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An investigation into the evolution of coral rubble deposits on a coral reef platform is assessed using high-resolution remote sensing data and geospatial analysis. Digital change detection analysis techniques are applied to One Tree Reef in the southern Great Barrier Reef by analysing aerial photographs and satellite images captured between 1964 and 2009. Two main types of rubble deposits were identified: (1) rubble flats that are featureless mass accumulations of coral rubble; and, (2) rubble spits that are shore-normal linear features. While both deposits prograde in a lagoon-ward direction, rubble spits move faster (∼2 m/yr) than rubble flats (∼0.5 m/yr). The volume of rubble, the underlying substrate, the energy regime, and storm frequency control the rate of progradation. Rubble flat occurrence is restricted to the high-energy (windward) margin of the coral reef platform, while rubble spits are distributed reef wide, both in modal high energy and modal low energy regions of the reef. Rubble spit deposition is considered to be a result of enlarged spur and groove morphology of the forereef, whereby wave energy is focused through the enlarged groove formations causing the preferential deposition of coral rubble in particular zones of the adjacent reef flat. One last control is thought to be the elevation of the reef crest whereby lower areas are more prone to rubble flat development. A vertical and ocean-ward accumulation of rubble is occurring on the windward margin of the reef leading to a build-up and build-out of the reef, governing the expansion of the reef footprint. This study shows for the first time the evolution of a coral reef rubble flat and rubble spits over decadal time scales as detected through remotely sensed images spanning 45 years. [ABSTRACT FROM AUTHOR]

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The objective of this work is to determine the relative importance of different processes (swash, overwash, aeolian, and lagoon) affecting sediment dynamics in barriers with frequent overwash. Aerial photographs, topographic surveys (with real-time kinematic global positional system), and sample collection were undertaken on Barreta Island (Ria Formosa barrier islands, Portugal). Wave and tidal data were also analysed during a 3-year monitoring period (2001-2004). This 3-year period represents the typical wave conditions of this region. Historical barrier evolution was characterised for the 1947-2001 period. Up until 1996, the supratidal barrier changed from barred sand to fully developed dunes. In 2001, a wide washover plain had developed and an updrift tidal inlet (Ancão Inlet) was migrating toward the study area. Between 2001 and 2004, the study area had three geomorphological sectors. Sector A (westernmost) showed the widest beach berms that developed because of welding of swash bars of Ancão Inlet. Sector B had the widest washover and developed embryonic dunes with well-sorted medium sands. In sector C (easternmost), overwash was more frequent, and induced the deposition of 57 m3/m of sediments (the coarsest of the barrier). Globally, barrier volume variation was mostly due to swash (83%); however, net barrier accretion was 88% induced by overwash. Barrier volume variation was more related to Ancão Inlet evolution than to the seasonality of wave climate. A four-stage conceptual model of barrier evolution under frequent overwash is presented: beach erosion; dune destruction, and formation of washover plain; frequent overwash; and dune development. [ABSTRACT FROM AUTHOR]

Waves interact with headlands on embayed beaches through refraction, diffraction and attenuation and can create alongshore gradients in wave energy and beach response. This energy gradient shifts alongshore with changes in wave direction, especially during storms when waves may come from a direction different to average conditions. To better predict embayed beach storm responses, improved identification of exposed and headland shadowed zones is needed. Here we present a generalised geometric approach to quantify the alongshore distance shadowed by headlands, called the shadow edge (Y sh), differentiating exposed and headland shadowed zones. Our approach uses headland geometry collected from readily available imagery, combined with measured or modelled storm wave direction, reducing the reliance on complex nearshore wave models derived from relatively scarce bathymetric data. We use monthly topographic beach surveys (2015–2019) at nine embayed beaches in SE Australia to investigate the impacts of headland and embayment geometry relative to storm wave direction (i.e., exposure). Further, beach responses were compared to erosion and recovery rates across different embayment geometries with varying levels of geological control on beach morphodynamics. We found that storm frequency, headland shadowing and embayment geometry control the alongshore magnitude of beach erosion and recovery rates. Mean subaerial beach volume losses to six high-energy storms were on average 3.5 times higher in exposed zones (42.7 ± 40.7 m3/m) than headland shadowed zones (12.1 ± 23.6 m3/m). Our geometric approach provides a simple alternative to predict headland influence on beach morphodynamics. The approach can be applied at a regional scale and could be integrated into early warning systems to predict coastal erosion. [Display omitted] • Generalised geometric approach defines headland shadow zones on beaches using open-access data. • Storm erosion is 3.5 times higher in exposed zones than headland shadow zones. • Results could help predict beach response to climate change-induced shifts in storm wave direction. • This approach could easily complement complex nearshore wave models. [ABSTRACT FROM AUTHOR]

