Gravel barrier systems are ubiquitous on mid- and high-latitude coasts and provide vital protection from coastal flooding and coastal erosion. They are highly dynamic systems that exhibit complex responses to hydrodynamic forcing over a range of timescales (hourly-monthly-decadal-centennial). Their dynamics differ greatly from those of sandy beaches yet have received considerably less attention in the literature, particularly at interannual to decadal scales. We use over four decades of satellite-derived shoreline (SDS) data to explore the long-term dynamic of 45 selected gravel systems around the United Kingdom and Ireland. Unlike most large-scale studies, we apply an SDS extraction methodology specifically tailored to gravel beaches to derive the long-term shoreline trends along 1554 shore-normal transects across our sites. Our findings indicate a great variability in shoreline trends, ranging from -4.73 m/year to + 10.5 m/year with the majority of transects (62

Coral atoll lagoons are typically connected to the bounding ocean by both deep channels and shallow reef flats. Reef flats are ubiquitous on atoll rims, interspersed between atoll islands, and flows here are primarily unidirectional; setup from wave breaking on reef flats create a lagoon-ward water level gradient that drives a flow into the lagoon. While this flow has a clear tidal variability, the magnitude is determined by the incoming wave energy and modified by the tide through depth-limited wave breaking. Results from a four-month deployment in Huvadhoo atoll in the Maldives, Indian Ocean, explore the relation between reef flat velocities and external forcing from offshore swell waves and tidal depth. Results show that tidal phase is an additional, important parameter, creating a lag between lagoon and ocean tides that opposes wave-generated setup on rising tides, and amplifies it on falling tides.

Rubble strips are shore-normal, elongated deposits on coral reef flats, composed of variably sized and shaped mobile coral clasts, which can contribute to the sediment supply on exposed low-lying coral reef islands. As climate change-driven ecological and physical disturbances increasingly impact coral reef systems and associated landforms, understanding the processes and controls governing their morphodynamics is critical. At an exposed intertidal rubble strip in Huvadhu Atoll, we examined: i) the morpho-sedimentological characteristics; ii) short-term (days-weeks) clast mobility using tracer experiments with in situ current and wave measurements; iii) intra-annual morphological changes using repeated UAV photogrammetric surveys; and iv) deposit age and rubble throughput using UTh dating to provide a chronological framework. This deposit is a modern reservoir of coral rubble (less than 250 years in age) which shows a lagoonward increase in clast age. Surface sediments displayed lagoonward surficial sorting with decreasing grain size (D50 = 5–45 mm) and a transition from tabular to branching clasts. Subsurface layers contained substantial sand fractions (>60%) indicating that these landforms are not exclusively coarse-grained. Tracer experiments revealed monthly clast mobility of 40–85% and displacements up to 16.6 m under modal hydrodynamic conditions (offshore Hs = 0.48–2.09 m). Mobility increased with decreasing clast size, from tabular to massive to branching forms, and higher wave-current velocities. Morphological analysis indicated that 15–43% of the rubble strip experienced significant elevation changes bi-weekly to intra-annually, reflecting localised erosion and accretion. Although the net sediment volume of the strip remained stable, sediment was actively redistributed from the algal rim towards the lagoon. Overall, rubble strips are morphodynamically active landforms under moderate forcing conditions, with mobilisation controlled by size and shape attributes, and wave-current velocities.

Transport of sand and coral-derived rubble (gravel/cobbles/boulders) across coral platforms could be a critical sediment supply for island maintenance under a rising sea level. While several studies have focused on cross-platform sand transport, less attention has been paid to understanding modes, mobility and rates of transport of rubble across different environments. Here we examine the distribution and morphodynamics of rubble tracts located on intertidal platforms associated with low lying atoll islands in the Huvadhu atoll, Maldives. Broad scale assessment of rubble tract distributions throughout the atoll (satellite images) identified the greatest abundance of rubble features in the high-energy SW of atoll. Local-scale morphodynamic assessment of a representative rubble tract was conducted by collecting 6 repeat RTK UAV flights over a 5-month period, combined with bed current measurement (ADV) at the tract and offshore wave observations (ADCP). Offshore significant wave heights and mean current conditions on the tract ranged from 0.48 to 3.91 m, and from −0.22 to 0.65 m/s, respectively, during the study period. DEMs of difference indicated that the studied rubble tract feature was geomorphologically stable (no significant change in total sediment budget) over a 5-month period. Although large portions (10–50

