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Harris, D.L. and Vila-Concejo, A., 2013. Wave transformation on a coral reef rubble platform Wave transformation across coral reef platforms is the primary process affecting changes in coral reef geomorphology. Transformation regulates the amount of wave energy entering reef systems, however there have been relatively few hydrodynamic assessments conducted on coral reefs when compared to siliciclastic environments with the effects of common geomorphic features like rubble platforms on wave transformation never specifically examined. This study focuses on the changes in wave characteristics across a rubble platform in a high energy environment (One Tree Reef, southern Great Barrier Reef). Wave conditions were measured at five locations over two days along a cross-reef transect from the reef rim to lagoon. Most of the wave energy was dissipated during wave breaking with energy attenuation due to bottom friction a secondary process. Wave energy attenuation was between 60–99% of the offshore wave conditions only during high tide would wave propagation across the reef platform be capable of affecting reef geomorphology. The wave spectrum also changed with the shorter period gravity wave energy (3 – 20 s) almost completely expending during transformation while longer period infragravity waves (20 – 300 s) were capable of propagating across the reef platform. Wave heights were depth limited and primarily controlled by water depth which suggests that water depth over the reef platform and subsequently elevation of the reef platform above mean sea level govern the amount of wave energy transferred across into reef systems, with most of the gravity wave energy removed during propagation over coral rubble platforms.

Wave transformation across coral reef platforms is the primary process affecting changes in coral reef geomorphology. Transformation regulates the amount of wave energy entering reef systems, however there have been relatively few hydrodynamic assessments conducted on coral reefs when compared to siliciclastic environments with the effects of common geomorphic features like rubble platforms on wave transformation never specifically examined. This study focuses on the changes in wave characteristics across a rubble platform in a high energy environment (One Tree Reef, southern Great Barrier Reef). Wave conditions were measured at five locations over two days along a cross-reef transect from the reef rim to lagoon. Wave heights were depth limited and primarily controlled by water depth, which suggests that water depth over the reef platform and subsequently elevation of the reef platform above mean sea level govern the amount of wave energy transferred across into reef systems, with most of the gravity wave energy removed during propagation over coral rubble platforms.

This article investigates sand apron progradation on decadal and daily scales on a platform reef (One Tree Reef, OTR) located in the southern Great Barrier Reef. The decadal scale is addressed by analysing sand apron progradation using remotely sensed images (aerial photos and satellite imagery) coupled with wind data and cyclone events. The daily scale is addressed through a field campaign that was undertaken in September 2011-October 2011. The campaign consisted of hydrodynamic measurements in three stations over the southern sand apron in OTR. It was found that, while there was a small overall progradation over the last 31 years, the progradation had not occurred continuously or consistently along the entire sand apron. Additionally, the effect of cyclones was not clear on the decadal scale. On the daily scale, it was found that currents are generally weak and that currents during conditions at which suspended sediment is maximized are ocean-ward directed on the central part of the sand apron and lagoon-ward directed on the easternmost end. As such, daily sediment transport does not represent a gross contribution to lagoon infilling by sand apron progradation. The results of this study show that sand apron progradation does not occur continuously on the decadal or the daily scale.

Vila-Concejo, A. Harris, D.L., Shannon, A.M., Webster, J.M., and, Power, H.E., 2013. Coral reef sediment dynamics: evidence of sand-apron evolution on a daily and decadal scale This paper investigates sand apron progradation on decadal and daily scales on a platform reef (One Tree Reef, OTR) located in the southern Great Barrier Reef. The decadal scale is addressed by analysing sand apron progradation using remotely sensed images (aerial photos and satellite imagery) coupled with wind data and cyclone events. The daily scale is addressed through a field campaign that was undertaken in September-October 2011. The campaign consisted of hydrodynamic measurements in three stations over the southern sand apron in OTR. It was found that while there was a small overall progradation over the last 31 years, the progradation had not occurred continuously or consistently along the entire sand apron. Additionally, the effect of cyclones was not clear on the decadal scale. On the daily scale, it was found that currents are generally weak (<0.4 m/s) and that currents during conditions at which suspended sediment is maximized are ocean-ward directed on the central part of the sand apron and lagoon-ward directed on the easternmost end. As such, daily sediment transport does not represent a gross contribution to lagoon infilling by sand apron progradation. Our results show that sand apron progradation does not occur continuously on the decadal or the daily scale.

