Rubble islands are dynamic sedimentary features present on reef platforms that evolve under a var... more Rubble islands are dynamic sedimentary features present on reef platforms that evolve under a variety of morphodynamic processes and controlling mechanisms. They provide valuable inhabitable land for small island nations, critical habitat for numerous species, and are threatened by climate change. Aiming to investigate the controlling mechanisms dictating the evolution of One Tree Island (OTI), a rubble island in the Southern Great Barrier Reef, we combined different remotely-sensed data across varying timescales with wave data extracted from satellite altimetry and cyclone activity. Our findings show that (1) OTI had expanded by 7% between 1978 and 2019, (2) significant gross planform decadal adjustments were governed by the amount, intensity, proximity, and relative position of cyclones as well as El Niño Southern Oscillation (ENSO) phases, and (3) the mechanisms of island growth involve rubble spits delivering and redistributing rubble to the island through alongshore sediment tr...
Understanding the effects of climatic variability on sediment dynamics is hindered by limited abi... more Understanding the effects of climatic variability on sediment dynamics is hindered by limited ability of current models to simulate long-term evolution of sediment transfer from source to sink and associated morphological changes. We present a new approach based on a reduced-complexity model which computes over geological time: sediment transport from landmasses to coasts, reworking of marine sediments by longshore currents, and development of coral reef systems. Our framework links together the main sedimentary processes driving mixed siliciclastic-carbonate system dynamics. It offers a methodology for objective and quantitative sediment fate estimations over regional and millennial time-scales. A simulation of the Holocene evolution of the Great Barrier Reef shows: (1) how high sediment loads from catchments erosion prevented coral growth during the early transgression phase and favoured sediment gravity-flows in the deepest parts of the northern region basin floor (prior to 8 ka ...
The response of platform reefs to sea-level stabilization over the past 6 ka is well established ... more The response of platform reefs to sea-level stabilization over the past 6 ka is well established for the Great Barrier Reef (GBR), with reefs typically accreting laterally from windward to leeward. However, these observations are based on few cores spread across reef zones and may not accurately reflect a reef's true accretional response to the Holocene stillstand. We present a new record of reef accretion based on 49 U/Th ages from Heron and One Tree reefs in conjunction with re-analyzed data from 14 reefs across the GBR. We demonstrate that hydrodynamic energy is the main driver of accretional direction; exposed reefs accreted primarily lagoon-ward while protected reefs accreted seawards, contrary to the traditional growth model in the GBR. Lateral accretion rates varied from 86.3 m/ka–42.4 m/ka on the exposed One Tree windward reef and 68.35 m/ka–15.7 m/ka on the protected leeward Heron reef, suggesting that wind/wave energy is not a dominant control on lateral accretion rate...
ABSTRACT Spur and groove features occur on the seaward reef slope of coral reefs around the world... more ABSTRACT Spur and groove features occur on the seaward reef slope of coral reefs around the world. They are believed to act as important natural breakwaters, regulating the hydrodynamic energy and nutrients received by reef platforms. They also represent one of the most diverse and productive zones of modern reefs. However, the formation processes and morphodynamics of spur and groove systems are poorly understood, particularly in the Great Barrier Reef (GBR). This paper constitutes the first broad scale analysis of spur and groove systems in the Capricorn Bunker Group (CBG) in the southern GBR. It uses remotely sensed imagery coupled with ground truthed data to measure groove length at four reefs (Wreck, Heron, One Tree and Lady Elliot). A total of 2621 grooves were digitised across the four study reefs. Groove length was found to vary both between and within the study reefs. The maximum groove length was 536 m. Average groove length ranged from 93 m at Wreck Reef to 32 m at Heron Reef. This data was compared to relative wave exposure estimates derived from the fetch scenario at each reef. Strong positive correlation was found with groove length increasing as wave exposure increased. Groove length was highly spatially dependant and varied around the reef platforms according to the degree of wave exposure. The longest grooves were found on the most exposed, eastern sides of all reefs. These results provide valuable insight into spur and groove function, formation and likely response to future environmental changes in the CBG and further afield.
