Resolving the complex geometry of coastal areas is of utmost importance to numerically reproduce the physical processes involved in storm surges. In recent years, a few numerical models using unstructured meshes have been developed to seamlessly reproduce these small-scale features as well as larger scale processes such as tides, while retaining limited computational costs (e.g. Dietrich et al. 2012, Zhang et al 2016). Whilst providing a major step forward, these models suffer from a number of shortcomings, such as strong numerical stability constraints, or over-dissipation of short wavelengths by the numerical scheme. Here we present TOLOSA-SW, a new finite- volume model for predicting storm surges.

The French Flooding Prevention Action Program of Saint-Malo, France, requires the assessment of coastal flooding risks and the development of a local flood warning system. The first prerequisite is knowledge of the topography and bathymetry of the bay of Saint-Malo; the acquisition of new multibeam bathymetric data was performed in 2018 and 2019 to increase the resolution of the existing topo-bathymetric datasets and to produce two high-resolution (20 and 5 m) topo-bathymetric digital terrain models. Second, the hydrodynamics associated with coastal flooding were investigated through a dense and extensive oceanographic field experiment conducted during winter 2018–2019 using a network of 22 moorings with 37 sensors: the network included 2 directional buoys, 2 pressure tide gauges, 18 wave pressure gauges, 4 single-point current meters, 7 current profilers, and 4 acoustic wave current profilers from mid-depth (25 m) up to the upper beach and the dike system. The oceanographic dataset thus provides an extended overview of the hydrodynamics and wave processes in the bay of Saint-Malo from the coast up to over-flooding and over-topping areas. This dataset helps to identify the physical drivers of the coastal flooding and provides a quantification of their respective contributions. In particular, the wave processes at the foot of the protection structures can be observed: in this macro-tidal environment, during high spring tides, short and infragravity waves propagate up to the protection structures, while the wave set-up remains negligible, and over-topping by sea packs can occur. The combination of high-resolution topo-bathymetric and oceanographic datasets allows the construction, calibration and validation of a wave and hydrodynamic coupled model that is used to investigate flooding processes more deeply and might be integrated into a future local warning system by means of Saint-Malo inter-communality. The topo-bathymetric and oceanographic datasets are available freely at https://doi.org/10.17183/MNT_COTIER_GNB_PAPI_SM_20m_WGS84, https://doi.org/10.17183/MNT_COTIER_PORT_SM_PAPI_SM_5m_WGS84 and https://doi.org/10.17183/CAMPAGNE_OCEANO_STMALO (Shom, 2020a, b, 2021).

The HOMONIM-3 project is driven by Meteo-France and Shom under the project management of the DGPR (Direction Generale de la Prevention des Risques) and is aiming at improving the operational capacity to model the coastal sea level and waves to help the crisis management of the marine flooding. The main project developments will be presented, concerning the new coastal atlantic configuration of the TOLOSA-SW sea level model, the calibration of the storm surges ensemble forecast, the implementation of a waves ensemble forecasting system for the coasts, the coupled modelisation of the waves and the ocean at a very high resolution for the North Aquitaine shore, and the non-hydrostatic modelisation. All these results may be useful for the flood forecasting models, near the estuaries or the river mouths, by providing more realistic maritime boundary conditions that will be consistent with the forecasts used by the national VVS (early warning for waves and marine flooding) system, as well as an information on the previsibility of the models for a given day or on the existence of alternative maritime scenarios for the same day.

