Reef-island topography and the vulnerability of atolls to sea-level rise

Woodroffe, Colin D.

doi:10.1016/j.gloplacha.2007.11.001

Cited by 280 publications

(225 citation statements)

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“…It is difficult to compare these findings to shoreline change elsewhere as these vary greatly across different localities. For example, topographic and radiocarbon evidence indicates net accretion rather than erosion of oceanward atoll island shores at Cocos-Keeling, Kiribati, Funafuti and the Maldives [24], whereas reduced shoreline protection from coral mortality, has been observed to erode beaches on West Indian Ocean continental islands in the Seychelles [9].…”

Section: Discussionmentioning

confidence: 99%

“…The degree to which these represent a recent, climate change-driven departure from a longer-term accretion regime, is unclear. In a study of five Indo-Pacific atoll islands including Diego Garcia, Woodroffe suggests that long-term accretion is likely to outweigh short-term observable erosion on atoll rim islands [24]. Lines of evidence for oceanward beaches acting as net sediment sinks include radiocarbon chronologies indicating incremental accretion, the unidirectional progression of waves that transport reef crest sediment across reef flats and multitemporal image and photographic analysis indicating shoreline progradation elsewhere [24].…”

Section: Discussionmentioning

confidence: 99%

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A Geospatial Appraisal of Ecological and Geomorphic Change on Diego Garcia Atoll, Chagos Islands (British Indian OceanTerritory)

Hamylton

East

2012

Remote Sensing

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This study compiled a wide range of modern and historic geospatial datasets to examine ecological and geomorphic change at Diego Garcia Atoll across a 38-year period . This remarkable collection of spatially referenced information offered an opportunity to advance our understanding of the nature and extent of environmental change that has taken place with the construction of the military airbase at Diego Garcia. Changes assessed included movements of the lagoon rim shorelines, changes in the terrestrial vegetation on the lagoon rim and amendments to the bathymetry of the lagoon basin through dredging activities. Data compiled included detailed shoreline and vegetation maps produced as part of the H.M.S. Vidal Indian Ocean Expedition (1967), three Ikonos satellite images acquired in 2005 that collectively covered the complete Atoll area, a ground truthing field dataset collected in the northern section of the lagoon for the purpose of seafloor mapping (2005), observational evidence of shoreline erosion including photographs and descriptions of seawater inundations and bathymetric soundings from five independent surveys of the lagoon floor (1967, 1985, 1987, 1988 and 1997). Results indicated that much of the change along the lagoon rim is associated with the expansion of the inner lagoon shoreline as a result of the construction of the military airbase, with an estimated increase in land area of 3.01 km 2 in this portion of the atoll rim. the lagoon, particularly in the vicinity of the Main Passage. As no previous record of benthic character exists, a complete benthic habitat map of the atoll was derived through classification of the three IKONOS satellite images. Management implications arising from this overall appraisal of geomorphic and ecological change at Diego Garcia included the need for ongoing monitoring of shoreline change at a representative set of sites around the atoll rim, monitoring of the water flow regime through the northern channels between the open ocean and the lagoon basin and an ongoing mapping campaign to record periodic changes in the character of the benthic surface ecology.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

A Geospatial Appraisal of Ecological and Geomorphic Change on Diego Garcia Atoll, Chagos Islands (British Indian OceanTerritory)

Hamylton

East

2012

Remote Sensing

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“…We also draw attention to the earlier comments from David Stoddart that begin his descriptions of sand dunes on the aggrading coasts of Diego Garcia, and for which the full sentence reads: "while most of the seaward and lagoonward coasts of Diego Garcia are slightly retreating, as shown by the presence of erosion ramps and cliffing, sectors totalling about 17 km possess coastal dune belts" ([2], p. 15). Woodroffe [7] provides profiles of these dunes at Diego Garcia and describes similar ones on the coasts of other Chagos islands, including Brother, Danger and Egmont. These are observed at other Indo-Pacific atolls, including Cocos (Keeling), the Maldives, Kiribati and Tuvalu and are interpreted as a product of persistent winds in exposed locations [8].…”

Section: Received: 13 March 2014 / Accepted: 17 March 2014 / Publishementioning

confidence: 96%

“…Variable rim dynamics incorporate both erosion and accretion around different atoll aspects, which themselves are a function of varying local environmental controls (e.g., incident wave energy). These alter shoreline configuration and elevation, resulting in spatially heterogeneous atoll rims [7]. Such heterogeneity is evident around the lagoon rim in the distinction between "narrow" and "normal" atoll rim types [2,4].…”

Section: Received: 13 March 2014 / Accepted: 17 March 2014 / Publishementioning

confidence: 99%

Response to Sheppard, C. Atoll Rim Expansion or Erosion in Diego Garcia Atoll, Indian Ocean? Comment on Hamylton, S.; East, H. A Geospatial Appraisal of Ecological and Geomorphic Change on Diego Garcia Atoll, Chagos Islands (British Indian Ocean Territory). Remote Sens. 2012, 4, 3444–3461