ABSTRACT Waves are the main hydrodynamic force acting on coral reefs and are crucial in driving t... more ABSTRACT Waves are the main hydrodynamic force acting on coral reefs and are crucial in driving the evolution of reef systems. Previous research has mainly focused on wave breaking and transformation on the reef flat but often neglected the spatial variation of reformed wave characteristics once they have propagated into the back-reef environment. This study examines wave conditions on a reef flat and the adjacent back-reef sand apron specifically focusing on the transformation of wave height, period and spectra (including changes to long-period and incident wave height). The ability of reformed waves in the back-reef environment to entrain sediment is also investigated. Wave conditions were found to be distinctly different on the sand apron compared to the reef flat, with the majority of wave energy dissipated during initial breaking and transformation. Almost all incident waves are dissipated on the reef flat under a depth threshold of 0.5 m before reaching the back-reef with long-period motions dominating in the back-reef during these tidal stages. At higher tidal stages incident waves are capable of propagating into the back reef but they are very low energy with under 1% of all waves capable of entraining sediment. This suggests that higher energy events, such as high frequency storms, are required to significantly transport sediment and change reef geomorphology. Smaller scale spatial changes in wave height were observed on the sand apron that show the influence of both cross- and along-reef attenuation processes. A distance parameter (Xpd) is introduced that combines the cross-reef distance from the reef crest (Xd) and the temporally specific along-reef distance from the first point of wave breaking on the reef rim (Xp, where Xpd = Xd + Xp). Xpd is shown to accurately describe the changes in wave height and sediment entrainment if deep water significant wave height, wave direction, and depth over the reef flat are known. The results in this study shows that wave conditions, sediment entrainment, and longer term trends in sediment characteristics can be predicted in back-reef environments from a few simple geomorphic inputs.
ABSTRACT This paper investigates sand apron progradation on decadal and daily scales on a platfor... more ABSTRACT 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.
Rubble islands are dynamic sedimentary features present on reef platforms that evolve under a var... more Rubble islands are dynamic sedimentary features present on reef platforms that evolve under a variety of morphodynamic processes and controlling mechanisms. They provide valuable inhabitable land for small island nations, critical habitat for numerous species, and are threatened by climate change. Aiming to investigate the controlling mechanisms dictating the evolution of One Tree Island (OTI), a rubble island in the Southern Great Barrier Reef, we combined different remotely-sensed data across varying timescales with wave data extracted from satellite altimetry and cyclone activity. Our findings show that (1) OTI had expanded by 7% between 1978 and 2019, (2) significant gross planform decadal adjustments were governed by the amount, intensity, proximity, and relative position of cyclones as well as El Niño Southern Oscillation (ENSO) phases, and (3) the mechanisms of island growth involve rubble spits delivering and redistributing rubble to the island through alongshore sediment tr...
Understanding the effects of climatic variability on sediment dynamics is hindered by limited abi... more Understanding the effects of climatic variability on sediment dynamics is hindered by limited ability of current models to simulate long-term evolution of sediment transfer from source to sink and associated morphological changes. We present a new approach based on a reduced-complexity model which computes over geological time: sediment transport from landmasses to coasts, reworking of marine sediments by longshore currents, and development of coral reef systems. Our framework links together the main sedimentary processes driving mixed siliciclastic-carbonate system dynamics. It offers a methodology for objective and quantitative sediment fate estimations over regional and millennial time-scales. A simulation of the Holocene evolution of the Great Barrier Reef shows: (1) how high sediment loads from catchments erosion prevented coral growth during the early transgression phase and favoured sediment gravity-flows in the deepest parts of the northern region basin floor (prior to 8 ka ...