RÉSUMÉLe projet HOMONIM phase 3 est mené par MétéoFrance et le Shom sous la maîtrise d’ouvrage de la Direction Générale de la Prévention des Risques (DGPR). Il vise à améliorer les capacités opérationnelles de modélisation des niveaux marins et des vagues à la côte pour aider la gestion du risque de submersion marine. Les principaux travaux de R&D en cours et à venir dans le projet HOMONIM3 sont abordés dans cet article : configuration de façade Manche – Atlantique du nouveau modèle de niveau marin TOLOSASW, calibration de la prévision d’ensemble (PE) de surcotes, mise en place d’une PE des vagues en côtier, modélisation couplée vagues/niveaux/courants à très haute résolution, pour le littoral Nord-Aquitain, et la modélisation non hydrostatique. Ces résultats sont susceptibles de servir aux modèles de prévision des inondations à proximité des estuaires ou des fleuves près de leur embouchure, en leur fournissant, sur leur frontière maritime, une condition limite plus réaliste, et cohérente avec les prévisions utilisées pour le dispositif de la Vigilance Vagues-Submersion (VVS), ainsi qu’une information, pour un jour donné, sur la prévisibilité des modèles ou sur l’existence de scénarios océaniques alternatifs.

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).

Abstract. The French Flooding Prevention Action Program of Saint-Malo requires assessment of coastal flooding risks. The first prerequisite is a knowledge of the topography and bathymetry of the bay of Saint-Malo. In addition to existing topo-bathymetric data, the acquisition of new multibeam bathymetric data is performed. The combination of these datasets allows the generation of two high resolution topo-bathymetric digital terrain models. Then, to understand the hydrodynamic conditions which cause coastal flooding, a dense and extensive oceanographic field experiment is conducted. Oceanographic data were acquired using a network of 22 moorings with 37 sensors, during winter 2018–2019. The network included 2 directional buoys, 2 pressure tide gauges, 18 wave pressure gauges, 4 single-point current meters, 7 current profilers and 4 acoustic wave-current profilers from mid-depth (25 m) up to the upper beach and the dike system. The oceanographic dataset provides an overview of hydrodynamics in Saint-Malo bay and wave processes leading to coastal flooding. The combination of high-resolution topo-bathymetric and oceanographic datasets provides a unique capability for model validation and process studies. The topo-bathymetric and oceanographic datasets are available freely at doi : https://doi.org/10.17183/MNT_COTIER_GNB_PAPI_SM_20m_WGS84, https://doi.org/10.17183/MNT_COTIER_PORT_SM_PAPI_SM_5m_WGS84, and https://doi.org/10.17183/CAMPAGNE_OCEANO_STMALO.

This paper reviews the state of the art in storm surge forecasting and its particular application in the northern Adriatic Sea. The city of Venice already depends on operational storm surge forecasting systems to warn the population and economy of imminent flood threats, as well as help to protect the extensive cultural heritage. This will be more important in the future, with the new mobile barriers called MOSE (MOdulo Sperimentale Elettromeccanico, Experimental Electromechanical Module) that will be completed by 2021. The barriers will depend on accurate storm surge forecasting to control their operation. In this paper, the physics behind the flooding of Venice is discussed, and the state of the art of storm surge forecasting in Europe is reviewed. The challenges for the surge forecasting systems are analyzed, especially in view of uncertainty. This includes consideration of selected historic extreme events that were particularly difficult to forecast. Four potential improvements are identified: (1) improve meteorological forecasts, (2) develop ensemble forecasting, (3) assimilation of water level measurements and (4) develop a multimodel approach.

Résumé : À l’initiative de Saint-Malo Agglomération (SMA), un Programme d’Actions pour la Prévention des Inondations (PAPI) d’intention a été mis en place. Son axe 2 s’intéresse à l’amélioration de la connaissance des phénomènes météo-marins (vagues, houles, surcotes) au large de Saint-Malo. Pour y répondre, l’approche proposée est d’utiliser les moyens complémentaires que sont le recueil et l’utilisation de données in-situ d’une part, et la modélisation d’autre part. Dans un premier temps, une campagne de mesure océanographique d’envergure a été réalisée par le Shom durant l’hiver 2018-2019, en parallèle d’une campagne bathymétrique. Des données de courants, de vagues et de hauteur d’eau, ont été acquises à l’aide d’un réseau de 22 mouillages, pour un total de 37 instruments, dont 18 capteurs de pression, 7 profileurs de courant, 4 profileurs de courant houlographe, 4 courantomètres ponctuels, 2 marégraphes et 2 bouées houlographiques. L’ensemble du jeu de données acquis dans la baie de Saint-Malo, et leurs traitements sont présentés. Une première analyse des processus physiques observés est exposée. Les données acquises et les traitements effectués seront diffusés en open data sur le portail du Shom. Mots-clés : Campagne de mesure, Hydrodynamique côtière, Traitements de données de pression, PAPI, Submersion marine.

