Coral reefs serve as a highly effective natural barrier against incoming ocean waves. However, climate change and human-induced degradations are threatening reefs, reducing their capacity to protect coastal populations by diminishing the frictional processes that dissipate wave energy. The most common approach to represent wave energy dissipation through bottom friction relies on the estimation of a representative roughness length, whose definition on coral environments remains to be elucidated. As a consequence, a generic parametrization for friction processes on coral reef systems is still lacking. In this study, high-resolution hydrodynamical and topographical data collected at the South-West barrier reef of Mayotte, Indian Ocean, were used to perform an estimation of wave friction. The hydraulic roughness estimated across the reef varies spatially, and attempts are made to connect this information with the diverse bed morphologies observed in the field.

Estuaries serve as both reservoirs and gateways for microplastics, yet the processes that control particle trapping, release, and redistribution are still insufficiently understood. This study assesses the relative importance of key physical processes in a macrotidal, hyperturbid Gironde estuary, influencing the transport and trapping of floating and settling microplastics - namely, interactions with shorelines, vertical mixing, effect of water density on particle buoyancy, interactions with bottom sediments, and microplastic-sediment flocculation and examines seasonal transport trends. Results show that floating microplastics are mainly exported seaward, although a fraction remains trapped as a longitudinal line in the middle estuary. The intensity of microplastic export and accumulation varies seasonally with river discharge. Shoreline interactions via beaching-refloating dynamics, along with flocculation with fine sediments, represent the dominant processes modulating the distribution and fate of floating microplastics. In contrast, settling microplastics exhibit limited export to the ocean and remain confined within the estuary. Although this retention is primarily governed by hydrodynamics, interactions with bottom sediments through (temporary or permanent) deposition and resuspension, as well as intra-tidal variability in vertical diffusivity, have a relatively stronger influence on the transport and distribution of settling microplastics. Two distinct accumulation zones, or Estuarine Microplastic Maxima (EMPM), are identified: a surface EMPM, formed by low-density floating particles associated with surface convergence, and a water-column EMPM of denser particles in suspension near the Estuarine turbidity maxima (ETM) linked to tidal pumping. Overall, macrotidal estuaries act as significant sinks for microplastics, where hydro-sedimentary dynamics govern particle retention and redistribution.

Coastal hazards in tropical islands are likely to increase under the combined effects of sea-level rise, coral reef degradation and the potential intensification of extreme cyclones. In volcanic islands bordered by deep waters, storm surges are primarily driven by atmospheric pressure gradients and the development of a wave setup in the nearshore, which results from wave dissipation. Recent studies on the hydrodynamics of reef lagoon systems have demonstrated that the mean wave-driven circulation can either increase or decrease the wave setup along the shoreline, depending on the system configuration. This study combines the analysis of a unique dataset capturing a paroxysmal event (significant height exceeding 7 m at the breaking point) with phase-averaged numerical modelling to provide further insight on wave setup development in fringing reef environments under extreme conditions. When considering a 2DH modelling approach, the largest wave setup is underestimated by over 40

This paper presents a new set of experiments conducted at near-prototype scale to investigate the impact of artificial reef flat restoration on wave transformation and runup. The experiments were carried out in the 300m-long Delta Flume, which allowed the representation of the entire fringing reef profile and sandy island at a 1: 3 scale. The reef restoration consisted of rows of complex-shaped roughness elements arranged in different layouts. In total, three configurations were tested under five irregular wave conditions and two water levels. All tests were also repeated on a bare (unrestored) reef flat to assess the impact of the restoration. Preliminary data analysis indicates that the artificial reef restoration reduced significant wave heights by up to 13

