In response to the challenges posed by coastal erosion and rising sea levels, bio-inspired strucutres represent an innovative solution by combining physical protection with ecological benefits. This study investigates how key structural parameters, including tortuosity, surface roughness, porosity, and structural diversity, affect near-bed shear stress and turbulence around bio-inspired coastal defense modules.Wave flume experiments were conducted using fifty-one different modules, organized in three rows and tested under five monochromatic wave conditions (heights 2.5–10 cm, periods 1–2 s), scaled for Mediterranean deployment. Measurements from resistive wave gauges and Vectrino velocimeters were used to analyse wave energy dissipation, vertical current profiles, turbulence, and bed shear stress.Preliminary results show that structural geometry appears to influence local hydrodynamics, with implications for a better understanding of how the selected parameters affect the surrounding hydrodynamic conditions. The effects of the parameters are ranked to guide the development of efficient, multifunctional, bio-inspired coastal defense solutions. A combination of several of these parameters, within a single module and then at the scale of an entire structure, allowed us to explore the potential benefits of structural complexity in coastal protection systems.

The sixth report published in March 2023, from the IPCC 2023, lists several alarming findings about the ocean (Lee et al., 2023). The rise in sea level has accelerated and is now three times faster than it was during the period 1901-1971. The increase in ocean levels and the multiplication of energetic oceanic events represent a real problem for the management of coastal infrastructure. The proximity of human activities near the seaside makes these areas particularly vulnerable to risks. The submersion risks are seriously considered, given the damage that certain storms can have on the coast. Real-time measurement of the offshore wave field makes it possible to improve coastal submergence warning systems and predictions of submersion. Nevertheless, maintaining hydraulic measurement stations is still a challenge since the ocean is a hostile and vast environment that induces high installation costs. Consequently, a significant portion of our oceans remains unmonitored, making it difficult to find effective solutions for managing coastal infrastructures. On the other hand, ocean warming has been faster in the last century than in about 11,000 years (medium). For instance, marine heatwaves will increase in number and intensity, compromising many ecosystems. These marine heat waves can be detected on the surface with satellites. Still, their dynamic can greatly differ from the evolution of marine heat waves at depths where they remain poorly documented.\\ Distributed Acoustic Sensing (DAS) technology is a new photonic method that can convert several tens of kilometer-long seafloor fiber-optic telecommunication cables into dense arrays of strain sensors. With such spatial and temporal resolution, DAS is a new transforming approach for in-situ oceanographic measurements. For a recent DAS experiment performed on seafloor cables along the French Mediterranean coast, we show that it is possible to measure ocean swell fields up to depths of about 100 meters, and track water temperature variability from the coast to the bottom of the Mediterranean sea with mK sensitivity. Because DAS data are acquired at the speed of light from the land termination of the cable, the technology also enables the establishment of effective and rapid submergence warning systems, capable of anticipating the impact of storms or marine heat waves in real-time. Considering the vast network of submarine telecommunications cables and the ability of DAS to operate on fiber optic cables with live traffic, DAS could be easily and rapidly implemented across the globe.

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 site of the “Étangs et Marais des Salins de Camargue” (EMSC) is located southern France in the Rhône Delta, facing Mediterranean sea, at the heart of the Camargue Regional Natural Park. This site, formerly dedicated to salt production, is now owned by the Coastal Conservatory and co-managed by the Camargue Regional Natural Park, Tour du Valat, and the French National Society for the Protection of Nature. A habitat restoration project was initiated in 2011 and continues as part of the development of a 2023-2032 management plan and the site's participation in the European H2020 Rest-Coast project. This restoration aims to facilitate the recolonization of the area by plant (seagrass, salt marsh) and animal species, reestablishing the natural hydrological processes of the site. An adaptive coastal management strategy is also applied on this site, with the decision to no longer maintain the historical dikes of the salt production site facing the sea, leading to their gradual collapsing. Protection against the risk of marine submersion is ensured by an landward seawall, which will be elevated, as part of the coastal strategy defined for the greater Rhône Delta. Natural hydro-morphodynamic processes are reestablishing locally, creating new connections with the sea and the formation of beach and barrier areas through overwash. This results in the emergence of a natural buffer zone that will better address the consequences of climate change or storms, such as marine submersion and salinization.