The response of platform reefs to sea-level stabilization over the past 6 ka is well established ... more The response of platform reefs to sea-level stabilization over the past 6 ka is well established for the Great Barrier Reef (GBR), with reefs typically accreting laterally from windward to leeward. However, these observations are based on few cores spread across reef zones and may not accurately reflect a reef's true accretional response to the Holocene stillstand. We present a new record of reef accretion based on 49 U/Th ages from Heron and One Tree reefs in conjunction with re-analyzed data from 14 reefs across the GBR. We demonstrate that hydrodynamic energy is the main driver of accretional direction; exposed reefs accreted primarily lagoon-ward while protected reefs accreted seawards, contrary to the traditional growth model in the GBR. Lateral accretion rates varied from 86.3 m/ka–42.4 m/ka on the exposed One Tree windward reef and 68.35 m/ka–15.7 m/ka on the protected leeward Heron reef, suggesting that wind/wave energy is not a dominant control on lateral accretion rate...
ABSTRACT Spur and groove features occur on the seaward reef slope of coral reefs around the world... more ABSTRACT Spur and groove features occur on the seaward reef slope of coral reefs around the world. They are believed to act as important natural breakwaters, regulating the hydrodynamic energy and nutrients received by reef platforms. They also represent one of the most diverse and productive zones of modern reefs. However, the formation processes and morphodynamics of spur and groove systems are poorly understood, particularly in the Great Barrier Reef (GBR). This paper constitutes the first broad scale analysis of spur and groove systems in the Capricorn Bunker Group (CBG) in the southern GBR. It uses remotely sensed imagery coupled with ground truthed data to measure groove length at four reefs (Wreck, Heron, One Tree and Lady Elliot). A total of 2621 grooves were digitised across the four study reefs. Groove length was found to vary both between and within the study reefs. The maximum groove length was 536 m. Average groove length ranged from 93 m at Wreck Reef to 32 m at Heron Reef. This data was compared to relative wave exposure estimates derived from the fetch scenario at each reef. Strong positive correlation was found with groove length increasing as wave exposure increased. Groove length was highly spatially dependant and varied around the reef platforms according to the degree of wave exposure. The longest grooves were found on the most exposed, eastern sides of all reefs. These results provide valuable insight into spur and groove function, formation and likely response to future environmental changes in the CBG and further afield.
ABSTRACT Waves are the main hydrodynamic force acting on coral reefs and are crucial in driving t... more ABSTRACT Waves are the main hydrodynamic force acting on coral reefs and are crucial in driving the evolution of reef systems. Previous research has mainly focused on wave breaking and transformation on the reef flat but often neglected the spatial variation of reformed wave characteristics once they have propagated into the back-reef environment. This study examines wave conditions on a reef flat and the adjacent back-reef sand apron specifically focusing on the transformation of wave height, period and spectra (including changes to long-period and incident wave height). The ability of reformed waves in the back-reef environment to entrain sediment is also investigated. Wave conditions were found to be distinctly different on the sand apron compared to the reef flat, with the majority of wave energy dissipated during initial breaking and transformation. Almost all incident waves are dissipated on the reef flat under a depth threshold of 0.5 m before reaching the back-reef with long-period motions dominating in the back-reef during these tidal stages. At higher tidal stages incident waves are capable of propagating into the back reef but they are very low energy with under 1% of all waves capable of entraining sediment. This suggests that higher energy events, such as high frequency storms, are required to significantly transport sediment and change reef geomorphology. Smaller scale spatial changes in wave height were observed on the sand apron that show the influence of both cross- and along-reef attenuation processes. A distance parameter (Xpd) is introduced that combines the cross-reef distance from the reef crest (Xd) and the temporally specific along-reef distance from the first point of wave breaking on the reef rim (Xp, where Xpd = Xd + Xp). Xpd is shown to accurately describe the changes in wave height and sediment entrainment if deep water significant wave height, wave direction, and depth over the reef flat are known. The results in this study shows that wave conditions, sediment entrainment, and longer term trends in sediment characteristics can be predicted in back-reef environments from a few simple geomorphic inputs.
ABSTRACT This paper investigates sand apron progradation on decadal and daily scales on a platfor... more ABSTRACT 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.
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Papers by Ana Vila-Concejo