The future of low-lying reef islands has been the subject of international concern, scientific de... more The future of low-lying reef islands has been the subject of international concern, scientific debate, and media interest in the last decade. As a result of sea-level rise, atoll islands are expected to become increasingly unstable and to be susceptible to potential depopulation by the end of the 21st century. Some have suggested that sea-level rise has already resulted in widespread erosion and inundation of atoll islands. Here, we analyze the physical changes in over 200 islands on 12 atolls in the central and western Pacific in the past few decades when sea level in the region increased at rates three to four times the global average. Results show little evidence of heightened erosion or reduction in island size. Instead island shores have adjusted their position and morphology in response to human impacts such as seawall construction and to variations in climate–ocean processes. These changes are reviewed and the role of sea-level rise is evaluated. The implications of this analysis are addressed in two parts. First, we consider the proposition that future sea-level rise will destabilize atoll islands to such an extent that their inhabitants will be forced to migrate offshore. And second, we identify a series of new challenges relating to risk reduction and adaptation policy for atoll island governments, international agencies, and island communities. These require a substantial shift away from the present adaptation paradigm of external migration and focus on the persistence of atoll islands and in-country solutions.
Storms are important drivers of geomorphic change within atoll settings, causing rapid island ero... more Storms are important drivers of geomorphic change within atoll settings, causing rapid island erosion while also initiating island building through the generation of vast quantities of reef-derived sediment. Due to the combined effects of storms and sea-level rise, reef islands are thought to be on an erosional trajectory. Jaluit Atoll in the Marshall Islands was struck by Typhoon Ophelia in A.D. 1958, causing significant geomorphic change to reef islands. Using aerial photographs as well as recent satellite imagery we track the impacts of the typhoon and the multidecadal recovery of islands. Ophelia caused a significant reduction in total land area of Jaluit Atoll, from 9.95 km 2 to 9.45 km 2 (–5.1%), with islands on the northeast rim collectively reducing in size from 4.72 km 2 to 4.14 km 2 (–12.2%). Between 1976 and 2006, 73 of 87 islands increased in size, with the total landmass exceeding the pre-typhoon area (10.25 km 2). However, we observe considerable spatiotemporal variability of impacts and relaxation following the typhoon. Results indicate that despite significant typhoon-driven erosion and a relaxation period coincident with local sea-level rise, islands have persisted and grown.
Waves are an important process responsible for the initiation and subsequent development of inter... more Waves are an important process responsible for the initiation and subsequent development of intertidal shore platforms. However, few field studies to date have described wave processes on shore platforms. A field experiment was conducted using an across-shore array of wave gauges, including a directional sensor, on a near-horizontal platform at Mahia Peninsula, North Island, New Zealand. Results show that the platform is very efficient in filtering wave energy at gravity wave frequencies (>0.05 Hz), with 40-80% reduction in wave height measured across the 140 m wide platform. By contrast, infragravity waves are present on the platform and increase in magnitude towards the cliff toe. Directional wave analysis indicates that wave reflection is mainly restricted to infragravity wave frequencies, demonstrating the differences in gravity and infragravity wave behaviour on the platform studied. Results indicate that, under fair weather conditions, the role of waves as an agent of cliff toe erosion is likely to be limited at present, although they are probably important for removing sediments accumulated at the cliff toe. The observed increase in infragravity wave energy towards the cliff toe implies that these long-period waves may be important geomorphic agents on shore platforms.
This paper examines the use and environmental consequences of engineered structures on reef platf... more This paper examines the use and environmental consequences of engineered structures on reef platform islands in the Maldives. The Maldives comprises 1 500 islands which are mostly low-lying sand cays. These cays are inherently unstable, changing their size, and position on reef platforms in response to short-term and seasonal adjustments in wind, wave and current patterns. Island instability combined with high population densities h as resulted in the proliferation of engineered structures to combat erosion and to provide boat access. In many instances introduction of hard engineered structures has exacerbated island erosion and degraded reef productivity. Reasons for the negative environmental consequences are twofold. First, the design, materials used in construction and the mode of construction contravene most standard measures of sound engineering design. Constraining sound design is a total lack of environmental information, particularly wave and current data. Second, structure...