Coral reefs act as natural breakwaters that significantly reduce coastal wave energy, erosion, and coastal flooding risks. Yet, future projections suggest that accelerating sea-level rise and widespread coral degradation driven by climate change and human activities will compromise their protective function. A phase-averaged wave energy model was calibrated on the SW barrier reef of Mayotte, Indian Ocean, to compute the key drivers of wave dissipation from the open ocean to the inner lagoon, namely friction, depth-induced breaking, and nonlinear energy transfers. This model was used to further infer the capacity of the barrier to dissipate waves under projected climate change effects. A literature review has allowed to classify the hydraulic roughness k_s value into health categories. This study is the first to establish a correspondence between changes in bottom roughness and IPCC scenarios. We showed that a significant loss of wave reduction is expected for the future (currently a 60.5 k_s ) would be necessary to maintain wave attenuation close to the current state, shifting corals from degraded to intermediate up to very complex ranges.

Rough rocky seabeds dominate world's coastlines, making accurate modeling of wave transformation in such environments essential for understanding coastal processes and hazards. Wave breaking and friction are critical drivers of wave dissipation over rough seabeds, especially in the surf zone. Phase-resolved wave models, often relying on classical bed shear stress or canopy drag approaches, can fail to capture these processes due to limitations in their physical assumptions, especially for large roughness elements typical of rocky seabeds. The present study adapts the Bulk Canopy Drag (BCD) parameterization, inherited from vegetation and porous media studies, into the 3D non-hydrostatic phase-resolved SYMPHONIE model to simulate wave dissipation over rough seabeds. The model was tested against laboratory experiments (LEGOLAS), using controlled irregular wave forcing propagating over a rough ramp. Results reveal that turbulent drag dominates the outer surf zone, while inertial drag plays a key role in the inner surf zone. A combined optimization of these influences achieved strong agreement with experimental measurements under one irregular wave forcing and a specific bottom configuration. These findings underscore the importance of accurately incorporating both turbulent and inertial contributions into wave dissipation models for the rocky surf zone. The proposed BCD approach provides a promising framework for improving phase-resolved modeling of wave dynamics, with potential applications for more complex field conditions. Future work will aim to extend the approach to diverse roughness configurations and refine empirical coefficients for broader scalability and in situ applicability.

In coastal hydrodynamics, the concept of resilience is mostly used by coastal managers with definitions closer to the field of risk assessment than to the study of dynamic systems (Masselink et al. 2019). This study proposes a new approach to quantify the hydrodynamical resilience closer to the ecological definition of Holling (1973) and is applied to the simplified case of small, cir- cular, micro-tidal atolls.

This study introduces a novel pixel-based satellite mapping approach for classifying coral island seabed. The model combines a pixel-based approach and a segmentation technique, to smooth the predictions into coherent objects. The model is applied to Maupiti Island (French Polynesia) and compared with an expert-based mapping mostly based on the Reef Cover classification. Results demonstrate high accuracy, ranging between 87% and 90% for various spatial resolutions. The developed tool is open-source and flexible, allowing users to retrain it for different classification schemes and environments. The study highlights the potential of automated satellite mapping for monitoring coral reef ecosystems and supporting conservation efforts.

The present paper proposes a unified view of the wave-driven amplification of the wave-averaged bottom shear stress in rough seabed contexts, covering both co- and opposing wave/current cases. The analysis is first based on a series of field observations performed over the Flysch rocky shore platform of Socoa. The momentum balance is examined locally, separating the net effect of the waves on the depth- and wave-averaged momentum budget, based on velocity and pressure measurements. The present observations confirm that, in the presence of complex seabed topography, the bed shear stress is an important component of the momentum balance. The results highlight two distinct regimes depending on the breaking activity due to the intricate composition between waves and mean currents in the wave averaged shear stress. In moderately developed undertow conditions, the bottom stress brings a negative contribution to the wave momentum balance, and acts to to promote wave setdown, while in conditions of depth-limited wave breaking saturation the bed friction acts to increase the wave setup. A novel empirical parameterization of the mean bottom stress under combined waves and current is proposed. The in-situ findings are complemented by a series of wave-resolving simulations on idealized closed and open beaches, confirming the complex effect of waves on the time-averaged water circulation.