Sandy beaches in estuaries and bays (BEBs) are ubiquitous around the world. While some are referred to as "low energy" beaches, there are large differences in wave climate due to fetch, swell exposure and shoreface morphology. In this study, wave-climate controls on BEB morphology are identified relative to bay geometry and focusing on the relative contributions of wind, ocean swell and infragravity waves to the wave signature of a BEB. We focus on two swash-aligned and two drift-aligned BEBs in an urbanized, semi-enclosed bay in SE Australia (Kamay/Botany Bay), with differences in proximity to the bay mouth, beach aspects and fetch, as well as different histories of storm erosion and recovery. Spectral analysis of local wave measurements (June 2018–January 2019) shows that wave signatures at all 4 BEBs are dominated by swell waves (>51%), followed by locally generated wind waves at 3 out of 4 BEBs (<43%), then infragravity waves (<35%). We identified five weather scenarios based on local wind speed/direction and resultant wind waves, combined with infragravity waves, and offshore swell wave conditions. BEBs close to the bay entrance are primarily controlled by swell waves, but at times even BEBs close to the entrance can be sheltered from swell waves depending on their incident angle. Conversely, BEBs farthest from the bay entrance (~8 km) alternated between periods where spectra were dominated by either wind waves or offshore swells. Infragravity waves were only important relative to wind waves and swell at the most sheltered BEBs, where there is limited fetch and swell dissipated over shallow areas. We also show that anthropogenic factors such as constructed revetments or groynes significantly alter wave signatures at BEBs in this bay. Thus, wave spectral signatures are highly variable across BEBs in the same bay and can represent a simple tool to study BEB morphodynamics including storm response and subsequent recovery. • Sandy beaches in estuaries and bays (BEBS) often assumed to be low wave energy. • But wave signatures (wind, swell and infragravity waves) vary greatly. • BEBs had generally >50% swell wave energy with infragravity only significant at sheltered BEBs. • Anthropogenic factors such as engineering and dredging altered wave signatures. • Wave signatures are useful tools to understand BEB morphodynamics [ABSTRACT FROM AUTHOR]

