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Given recent scientific advances, coastal flooding events can be modelled even in complex environments. However, such models are computationally expensive, preventing their use for forecasting. At the same time, metamodelling techniques have been explored for coastal hydrodynamics, showing promising results. Developing such techniques for predicting coastal flood information (e.g., inland water depths) requires large enough learning datasets providing such inland information. However, detailed inland coastal flood observations are scarce and—when available—only correspond to a limited number of events. This paper aims at demonstrating how we can fill this gap by introducing a publicly available dataset, presenting its setup, and providing examples of use and recommendations. It has been built for the site of Gâvres (France), relying on the joint use of spectral wave (WW3) and non-hydrostatic wave-flow (SWASH) models, accounting for wave overtopping. It compiles 250 scenarios (defined by time-varying forcing conditions; including real and stochastically generated events) and the resulting maximal flooded areas and water depths (on 64,618 inland points). Its construction required the equivalent of 2 years of simulations on 48 cores. The examples of use of the dataset focus on method developments (metamodelling, forecast), local knowledge, and risk management.

The 2011 Great East Japan earthquake and tsunami remind us once again that these types of cascade event can occur and cause considerable damage. The scientific community realizes the need for rapid theoretical and practical progress on cascade events to provide field teams with the necessary tools and information for action during these types of events. The earthquake damage scenario for Martinique and Guadeloupe islands (French West Indies) has already been performed within the framework of French governmental projects, but these areas, in the vicinity of the French West Indies subduction zone, are also subject to tsunami events. In this study, we propose to perform a combined scenario in which an earthquake is followed by a tsunami, as it could arrive one day, considering the seismic characteristics and potential of such a subduction zone. The vulnerability of the buildings is defined considering local specific information based on several years of field inventories and inspections and is later classified into one of the 36 model building types of HAZUS. The calculation of the damages due to tsunamis follows the HAZUS methodologies. The main novelty of our study is the calculation of damage due to the two phenomena occurring one after the other, not in parallel, as is calculated in the existing literature. Therefore, for the calculation of the damages due to the second event (i.e. the tsunami), the vulnerability characteristics of the initial structure are reduced, considering the damage state of the construction after the first event (i.e. the earthquake). Hence, in our case, this calculation approach allows us to update the number of exposed elements and their changed vulnerabilities considering the damages due to the earthquake, since certain structures are already damaged by the earthquake before the arrival of the tsunami wave. The results coming from our study and our manner of treating the cascading hazards are putting into perspective with the Hazus method for combining damages coming from earthquake and the damages coming from consequently tsunami. The results expressed as the sum of the damages in both most damaged states, Extensive and Complete, are more or less in the same range of values for both studies (our study and HAZUS 2017). However, a trend of having more percentage of complete damages (and hence, less the Extensive damages) with our method than the ones obtained with the Hazus combination can be important information for crisis managing. This is a first result for the French West Indies territory, but anyway, more studies should be carried out in order to check this trend and eventually to confirm and validate this issue for others territories with others bathymetries, vulnerabilities and seismological features.

Pedreros, R.; Idier, D.; Le Roy, S.; David, A.; Schaeffer, D.; Durand, J., and Desmazes, F., 2020. Infragravity waves in a complex macro-tidal environment: High frequency hydrodynamic measurements and modelling. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1235–1239. Coconut Creek (Florida), ISSN 0749-0208.Nearshore high frequency (> 1 Hz) hydrodynamic measurements over month to year scale face many issues, especially in inter-tidal areas. We present measurements done in the intertidal area of a macro-tidal site (Gâvres, France), and on a time span of 2 years. The measurements are done with a WiSens NKE pressure sensor (burst mode of 17 minutes). Infragravity (IG) waves are present in the signal with IG wave heights reaching 0.5 m. The hydro-meteorological conditions (tide, surge, offshore short wave, wind) are characterized for each burst, showing for instance a linear increase of IG wave height with the offshore wave height (Hs) as long as Hs remains smaller than 4 m. To further investigate the temporal and spatial variability of IG waves, the analysis is completed using an unsteady phase resolving model (SWASH). We show how the IG wave height depend on the offshore wave conditions, the still water level, local wind conditions, and the local configuration, and provide hypothesis on the IG wave mechanisms generation on Gâvres.