Oxygen, nutrients, and pollutants fluxes at the sediment-water interface are involved in the water quality and ecological status of shallow coastal environments. The short-term variabilities of these fluxes, in response to hydrodynamic and benthic biogeochemical processes in the field, make it difficult to assess the net balance of these fluxes. Here we evaluate the ability of using concentration gradients in the benthic boundary layer to better understand temporal variations in these fluxes. Three shallow Mediterranean coastal lagoons (Berre, Thau and Prevost lagoons), affected by deoxygenation events and depicting contrasting water quality status, were investigated. A Benthic Oxygen Gradient Observatory System (BOGOS) was set-up and deployed for two weeks in each lagoon to obtain continuous oxygen gradient time-series. In addition, concentration gradients of nutrients (Si, NH4+, PO43-) and trace elements (Fe, Mn, Co, As, Cu, Mo, MeHg) were obtained using a specifically dedicated benthic water sampler (SUSANE) deployed under contrasting oxygenation conditions (oxic, hypoxic, anoxic, euxinic) and diurnal cycles (day and night). The results enabled us firstly to better define the limits of applying the gradient method in shallow coastal areas, considering environmental conditions (water stratification, intense water mixing, low flux intensity) as well as technical limitations (concentration uncertainty). This approach then allowed to accurately capture contrasted benthic oxygen dynamics: diurnal cycles in sediments colonized by macrophytes in Prevost and Thau lagoons, and a dominant high oxygen demand in bare sediments in Berre lagoon. Benthic solute gradients in Berre lagoon indicated release of nutrients from the sediment under normoxic conditions after a long-term anoxic event, as well as release of dissolved Mn, Fe, As and Co under anoxic conditions. Under euxinic conditions, gradients reversed for most trace elements that were trapped in sediment due to (co)precipitation with sulfide minerals, while nutrient gradients indicated continuing sizeable releases to the water column. The benthic concentration gradient approach can thus be seen as a promising approach in assessing the dynamics of benthic fluxes at an appropriate time scale in highly dynamic shallow coastal lagoons. This effort should be completed by vertical turbulent diffusivity measurements, to obtain time series of turbulent fluxes of various solutes.

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.

Hard structures like dykes or groins have been recognized for their negative environmental impact and limited durability in the face of climate change (Sutton-Grier et al., 2015). The concept of Shore Soft Engineering (SSE) has allowed ecological considerations to be embedded in the design of coastal protection (Hartig et al., 2011). Among emerging defense structures, nature-based solutions are built with natural materials and rely on physical properties and mechanisms observed in nature. They aim to protect, manage and restore ecosystems while providing some benefit to the human-being and biodiversity (Cohen-Shacham et al., 2016); they offer an interesting alternative to hard structures. However, implementing these solutions in urbanized coastal areas where ecosystems are vulnerable can be challenging. Another alternative approach is using biomimetic solutions, which can provide the functions of natural systems like seagrass, dunes, or coral through robust human-made constructions. Natural habitats exist in a variety of more or less complex forms, ranging from rigid (mangroves, coral) to flexible (seagrass) (Mullarney and Henderson, 2018); and they drive changes in the current profile, in wave dissipation or in sediment motion. Understanding how to mimic these systems, in particular their internal geometry and hydrodynamic effects, is challenging. This complexity makes the development of biomimetic solutions difficult (Perricone et al., 2023). In this context, this study strictly focuses on wave dissipation by flexible systems. Wave dissipation by natural habitats has been extensively studied through laboratory experiments (Houser et al., 2015) and in-situ measurements (Bradley and Houser, 2009). The pioneer analytical model (Dalrymple et al., 1984) depicted aquatic vegetation as rigid cylinders; however, the rigidity assumption fails to capture the inherent flexibility of plants. Alternative strategies emerged to account for this flexibility, such as using an empirical drag coefficient (Mendez and Losada, 2004) or introducing an effective length, representing the length that a rigid cylinder would have to dissipate the same wave height as flexible cylinders (Luhar and Nepf, 2016). However, to define this effective length or an empirical drag coefficient, it is necessary to carry out in-situ measurements. Numerous analytical models based on force balance (Luhar and Nepf, 2016; Leclercq and de Langre, 2018) have been developed to represent the movement of flexible vegetation like seagrass. These models depict the flexible vegetation as a series of segmented rigid stems attached to each other and subjected to oscillating flows. Some models attempt to link stem motion with wave dissipation to integrate this coupling into numerical models (Yin et al., 2022). However, these models still require unknown quantities such as drag and inertial coefficients. The present study aims to develop an analytical model for determining drag and inertial coefficients based solely on the geometry, the structure flexibility and wave forcing. At last, the method could help better represent the dissipation into numerical simulations without the need for prior parametrization.