The geological stability and existence of low-lying atoll nations is threatened by sea-level rise... more The geological stability and existence of low-lying atoll nations is threatened by sea-level rise and climate change. Funafuti Atoll, in the tropical Pacific Ocean, has experienced some of the highest rates of sea-level rise (~5.1 ± 0.7 mm/yr), totaling ~0.30 ± 0.04 m over the past 60 yr. We analyzed six time slices of shoreline position over the past 118 yr at 29 islands of Funafuti Atoll to determine their physical response to recent sea-level rise. Despite the magnitude of this rise, no islands have been lost, the majority have enlarged, and there has been a 7.3% increase in net island area over the past century (A.D. 1897-2013). There is no evidence of heightened erosion over the past half-century as sea-level rise accelerated. Reef islands in Funafuti continually adjust their size, shape, and position in response to variations in boundary conditions, including storms, sediment supply, as well as sea level. Results suggest a more optimistic prognosis for the habitability of atoll nations and demonstrate the importance of resolving recent rates and styles of island change to inform adaptation strategies.
To evaluate contamination of coastal sediments along Fongafale Islet, Central Pacific, a field su... more To evaluate contamination of coastal sediments along Fongafale Islet, Central Pacific, a field survey was conducted in densely populated, sparsely populated, open dumping and undisturbed natural areas. Current measurements in shallow water of the lagoon indicated that contaminants from the densely populated area would only be transported for a small proportion of a tidal cycle. Acid-volatile sulfides were detected in both the intertidal beach and nearshore zones of the densely populated area, whereas these were no detection in the other areas. This observation lends support to argument that the coastal pollution mechanism that during ebb tide, domestic wastewater leaking from poorly constructed sanitary facilities seeps into the coast. The total concentrations of Cr, Mn, Ni, Cu, Zn, Cd and Pb were relatively high in all of the areas except the undisturbed natural area. The indices of contamination factor, pollution load index and geoaccumulation index were indicative of heavy metal ...
Reef islands are unique landforms composed entirely of sediment produced on the surrounding coral... more Reef islands are unique landforms composed entirely of sediment produced on the surrounding coral reefs. Despite the fundamental importance of these ecological-sedimentary links for island development and future maintenance, reef island sediment production regimes remain poorly quantified. Using census and sedimentary data from Vakkaru island (Maldives), a sand-dominated atoll interior island, we quantify the major sediment-generating habitats, the abundance of sediment producers in these habitats, and the rates and size fractions of sediment generated by different taxa. The estimated annual sediment production is 685,000 kg (or 370 m 3 ), ~75% of which is produced on the narrow outer reef flat, despite composing only 21% of the total platform area. Approximately 65% of the platform acts solely as a sediment sink. Census data identify parrotfish as the major sediment producers, generating >85% of the 5.7 kg m -2 of new sand-grade sediment produced on the outer reef flat each year. Halimeda (macroalgae) produce a further 10%, most as gravelgrade material. Comparisons between production estimates and sedimentary data indicate that reef ecology and island sedimentology are tightly linked; reef flat and lagoon sediments are dominated by coral and Halimeda, although fine-to medium-grained coral sand is the dominant (~59%) island constituent. The generation of sediment suitable for maintaining this reef island is thus critically dependent on a narrow zone of high-productivity reef, but most especially on the maintenance of healthy parrotfish populations that can convert reef framework to sand-grade sediment.
