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Sand aprons are distinctive landforms that offer important insights into sedimentary dynamics for reef platform development. Here we link temporal and spatial scales of 21 sand aprons in the southern Great Barrier Reef to understand their Holocene formation and evolution in response to relative sea-level changes, the depth of the Pleistocene base, and contemporary morphodynamics. Our results show that lagoon infilling is a function of reef size and is a self-limiting process controlled by hydrodynamics and relative sea-level changes. Lagoon infilling does not depend on the type of reef or degree of exposure to waves, but it could reflect past wave climates. Our carbonate productivity estimates based on lagoon infilling are remarkably similar to those inferred from habitat classification. Finally, we hypothesize that current carbonate productivity has slowed because of the effects of climate change.

Coral reef islands are under threat from warming and rising seas, ocean acidification and increased storminess. These islands are low-lying accumulations of sediment derived from a continual supply of shells and skeletons from calcifying reef organisms. Over 200 million people rely on reefs and their islands for their livelihoods, including Small Island Developing States (SIDS). Coastal States and SIDS commonly use coral islands to support and extend their maritime jurisdictions. Coral islands are morphologically active (e.g., erode, accrete, migrate) across broad spatial-temporal scales and have been extensively studied using historical aerial imagery, satellite imagery and satellite derived bathymetry (SDB). The future of coral islands and the reefs that support them is not certain, and it is unclear what eco-morphological tipping points may cause constructive and destructive impacts. A better understanding of coral island stability and evolution to changing conditions is urgently needed. Here we focus on 31 coral islands on 10 offshore coral reefs that extend Australia’s maritime jurisdictions in the Coral Sea (SW Pacific) and NW continental shelf (Timor Sea). We digitised island morphology (e.g., shoreline positions, island area and shape) from imagery and SDB (1976-2022) and compared this to local ocean and climate data (e.g., cyclone tracks, sea surface temperatures (SST), sea-level rise) to identify potential tipping points and processes. Initial results show that since the 1970s a third of the coral islands were stable (n=9

Introduction Sand aprons are ubiquitous depositional sedimentary features that offer insights into the sediment dynamics of coral reef environments. Global studies found that the extent of sand aprons are not related to reef platform size and their widths are a function of environmental factors such as swell period and height, tidal amplitude, latitude, and exposure to wind and waves (Rankey and Garza-Perez 2012). Recent studies using numerical modelling have found that sand aprons in reef flats attain a critical water depth resulting in constant depth (Ortiz and Ashton 2019), and that lagoon infilling through sand apron progradation is a self-limiting process (Rankey 2021). Sand apron progradation is an eco-morphodynamic process and climate change, including intensification and increased frequency of marine heatwaves, ocean and coastal acidification, and changes in wave and tropical storm climates are triggering changes that need to be understood to inform sound management of coral reefs.This paper presents data on the Holocene evolution of the sand aprons on 21 offshore platform reefs located on the southern Great Barrier Reef and how it can be used to infer past wave climates. We then present the recent wave climate for the study area (Smith et al. 2022) and analyse sand apron evolution accordingly. Methods The sand aprons on 21 reefs located on the Capricorn Bunker Group (Southern Great Barrier Reef) were assessed from high-resolution satellite imagery, obtaining digital bathymetric models and digitizing the reef and lagoon contours. We then measured reef area and lagoon area to calculate the percentage of lagoon infilling. The wave climate for the study area was obtained from satellite altimetry using an open-source Python tool (Smith et al. 2020). Preliminary findings Our results showed that the most important factor for lagoon infilling was the size of the reef, with larger reefs typically appearing less infilled than smaller reefs. Wave incidence seemed to be unimportant: the three reefs with less than 50% infill were all medium-sized and exposed to incident waves while all six protected reefs had infilling above 50%. While some authors had pointed out at relative sea-level changes to explain current sand apron stability (Harris et al. 2015), our results show that the self-limiting nature of the sand apron progradation, combined with relative sea-level changes, is a better explanation for sand apron stability. In any case, the extent of the sand apron can be used to infer wave climate at the time of sand apron progradation. For example, for One Tree Reef, one can argue the sand apron could had stopped prograding 4 ka BP because of the self-limiting sediment transport and remained stable until the sea-level fell.The wave climate on the Southern Great Barrier Reef is characterized by significant wave heights (Hs) of 1.7 m and has been stable for the past 33 years (Smith et al. 2022). Future changes in the wave climate, storm frequency, increases in sea level and changes to sediment availability caused by anthropogenic climate change will modify the eco-morphodynamics of the sand aprons and the percentage infilling of the lagoons.