In addition to external hydrodynamic forcing, the behaviour of beach-dune systems is strongly controlled by nearshore sediment availability and coastal transport pathways. Here we integrate new sedimentological observations, shoreface geomorphological analysis, and regional hydrodynamic modelling to improve understanding of the distribution, characteristics, and connectivity of mobile sediments along a macro-tidal, highenergy, sediment-limited embayed coastline in Cornwall, United Kingdom. We first map the spatial extent of mobile sand bodies along a 45-km coastline and assess headland bypassing potential to define coastal embayments. Coast-wide sediment samples (n = 348), spanning dunes to the lower shoreface, were classified using Kmeans clustering of grain-size and mineralogical data to quantify spatial variability in sediment character, refine the identification of closed/constrained embayments, and evaluate existing regional sediment transport modelling. Eight closed coastal cells and four major sediment sinks, associated with large dune systems, were identified, together with a dominant southwest-to-northeast transport pathway. Sediment-starved regions exhibited the highest calcium carbonate contents (45-57%), consistent with long-term sediment supply limitations. These results demonstrate the value of spatially comprehensive sediment resource and geomorphological datasets for constraining sediment pathway modelling and provide a scientific basis for evaluating long-term coastal evolution.

The seasonal feeding aggregations of reef manta rays (Mobula alfredi) in Hanifaru Bay, Maldives, are the largest known of their kind, yet the mechanisms facilitating this phenomenon remain poorly understood. Understanding these drivers is crucial for effective conservation and management, as protecting key foraging habitats requires knowledge of the physical processes influencing prey availability. While previous studies highlight the role of zooplankton in attracting manta, the oceanographic mechanisms responsible for concentrating prey within the bay remain hypothesised but unproven. Specifically, the influence of eddy dynamics, tidal forcing, and regional circulation on zooplankton distribution has not been thoroughly examined. A 2DH hydrodynamic model (Delft3D-FM) was used to simulate ocean circulation patterns and analyse their evolution throughout the tidal cycle. Results reveal that topographic eddies, modulated by tidal flows, create retentive zones within Hanifaru Bay capable of retaining particles. These eddies form midway through the flood tide-aligning with manta aggregation-and dissipate approximately 2 h after high tide, when the manta disperse. The combination of this timing and the ability of eddies to retain particles suggests these features are the dominant mechanism driving feeding aggregations in the bay. By identifying the physical processes that promote prey availability, this study provides critical insight into the environmental conditions underpinning manta foraging dynamics. Given the potential impacts of climate change and coastal development on local hydrodynamics, these results may help inform conservation strategies aimed at preserving key foraging habitats for Mobula alfredi.

Abstract. Rip currents are dangerous flows in the surfzone of wave-exposed coasts and can take bathers from the shallows into deeper water. They cause hundreds of drownings globally each year and are the leading cause of all beach lifeguard rescues. In New Zealand, with a population of approx. 5 million people, rip currents typically cause 500–1000 lifeguard rescues each year and are attributed to 53 % of all Surf Life Saving New Zealand rescues. This study aims to identify environmental conditions associated with rip current incidents and develop a simple algorithm for forecasting rip current risk and hazard. A dataset of ~9,000 recorded rip current rescues along with water user head counts made at 58 beaches by lifeguards around the coast of New Zealand between 2001 and 2022 was used to assess rip current risk (parameterised from the total number of incidents) and rip current hazard (parameterised as the likelihood of an individual being in a rip incident) under different wave, tide, and wind conditions. In concurrence with previous findings, most rip incidents in New Zealand were recorded at beaches with intermediate ‘bar-rip’ beach morphology and occurred disproportionately during wave conditions at or above average breaker height with tide level at or below average low tide. Although rip incidents were also recorded at dissipative and reflective beaches lacking in bar-rip morphology, water users were 4 and 24 times more likely, respectively, to be in a rip-related incident at intermediate beaches with bar-rip morphology. A simple, threshold-based algorithm was developed using only breaker height, relative tide level, and a binary bar-rip morphology variable as predictors for use as a national-scale rip forecast across New Zealand. The algorithm achieves a high incident hit rate, capturing 98 % of historic rip incidents across New Zealand, and captures exponentially increasing hazard at each of its five Rip Index levels, with a water user 6 times more likely to be in a rip incident at the highest Rip Index (~1-in-200) compared to the lowest (~1-in-1200). It also conservatively replicates a lifeguard’s perception of rip hazard, with an overall agreement rate of approximately 81 %, indicating it could provide useful forewarnings to the public especially at non-lifeguarded beaches or outside lifeguard patrol hours. To our knowledge, this represents the longest running rip incident data set analysed, and most widely validated rip forecast in the literature to date.