ABSTRACT Few studies of wave processes on shore platforms have addressed the hydrodynamic thresho... more ABSTRACT Few studies of wave processes on shore platforms have addressed the hydrodynamic thresholds that control wave transformation and energy dissipation, especially under storm conditions. We present results of a field experiment conducted during a storm on a sub-horizontal shore platform on the east coast of Auckland, New Zealand. Small (<0.5 m) locally generated waves typically occur at the field site, whereas during the experiment the offshore wave height reached 2.3 m. Our results illustrate the important control that platform morphology has on wave characteristics. At the seaward edge of the platform a scarp abruptly descends beneath low tide level. Wave height immediately seaward of the platform was controlled by the incident conditions, but near the cliff toe wave height on the platform was independent of incident conditions. Results show that a depth threshold at the seaward platform edge >2.5 times the gravity wave height (0.05 – 0.33 Hz) is necessary for waves to propagate onto the platform without breaking. On the platform surface the wave height is a direct function of water depth, with limiting maximum wave height to water depth ratios of 0.55 and 0.78 at the centre of the platform and cliff toe respectively. A relative ‘platform edge submergence’ (water depth/water height ratio) threshold of 1.1 is identified, below which infragravity (<0.05 Hz) wave energy dominates the platform energy spectra, and above which gravity waves are dominant. Infragravity wave height transformation across the platform is governed by the relative platform edge submergence. Finally, the paper describes the first observations of wave setup on a shore platform. During the peak of the storm, wave setup on the platform at low tide (0.21 m) is consistent with measurements from planar sandy beaches, but at higher tidal stages the ratio between incident wave height and maximum setup was lower than expected. This article is protected by copyright. All rights reserved.
Eos, Transactions American Geophysical Union, 2008
... com Page 6. For my wife Lisa Young with love Page 7. ... Riegl (Chapter 10) examines the exte... more ... com Page 6. For my wife Lisa Young with love Page 7. ... Riegl (Chapter 10) examines the extent to which extreme climatic events, particularly tropical cyclones and El Niño–Southern Oscillation (ENSO) events, can be expected to damage corals and coral reefs in a warming world. ...
Proceedings. Biological sciences / The Royal Society, Jan 7, 2014
Coral cover has declined rapidly on Caribbean reefs since the early 1980s, reducing carbonate pro... more Coral cover has declined rapidly on Caribbean reefs since the early 1980s, reducing carbonate production and reef growth. Using a cross-regional dataset, we show that widespread reductions in bioerosion rates-a key carbonate cycling process-have accompanied carbonate production declines. Bioerosion by parrotfish, urchins, endolithic sponges and microendoliths collectively averages 2 G (where G = kg CaCO3 m(-2) yr(-1)) (range 0.96-3.67 G). This rate is at least 75% lower than that reported from Caribbean reefs prior to their shift towards their present degraded state. Despite chronic overfishing, parrotfish are the dominant bioeroders, but erosion rates are reduced from averages of approximately 4 to 1.6 G. Urchin erosion rates have declined further and are functionally irrelevant to bioerosion on most reefs. These changes demonstrate a fundamental shift in Caribbean reef carbonate budget dynamics. To-date, reduced bioerosion rates have partially offset carbonate production declines,...
Figure 1. Present-day beach ridge composed of shells of Fragum erugatum. Note evidence of recent ... more Figure 1. Present-day beach ridge composed of shells of Fragum erugatum. Note evidence of recent wave erosion. Hamelin Pool, Shark Bay.
A field experiment was conducted at Warraber Island, Torres Strait, Australia to investigate spat... more A field experiment was conducted at Warraber Island, Torres Strait, Australia to investigate spatial and temporal variations in wave characteristics and energy across a mesotidal coral reef platform. Measurements of water depth were obtained using five pressure sensors deployed across a 2.7-km section of reef flat from July 3 -5, 2001. The reef surface was uneven and consisted of an outer reef flat, a central reef flat depression, an inner reef ramp, a palaeo-reef surface and the shoreline. Water levels decreased landward across the platform with tide ranges at the shoreline being almost 50% lower than at the outer reef flat. Rising and falling tides were characterised by a bimodal energy distribution with both short-period (0 -3 s) and wind (3 -8 s) waves present. Higher water levels were dominated by wind waves. The highest waves occurred at high tides associated with nocturnal tidal cycles with H s decreasing from 0.5 to 0.2 m from the outer reef flat to the shoreline. Wave energy at swell (8 -20 s) and infragravity (>20 s) frequencies was negligible across the reef platform although there was evidence of wave groups at higher water levels.