Le Shom a été sollicité par la communauté de communes de Saint-Malo pour participer à son Programme d'Actions de Prévention des Inondations (PAPI) d'intention.L'un des axes de ce programme vise à améliorer la connaissance des phénomènes météo-marins (vagues et surcotes) au large et dans la zone littorale de Saint-Malo.Afin de répondre à cette problématique, le Shom a mis en place une approche combinant l'observation et le recueil de données in situ océanographiques et bathymétriques d'une part, et la mise en place et l'utilisation de la modélisation d'autre part.Deux modèles numériques de terrain (MNT) topo-bathymétriques à haute résolution ont ainsi été produits spécifiquement, par le Shom, pour les besoins du PAPI Saint-Malo.Ces modèles bénéficient de levés lidars conduits dans le cadre du programme Litto3D ® sur l'intégralité de la zone d'étude, ainsi que des données bathymétriques récemment acquises dans le cadre du PAPI.Ce travail se focalise sur l'exploitation de ces nouvelles données topo-bathymétriques ainsi que sur la méthodologie de production et les solutions retenues.Les MNT produits dans le cadre du PAPI seront diffusés en open data sur le portail du Shom, permettant à la communauté d'utilisateurs de disposer de produits de référence à usage civil.

Résumé : Le projet HOMONIM Historique, Observation, MOdélisation des NIveaux Marins est un projet commun du SHOM et de Météo-France, sous maîtrise d’ouvrage de la Direction Générale pour la Prévention des Risques (DGPR). Il s’attache depuis plusieurs années à développer une capacité de prévision multi-échelle des surcotes et des vagues jusqu’à la côte, à l’état de l’art, compatible avec les contraintes de mise en œuvre opérationnelle, et avec pour applications principales d’alimenter le dispositif de vigilance vagues-submersion de Météo-France et de mieux anticiper le risque de submersion marine. La phase-3 du projet, qui va débuter au premier semestre 2020, a pour objectif : de contribuer à enrichir le patrimoine de données de référence existant (données bathymétriques, forçage météorologiques,...), d’améliorer le réalisme des modèles opérationnels à proximité immédiate de la côte, de fournir un contexte de modélisation très haute résolution, de référence, adapté à l’alimentation et l’interfaçage d’autres capacités de modélisation locale, des modèles de débordement ou d’outils empirique d’estimation d’impact à terre, de mieux appréhender les incertitudes sur les prévisions liées aux forçages atmosphériques et aux modèles mis en œuvre, de préparer la modélisation fine des processus littoraux pour un usage opérationnel ultérieur, tout en maîtrisant les enjeux de production opérationnelle temps réel en termes de temps calcul, de robustesse et de volumes de données générés par les chaînes opérées. Mots-clés : Submersions marines, Surcote, Vagues, Prévision, Vigilance, Opérationnalité