This work examines the physics of free-surface flow and groundwater flow within a coupled model. Coupled models for such phenomena are not clearly justified, and there is a lack of precision in the derivation of such models. The primary objective of this work is to derive a coupled model of the shallow water equations (SWE) and Richards’ equation (RE) using asymptotic considerations. The numerical coupling approach chosen for the unified model will be described as a parallel coupling. Additionally, numerical considerations regarding how to solve this model using the discontinuous Galerkin (DG) methods will be provided. Furthermore, the exchange of information between the two models, which are time-synchronized, will be explained. The solution of RE coupled with SWE, following the described procedure and implemented using the DG formulation, is integrated into RIVAGE (an in-house numerical code based on the DG method). This implementation is then tested on a numerical problem and validated against an experimental benchmark.

Climate change is driving sea-level rise and potentially intensifying extreme events in the tropical belt, thereby increasing coastal hazards. On tropical islands, extreme sea levels and subsequent marine flooding can be triggered by cyclones but also distant-source swells. Knowledge of sea states in the tropical ocean is thus of key importance, and their study is usually based on spectral wave models. However, existing global wave models typically employ regular grids with a coarse resolution, which fail to accurately represent volcanic archipelagos, a problem usually circumvented by the use of obstruction grids but typically resulting in large negative biases. To overcome this problem, this study presents a new global wave model with a focus on distant-source swells, which have received less attention than waves generated by cyclones. To accurately simulate sea states in tropical areas, we implemented the spectral wave model WAVEWATCH III© (WW3) over a global unstructured grid with a spatial resolution ranging from 50 km to 100 m. The model is forced by ERA5 wind fields, corrected for negative biases through a quantile–quantile approach based on satellite radiometer data. The wind input source terms adjusted accordingly and the explicit representation of tropical islands result in improved predictive skills in the tropical ocean. Moreover, this new simulation allows for the first time direct comparisons with the in situ data collected on volcanic islands at water depths ranging from 10 to 30 m, which corresponds to a few hundred meters from the shore.

In order to understand and predict coastal flooding processes in rocky environments, it is necessary to take into account bottom roughness, which plays a key role in wave transformation processes and in general coastal dynamics. The present work aims to implement a parameterisation of roughness-induced dissipation in 3D non-hydrostatic phase-resolved wave models, based on the Symphonie code (MARSALEIX et al., 2019). The modified model is compared with laboratory experiments carried out on a surf zone with a linear slope (DEALBERA et al., 2024). Different irregular waves were tested on various configurations of bottom roughness (represented by block configurations of different sizes and distributions). Wave generation and dissipation induced by breaking were first parametrised with respect to laboratory data on smooth bottoms. The different roughness cases were then studied on the basis of two distinct strategies for parameterising bottom friction, namely the bottom stress approach and the canopy drag approach. The performance of these two approaches is assessed by comparing the model results with measurements of the cross-shore significant wave height profile for different bottom configurations. On the basis of this work, recommendations on the choice of dissipation parameterisations will be made.

Studying microplastic transport in estuaries is challenging due to the dynamic interplay between river and ocean, compounded by the diverse properties exhibited by these particles. Lagrangian particle-tracking numerical modelling is a relevant tool for investigating microplastic transport dynamics, dispersion patterns, and vertical distribution. However, these models oversimplify the parametrizations of crucial estuarine processes by ignoring the effect of varying water density or vertical diffusion coefficients. In this study, we implement a hydrodynamic and improved particle tracking model in the macrotidal Gironde estuary (SW France) to explore the relative importance of different physical processes (time-space varying vertical diffusivity and water density, beaching-refloating, bottom resuspension) and provide a better understanding of microplastic dispersion and potential trapping. The simulated particle trajectories and density distributions from our findings indicate a limited influence of the spatio-temporal variability of vertical turbulence on floating particles, with a notable impact observed for settling particles, showing its significance in particle resuspension. Despite the time-space-varying water density, the effect on the transport patterns of both floating and settling microplastics is relatively lower, while the phenomenon of beaching-refloating increases the particle's residence time within the upper estuary. The higher river discharge during the spring season flushes floating particles downstream, with a portion reaching the open sea, while settling particles persist within the estuary during both seasons. Notably, denser microplastic particles tend to accumulate in the upper estuary region during summer, where the estuarine turbidity maxima have been identified.