Sandy beaches and dunes are vital for protecting coastal communities from erosion and flooding, particularly along high-energy, sediment-limited coastlines. This rugged coastline, defined by rocky headlands and sediment-starved systems, requires a comprehensive understanding of sediment dynamics to maintain its natural defenses. This study examines sediment characteristics, dynamics, and connectivity along a 65-km stretch of coastline in southwest England using a comprehensive dataset comprising ground-truth sediment analysis, high-resolution remote sensing (bathymetric and LiDAR), and hydrodynamic modeling outputs. Four sediment clusters, distinguished by grain size and carbonate content, served as fingerprints to track sediment variability. Remote sensing data were used to create a seabed roughness map, differentiating flat sediment areas from rocky reef-dominated zones. By integrating sediment characteristics, geological features, and regional hydrodynamic models, the study identifies seven sediment cells dominated by a strong eastward-northward sediment transport pathway, with sediment settling downstream on the constraining headlands. These results provide a basis for evaluating the impacts of sediment supply and regional transport pathways on the future evolution of sand beaches and dune systems in sediment-starved embayed regions.

Coral reef systems are highly vulnerable to anthropogenic climate change influencing sediment dynamics between islands and the surrounding reef. This study focuses on coral rubble mobilization of rubble sheet deposits on an intertidal platform in the Maldives. Rubble clasts display an overall migration and fining in a lagoonward direction, potentially contributing to island formation. To further understand mobilization patterns, a rubble tracer experiment was conducted using 180 rubble clasts of different sizes and shapes released at two locations Ocean and Lagoon. Their movements were tracked over 30 days under varying hydrodynamic conditions. Rubble clast size, shape and releasing location influenced displacement and mobility. The proportion of rubble mobilized during the experiment varied between 60–100

Gravel beach systems provide vital protection from coastal flooding and erosion. They are highly dynamic and exhibit complex responses to hydrodynamic forcing over a range of temporal (hourly centennial) and spatial scales (m to km). Yet gravel beach evolution, particularly at interannual to decadal scales, across the spectrum of coastal settings, remains poorly understood. We use four decades of Satellite-Derived Shoreline (SDS) data to explore the morphodynamic behaviour of 45 selected gravel beach systems around the United Kingdom and Ireland. We apply a site-specific SDS extraction methodology and derive shoreline trends along 1554 shore-normal transects. Our findings indicate significant variability in decadal trends between sites ranging from 0.60 m/year retreat to 2.24 m/year progradation, with 36% of sites showing significant long-term trends over the study period. Nesses and spits were by far the most dynamic systems exhibiting the largest changes at transect level (from 4.73 m/year retreat to 10.5 m/year progradation), and the most significant changes in planform shape, while most constrained and unconstrained sites remained stable. We classify the observed behaviours, providing a first inventory of morphodynamic behaviours across different gravel beach systems in the United Kingdom and Ireland. We find that leading regional winter-averaged atmospheric indices provide some insight into planform behaviour over the entire domain, with 16 sites (35.6%) showing at least moderate (R >= 0.4) statistically significant correlations (p <= 0.05). Our results provide a deeper understanding of the long-term behaviour of gravel beach systems that can inform more effective coastal management strategies.

Dunes provide a range of ecosystem services, including natural resilience against coastal flooding, and it is essential for proactive coastal zone management to forecast future dune behaviour as sea levels rise. However, dunes exhibit dynamic behaviours over a wide range of spatial and temporal scales, posing challenges for predicting their evolution. Here we demonstrate a high degree of variability in contemporary dune behaviour along a single coastal region, across 31 sites in Cornwall, southwest England. Rates of change varied widely, with sediment loss ranging from-12.4 m3/m/yr to a gain of +5.1 m3/m/yr, and dune retreat/ advancement ranging from-3.7 m/yr to +1.6 m/yr. The results highlight four distinct modes of dune evolution: (1) volume gain with advancement, (2) volume loss with retreat, (3) volume gain with retreat, and (4) volume loss with advancement. These findings validate scepticism regarding the applicability of the Bruun rule over long timescales and underscore the importance of incorporating site-specific factors, such as sediment supply, aeolian transport, underlying rock surface and estuarine channel dynamics, into future model development. Nonetheless, this study demonstrates that combining contemporary dune behaviour analysis with the Bruun Rule offers a practical approach for initial predictions of future dune evolution. When combined with observed data, the Bruun Rule can effectively identify coastal change 'hot-spots' and guide coastal management decisions. Until more sophisticated models are developed, we recommend a staged approach for coastal managers: starting with regional-scale projections using observed trends and the Bruun Rule, followed by targeted application of reduced complexity and process- based models in areas of high concern. Incorporating probabilistic assessments is essential for managing uncertainties in these predictions, ensuring more informed and effective coastal management strategies.