Le projet HOMONIM (Historique, Observation, MOdélisation des NIveaux Marins) est un projet commun du SHOM et de Météo-France, dont l'objectif est l'amélioration du système français de vigilance et d'alerte vagues-submersions, opéré par Météo-France sur les côtes métropolitaines et les départements d'outre-mer.Ce système combine les capacités de prévisions du modèle océanique HYCOM (BARAILLE & FILATOFF, 1995) dans un formalisme barotrope, et du modèle d'état de mer WaveWatch3 ® -WW3 (TOLMAN, 2016).Depuis 2017, deux nouvelles configurations ont été développées, implémentées et opérées sur les territoires d'outre-mer : -Pour la configuration Antilles-Guyane, une grille curvilinéaire est utilisée pour atteindre une résolution de 2.5 km près des côtes de Guyane et de 900 m autour des iles caribéennes.-Pour la configuration Sud-Ouest Océan Indien, une grille régulière de 3.2 km est présentée.Des configurations imbriquées, atteignant une résolution d'une centaine de mètres autour des iles françaises de Mayotte et La Réunion, sont en cours de développement.Dans cette future version, les échanges entre les grilles HYCOM sont réalisés avec le coupleur Oasis-MCT.Ces systèmes bénéficient d'optimisations numériques et développements physiques tels qu'un nouveau solveur barotrope, une friction sur le fond optimisée par une approche stochastique et des mises à jour des MNT.Ces deux configurations ainsi que leurs résultats sur quelques évènements tempétueux seront présentés.La sensibilité du modèle à la paramétrisation du stress de vent sera discutée.

The dampening effect of waves by soft mud layers is observed throughout the spectrum, in laboratory (WELLS & KEMP, 1986) as on the Louisiana or Guyana coasts (WINTERWERP et al., 2007; GENSAC, 2012). Since the bi-layer theoretical approach of GADE (1958), multi parameterizations have been proposed and implemented in wave numerical models (e.g. ROGERS & HOLLAND, 2009) but many efforts of calibrations and additional works are still required to obtain realistic representations of in situ processes. The Guyanese's coast are impacted by the Amazon sediments discharge whose 20 to 30 % migrate longshore either in turbid or in mud banks forms due to the waves and current combined actions (BISCARA, 2016). These mud banks cause rapid coastline variations, leading to accretion, erosion and submersion risks. The French operational wave forecasting system at coastal scale is based on WAVEWATCH III ® (WW3, TOLMAN, 2016), using an unstructured grid that covers Guiana with a resolution of 200 m nearshore. An implementation has been realized in 2017 in the framework of the HOMONIM project (History, Observation, Modeling sea levels, joint SHOM and Meteo-France project) for the French Guiana coasts however this version didn't include the effects of the mud and sand banks on waves. In this paper, we evaluate the mud effect on the wave propagation in order to improve the future version of the operational French Guiana configuration. Numerical tests on different parameterizations are performed on a laboratory case, to assess the behaviour of WW3. A more specific application on Guiana is carried out via the creation of a seabed map (grain size) as well as a fine description of the characteristics and location of the mud banks, thanks to high resolution satellite imagery and in-situ data. 1. Introduction Guyana shoreline is characterised by muddy sedimentation fed continuously by deposits brought to the ocean by the Amazon, 800 km further south. At the mouth of the river, this intense sediment load is set in motion by the North Brazilian current and swell, and spread along the coast of Guyana during its ascent to the north. The sediments that are deposited form huge mud banks (up to 5 m thick, 10 to 60 km long, 20 to 30 km wide and 15 to 25 km apart) that migrate rapidly (1 to 5 km.y-1) in low water depth (< 20 m) causing rapid coastline morphological changes which are difficult to predict. The mud banks present on the entire coast of Guyana quickly absorb and dissipate wave energy across the full spectrum (about 70% and more, (WELLS & KEMP, 1986)) and in particular short waves to long waves as long as they pass through the sedimentary body. In the HOMONIM project, the objective is to develop a wave forecasting model in order to better anticipate flooding from the sea and to improve warning systems on French metropolitan and overseas coasts. Initial configurations have been delivered since 2014. For Guiana, a first version was produced in 2016 (V1), based on the WAVEWATCH III ® model using an unstructured grid with a resolution of 200 m nearshore and 8 km offshore. However, this version does not include the effects of sandy and mud banks or current and water level variations on the waves. The objective of this paper is therefore to evaluate the effects of seabed sedimentary characteristics on wave propagation in order to improve the future version of the operational configuration for the French Guiana coastal area. Numerical tests on different parameterizations are performed on the laboratory case of DE WIT (1995), to assess the behaviour of WW3. A more specific application on Guiana during winter storm 2016 is then carried out via the creation of a seabed map as well as a fine description of the characteristics and location of the mud banks, thanks to high resolution satellite imagery and in-situ data.