Coastal protection solutions can be categorised as grey, hybrid or natural. Grey infrastructure includes artificial structures like dykes. Natural habitats like seagrasses are considered natural protection infrastructure. Hybrid solutions combine both natural and grey infrastructure. Evidence suggests that grey solutions can negatively impact the environment, while natural habitats prevent flooding without such adverse effects and provide many ecosystem services. New types of protective solutions, called biomimetic solutions, are inspired by natural habitats and reproduce their features using artificial materials. Few studies have been conducted on these new approaches. This study aims to quantify wave dissipation observed in situ above a biomimetic solution inspired by kelps, known for their wave-dampening properties. The solution was deployed in a full water column near Palavas-les-Flots in southern France. A one-month in situ experiment showed that the biomimetic solution dissipates around 10% of total wave energy on average, whatever the meteo-marine conditions. Wave energy dissipation is frequency-dependent: short waves are dissipated, while low-frequency energy increases. An anti-dissipative effect occurs for forcing conditions with frequencies close to the eigen mode linked to the biomimetic solution’s geometry, suggesting that resonance should be considered in designing future biomimetic protection solutions.

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.

Distributed Acoustic Sensing (DAS) is a photonics technology converting seafloor telecommunications and optical fiber cables into dense arrays of strain sensors, allowing to monitor various oceanic physical processes. Yet, several applications are hindered by the limited knowledge of the transfer function between geophysical variables and DAS measurements. This study investigates the quantitative relationship between surface gravity DAS-recorded wave-generated strain signals along the seafloor and the pressure at a colocated sensor. A remarkable linear correlation is found over various sea conditions allowing us to reliably determine significant wave heights from DAS data. Utilizing linear wave potential theory, we derive an analytical transfer function linking cable deformation and wave kinematic parameters. This transfer function provides a first quantification of the effects related to surface gravity waves and fiber responses. Our results validate DAS's potential for real-time reconstruction of the surface gravity wave spectrum over extended coastal areas. It also enables the estimation of waves hydraulic parameters at depth without the need from offshore deployments.

The present paper reports on a field experiment performed over a shallow, roughness-varying barrier reef at Maupiti island, French Polynesia. The spectral wave energy balance is examined, outside the breaking zone and accounting for non-linear transfers and mean current, to estimate the wave friction factor. This latter varies from 0.05 to 4, with dependence on the ratio between near-bed orbital amplitude and roughness height well predicted by an adjusted parameterization from Madsen (1995). The present results are discussed with respect to other field data recovered on coral and rocky grounds.

Temperature is an essential oceanographic variable (EOV) that still today remains coarsely resolved below the surface and near the seafloor. Here, we gather evidence to confirm that Distributed Acoustic Sensing (DAS) technology can convert tens of kilometer-long seafloor fiber-optic telecommunication cables into dense arrays of temperature anomaly sensors having millikelvin (mK) sensitivity, thus allowing to monitor oceanic processes such as internal waves and upwelling with unprecedented detail. Notably, we report high-resolution observations of highly coherent near-inertial and super-inertial internal waves in the NW Mediterranean sea, offshore of Toulon, France, having spatial extents of a few kilometers and producing maximum thermal anomalies of more than 5 K at maximum absolute rates of more than 1 K/h. We validate our observations with in-situ oceanographic sensors and an alternative optical fiber sensing technology. Currently, DAS only provides temperature changes estimates, however practical solutions are outlined to obtain continuous absolute temperature measurements with DAS at the seafloor. Our observations grant key advantages to DAS over established temperature sensors, showing its transformative potential for the description of seafloor temperature fluctuations over an extended range of spatial and temporal scales, as well as for the understanding of the evolution of the ocean in a broad sense (e.g. physical and ecological). Diverse ocean-oriented fields could benefit from the potential applications of this fast-developing technology.

Ocean water temperature measurements are fundamental to atmospheric and ocean sciences. Obtaining them, however, often comes along with major experimental and logistic challenges. Except for the uppermost ocean surface temperature, which can be measured from satellites, temperature data of the ocean is often poorly sampled or nonexistent, especially in deep-water regions. Although Distributed Acoustic Sensing (DAS) technology has become popular because its high sensitivity to strains and mechanical vibrations, our work focuses on its usage on tens-of-kilometer-long underwater fibre-optic (FO) telecommunication cables to measure temperature anomalies at the seafloor at millikelvin (mK) sensitivity. This is possible because of the lack of dominant strain signals at frequencies less than about ∼1 mHz, as well as the poor coupling of the fibre with these signals while remaining highly sensitive to slow ambient temperature variations that locally affect its optical path length. DAS allows us to observe significant temperature anomalies at the continental shelf and slope of the Mediterranean sea, South of Toulon, France over periods of several days, with variability remaining relatively low at the deep ocean. By means of this approach, oceanic processes such as near-inertial internal waves and upwelling can be monitored at unprecedented detail.Our observations are validated with oceanographic in-situ sensors and alternative Distributed Fibre Optic Sensing (DFOS) technologies established for temperature sensing. We outline key advantages of DAS thermometry over the aforementioned sensors in terms of spatial coverage, sensitivity, versatility and highest attainable frequency. At the current state of the art, DAS can only measure temperature anomalies as opposed to absolute temperature, a drawback that could be compensated via single temperature calibration measurements.