In the presence of large roughness, such coral reef or rocks, frictional wave dissipation is expected to play a crucial role in wave transformation, possibly dominating over depth-induced breaking dissipation (Monismith et al. 2015). The wave friction factor, f w, is a key parameter for the representation of frictional wave dissipation in phase-averaged numerical models. Based on laboratory experiments of surf zone dynamics over a series of roughness layouts, the present study aims to provide further insight on (I) the validity of the existing fwin low ranges of Ao/Kr, i.e. in the presence of large roughness, and (ii) to decipher the connection between wave frictional dissipation and the architectural structure of the seabed roughness.

Despite their global importance, the understanding of wave transformation processes and marine flooding has been much less studied on rocky beaches than on sandy beaches. Thus, the representation of bottom dissipation on this type of environment is generally based on site-specific parameterization in operational wave models, with mixed performance. The main aim of this study is to improve wave propagation and transformation on rough bottoms in the phase-resolved 3D non-hydrostatic model Symphonie (Marsaleix et al., 2019), using comparative confrontations with wave channel experiments and in situ data on the rocky beach of Ars-en-Ré, France. Different types of parameterization are implemented and evaluated against controlled laboratory experiments and in situ observations. The second main objective is to improve the existing representation of frictional dissipation, targeting the establishment of more generic parameterization based on the actual statistical properties of the seabed structure. To date, most parameterizations are entirely or semi-empirical, site-specific and almost never include appropriate measurements of roughness topography.

Wave frictional dissipation is a key process in rough seabed's environments such as coral reefs, expected to significantly reduce incoming wave energy. In phase-averaged models, wave dissipation is typically estimated through a wave friction factor which is dependent of the near-bed orbital excursion and the hydraulic length, a proxy for the seabed substrate roughness. A field experiment was conducted in the South-Western coral reef barrier of Mayotte (Indian Ocean) to compute through a frequency-integrated wave energy balance. Significant time and space variations of have been observed, driven by the evolution of the wavefield and the diversity of coral geometry and scales found along the barrier. The fine reef architecture has been examined thanks to a high-resolution multi-beam echo sounder survey. This study has reassessed the established link between hydraulic roughness length and roughness standard deviation; it also indicates that second-order roughness metrics may also significantly explain variations in . Future challenges remain in the proper definition of the length scales of seabed variability attributed to roughness and to bathymetry.

Tropical coastlines and small islands are particularly vulnerable because of their physical exposure to climate change hazards and their limited adaptative capacity. Located in oceanic regions, low-latitude low-lying islands are subject to tropical cyclones, storms, flooding and drought, leading the UNESCO (2014) initiative to carry strategies for building Small Islands resilience in the context of global change. Many of these tropical islands and coastlines are bordered by coral reefs that can provide substantial defense by reducing as much as 98percent of incident wave energy. Coral reefs reduce the risk by inducing the breaking at the reef crest and by dissipating the remaining energy by bed friction linked to the architectural complexity of the reef system. The predictions regarding future climates over the global ocean point towards a rise in sea levels, an increase of storm intensities, and the alteration of coral reefs induced by the climate change in combination to anthropogenic stresses at global and local scales, contributing to more severe wave-driven flooding events in tropical islands and atolls. In order to predict submersion risks in such conditions, numerical modelling is used, such as BEWARE dataset (Pearson et al. 2020), knowing water depth over the reef flat, incident wave conditions, reef flat width, beach slope and bed friction. The focus of this study is La Saline coral reef, located in the West of La Reunion Island (France) in the Indian Ocean.

