, shape, elevation, volume and position) to any changes in boundary conditions through sediment reworking (McLean and Kench, 2015; Tuck et al., 2019). Using a sample of twelve islands that are considered key by the local population and public authorities to the maintenance of the habitability of Rangiroa Atoll (Tuamotu Archipelago, French Polynesia) in the future, and that exhibit contrasting physical configurations, we assess atoll island physical robustness using six parameters: (1) island size; (2) island elevation; (3) island shape; (4) island structure; (5) island vegetation; and (6) the influence of local human activities on island dynamics and natural capacity to adjust to climate-ocean changes.

Section snippets

Study area

The Tuamotu Archipelago is one of the five archipelagos composing French Polynesia (Fig. 1A), and the largest group of atolls in the world. It consists of seventy-seven atolls stretching >1500 km from northwest to southeast and extending from 14°21′S to 23°22′S and from 134°28′W to 148°43′W. This study focuses on Rangiroa, which is the largest atoll in the archipelago, with maximal dimensions of 79 by 32 km (Fig. 1B), and a total landmass of approximately 66 km2 (Andrefouët et al., 2008) made

Justification of island sample

The main objective of this study is to assess the physical robustness of twelve islands that are considered key by the local population and public authorities to the maintenance of the atoll's habitability because they are already settled, cultivated or targeted for future development. These islands constitute a relevant island sample to assess atoll island physical robustness because they are geomorphologically (in terms of size, geomorphic features, shape and elevation) and ecologically

Variable 1: island size

The twelve study islands have land areas ranging from 0.69 ha for island no. 7, which is a rim-perpendicular elongated islet that formed between 1966 and 2013, to 464.20 ha for island no. 1, both of which are settled (Table 3). Islands nos. 6 and 7 (inhabited) rank 1 (x < 10 ha), while islands nos. 3 and 4 (inhabited) rank 2 (10 ≤ x < 50 ha); islands nos. 8 to 10 (inhabited) rank 3 (50 ≤ x < 100 ha); islands nos. 2, 5 (inhabited) and 11 (uninhabited) rank 4 (100 ≤ x < 200 ha); and islands nos.

Discussion

Using the example of Rangiroa Atoll, this study reveals that atoll islands exhibit contrasting levels of physical robustness, with some unsettled agricultural islands showing higher levels of physical robustness than the settled islands. This is, firstly, because inhabited islands were settled for economic reasons (proximity to passes allowing the promotion of commercial activities) regardless of their physical characteristics; and secondly, because local human activities have reduced the

Conclusions

Atoll islands exhibit extremely diverse physical and human features, which together influence their current and future habitability. Recognizing that these islands are geomorphologically diverse, this article proposes a comprehensive assessment of their physical robustness that considers not only geomorphic, but also ecological and human determinants of this robustness.

Study islands exhibit contrasting levels of physical robustness, both at the variable scale and at the aggregated index scale.

Declaration of competing interest

The authors wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

Acknowledgements

This work was supported by the Agence Nationale de la Recherche (France) under the STORISK research project (grant no. ANR-15-CE03-0003).

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