This paper focuses on a new approach to describe coastal morphodynamics, based on optimization theory, and more specifically on the assumption that a sandy beach profile evolves in order to minimize a wave-related function, the choice of which depends on what is considered the driving force behind the coastal morphodynamic processes considered. The numerical model derived from this theory uses a gradient descent method and allows us to account for physical constraints such as sand conservation in wave flume experiments. Hence, the model automatically adapts to either wave flume or open sea settings and only involves two hyper-parameters: a sand mobility and a critical angle of repose. The ability of OptiMorph to model cross-shore beach morphodynamics is illustrated on a flume configuration. Comparison of the beach profile changes computed with OptiMorph with experimental data as well as the results from the coastal morphodynamic software XBeach demonstrates the potential of a model by wave energy minimization.

This paper focuses on a new approach to describe coastal morphodynamics, based on optimization theory, and more specifically on the assumption that a sandy seabed evolves in order to minimize a wave-related function, the choice of which depends on what is considered the driving force behind coastal morphodynamics. The numerical model derived from this theory uses a gradient descent method and permits to accounts for physical constraints such as sand conservation in basin experiments. Hence, the model automatically adapts to either basin or open sea settings and only involves two hyper-parameters: the sand abrasion and the critical angle of repose. The model behavior is illustrated on a flume configuration. Comparison of the resulting seabed with experimental data as well as with the results of the widely distributed coastal morphodynamic software XBeach demonstrate the potential of a model by wave energy minimization.

Improving knowledge of the processes driving coastal flooding is a key issue in the era of climate change, especially for decision support of public policies and operational forecasting. From regional to city scales, different processes interact, such as tide, storm surge, waves, wave setup, or infra-gravity waves, and further depend on the area topography, bathymetry, sedimentology and external forcings. While sandy beaches have been the subject of many studies, rocky beaches are still poorly understood compared to sandy beach context, despite the important part of rocky French and world coasts. In this paper, we investigate the swell transformation on the rocky beach of Ars-en-Ré with an extensive field campaign and a numerical modelling with a 3D non-hydrostatic circulation model Symphonie NH (Marsaleix et al., 2019).

Cette étude évalue la dissipation de la houle engendrée par un réseau de structures biomimétiques artificielles souples.Les structures sont pensées comme des solutions de contrôle de l'hydrodynamisme et indirectement du transport sédimentaire.Les structures biomimétiques sont généralement de formes complexes et la physique de leur effet dissipatif sur les vagues n'est pas directement intégrable dans les modèles numériques existants.Des modèles analytiques décrivant la dissipation de la houle présents dans la littérature scientifique sont testés sur le dispositif considéré.Ces formulations se basent sur l'hypothèse selon laquelle la structure est représentée par un ensemble d'éléments cylindriques rigides.Pour quantifier la dissipation réelle induite par cette solution, une campagne de mesure a été effectuée en juillet 2021 à Palavas-Les-Flots (Hérault, France).Le réseau déployé comprenait 16 structures souples réparties sur une surface totale de 16 m 2 .Pour tous les forçages de houle enregistrés, l'analyse des données montre que la dissipation moyenne mesurée est de 12,1%.Pour des conditions de forçage similaires, la valeur de dissipation moyenne obtenue à partir des différentes formulations théoriques est de l'ordre de 1,5%.Ce premier résultat montre que les modèles analytiques existants sous-estiment l'atténuation de la houle au-dessus du réseau de structures souples et sont totalement inadaptés.Il est donc envisagé de développer des formulations analytiques plus adaptées à la solution de protection étudiée pour permettre sa meilleure prise en compte dans les modèles numériques traditionnels.

The present paper reports of a field experiment over a shallow, roughness‐varying barrier reef at Maupiti island, French Polynesia. The depth‐averaged momentum balance is used to estimate the bottom drag coefficient C d , which varies from 0.01 to 0.3, with dependence on both depth and reef structure. The depth effect on C d is well predicted by a log dependency, as used in previous laboratory and fields observations. The present results extend the approach to a system with more wave exposure and higher roughness‐to‐depth ratio. Additionally, the statistical relationship between high‐resolution reef topography and hydrodynamical parameters is discussed.