Seagrasses impact on sedimentary intertidal and subtidal ecosystems by affecting local hydrodynamics, geomorphology and sediment properties. Their influence on hydrodynamics is to reduce flow velocities in their canopies, and this leads to increased net sedimentation rates and reduction of the grain size. Most investigations of the seagrass-hydrodynamics-sediment feedback system has been carried out over silt and fine-sand beds under tide-dominated conditions, mostly in the intertidal zone. Here, we use sedimentological data from a relatively wave-exposed and subtidal seagrass (Zostera marina) meadow in the Isles of Scilly with a fine-to-medium sand bed and show that the sand within the seagrass meadow is indeed finer than in adjacent, unvegetated regions. However, in contrast to previous studies, this is not due to increased mud/silt content within the seagrass meadow, but an almost 0.1-mm shift in the median sediment size across the sand fraction from 0.25 mm (fine to medium sand) to 0.35 mm (medium sand). The studied seagrass meadow is characterised by a distinct spatial structure comprising of vegetated 'ridges' and bare sand 'valleys'. Even the bare sand valleys within the seagrass meadow are characterised by significantly coarser sand than the adjacent vegetated ridges, providing further confirmation of the efficiency of sediment sorting by wave processes that takes place within seagrass meadows. It is concluded that any numerical modelling involving sediment transport processes associated with seagrass environments must account for variability in the textural characteristics.

Monitoring sandy shoreline evolution from years to decades the closest coast. Because spatial averaging can help to is critical to understand the past and predict the future of our smooth out uncertain and noisy SDS, we also defined a coasts. Interannual shoreline variability is often primarily moving average distance L. enforced by large-scale climate patterns of atmospheric or coupled ocean-atmospheric variability (e.g. El Niño– Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO)). The seasonal to decadal predictability of these climate patterns recently showed increasing skill (Athanasiadis et al., 2020), which may allow a reduction in shoreline evolution uncertainties in the next decades. Understanding shoreline evolution on interannual to decadal timescales is therefore critical, particularly along the Atlantic coast of Europe, where previous work on a limited number of intensively monitored sites showed a strong control of large-scale climate modes of atmospheric variability on shoreline change (e.g., Dodet et al., 2019, Masselink et al., 2023). In this contribution, we consult an improved state-of-the-art global SDS dataset (Luijendijk et al., 2018) to address the spatial distribution of interannual variability of sandy shores along the Atlantic coast of Europe, and to further identify the primary drivers and coastal settings affecting this spatial variability.

SCOTT, T., RUSSELL, P., MASSELINK, G., WOOLER, A., and SHORT, A., 2007. Beach rescue statistics and their relation to nearshore morphology. and hazards: a case study for southwest England. Journal of Coastal Research, SI 50 (Proceedings of the 9th International Coastal Symposium), 1 - 6. Gold Coast, Australia, ISSN 0749.0208The coasts of Devon and Cornwall in the southwest of England experience some of the most energetic wave conditions (H(s,10%) = 2-3 m) and largest tide ranges MSR = (4.2-8.6 m) in the UK. They are also a popular tourist destination during the summer months with over 10 million visitors per year. The energetic wave/tide conditions pose a considerable physical risk to beach users and 62 beach environments in this region are therefore patrolled by Royal National Lifeboat Institution (RNLI) lifeguards. Beach rescue statistics collected by the RNLI during spring and summer (1 May to 1 October) were analysed to examine and quantify the risk posed by physical beach hazards to beach users. Rip currents were found to be the main hazard and were responsible for 71% of all recorded incidents. The most hazardous beaches were found on the exposed west coast of the study area. Beaches here can be classified as morphodynamically intermediate and are characterized by low-tide tide bar and rip systems, often topographically-constrained by intertidal geology. The rip currents are generally most active around low tide. Beaches in Devon and Cornwall exhibit morphologies that are significantly different from previously studied beaches in Australia due to the combination of high energy surf zones, large tides and variable coastal geology. This work represents a first step towards the generation of standardized beach risk assessments in the UK.