The future of low-lying reef islands has been the subject of international concern, scientific de... more The future of low-lying reef islands has been the subject of international concern, scientific debate, and media interest in the last decade. As a result of sea-level rise, atoll islands are expected to become increasingly unstable and to be susceptible to potential depopulation by the end of the 21st century. Some have suggested that sea-level rise has already resulted in widespread erosion and inundation of atoll islands. Here, we analyze the physical changes in over 200 islands on 12 atolls in the central and western Pacific in the past few decades when sea level in the region increased at rates three to four times the global average. Results show little evidence of heightened erosion or reduction in island size. Instead island shores have adjusted their position and morphology in response to human impacts such as seawall construction and to variations in climate–ocean processes. These changes are reviewed and the role of sea-level rise is evaluated. The implications of this analysis are addressed in two parts. First, we consider the proposition that future sea-level rise will destabilize atoll islands to such an extent that their inhabitants will be forced to migrate offshore. And second, we identify a series of new challenges relating to risk reduction and adaptation policy for atoll island governments, international agencies, and island communities. These require a substantial shift away from the present adaptation paradigm of external migration and focus on the persistence of atoll islands and in-country solutions.
Storms are important drivers of geomorphic change within atoll settings, causing rapid island ero... more Storms are important drivers of geomorphic change within atoll settings, causing rapid island erosion while also initiating island building through the generation of vast quantities of reef-derived sediment. Due to the combined effects of storms and sea-level rise, reef islands are thought to be on an erosional trajectory. Jaluit Atoll in the Marshall Islands was struck by Typhoon Ophelia in A.D. 1958, causing significant geomorphic change to reef islands. Using aerial photographs as well as recent satellite imagery we track the impacts of the typhoon and the multidecadal recovery of islands. Ophelia caused a significant reduction in total land area of Jaluit Atoll, from 9.95 km 2 to 9.45 km 2 (–5.1%), with islands on the northeast rim collectively reducing in size from 4.72 km 2 to 4.14 km 2 (–12.2%). Between 1976 and 2006, 73 of 87 islands increased in size, with the total landmass exceeding the pre-typhoon area (10.25 km 2). However, we observe considerable spatiotemporal variability of impacts and relaxation following the typhoon. Results indicate that despite significant typhoon-driven erosion and a relaxation period coincident with local sea-level rise, islands have persisted and grown.
Waves are an important process responsible for the initiation and subsequent development of inter... more Waves are an important process responsible for the initiation and subsequent development of intertidal shore platforms. However, few field studies to date have described wave processes on shore platforms. A field experiment was conducted using an across-shore array of wave gauges, including a directional sensor, on a near-horizontal platform at Mahia Peninsula, North Island, New Zealand. Results show that the platform is very efficient in filtering wave energy at gravity wave frequencies (>0.05 Hz), with 40-80% reduction in wave height measured across the 140 m wide platform. By contrast, infragravity waves are present on the platform and increase in magnitude towards the cliff toe. Directional wave analysis indicates that wave reflection is mainly restricted to infragravity wave frequencies, demonstrating the differences in gravity and infragravity wave behaviour on the platform studied. Results indicate that, under fair weather conditions, the role of waves as an agent of cliff toe erosion is likely to be limited at present, although they are probably important for removing sediments accumulated at the cliff toe. The observed increase in infragravity wave energy towards the cliff toe implies that these long-period waves may be important geomorphic agents on shore platforms.
This paper examines the use and environmental consequences of engineered structures on reef platf... more This paper examines the use and environmental consequences of engineered structures on reef platform islands in the Maldives. The Maldives comprises 1 500 islands which are mostly low-lying sand cays. These cays are inherently unstable, changing their size, and position on reef platforms in response to short-term and seasonal adjustments in wind, wave and current patterns. Island instability combined with high population densities h as resulted in the proliferation of engineered structures to combat erosion and to provide boat access. In many instances introduction of hard engineered structures has exacerbated island erosion and degraded reef productivity. Reasons for the negative environmental consequences are twofold. First, the design, materials used in construction and the mode of construction contravene most standard measures of sound engineering design. Constraining sound design is a total lack of environmental information, particularly wave and current data. Second, structure...