Sandy beaches in estuaries and bays (BEBs) are common landforms on the coasts of many major cities. They exist under a wide range of settings and their morphology is controlled by their distance from the estuary/bay entrance, exposure to different types of waves (e.g., ocean swells vs locally generated wind waves), proximity to flood-tide delta/shoals, and anthropogenic interventions (e.g., dredging, groynes). Both swell waves propagating into estuaries/bays and locally generated wind waves can erode BEBs. However, more understanding of BEB storm erosion and recovery over decadal timescales is needed, as they typically respond slower than open coast beaches. Here we present decadal shoreline behaviours of nine BEBs from two estuarine systems in SE Australia are presented in this study, using 76 years of aerial imagery (1941–2017). We quantify and compare decadal behaviour between beaches, developing a new typology of BEBs based on shoreline evolution. We identify four decadal behaviours: prograding , quasi-stable , retreating and storm relict – and assess the influence of flood-tide deltas, river mouths, distance from the ocean entrance, and anthropogenic interventions. Swell-exposed BEBs near the entrance are quasi-stable and recover after storms at rates comparable with open coast beaches (<3 years). In contrast, BEBs further from the entrance and those with less swell exposure, have slower recovery timescales (3–15 years) and will only be quasi-stable if storms are sufficiently infrequent. Thus, long-term behaviour is controlled by storm return timescales. Prograding BEBs are typically far from the entrance, where fluvial and tidal processes dominate and erosion events due to wind waves are less pronounced. Whether BEBs recover (quasi-stable), partially recover between storms (retreating) or never recover (storm relict) relates to storm frequency, recovery rates and proximity to sediment sources and sinks (e.g., dredge sites, flood-tide deltas, tidal channels) and anthropogenic interventions. Findings will help to better understand and manage BEB shorelines in major cities. [Display omitted] • Decadal beach behaviours are prograding , quasi-stable , retreating or storm relict. • Some estuarine beaches recover in <15 years, while others never recover. • Behaviour dictated by exposure, entrance proximity, rivers and flood-tide delta. • Anthropogenic interventions can benefit or be a detriment to estuarine beaches. • Decadal behaviours will assist coastal management and planning in major cities. [ABSTRACT FROM AUTHOR]

Estuarine beaches are ubiquitous, yet understudied, coastal systems. The mixed hydrodynamic processes – such as combined tidal and wave forcing – and the influence of adjacent sedimentary features – such as flood-tide deltas (FTDs) – leads to complex morphodynamic processes. As such, the dynamics and evolution of these important coastal systems are poorly understood. This study synthesises a set of analyses conducted on an estuarine beach, in southeast Australia, over multiple temporal scales. Nearshore waves and currents were measured and used to determine the processes driving seasonal to yearly beach change between 2007 and 2010. These results were compared to decadal scale beach change determined from aerial photos between 1963 and 2006. We found the westward transport of sediment towards the inner estuary was the dominant nearshore process leading to erosion in the eastern regions of the beach (near the estuary entrance) and accretion in the west over yearly time scales. Cross-shore sediment transport occurred during winter storm swell conditions leading to erosion in more exposed sites and some limited accretion in sheltered zones, most likely due to sediment input from the flood-tide delta. However, severe storm swell events that propagated into the estuary led to a loss of sediment and erosion across the entire beach which was not recovered during the study period. It is likely that the erosion processes observed in the short- to medium-term (days to years) analyses are the conditions that led to the long-term shoreline retreat observed in the aerial photograph record. The long-term shoreline retreat since 1963 is likely a result of a negative sediment balance due to movement of the FTD and a lack of sediment input in the eastern regions of Shoal Bay. Shoreline retreat will likely continue unless increased sediment input occurs from marine sources or shoreline interventions are maintained. • Multi-scaled morphodynamics of an estuarine beach was investigated. • Beach rotation was observed driven by seasonal cross and long-shore processes. • Over 40 years of chronic storm driven erosion underlies seasonal beach dynamics. • Shoreline retreat is likely driven by a lack of sediment input from marine sources. [ABSTRACT FROM AUTHOR]