Rip currents are the single largest cause of beach safety incidents globally, but where an estuary mouth intersects a beach, additional flows are created that can exceed the speed of a typical rip current, significantly increasing the hazard level for bathers. However, there is a paucity of observations of surfzone currents at estuary mouth beaches, and our understanding and ability to predict how the bathing hazard varies under different wave and tide conditions are therefore limited. Using field observations and process-based XBeach modelling at an embayed, estuary mouth beach, we demonstrate how surfzone currents can be driven by combinations of estuary discharge and wave-driven bathymetric and boundary rip currents under various combinations of wave and tide forcing. While previous studies have demonstrated the high hazard that rip currents pose, typically during lower stages of the tide, here we demonstrate that an estuary mouth beach can exhibit flows reaching 1.5 m s(-1) - up to 50 % stronger than typical rip current flows - with a high proportion (> 60 %) of simulated bathers exiting the surfzone during the upper half of the tidal cycle. The three-dimensional ebb shoal delta was found to strongly control surfzone currents by (1) providing a conduit for estuary flows that connect to headland boundary rips and (2) acting as a nearshore bar system to generate wave-driven "river channel bathymetric rips". Despite significant spatio-temporal variability in the position of the river channels on the beach face, it was possible to hindcast the timing and severity of past bathing incidents from model simulations, providing a means to forewarn bathers of hazardous flows.

Surf zones of sandy beaches are among the most heavily impacted aquatic ecosystems, yet are of critical ecological importance for inshore fish and fisheries. Knowledge of the drivers of fish habitat use in surf zones is needed across broad scales to advise conservation and fisheries management, but sampling capabilities can be limited in spatio-temporal extent and resolution. The lesser weever Echiichthys vipera a is a small, benthic, venomous fish that dominates surf zone fish assemblages in Northwest Europe and inflicts painful stings on beachgoers. This study capitalises on an extensive record of E. vipera sting incidents to characterise variations in surf zone habitat use in relation to key physical environmental factors. Sting incidents, standardised by water user numbers, are used as a proxy for E. vipera abundance across 77 beaches throughout Southwest England, with 2 h resolution, from April-November 2018. General Additive Models indicated a clear peak in E. vipera abundance at spring low tides, in the afternoons of summer months, under calmer wave conditions and at higher levels of solar irradiance. Although the order of significance differed, human water users were also driven by the same variables, compounding sting interactions over time. Key physical variables did not explain spatial variation in E. vipera abundance, although there was a weak relationship with sea surface temperature, and some evidence that reflective beaches are unsuitable. Physical factors explained more spatial variation in human water users, who gathered at more dissipative beaches with greater wave heights. This detailed study of an important surf zone fish reveals clear drivers of temporal variation in habitat use, yet infers wide suitability of beaches varying in the key physical drivers of sandy shore ecology.

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Climate change is resulting in global changes to sea level and wave climates, which in many locations significantly increase the probability of erosion, flooding and damage to coastal infrastructure and ecosystems. Therefore, there is a pressing societal need to be able to forecast the morphological evolution of our coastlines over a broad range of timescales, spanning days-to-decades, facilitating more focused, appropriate and cost-effective management interventions and data-informed planning to support the development of coastal environments. A wide range of modelling approaches have been used with varying degrees of success to assess both the detailed morphological evolution and/or simplified indicators of coastal erosion/accretion. This paper presents an overview of these modelling approaches, covering the full range of the complexity spectrum and summarising the advantages and disadvantages of each method. A focus is given to reduced-complexity modelling approaches, including models based on equilibrium concepts, which have emerged as a particularly promising methodology for the prediction of coastal change over multi-decadal timescales. The advantages of stable, computationally-efficient, reduced-complexity models must be balanced against the requirement for good generality and skill in diverse and complex coastal settings. Significant obstacles are also identified, limiting the generic application of models at regional and global scales. Challenges include the accurate long-term prediction of model forcing time-series in a changing climate, and accounting for processes that can largely be ignored in the shorter term but increase in importance in the long term. Further complications include coastal complexities, such as the accurate assessment of the impacts of headland bypassing. Additional complexities include complex structures and geology, mixed grain size, limited sediment supply, sources and sinks. It is concluded that with present computational resources, data availability limitations and process knowledge gaps, reduced-complexity modelling approaches currently offer the most promising solution to modelling shoreline evolution on daily-to-decadal timescales.