The geological stability and existence of low-lying atoll nations is threatened by sea-level rise... more The geological stability and existence of low-lying atoll nations is threatened by sea-level rise and climate change. Funafuti Atoll, in the tropical Pacific Ocean, has experienced some of the highest rates of sea-level rise (~5.1 ± 0.7 mm/yr), totaling ~0.30 ± 0.04 m over the past 60 yr. We analyzed six time slices of shoreline position over the past 118 yr at 29 islands of Funafuti Atoll to determine their physical response to recent sea-level rise. Despite the magnitude of this rise, no islands have been lost, the majority have enlarged, and there has been a 7.3% increase in net island area over the past century (A.D. 1897-2013). There is no evidence of heightened erosion over the past half-century as sea-level rise accelerated. Reef islands in Funafuti continually adjust their size, shape, and position in response to variations in boundary conditions, including storms, sediment supply, as well as sea level. Results suggest a more optimistic prognosis for the habitability of atoll nations and demonstrate the importance of resolving recent rates and styles of island change to inform adaptation strategies.
To evaluate contamination of coastal sediments along Fongafale Islet, Central Pacific, a field su... more To evaluate contamination of coastal sediments along Fongafale Islet, Central Pacific, a field survey was conducted in densely populated, sparsely populated, open dumping and undisturbed natural areas. Current measurements in shallow water of the lagoon indicated that contaminants from the densely populated area would only be transported for a small proportion of a tidal cycle. Acid-volatile sulfides were detected in both the intertidal beach and nearshore zones of the densely populated area, whereas these were no detection in the other areas. This observation lends support to argument that the coastal pollution mechanism that during ebb tide, domestic wastewater leaking from poorly constructed sanitary facilities seeps into the coast. The total concentrations of Cr, Mn, Ni, Cu, Zn, Cd and Pb were relatively high in all of the areas except the undisturbed natural area. The indices of contamination factor, pollution load index and geoaccumulation index were indicative of heavy metal ...
Reef islands are unique landforms composed entirely of sediment produced on the surrounding coral... more Reef islands are unique landforms composed entirely of sediment produced on the surrounding coral reefs. Despite the fundamental importance of these ecological-sedimentary links for island development and future maintenance, reef island sediment production regimes remain poorly quantified. Using census and sedimentary data from Vakkaru island (Maldives), a sand-dominated atoll interior island, we quantify the major sediment-generating habitats, the abundance of sediment producers in these habitats, and the rates and size fractions of sediment generated by different taxa. The estimated annual sediment production is 685,000 kg (or 370 m 3 ), ~75% of which is produced on the narrow outer reef flat, despite composing only 21% of the total platform area. Approximately 65% of the platform acts solely as a sediment sink. Census data identify parrotfish as the major sediment producers, generating >85% of the 5.7 kg m -2 of new sand-grade sediment produced on the outer reef flat each year. Halimeda (macroalgae) produce a further 10%, most as gravelgrade material. Comparisons between production estimates and sedimentary data indicate that reef ecology and island sedimentology are tightly linked; reef flat and lagoon sediments are dominated by coral and Halimeda, although fine-to medium-grained coral sand is the dominant (~59%) island constituent. The generation of sediment suitable for maintaining this reef island is thus critically dependent on a narrow zone of high-productivity reef, but most especially on the maintenance of healthy parrotfish populations that can convert reef framework to sand-grade sediment.