This paper deals with the extension of mixed water plumes in a coastal environment. This problem is connected with the dynamics of tidal fronts and is studied using simplified academic configurations where tidal mixing is represented as fixed areas where stratified waters are continuously homogenized in the vertical. The sensitivity of dispersive processes to seasonal and local parameters is analyzed.Localized mixing produces baroclinically unstable structures which are shown to generate vortices that transport mixed waters trapped in their core away from the mixing area. New stratified waters then enter the mixing area so that the homogenization process can be pursued. The production rate of homogenized waters is thus closely related to the ability of vortices, or other dispersive effects, to propagate away from the mixing area and disperse homogenized waters. A way to quantify dispersion is then proposed, based on this principle.Several mechanisms leading to vortex propagation and dispersion of mixed waters are identified. We use a four layer configuration to study their sensitivity to different parameters such as topography, the presence of a coast near the mixed water plume, the vertical mixing rate, bottom friction, stratification or the existence of a background current. Potential vorticity anomaly is used as a tracer of the mixed waters and to calculate production rates as a function of the latter parameters, and to analyze the dispersive mechanisms. It is shown that:center dot Baroclinic instability represents the most efficient mechanism for dispersing the homogenized waters. The emerging vortices are indeed mainly constituted of baroclinic dipoles (or hetons) with self propagating capacities.center dot On the f-plane, a bottom slope perturbs baroclinic instability and reduces the production rate. The homogenized waters are also funneled in a plume along lines of constant depth.center dot When mixing is produced against a wall, smaller vortices emerge, which drastically reduces the baroclinic instability efficiency. Mirror effects and a thin coastal (Kelvin) current developing along shore also favor dispersion but are of weaker influence. They also funnel dispersion along the coast (an effect which is strengthened by a bottom slope) so that the mixed waters form a plume extending Northward when the wall is located on the Eastern boundary. However these effects also counteract baroclinic instability so that the production rate and dispersion efficiency globally decrease when homogenization occurs along a coast.center dot An enhancement of mixing has a limited effect on the production rate and dispersion of homogenized waters because the production of mixed waters is strongly constrained by dispersive processes rather than the ability of mixing to renew the homogenized plume.center dot A bottom friction establishes a reduced gravity dynamics and inhibits the barotropic mode. This induces a reduction of the size of vortices emerging from baroclinic instability, which strongly diminishes the production rate and dispersion efficiency. It also modifies the vertical structure of the coastal (Kelvin) current which can significantly alter the dispersion pattern, but the effect associated with this process remains difficult to predict.center dot A decrease of stratification has two counteracting effects: it reduces the size of the emerging vortices but also reinforces the coupling between layers. We have found that the latter effect is not strong enough to overcome the reduction of the size of the emerging vortices, so that baroclinic instability and dispersion of homogenized waters are inhibited when stratification is reduced.center dot Finally, when a background barotropic current is added and if mixing occurs away from boundaries, dispersion increases linearly with the velocity field above a background velocity threshold. However a tidal front developing along a coast exhibits strong asymmetries depending on the direction of the current: production and dispersion increases linearly (above a threshold) with a Southward current, whereas it is shown that a Northward current inhibits baroclinic instabilities so that the average production rate remains almost constant. In that case, dispersion exhibits specific patterns with periodic release of homogenized water.The application of these results to the real ocean is finally discussed. (C) 2012 Elsevier Ltd. All rights reserved.

The MOUTON project aims at constructing a numerical system to reproduce the oceanic circulation in restricted areas. In this framework, the HYCOM (HYbrid Coordinate Ocean Model) numerical model was improved and used to reproduce many oceanic processes that are listed here. The validation of the system is based on observations obtained during dedicated campaigns at sea.The area over which the system was tested is the Manche (English Channel), the bay of Biscay,West Portugal and the gulf of Cadiz. The results from the numerical model agree well with the observations.