Describing the structural complexity of seabeds is of primary importance for a number of geomorphological, hydrodynamical and ecological issues. Aiming to bring a decisive insight on the long-term development of a unified view, the present study reports on a comparative multi-site analysis of high resolution topography surveys in rough nearshore environments. The nine study sites have been selected to cover a wide variety of topographical features, including rocky and coral seabeds. The topography data has been processed to separate roughness and bathymetry-related terrain features, allowing to perform a comprehensive spectral and statistical analysis of each site. A series of roughness metrics have been tested to identify the most relevant estimators of the bottom roughness at each site. The spectral analysis highlights the systematic presence of a self-affine range of variable extension and spectral slope. The standard deviation of the seabed elevation varies from 0.04 to 0.77 m. The statistical and multi-scale analysis performed on the whole set of roughness metrics allows to identify connection between metrics and therefore to propose a reduced set of relevant roughness estimators. A more general emphasis is placed on the need to properly define a unified framework when reconstructing roughness statistics and bathymetry from fine seabed topographical data.

Reef barriers play a major role many coral islands, by sheltering the lagoon from the ocean wave energy and then creating a unique habitat for many species. This filtering action becomes increasingly crucial for ecosystems health and shoreline protection in the context of climate change and related sea level rise, degradation of coral systems and modification of wave conditions. A strong research effort has therefore been engaged by the coastal oceanographers community for the last two decades to improve our knowledge and prediction skills of wave dynamics over coral reef systems. A widely reported observation is the importance of infragravity waves (IG) over wave-driven reef systems, whether fringing or barrier reefs. IG are primarily forced by groups in the incoming short-wave (SW) field, either by the release of bound waves or the breakpoint oscillations (Bertin et al. 2018). IG period typically ranges between 30 and 200s, which makes them prone to excite or interact with natural seiching modes in reef-lagoon systems often ranging in the Very Low Frequency (VLF) band. Further research efforts are now necessary to better understand the interaction between long IG/VLF oscillations and SW field. In particular, long waves are expected to play a dynamic depth-filtering role on SW energy, acting as long carrier wave able to promote the propagation of larger SW groups by IG/VLF crests. More generally, the spectral energy transfers over the reef crest-flat system and their relative importance w.r.t. frictional and breaking dissipation are not fully understood over the complete range of surface waves. The aim of the present study is to analyse and to discuss a series of field observations performed on the barrier reef of Maupiti Island, French Polynesia. A particular focus is placed on the interaction between SW and IG wave fields across the reef crest-flat system.

Abstract. Potentially acting as a source or a sink for plastic pollution to the open ocean, nearshore waters remain a challenging context for predicting the transport and deposition of plastic debris. In this study, we present an advanced modelling approach based on the SWASH wave model and the TrackMPD (v3.0) particle transport model to investigate the transport dynamics of floating and sinking microplastics in wave-dominated environments. This approach introduces novel features such as coupling with advanced turbulence models, simulating resuspension and bedload processes, implementing advanced settling and rising velocity formulations, and enabling parallel computation. The wave laboratory experiments conducted by Forsberg et al. (2020) were simulated to validate the model's ability to reproduce the transport of diverse microplastics (varying in density, shape, and size) along a comprehensive beach profile, capturing the whole water column. Our results underscore the robustness of the proposed model, showing good agreement with experimental data. High-density microplastics moved onshore near the bed accumulating in proximity to the wave-breaking zone, while the distribution of low-density particles varied along the coastal profile depending on the particle properties. The study also sheds light on the primary mechanisms driving microplastic transport, such as Stokes drift, wave asymmetry and settling/rising velocities. Sensitivity analyses on calibration parameters further confirm the robustness of the model results and the influence of these factors on transport patterns. This research establishes the SWASH-TrackMPD approach as a valuable tool, opening avenues for future studies to contextualize laboratory findings within the complexities of real-world nearshore environments and further refine our comprehension of microplastic dynamics across different beaches and wave-climate conditions.