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Matheen, N. and Kench, P.S., 2024. Morphological response of coral reef islands to the 2019 extreme positive Indian Ocean Dipole event. Journal of Coastal Research, 40(2), 246–256. Charlotte (North Carolina), ISSN 0749-0208. The influence of the Indian Ocean Dipole (IOD) on winds, waves, and water levels has been studied in considerable detail, but the effect of these anomalous IOD-induced metocean conditions on coastal zones has so far received limited attention. This study analyses the impact of IOD events on the morphological response of coral reef islands by undertaking a case study of five islands in the Maldivian archipelago during the 2019 extreme positive IOD (pIOD) event. A 6-year dataset of shorelines before, during, and after the pIOD event was analysed to establish the magnitude of year-on-year variability and seasonal oscillation trends in the shorelines and thereby identify variability induced by the pIOD event. The results indicate a departure in the shorelines beyond the magnitude of the year-on-year variability during the mature phase of the pIOD event (September–November 2019) and a substantial change in the morphodynamics of the shoreline in the months immediately following the end of the pIOD event (November 2019–April 2020). It was identified that the anomalous metocean conditions during the pIOD event caused a change in the seasonally oscillating shoreline buffer around the island, with potential implications on medium-term island change and overall stability for the vegetated island core. The results showed that it took up to 2 years after the end of the pIOD event for the shorelines to return to their pre-pIOD state. [ABSTRACT FROM AUTHOR]

Existing principles for perched-beaches and rock-truncated shorefaces were used to modify Bruun-type concepts in developing principles for prediction of atoll-island response to sea-level rise. The principles incorporate the effects of a non-erodable reef substrate and how these influence profile movements, with an appropriate modification to the key concept of closure depth. This influence is exerted through sediment-continuity constraints: sediment-displacement volumes are limited due to shoreface truncation by the reef. The elevation of the reef thus defines profile closure, because of which the closure-depth problem involves less uncertainty than on continental siliclastic coasts. The revised principles were incorporated numerically into an existing profile-based computer model: the Shoreface Translation Model (STM). The modified STM was applied to idealized settings for islands with different types of substrates, but ignoring sedimentsupply effects and other factors such as changes in the wave climate. Simulation results confirm the extreme sensitivity of shoreline recession on islands due to barrier-rollover effects in the absence of sediment supply. Previous ideas that islands located on raised conglomerate platforms have enhanced stability prove unfounded with respect to sea-level rise, with such islands responding in simulations by the same amount as islands located directly on the reef flat. However, simulations of islands located on prograded sediment terraces were more stable under conditions of sea-level rise: the sediments from the underlying terrace are reworked into the retreating island, thus providing an additional sediment source. Overall, the critical importance of carbonate-sediment production and supply can be inferred from the ultrasensitivity of islands to sea-level rise evident in simulations. Because of this, secondary effects of sea-level rise are likely to be of first-order importance to island response. These secondary effects involve changes in littoral sediment-transport patterns caused in turn by changes to wave transformation due to increased water depths over reefs. These effects may be compounded by directional shifts in regional wave regime: like sea-level rise, a potential by-product of global climate change. Profile-response models, such as the STM, incorporate these effects by including sediment gains and losses as boundary conditions in the mass balance solution. [ABSTRACT FROM AUTHOR]

The morphological response of atoll islands to sea-level rise is of paramount importance to low-lying atoll nations, which have limited land area. These islands are situated on horizontal non-erodable reef platforms backed by deep lagoons. To date, there have been few attempts to model atoll island response to changes in sea level. This paper uses the modified shoreline translation model (STM) for non-erodable and horizontal substrate surfaces to examine the likely mode of island response to changing sea level using morphological surveys from two islands on Tarawa atoll, Kiribati. Model simulations using a range of sea-level rise scenarios show that atoll islands can undergo a range of morphological responses from the standard Bruun response to Barrier rollover. The mode and relative magnitude of response was shown to be sensitive to island morphology. Low elevation and narrow islands were shown to be most sensitive to changes in sea level. These islands undergo barrier rollover and migrate across the reef flat toward the lagoon and become increasingly unstable as sea-level rise increases. That is, since islands are low in elevation and have lagoons behind them, beach sediments will be displaced to the top of islands or through inlets into lagoons. Higher elevation islands are shown to be more stable and undergo a combination of Bruun and barrier recession responses as sea level increases. Simulations indicate that washover processes allow the island surface to keep pace with sea level rise. Under total washover barrier rollover also implies no change in island width although islands become very unstable rapidly migrating across the reef flat. Under restricted washover conditions (wide islands) simulations indicate upgrowth of the island margin and island narrowing as the barrier recedes. All simulations indicate recession of the island shoreline away from the reef edge. Relative differences in magnitude of change indicate that while low narrow islands experience significant recession, higher islands are relatively stable and are able to cope with sea level rise. Significantly results dispel a number of misconceptions regarding possible atoll island response to sea level showing that: atoll islands will respond in a variety of ways depending on the physical characteristics of islands; conglomerate platform does not necessarily provide stability for islands; and that total loss of atoll islands is unlikely as islands will be able to morphologically adjust to changes in sea level ensuring their future persistence. [ABSTRACT FROM AUTHOR]

Lagoon infill from autochthonous sediment, derived from reef flats, is the dominant constructional processes in coral reef environments once reefs attain a stable elevation with respect to sea-level. Tide- and wave-induced currents were measured and the hydraulic response ofbioclastic sediments examined to assess physical controls on development of lagoon sand bodies and infill of the Cocos (Keeling) Islands atoll, Indian Ocean. Results show that the sediment transport system is active during mean energy conditions. Peak wave-induced currents (140 cms-1 east, 97 cms-1 South) entrain 100% and 97% of reef flat sediment respectively. Potential mobility values decrease through shallow passages (100%, 73% east and south) to sand aprons/flats (73%, 11%, east and south) and shallow lagoon (11% PM) reflecting the lagoonward decline in wave-induced current energy. Unidirectional ocean-side reef flat to lagoon currents transport sediment through passages to sand bodies. This study identifies a hydrodynamicblock, generated by opposing passage and lagoon currents at higher tidal stages, preventing the transport of sediment to the shallow lagoon. This is of great significance as it constrains the active sediment transport system between the reef flat and sand bodies and controlsthe areal expansion of sand bodies. Measured higher rates of sediment transport on the east and deposition in a spatially limited area are responsible for vertical accretion of sand aprons and the development of the flood delta morphology. Lower rates of transport and deposition in a greater spatial area are responsible for the subtidal nature and low relief of southern sand flats. Physical constraints on sandbody development and lateral extent have several important implications for infilling the Cocos lagoon: 1) sand aprons have reached theirlateral progradation limit; 2) lagoon infill. will become more dependent on allochthonous supplies of sediment than autochthonous delivery of [ABSTRACT FROM AUTHOR]