Robust data are the base of effective gender diversity policy. Evidence shows that gender inequality is still pervasive in science, technology, engineering and mathematics (STEM). Coastal geoscience and engineering (CGE) encompasses professionals working on coastal processes, integrating expertise across physics, geomorphology, engineering, planning and management. The article presents novel results of gender inequality and experiences of gender bias in CGE, and proposes practical steps to address it. It analyses the gender representation in 9 societies, 25 journals, and 10 conferences in CGE and establishes that women represent 30% of the international CGE community, yet there is under-representation in prestige roles such as journal editorial board members (15% women) and conference organisers (18% women). The data show that female underrepresentation is less prominent when the path to prestige roles is clearly outlined and candidates can self-nominate or volunteer instead of the traditional invitation-only pathway. By analysing the views of 314 survey respondents (34% male, 65% female, and 1% "other"), we show that 81% perceive the lack of female role models as a key hurdle for gender equity, and a significantly larger proportion of females (47%) felt held back in their careers due to their gender in comparison with males (9%). The lack of women in prestige roles and senior positions contributes to 81% of survey respondents perceiving the lack of female role models in CGE as a key hurdle for gender equality. While it is clear that having more women as role models is important, this is not enough to effect change. Here seven practical steps towards achieving gender equity in CGE are presented: (1) Advocate for more women in prestige roles; (2) Promote high-achieving females; (3) Create awareness of gender bias; (4) Speak up; (5) Get better support for return to work; (6) Redefine success; and, (7) Encourage more women to enter the discipline at a young age. Some of these steps can be successfully implemented immediately (steps 1–4), while others need institutional engagement and represent major societal overhauls. In any case, these seven practical steps require actions that can start immediately. [ABSTRACT FROM AUTHOR]

A decline in coral reef surface roughness may indicate that a coral reef ecosystem is approaching functional collapse. This is because surface roughness underpins many critical ecosystem metrics, such as, live coral cover and high fish biomass. Yet, we understand very little about how surface roughness changes at ecosystem scales due to the limited spatial extents of coral reef ecological surveys. Here, we investigate the surface roughness of a coral reef across multiple spatial scales on a high-resolution Light Detection and Ranging (LiDAR) derived digital elevation model (DEM, 0.25 m cells). We found that complex features dominated regions with high roughness (e.g., spur and groove systems on the reef slope) and increased the average roughness of geomorphic zones that are otherwise flat and featureless (e.g., lagoonal patch reefs in the sandy deep lagoon). Each geomorphic zone had a unique signature with high values of roughness usually expressed over finer spatial scales and, conversely, lower values of roughness observed over broader spatial scales. We allocated each geomorphic zone into one of four roughness equivalent habitats (REHs 1–4) that share similar surface roughness properties but are not necessarily geographically contiguous regions. These results identify fine scale features and geomorphic zones that are important for providing roughness to coral reef systems, such as spur and groove systems and the fore-reef slope respectively. They also suggest that morphodynamic processes operating at broader spatial scales influence the physical structure of coral reef ecosystems. The quantification of coral reef surface roughness is becoming increasingly important due to the wider availability of high (<1 m) and hyper (<0.1 m) resolution DEMs. Our approach and findings here can be used to provide greater surface textural information for common coral reef geomorphic zones and aid future management and research efforts, such as, monitoring of coral reef ecosystem response to environmental change and reef restoration and adaptation programs. [ABSTRACT FROM AUTHOR]

Carbonate production on coral reefs is responsible for the provision of beach sands, for the maintenance of seawater chemical balances and for the growth of reef structure and associated habitat complexity. Key carbonate producers including hard coral, crustose coralline algae, foraminiferal sand and Halimeda were mapped from satellite imagery (spatial resolution 2.5 m, mean overall accuracy = 81%) and an upscaling model was applied to estimate carbonate production. A sensitivity analysis was conducted to evaluate the influence of employing different calcification rates for live coral on the upscaling model. Contemporary coral reef carbonate production for the 21 reef platforms of the Capricorn-Bunker Group (southern Great Barrier Reef) is estimated to be between 489,000 and 659,000 t per year based on seawater chemistry, community composition, calcification rates and reef structural complexity (rugosity). The upscaling model was relatively insensitive to different parameterisations of live coral calcification employed, probably due to live coral being a relatively minor contributor by area (approximately 18% of total reef area throughout the study region). This suggests regional scale seafloor characteristics, such as percentage of area dominated by substrates prone to dissolution (e.g. coral rubble), have a strong bearing on calcium carbonate production and need to be given greater consideration The upscaling framework presented provides a new method for quantifying regional carbonate production that could be applied globally, and provides a valuable baseline against which future changes to carbonate production in this region can be assessed. [ABSTRACT FROM AUTHOR]