Sand aprons are ubiquitous formations in coral reef systems that are important components of reef evolution and ecological functioning. Despite this little is known of the morphodynamic processes that drive their evolution. One Tree reef is a mature lagoonal patch reef located in the southern Great Barrier Reef (GBR) that includes a well developed southern sand apron. Current knowledge of sand apron evolution suggests that they are active progradational features which are predominately driven by wave generated swash. However, there have been few process based assessments that actively measure the evolution of these formations. This study investigates the sediment transport processes on the One Tree reef southern sand apron through direct hydrodynamic measurement, morphological surveying and analysis of sediment textural parameters. Results indicate that sediment transport pathways and current flow direction are associated with morphological formations. Channel depressions in the sand apron are ebb dominated with sediment transport pathways and residual currents moving in the direction of these ebb channels. Shallower sections of the sand apron are flood/swash dominated with sediment transport in the direction of the lagoon. These results show that sediment transport on the sand apron is complex and can occur under average hydrodynamic conditions. They also indicate that there are similarities between the processes acting in coral reef settings and typical siliciclastic systems such as tidal inlets and estuaries.

The persistence and habitability of coral reef islands in future extreme oceanographic conditions has received increasing attention in the recent decade, concerning that the sea level rise (SLR) and more frequent and intense storms in the context of global climate change are expected to destabilize those islands. Here, we conduct a set of wave-flume laboratory experiments focusing on the morphodynamic change of reef islands to varying ocean forcing conditions (wave height and SLR). Subsequently, a phase-resolving XBeach numerical model is adopted to simulate the monochromatic wave process and its associated sediment dynamics. The adopted model is also firstly validated by laboratory experimental results as reported in this study. It is then used to examine the impacts of island morphological factors (island width, island height, island location and island side slope) on the island migration. The combined laboratory/physical and numerical experiment outputs suggest that reef islands can accrete vertically in response to the sea level rise and the increased storminess.

Mahabot, M.-M.; Pennober, G.; Suanez, S.; Troadec, R., and Delacourt, C., 2017. Effect of tropical cyclones on short-term evolution of carbonate sandy beaches on Reunion Island, Indian Ocean. Carbonate sandy beaches in Reunion Island show various forms of evidence of erosion. Extreme waves associated with tropical cyclones (TCs) play a major role in beach dynamics. The present study analyzes and quantifies back-reef beach response and recovery from forcing generated by TCs Dumile, Felleng, and Bejisa, which occurred in 2013 and 2014. The study focuses on carbonate beaches of Reunion Island from Cap Champagne to the Passe de Trois-Bassins. Morphological and volumetric changes on beaches were analyzed by comparing 19 beach profiles. The results show that TCs are able to cause significant morphosedimentary change on the back-reef beaches of Reunion Island. These changes affect beach topography and involve longshore and cross-shore sediment transport. An alongshore variation in beach response is observed, which varies according to tropical storm intensity and coastal morphology. The intensity of impact seems to be related to reef width. The most severe erosion occurred at Boucan Canot, where reef is absent with a loss of −24 ± 2 m3 after TC Dumile, −38.7 ± 1.2 m3 after Felleng, and −42.5 ± 1.6 m3 after Bejisa. Elsewhere, the volumetric changes is less than 5 m3 under TC Dumile and vary between 2 and 11 m3 under TC Felleng and between 2 and 23 m3 under TC Bejisa. No significant impact occurred at La Saline where the reef flat is large and provides good protection for the beach; however, relative coastline orientation and prestorm beach-profile morphology also play an essential role in storm impact. Wave height and water level are also determinant factors of storm erosion potential. After storms, the beaches show a relative capacity for recovering because of calm conditions; however, different behaviours are observed along the same beach compartment. This suggests local influence of coastal structure and/or reef geomorphology in sediment transport processes.