The characterisation of past coastal flood events is crucial for risk prevention. However, it is limited by the partial character of historical information on flood events and the lack or limited quality of past hydro-meteorological data. In addition coastal flood processes are complex, driven by many hydro-meteorological processes, making mechanisms and probability analysis challenging. These issues are tackled by joining historical, statistical and modelling approaches. We focus on a macrotidal site (Gâvres, France) subject to overtopping and investigate the 1900-2010 period. A continuous hydro-meteorological database is built and a damage event database is set up based on archives, newspapers, maps and aerial photographies. Using together historic information, hindcasts and hydrodynamic models, we identified 9 flood events, among which 5 significant flood events (4 with high confidence: 1924, 1978, 2001, 2008; 1 with a lower confidence: 1904). These flood events are driven by the combination of sea-level rise, tide, atmospheric surge, offshore wave conditions and local wind. The critical conditions leading to flood are further analysed, including the effect of coastal defences, showing that the present coastal defences would not have allowed to face the hydro-meteorological conditions of 09/02/1924 for instance, whose bi-variate return periods of exceedance Tr (still water level relative to the mean sea level and significant wave height) is larger than 1000 y. In addition, Tr is expected to significantly decrease with the sea-level rise, reaching values smaller than 1 y, for 8 of the 9 historical events, for a sea-level rise of 0.63 m, which is equal to the median amount of sea-level rise projected by the 5th Assessment Report of the IPCC in this region for RCP8.5 in 2100.

The characterisation of past coastal flood events is crucial for risk prevention. However, it is limited by the partial nature of historical information on flood events and the lack or limited quality of past hydro-meteorological data. In addition, coastal flood processes are complex, driven by many hydro-meteorological processes, making mechanisms and probability analysis challenging. Here, we tackle these issues by joining historical, statistical and modelling approaches. We focus on a macrotidal site (Gâvres, France) subject to overtopping and investigate the 1900–2010 period. We build a continuous hydro-meteorological database and a damage event database using archives, newspapers, maps and aerial photographs. Using together these historical information, hindcasts and hydrodynamic models, we identify nine flood events, among which five are significant flood events (four with high confidence: 1924, 1978, 2001, 2008; one with a lower confidence: 1904). These flood events are driven by the combination of sea-level rise, tide, atmospheric surge, offshore wave conditions and local wind. We further analyse the critical conditions leading to flood, including the effect of coastal defences, showing, for instance, that the present coastal defences would not have allowed to face the hydro-meteorological conditions of 09/02/1924, whose bi-variate return periods of exceedance $$T_\mathrm{R}$$ (still water level relative to the mean sea level and significant wave height) are larger than 1000 year. In the coming decades, $$T_\mathrm{R}$$ is expected to significantly decrease with sea-level rise, reaching values smaller than 1 year, for eight of the nine historical events, for a sea-level rise of 0.63 m, which is equal to the median sea-level rise projected by the 5th Assessment Report of the IPCC in this region for RCP8.5 in 2100.

Idier, D.; Aurouet, A.; Bachoc, F.; Baills, A.; Betancourt, J.; Durand, J.; Mouche, R.; Rohmer, J.; Gamboa, F.; Klein T.; Lambert, J.; Le Cozannet, G.; Le Roy, S.; Louisor, J.; Pedreros, R., and Veron, A.L., 2020. Toward a user-based, robust and fast running method for coastal flooding forecast, early warning, and risk prevention. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1111–1116. Coconut Creek (Florida), ISSN 0749-0208.Scientific progresses now allow properly modelling coastal flooding events. Such models are nevertheless very expensive in terms of computation time (>hours) which prevents any use for forecast and warning or even for estimating the coastal flood hazard return period together with uncertainties. In addition, there is a gap between model outputs and information actually needed by the decision makers. Within the RISCOPE project, we aim at developing a user-based method contributing to forecast, early-warning and prevention of coastal flooding risks. The method should be robust, fast and integrate the complexity of coastal flood processes. To build such coastal flooding models, the solution explored relies on meta-models, i.e. mathematical functions which estimate, with good precision and at a negligible computational cost (