ABSTRACT Few studies of wave processes on shore platforms have addressed the hydrodynamic thresho... more ABSTRACT Few studies of wave processes on shore platforms have addressed the hydrodynamic thresholds that control wave transformation and energy dissipation, especially under storm conditions. We present results of a field experiment conducted during a storm on a sub-horizontal shore platform on the east coast of Auckland, New Zealand. Small (<0.5 m) locally generated waves typically occur at the field site, whereas during the experiment the offshore wave height reached 2.3 m. Our results illustrate the important control that platform morphology has on wave characteristics. At the seaward edge of the platform a scarp abruptly descends beneath low tide level. Wave height immediately seaward of the platform was controlled by the incident conditions, but near the cliff toe wave height on the platform was independent of incident conditions. Results show that a depth threshold at the seaward platform edge >2.5 times the gravity wave height (0.05 – 0.33 Hz) is necessary for waves to propagate onto the platform without breaking. On the platform surface the wave height is a direct function of water depth, with limiting maximum wave height to water depth ratios of 0.55 and 0.78 at the centre of the platform and cliff toe respectively. A relative ‘platform edge submergence’ (water depth/water height ratio) threshold of 1.1 is identified, below which infragravity (<0.05 Hz) wave energy dominates the platform energy spectra, and above which gravity waves are dominant. Infragravity wave height transformation across the platform is governed by the relative platform edge submergence. Finally, the paper describes the first observations of wave setup on a shore platform. During the peak of the storm, wave setup on the platform at low tide (0.21 m) is consistent with measurements from planar sandy beaches, but at higher tidal stages the ratio between incident wave height and maximum setup was lower than expected. This article is protected by copyright. All rights reserved.
Eos, Transactions American Geophysical Union, 2008
... com Page 6. For my wife Lisa Young with love Page 7. ... Riegl (Chapter 10) examines the exte... more ... com Page 6. For my wife Lisa Young with love Page 7. ... Riegl (Chapter 10) examines the extent to which extreme climatic events, particularly tropical cyclones and El Niño–Southern Oscillation (ENSO) events, can be expected to damage corals and coral reefs in a warming world. ...
Proceedings. Biological sciences / The Royal Society, Jan 7, 2014
Coral cover has declined rapidly on Caribbean reefs since the early 1980s, reducing carbonate pro... more Coral cover has declined rapidly on Caribbean reefs since the early 1980s, reducing carbonate production and reef growth. Using a cross-regional dataset, we show that widespread reductions in bioerosion rates-a key carbonate cycling process-have accompanied carbonate production declines. Bioerosion by parrotfish, urchins, endolithic sponges and microendoliths collectively averages 2 G (where G = kg CaCO3 m(-2) yr(-1)) (range 0.96-3.67 G). This rate is at least 75% lower than that reported from Caribbean reefs prior to their shift towards their present degraded state. Despite chronic overfishing, parrotfish are the dominant bioeroders, but erosion rates are reduced from averages of approximately 4 to 1.6 G. Urchin erosion rates have declined further and are functionally irrelevant to bioerosion on most reefs. These changes demonstrate a fundamental shift in Caribbean reef carbonate budget dynamics. To-date, reduced bioerosion rates have partially offset carbonate production declines,...
Figure 1. Present-day beach ridge composed of shells of Fragum erugatum. Note evidence of recent ... more Figure 1. Present-day beach ridge composed of shells of Fragum erugatum. Note evidence of recent wave erosion. Hamelin Pool, Shark Bay.
A field experiment was conducted at Warraber Island, Torres Strait, Australia to investigate spat... more A field experiment was conducted at Warraber Island, Torres Strait, Australia to investigate spatial and temporal variations in wave characteristics and energy across a mesotidal coral reef platform. Measurements of water depth were obtained using five pressure sensors deployed across a 2.7-km section of reef flat from July 3 -5, 2001. The reef surface was uneven and consisted of an outer reef flat, a central reef flat depression, an inner reef ramp, a palaeo-reef surface and the shoreline. Water levels decreased landward across the platform with tide ranges at the shoreline being almost 50% lower than at the outer reef flat. Rising and falling tides were characterised by a bimodal energy distribution with both short-period (0 -3 s) and wind (3 -8 s) waves present. Higher water levels were dominated by wind waves. The highest waves occurred at high tides associated with nocturnal tidal cycles with H s decreasing from 0.5 to 0.2 m from the outer reef flat to the shoreline. Wave energy at swell (8 -20 s) and infragravity (>20 s) frequencies was negligible across the reef platform although there was evidence of wave groups at higher water levels.
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