Australian reef flats on the Cocos (Keeling) Islands atoll, Indian Ocean; Warraber Reef, Torres Strait; and Lady Elliot Island, Great Barrier Reef vary greatly in morphology (width, elevation) and hydrodynamic setting (wave and tidal regime). This study describes results from detailed wave and current measurements, under nonstorm conditions, along five reef flat transects on these reef systems and examines implications for surface geomorphic processes. Results show that wave frequency and transformation varies between reefs in a consistent manner dependent on tidal elevation, reef elevation, and reef width. A nondimensional reef energy window index (Ψ) is developed that incorporates these critical factors (water depth at spring high tide and reef width). A statistically significant relation (95% confidence interval) between Ψ and the proportion of time that wave energy propagates across reefs illustrates the index ability to characterise the wave process regime of reef flats and provide a physically meaningful descriptor of the efficacy of geomorphic processes on reefs. High values of Ψ indicate narrow and low-elevation reef flats, which are exposed to high wave energy and are geomorphically active. Low values reflect wide and high-elevation reef flats associated with less active wave and geomorphic processes. Results show that while incident energy is undoubtedly an important factor for reef geomorphology, the nature of wave modification across reef flats is equally important in governing levels of geomorphic activity that control development of surface geomorphic features on reef platforms.

Jeansson, M.; Dolique, F.; Anthony, E.J., and Aubry, A., 2019. Decadal-scale dynamics and morphological evolution of mangroves and beaches in a reef-lagoon complex, Mayotte Island. In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 195–208. Coconut Creek (Florida), ISSN 0749-0208. Mayotte Island is characterized by a vast coral reef-lagoon complex comprising significant mangrove development and numerous pocket beaches nested between volcanic headlands. Since 2005, field experiments involving topographic surveys, observations and hydrodynamic measurements have been coupled with the analysis of aerial photographs (1950-2016) in order to improve understanding of the morphodynamic interactions between mangroves, beaches and the coral reefs. The results, integrated in a coastal observatory of Mayotte Island, highlight a remarkably variable mangrove system subject to advance or stability in the north and east of the island but exhibiting a clearly regressive pattern along the southern and western shores. This variability largely reflects the impact of humans on mangrove stability. The hydrodynamic data acquired during the field experiments clearly bring out the spatial and temporal variations in wave patterns involved in these differences. These data also shed light on the short-term morphodynamics of small pocket beaches associated with these mangroves. Patterns of beach morphological change driven by residual wave energy following reef attenuation are strongly affected by the importance of beach embayment. These patterns affect, in turn, mangrove resilience, which is weak on the more exposed south and west coasts of the island, where all the mangrove stands fronting the lagoon have retreated, compared to the more stable stands on the north coast. These results highlight the complex time and space-varying morphodynamic interactions involved in a reef shoreline environment, and how these can also reflect the consequences of humans on mangrove stability. The results should, within the framework of the Mayotte coastal observatory, contribute to the management and conservation of the coast.

The shape of the coast and the processes that mold it change together as a complex system. There is constant feedback among the multiple components of the system, and when climate changes, all facets of the system change. Abrupt shifts to different states can also take place when certain tipping points are crossed. The coupling of rapid warming in the Arctic with melting sea ice is one example of positive feedback. Climate changes, particularly rising sea temperatures, are causing an increasing frequency of tropical storms and “compound events” such as storm surges combined with torrential rains. These events are superimposed on progressive rises in relative sea level and are anticipated to push many coastal morphodynamic systems to tipping points beyond which return to preexisting conditions is unlikely. Complex systems modeling results and long-term sets of observations from diverse cases help to anticipate future coastal threats. Innovative engineering solutions are needed to adapt to changes in coastal landscapes and environmental risks. New understandings of cascading climate-change-related physical, ecological, socioeconomic effects, and multi-faceted morphodynamic systems are continually contributing to the imperative search for resilience. Recent contributions, summarized here, are based on theory, observations, numerically modeled results, regional case studies, and global projections.