Murray R. Ford

Geological effects of tsunami on mid-ocean atoll islands: The Maldives before and after the Sumatran tsunami

Low-lying coral islands are fragile landforms susceptible to long-term sea-level rise and extreme events, such as hurricanes and tsunamis. The Sumatran earthquake of 26 December 2004 generated waves that reached the Maldives 2500 km away. Observations of the effects of the tsunami are presented here, based on pre- and post-tsunami topographic and planform surveys of 13 uninhabited Maldivian islands. The surveys showed there was no substantial island erosion and no significant reduction in island area. Rather, the tsunami accentuated predictable seasonal oscillations in shoreline change, including localized retreat of exposed island scarps by up to 6 m, deposition of cuspate spits to leeward, and vertical island building through overwash deposition of sand sheets up to 0.3 m thick, covering up to 17% of island area. These results have implications for island stability indicating that low-lying reef islands are physically robust and the geological signature of tsunamis on atoll island development is minor.

Water level effects on breaking wave setup for Pacific Island fringing reefs

The effects of water level variations on breaking wave setup over fringing reefs are assessed using field measurements obtained at three study sites in the Republic of the Marshall Islands and the Mariana Islands in the western tropical Pacific Ocean. At each site, reef flat setup varies over the tidal range with weaker setup at high tide and stronger setup at low tide for a given incident wave height. The observed water level dependence is interpreted in the context of radiation stress gradients specified by an idealized point break model generalized for nonnormally incident waves. The tidally varying setup is due in part to depth-limited wave heights on the reef flat, as anticipated from previous reef studies, but also to tidally dependent breaking on the reef face. The tidal dependence of the breaking is interpreted in the context of the point break model in terms of a tidally varying wave height to water depth ratio at breaking. Implications for predictions of wave-driven setup at reef-fringed island shorelines are discussed.

Coral islands defy sea-level rise over the past century: Records from a central Pacific atoll

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.

Observations and estimates of wave‐driven water level extremes at the Marshall Islands

Wave-driven extreme water levels are examined for coastlines protected by fringing reefs using field observations obtained in the Republic of the Marshall Islands. The 2% exceedence water level near the shoreline due to waves is estimated empirically for the study sites from breaking wave height at the outer reef and by combining separate contributions from setup, sea and swell, and infragravity waves, which are estimated based on breaking wave height and water level over the reef flat. Although each component exhibits a tidal dependence, they sum to yield a 2% exceedence level that does not. A hindcast based on the breaking wave height parameterization is used to assess factors leading to flooding at Roi-Namur caused by an energetic swell event during December 2008. Extreme water levels similar to December 2008 are projected to increase significantly with rising sea level as more wave and tide events combine to exceed inundation threshold levels.

Evidence for coral island formation during rising sea level in the central Pacific Ocean

The timing and evolution of Jabat Island, Marshall Islands, was investigated using morphostratigraphic analysis and radiometric dating. Results show the first evidence of island building in the Pacific during latter stages of Holocene sea level rise. A three-phase model of development of Jabat is presented. Initially, rapid accumulation of coarse sediments on Jabat occurred 4800–4000 years B.P. across a reef flat higher than present level, as sea level continued to rise. During the highstand, island margins and particularly the western margin accreted vertically to 2.5–3.0 m above contemporary ridge elevations. This accumulation phase was dominated by sand-size sediments. Phase three involved deposition of gravel ridges on the northern reef, as sea level fell to present position. Jabat has remained geomorphically stable for the past 2000 years. Findings suggest reef platforms may accommodate the oldest reef islands in atoll systems, which may have profound implications for questions of prehistoric migration through Pacific archipelagos.