Over 85 % of oyster reefs have been lost globally due to disease, overharvesting, global warming, and pollution. Consideration of the ecosystem services provided by healthy oyster reefs (e.g., coastal protection, water purification and carbon burial) has driven recent research and restoration efforts worldwide. However, hydrodynamic studies, specifically looking at the effects of different levels of wave exposure on the ecomorphodynamics of oyster reefs, are scarce. In this study, we consider oyster reefs in microtidal estuaries under different levels of relative wave exposure to determine how hydrodynamics may shape reef morphology and how reef morphology affects wave dissipation. We quantify oyster reef morphology through spatial analysis, using morphometrics and spatial density and relate these to the ability of oyster reefs to dissipate wave energy. Field campaigns were undertaken at three microtidal sites in southeast Australia with different hydrodynamic exposure and morphology: Gamay (Botany Bay), Port Hacking and Crookhaven River. We found that reef morphology and orientation is related to estuarine hydrodynamic conditions and thus we propose an ecomorphodynamic model with a continuum of morphologies from sparse reefs aligned perpendicular to the tidal currents and incoming waves (patch reefs), through broken up barriers semi-aligned or obliquely to the tidal flows (string reefs), to the total barrier that exists under the lowest hydrodynamic conditions (fringing reefs). The highest dissipative ability of locally generated wind waves occurred at Crookhaven (patch reef, 165 kW/m2), and lowest at Gamay (string reef, 11.66 kW/m2). Our results suggest that reef morphology, and orientation to currents and waves, influence wave dissipation, and that hydrodynamic conditions in turn influence reef morphology. These findings are important to inform future reef restoration under increasingly severe climate change conditions to optimise ecosystem services on restored oyster reefs. • We research ecomorphodynamics of oyster reefs. • We compare three oyster reefs under different levels of wave exposure. • We quantify the morphology of oyster reefs using spatial analysis. • Findings show links between reef morphology, orientation, and wave energy. • Oyster reefs play an important structural role influencing estuarine processes. [ABSTRACT FROM AUTHOR]

Abstract Wave breaking and transformation on coral reef flats is an important process protecting tropical coastlines and regulating the energy regimes of coral reefs. However, the high hydraulic roughness, shallow water, and steep bathymetries of coral reefs may confound common surf zone assumptions, such as a depth-limited and saturated surf zone with a constant wave height to water depth ratio (γ). Here, we examine wave transformation across a coral reef flat, during three separate swell events, on both a time-averaged and a wave-by-wave basis. We use the relationship between significant wave height and water depth (γ s) to examine the change in surf saturation across the reef flat and compare the measured wave height decay to results of modelled wave energy dissipation in the surf zone. Our results show that γ s was not cross-reef constant and varied according to location on the reef flat and local water depth. On average, γ s was greatest at the outer reef flat, near the reef crest, and progressively reduced towards the inner reef flat, near the reef lagoon. This was most pronounced in shallow water with large γ s values (γ s > 0.85) at the outer reef flat and small γ s values (γ s < 0.1) at the inner reef flat. This indicates that there is an increase in wave energy dissipation in shallow water, most likely due to increased breaker and bed frictional dissipation. The measured wave energy dissipation across the entire reef flat could, on average, be modelled accurately; however, this required location specific calibration of the free parameters, the wave friction factor (f w) and γ , and further suggests that there is no value for either parameter that is universally applicable to coral reef flats. Despite model calibration inaccuracies were still observed, primarily at the outer reef flat. These inaccuracies reflected the observed cross-reef variation of γ on the reef flat and potentially the limitations of random wave breaker dissipation models in complex surf zones. Our results have implications for the use of wave energy dissipation models in predicting breaker dissipation and subsequent benthic community change on coral reef flats, and suggest that careful consideration of the free parameters in such models (such as f w and γ) is required. Highlights • Saturated surf zone waves were measured and compared to a common wave energy dissipation model. • Surf zone saturation and depth limited wave conditions varied depending on water depth and location on coral reef flats. • Errors observed in the dissipation model correlated with variation in wave height to water depth ratios in the surf zone. • Some errors were corrected by spatially varying bed frictional dissipation on the reef flat. • Model calibration from surf zone data was required to accurately describe wave height decay on coral reef flats. [ABSTRACT FROM AUTHOR]