Hamylton

East

2014

Remote Sensing

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We welcome Sheppard's comments on our recent assessment of both ecological and geomorphic change at Diego Garcia Atoll in the central Indian Ocean [1]. Whilst our assessment incorporated numerous aspects of change, including movements of the lagoon rim shorelines, changes in the terrestrial vegetation on the lagoon rim and amendments to the bathymetry of the lagoon basin through dredging activities [2], this comment solely addresses the estimates of shoreline change. Here we make some brief remarks relating to this shoreline assessment of Diego Garcia and elaborate on some of the complexities of the geomorphic processes that underpin shoreline dynamics. These complexities have important implications for understanding reef island shoreline dynamics, both at this site and globally.The contrast between our estimations of shoreline change at Diego Garcia and Sheppard's in situ observations appears to be overstated. Sheppard provided eight independent photorecords of his inter-annual observations for inclusion in our study ([3]) with which moderate correspondence was found when compared to our findings [2]. Four photographs exhibiting erosion were also associated with erosion in analysis of rim width transects. The remaining four photo records suggested no shoreline change, but our measurements suggested that rim widths had eroded over the longer 38-year period. Thus, our study [2] appears to indicate a greater amount of erosion than Sheppard's photographic evidence. Indeed, the wider study reported both erosion and accretion across 26 and 43 OPEN ACCESS

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“…Woodroffe, 2008), such that even small rises in water level from storm set-up or tsunami may inundate lagoon-facing coastlines during spring tides (Ford et al, 2014). In this context, inhabitants of atolls face different challenges from those of larger bedrock islands and mainland coasts, not least of which the lack of topographic relief, and in turn, potential limitations to emergency evacuation procedures.…”

Section: Introductionmentioning

confidence: 99%

Tsunami hazard potential for the equatorial southwestern Pacific atolls of Tokelau from scenario-based simulations

Orpin

Rickard

Gerring

et al. 2016

Nat. Hazards Earth Syst. Sci.

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Abstract. Devastating tsunami over the last decade have significantly heightened awareness of the potential consequences and vulnerability of low-lying Pacific islands and coastal regions. Our appraisal of the potential tsunami hazard for the atolls of the Tokelau Islands is based on a tsunami source-propagation-inundation model using Gerris Flow Solver, adapted from the companion study by Lamarche et al. (2015) for the islands of Wallis and Futuna. We assess whether there is potential for tsunami flooding on any of the village islets from a selection of 14 earthquake-source experiments. These earthquake sources are primarily based on the largest Pacific earthquakes of M w ≥ 8.1 since 1950 and other large credible sources of tsunami that may impact Tokelau. Earthquake-source location and moment magnitude are related to tsunami-wave amplitudes and tsunami flood depths simulated for each of the three atolls of Tokelau. This approach yields instructive results for a community advisory but is not intended to be fully deterministic. Rather, the underlying aim is to identify credible sources that present the greatest potential to trigger an emergency response.Results from our modelling show that wave fields are channelled by the bathymetry of the Pacific basin in such a way that the swathes of the highest waves sweep immediately northeast of the Tokelau Islands. Our limited simulations suggest that trans-Pacific tsunami from distant earthquake sources to the north of Tokelau pose the most significant inundation threat. In particular, our assumed worst-case scenario for the Kuril Trench generated maximum modelledwave amplitudes in excess of 1 m, which may last a few hours and include several wave trains. Other sources can impact specific sectors of the atolls, particularly distant earthquakes from Chile and Peru, and regional earthquake sources to the south. Flooding is dependent on the wave orientation and direct alignment to the incoming tsunami.Our "worst-case" tsunami simulations of the Tokelau Islands suggest that dry areas remain around the villages, which are typically built on a high islet. Consistent with the oral history of little or no perceived tsunami threat, simulations from the recent Tohoku and Chile earthquake sources suggest only limited flooding around low-lying islets of the atoll. Where potential tsunami flooding is inferred from the modelling, recommended minimum evacuation heights above local sea level are compiled, with particular attention paid to variations in tsunami flood depth around the atolls, subdivided into directional quadrants around each atoll. However, complex wave behaviours around the atolls, islets, tidal channels and within the lagoons are also observed in our simulations. Wave amplitudes within the lagoons may exceed 50 cm, increasing any inundation and potential hazards on the inner shoreline of the atolls, which in turn may influence evacuation strategies. Our study shows that indicative simulation studies can be achieved even with only basic field information. In part, this ...

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Reef-island topography and the vulnerability of atolls to sea-level rise

Cited by 280 publications

References 59 publications

A Geospatial Appraisal of Ecological and Geomorphic Change on Diego Garcia Atoll, Chagos Islands (British Indian OceanTerritory)

A Geospatial Appraisal of Ecological and Geomorphic Change on Diego Garcia Atoll, Chagos Islands (British Indian OceanTerritory)

Response to Sheppard, C. Atoll Rim Expansion or Erosion in Diego Garcia Atoll, Indian Ocean? Comment on Hamylton, S.; East, H. A Geospatial Appraisal of Ecological and Geomorphic Change on Diego Garcia Atoll, Chagos Islands (British Indian Ocean Territory). Remote Sens. 2012, 4, 3444–3461

Tsunami hazard potential for the equatorial southwestern Pacific atolls of Tokelau from scenario-based simulations

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