Reef Flat Wave Processes and Excavation Pits: Observations and Implications for Majuro Atoll, Marshall Islands

ABSTRACT Ford, M.R.; Becker, J.M., and Merrifield, M.A., 2013. Reef Flat wave processes and excavation pits: observations and implications for Majuro Atoll, Marshall Islands. An experimental deployment of pressure sensors was undertaken to assess the impact of reef flat excavation pits on wave processes at Majuro Atoll, Marshall Islands. Experiments were undertaken on two sections of an 80-m-wide fringing reef flat, one modified by the excavation of a 17-m wide, 4- to 5-m deep pit and the other an unmodified reef flat of comparable width, topography, and incident wave energy. A wave-driven inundation event during the experiment led to minor amounts of debris overwashing the road surface. The event was associated with southerly swell that was normally incident to the study site at high tide, resulting in enhanced shoreline energy in both the sea and swell and the infragravity frequency bands. The shoreline with the excavation pit received slightly smaller wave heights (∼8%) for all wave conditions, including the overwash event, compared to the unmodified shoreline. The statistically significant difference is largely a result of a decrease in infragravity wave energy over and shoreward of the pit that compensates for a weak increase in sea and swell energy. The influence on infragravity energy levels is likely to depend on the pit geometry and position, as well as the wave forcing; hence, not all pits are likely to lead to net wave energy dissipation. Implications of the findings are discussed with respect to the impacts of reef flat excavation on a highly developed, urbanised atoll.

Spatiotemporal variability of typhoon impacts and relaxation intervals on Jaluit Atoll, Marshall Islands

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.

Patterns of island change and persistence offer alternate adaptation pathways for atoll nations

Sea-level rise and climatic change threaten the existence of atoll nations. Inundation and erosion are expected to render islands uninhabitable over the next century, forcing human migration. Here we present analysis of shoreline change in all 101 islands in the Pacific atoll nation of Tuvalu. Using remotely sensed data, change is analysed over the past four decades, a period when local sea level has risen at twice the global average (~3.90 ± 0.4 mm.yr−1). Results highlight a net increase in land area in Tuvalu of 73.5 ha (2.9%), despite sea-level rise, and land area increase in eight of nine atolls. Island change has lacked uniformity with 74% increasing and 27% decreasing in size. Results challenge perceptions of island loss, showing islands are dynamic features that will persist as sites for habitation over the next century, presenting alternate opportunities for adaptation that embrace the heterogeneity of island types and their dynamics.Inundation and erosion could make many atoll islands uninhabitable over the next century. Here the authors present an analysis of change in the atoll nation of Tuvalu that shows a 2.9% increase in land area over the past four decades, with 74% of islands increasing in size, despite rising sea levels.

Lagoonal reef sediment supply and island connectivity, Huvadhu Atoll, Maldives

ABSTRACT Liang, Y.; Kench, P.S., Ford, M.R., and East, H.K., 2016. Reef sediment supply and island building in the South Maldives. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 587–591. Coconut Creek (Florida), ISSN 0749-0208. Reef islands are small, low-lying islands composed mainly of bioclastic sands and gravels from the late Holocene. These islands, which support around 700,000 people, are vulnerable to potential threats including land submergence and shoreline erosion. Thus, the sediment supply of the surrounding coral reef is an important consideration for coastal maintenance but there is little research on the link between reef productivity and island building. This paper presents an examination of reef and island beach sediments (material and composition), as well as the eco-geomorphic zones of the contemporary reef. Two islands in Huvadhu Atoll of the Republic of Maldives were used in this investigation. Surficial sediment samples were collected from each reef zone, toe of beach, and beach berm for each island. Benthic surveys for each reef platform and aerial photo analyses were used to determine reef ecology and to constrain the eco-geomorphic zones on the reef. Prevalent organisms on both reefs were coral, crustose coralline algae, and Halimeda sp., while other sediment producers (molluscs and foraminifera) were less represented or absent. However, sediment samples taken from the reef flat show that all constituents are present, similarly to sand samples from the island beaches. The reef and beach sediments had similar compositions, suggesting that the island is still connected to the surrounding reef. This research provides insights into how these reef islands are maintained by the surrounding reef, which has implications for future island building and resilience to climate change.