Beaches in estuaries and bays (BEBs) are critical landforms along many coastlines, providing vital protection to human infrastructure and ecosystem services. Like open-ocean beaches, their morphodynamics are driven by the interaction of waves and tides with the boundary conditions determined by sediment supply and geology. However, as estuaries and bays tend to be lower energy than the open ocean, and are often fetch limited, BEB behaviour is typically controlled by high magnitude events rather than modal conditions. In this study, we assess the dynamics of a BEB in a semi-enclosed bay (Port Phillip Bay, Victoria, Australia). We analyse shoreline and subaerial cross sectional beach change across a range of time scales, including in response to storm events, using a combination of aerial photography, satellite imagery and UAV-based surveys spanning multiple decades. The study assesses the impact of geological controls on beach dynamics in this estuarine/embayed setting, investigating the impact of an intertidal rocky reef and the role of sediment budgets. The rocky outcrops throughout the compartment appear to exert little long-term control on beach morphology. We find that the location of the beach with respect to the sediment compartment boundaries is fundamental to determining long-term shoreline evolution. St Leonards Beach is located near the boundary between two longshore transport systems flowing in opposite directions. This results in low sediment flux across the compartment and therefore a paucity of sand supply for beach and dune development. As a consequence, the beach is relatively stable on a seasonal scale, but has a net erosional trend over the 75-year aerial photographic record. Erosion appears to be event driven as sand is transported out of the sediment compartment during storms. This work shows the importance of understanding local boundary conditions especially local sediment budgets in estuarine environments when analysing beach dynamics. • Storm events are the primary driver of longshore sediment movement. • The sediment starved beach has limited capacity to recover from storm events. • Littoral transport pathways affect the ability to respond to change. [ABSTRACT FROM AUTHOR]

Measurements of geomorphic change in the intertidal zones of coral reefs are made using a variety of remote sensing and in-situ techniques, where variations in the coverage and spatial-temporal precision achieved are directly related to the cost of data acquisition. We present a novel, low-cost technique for measuring high-resolution changes in reef environments based on Kite Aerial Photography (KAP) and photogrammetry/structure-from-motion post-processing. KAP images are used to measure fine-scale changes in intertidal topography and sediment texture characteristics, including rubble particle size, of a coral shingle cay at One Tree Island, Great Barrier Reef in the context of storm activity. Validation using Real Time Kinematic DGPS demonstrates the ability to measure topographic elevation with an error of 5.53 cm (RMSE) and a spatial resolution of 5 cm per point, an accuracy/resolution that is superior to airborne LiDAR and equivalent to terrestrial LiDAR, but at a fraction of the equipment cost. [ABSTRACT FROM AUTHOR]

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 rates. This represents the most comprehensive statement of lateral accretion direction and rates from the mid-outer platform reefs of the GBR, confirming great variability in reef flat growth both within and between reef margins over the last 6 ka, and highlighting the need for closely-spaced transects. [ABSTRACT FROM AUTHOR]

Widespread flooding occurred along the Amazon coast during 2009, starting in the early rainy season, when a La Niña event provoked an abnormal increase in rainfall levels. The present study analyzes hydrological data from ten 25-h campaigns conducted during spring tides between February, 2008, and September, 2009. This allows for the evaluation of the La Niña induced effects on the hydrological and hydrodynamic parameters in this equatorial zone that were caused primarily by this rainfall anomaly. The local climate is characterized by two seasons (rainy and dry). In 2009, the conditions of the coastal waters changed during the La Niña event because of the high rainfall rates (above 3000 mm). Under typical conditions, local coastal waters are well oxygenated, turbid and rich in nutrients, with relatively stable temperatures (27.5 °C–30.8 °C) and chlorophyll a concentrations of up to 30 mg m−3. During the La Niña event, hydrological and hydrodynamic values were exacerbated by increased rainfall levels, which left the water less saline (minimal of 5), but more turbid (maximum values above 100 NTU), and richer in dissolved nutrients and phytoplankton biomass (approximately 40 mg m−3) than normal. These results possible can be extrapolated to other equatorial macrotidal beaches influenced by major estuarine systems. [ABSTRACT FROM AUTHOR]