Coral reefs are the most biodiverse and productive ecosystems on Earth. Changes to global climate will alter the conditions required for coral reef survival. In atoll reefs, coral species distribution and survival is determined by waves and tidally induced flows. The outermost part of atoll reefs, the forereef, presents a key morphological control over wave attenuation and consequently nutrient distribution at the scale of entire reefs. These high-energy environments provide protection from the effects of wave inundation to over 200 million people globally. Despite this, the mechanisms of wave transformation over the complex bathymetries of forereefs have received little attention. Here we focus on processes of wave transformation and attenuation by the elongated troughs and depressions typical of forereef slopes known as spurs and grooves (SaGs) and provide a better understanding of the morphodynamics of SaGs. We combine a quantitative morphometric analysis with wave transformation modelling and hydrodynamic field data from One Tree Reef (OTR) of the southern Great Barrier Reef, Australia. We used a 50 cm resolution LiDAR bathymetry dataset and novel methods of morphometric spectral analysis at swell wave exposed, semi-exposed, and protected locations. The wave transformation model, XBeach, was calibrated and validated with published SaG field data and wave data obtained from a 33-year analysis of satellite altimeters. The effects of SaGs on wave energy dissipation were examined under various forecasted climate change scenarios (RCP 2.6, RCP 8.5, and a disaster scenario) to elucidate their role in preventing future coastal hazards. Finally, field data collected from a fourth site on at OTR over a three-day period in October 2022 used an array of two acoustic doppler velocimeters and an 8 Hz pressure transducer. Current and wave measurements were taken in a long (140 m) and deep (max spur height = 5.3 m) groove under typical wave conditions (Hs = 0.8 m, Tp = 7 s). Findings from the numerical models suggest SaGs play a critical role in dissipating wave energy, increasing wave dissipation by bed friction by 75%. Our results demonstrate that a decrease in wave energy dissipation results in an exponential increase in wave overtopping with storm waves producing overtopping > 3 m under worst-case scenarios. Additionally, we found that high bathymetric gradients in SaGs increase dissipation by wave breaking by up to 52% under RCP 8.5, leading to a 71% increase in mean wave energy dissipation despite an 89% reduction in bed friction factor and a 1 m increase in relative sea level. Field observations demonstrate the wave and tide-dominated flow regimes through forereef grooves. Future work aims to investigate the form and process feedbacks in forereef hydrodynamics and consider global to regional scale climate forecasts. To facilitate this, we provide field data and analytical codes open source. Understanding the morphodynamics of forereefs is critical to forecasting reef health, wave transformation, and coastal hazard reductions under climate change conditions.

Beaches in estuaries and bays (BEBs) are usually distinct from open coast beaches in terms of environment, process, and morphology, and their morphodynamics remain unresolved. The present study aims to establish a relationship between profile morphology, sediment characteristics, and hydrodynamic forces. We investigated four beaches of Gamay (Botany Bay) located in the south of Sydney, SE Australia. Profiles of these beaches were measured regularly from 2016 to 2022 and sediment samples were collected from each beach. Wave data were recorded from the Sydney Waverider buoy. We used principal component analysis (PCA) to ascertain the temporal and spatial scales of the data variability, and spectral analyses to investigate the waves. Sediment samples were analyzed by Laser Diffraction Particle Size Analyzer (Mastersizer 3000).Our preliminary results indicate that different profile morphotypes are associated with different sediment characteristics and wave climates on BEBs. BEBs with the least exposure to ocean swell waves have the narrowest and steepest profiles, with BEBs more exposed to ocean swells having moderately steep profiles. BEBs containing coarser sediments show steep profiles compared to beaches with fine to very fine-grained sediments. In general, the shape of profiles changes in order from straight-through concave to convex-concave with increasing wave energy. Current research aims to parameterize these relationships with offshore and locally generated wave parameters. The insights obtained from this study will assist to understand fundamental mechanisms governing the morphological evolution of BEBs.

The wave fields of coastal bays are comprised of waves generated by far-off storms that enter the bay to combine with waves generated locally by winds inside the bay and regionally outside the bay. In any given location, the resultant wave field varies spatially and temporally, and affects coastal features, such as beaches in estuaries and bays (BEBs). However, wave fields in enclosed bays with tidal shoals are poorly studied, limiting the efficacy of coastal protection and restoration projects in these systems, a critical focus in light of ongoing sea level rise. Here we present observations of the wave field in Tomales Bay, a 20-km-long, narrow, semi-enclosed embayment on the wave-dominated coast of Northern California (USA) with a spring-tide range of 2.5 m. We deployed pressure sensors near several beaches along the linear axis of the bay. Low-frequency waves ($$4*10^{-2}-2.5*10^{-1}$$ 4 ∗ 10 - 2 - 2.5 ∗ 10 - 1 Hz or 4–25-s period) were not observed further than 4 km of the mouth, delineating the “outer bay” region, where remotely generated swell and regionally generated wind waves could dominate. The wave spectrum of the landward “inner bay” was dominated by fetch-limited waves generated within the bay with frequency $$\ge 2.5*10^{-1}$$ ≥ 2.5 ∗ 10 - 1 Hz. The energy of both ocean waves and locally generated wind waves across all sites were controlled by the tide, but the former by changes in attenuation and the latter likely by modulation of wave generation. Wave energies were low at low tide and high at high tide, but high-frequency wind wave energy was increased during ebb tides while lower-frequency swell energy was reduced during ebb tides, suggesting different mechanisms of tidal influences. Thus, in addition to fluctuations in winds and the presence of ocean waves, tides exert a strong control on the wave energy spectra at coastal features in mesotidal regions. In general, events that may be impactful for BEB morphology are expected to occur when waves due to high winds or high-swell event arrive during high-tide periods. However, no such events were observed during our study and questions remain as to how rarely such events occur across the bay.