Contemporary understanding of Holocene coral reef development is based primarily on sub-surface investigations of reef flat, back reef and lagoon zones. Few studies of Holocene fore reef development exist, constituting a significant gap in our understanding of reef evolution. The spur and groove (SaG) zone is a distinct, understudied, feature of fore reefs worldwide. We review SaG development from previous studies and present 29 new SaG short cores with 52 14C and U–Th ages from six fore reef regions of Heron and One Tree reefs, the first such data from the Great Barrier Reef. Remarkably, we found that SaGs do not necessarily accrete in the same direction as their adjacent reef flat. We identified three modes of reef flat and SaG lateral accretion: Mode 1 – lagoonward accretion of both the reef flat and SaGs; Mode 2 –lagoonward accretion of the reef flat but seaward accretion of the SaGs; Mode 3 - seaward accretion of both the reef flat and SaGs. Most SaG zones (five of the six studied) accreted in a seaward direction (Modes 2 or 3). Hydrodynamic conditions and local topography appear to be the dominant factors determining which mode occurs. Episodic high-energy events are also likely to play an important role in SaG formation. Our findings suggest that traditionally held models of reef evolution whereby lagoonal, mature reefs fill, developing into senile platform reefs, may not hold. Rather, reef flats may continue to expand seaward on their leeward, and semi-exposed fronts to increase in size while maintaining their lagoons. • Cores show 3 modes of spur and groove development in southern Great Barrier Reef. • Hydrodynamic conditions and local topography likely determine which mode occurs. • Episodic high-energy events also play an important role in formation. • Most spur and groove zones cored were accreting seaward. • Reef flats may continue to expand seaward while maintaining their lagoons. [ABSTRACT FROM AUTHOR]

Global environmental change is identified as a driver of physical transformation of coral reef islands over the past half-century, and next 100 years, posing major adaptation challenges to island nations. Here we resolve whether these recent documented changes in islands are unprecedented compared with the pre-industrial era. We utilise radiometric dating, geological, and remote sensing techniques to document the dynamics of a Maldivian reef island at millennial to decadal timescales. Results show the magnitude of island change over the past half-century (±40 m movement) is not unprecedented compared with paleo-dynamic evidence that reveals large-scale changes in island dimension, shape, beach levels, as well as positional changes of ±200 m since island formation ~1,500 years ago. Results highlight the value of a multi-temporal methodological approach to gain a deeper understanding of the dynamic trajectories of reef islands, to support development of adaptation strategies at timeframes relevant to human security., (© 2023. The Author(s).)

Sediment transport calculations are used globally in the numerical models that coastal managers, scientists and engineers use to assess and forecast coastal change. Most of the existing sediment transport equations were defined based on experimental results using siliciclastic sands. Yet these equations are applied to all types of sand, including carbonate sands that have different characteristics and therefore, settling behaviour. A rigorous management of the transport of carbonate sand is essential for the present and future management of sedimentary features in coral reefs such as sandy beaches or reef islands. Here we present a new approach to estimating the drag coefficient of carbonate sands that considers both friction and pressure. Based on our new method, the calculated drag coefficients explain the great variability in settling velocities of carbonate sand observed in nature (from 0.025 m/s to 0.364 m/s in our database). Using our formula, we demonstrate that even small differences in the settling velocity obtained with the new drag coefficient can lead to substantial changes in sediment transport and call for an update of numerical models.