In this paper, we present a new source assessment of the La Palma collapse scenario previously described and studied in Abadie et al. (2012). Three scenarios (i.e., slide volumes of 20, 40 and 80 km3) are considered, from the initiation of the slide to the water waves generation, using THETIS, a 3D Navier–Stokes model. The slide is considered as a Newtonian fluid whose viscosity is adjusted to approximate a granular behavior. After 5 minutes of propagation with THETIS, the generated water wave is transferred into FUNWAVE-TVD for 15 minutes of Boussinesq model simulation. Then, four different depth-averaged codes are used to propagate the wave to the Guadeloupe area, Europe and French coasts. Finally, the wave impact in terms of run-up is evaluated through direct computations in specific areas or using theoretical formulas. Although the wave source appears reduced due to the rheology used compared to former works, the wave impact is still significant for the maximum slide volume considered on surrounding islands and coasts, as well as on remote most exposed coasts such as Guadeloupe. In Europe and in France, the wave impact is moderate (for specific areas in Spain and Portugal) to weak (Atlantic French coast). The comparison between the different wave models in overlapping computational regions shows an overall agreement in terms of first wave amplitude and time of arrival, but differences appear in the trailing waves.

In the framework of the French research project TANDEM dedicated to tsunami modelling, a series of benchmarks has been set up, addressing the various stages of a tsunami event: generation, propagation, run-up and inundation. We present here the results of five codes, involving both depth-averaged Boussinesq and fully 3D Navier-Stokes equations, aimed at being applicable to tsunami modelling. The codes are evaluated on a flow involving propagation, run-up, overtopping and reflection of the waves on two-dimensional reefs, and compared with the experimental data produced from a set of laboratory experiments carried out at the O.H. Hinsdale Wave Research Laboratory, Oregon State University (OSU, see Roeber et al., 2010 and Roeber and Chung, 2012).

Le projet ALDES a consiste a etudier, par des simulations numeriques detaillees, le risque tsunami sur 3 sites pilotes de la cote mediterraneenne francaise, pour des scenarios sismiques et gravitaires. Malgre l'intensite moderee des tsunamis susceptibles d'affecter ces cotes, Antibes apparait particulierement exposee, en particulier pour un scenario comme le glissement de terrain de l'aeroport de Nice de 1979.Cet evenement, a l'origine de nombreux dommages sur Antibes, a ete simule en s'appuyant sur des caracterisations du glissement menees par l'IFREMER. L'utilisation de calculs emboites (codes GEOWAVE et SURF-WB) a permis d'affiner la simulation jusqu'a une resolution de 3,75 m en integrant explicitement les elements urbains (modele numerique d'elevation integrant le bâti).Les resultats s'averent coherents avec les observations historiques, en termes de temps d'arrivee et de polarite de la vague sur la Baie des Anges et en termes d'inondation sur Antibes. La simulation montre que le quartier de la Salis est atteint au bout de 6 minutes par un tsunami depassant les 3 m a la cote. Cette premiere vague, la plus importante, entraine une submersion consequente avec des courants localement eleves et des run-up pouvant atteindre 4 m.Plusieurs tests de sensibilite ont ete menes sur ce cas, permettant de mettre en evidence l'importance du choix des equations (Boussinesq - Saint-Venant) et de la description du glissement pour simuler un tsunami d'origine gravitaire. Enfin, ces resultats permettent de souligner l'interet d'une prise en compte explicite du bâti pour la representativite des simulations de submersion en milieu urbain.

As the climate is changing, more applied information on resulting impacts are required to inform adaptation planning. Over the last decade, the amount of information relevant to climate change impact assessment has grown drastically. This can particularly be illustrated in coastal areas, threatened by sea-level rise due to climate change, where a key recent development has been the delivery of precise and accurate topography obtained by Light Detection and Ranging (Li-DAR) at regional and national scales, i.e., respectively, large and small scales. However, using such large, complex, and heterogeneous coastal data sets in a contextual manner is far from straightforward. It is the reason why these developments have not led to easier assessment of coastal climate change impacts so far. In this chapter, we address this interoperability challenge by developing and describing a prototype of Web service combining Li-DAR, tidal, and sea-level rise data to quickly communicate spatial information on the exposure to future coastal flooding along the French coastal zones. We discuss several issues related to data architecture, on-the-fly (geo)-processing capabilities, management of asynchronous workflows, and data diffusion strategies in the context of international standards such as Infrastructure for Spatial Information in Europe (INSPIRE). We believe that our flexible architecture mainly reusing off-the-shelf components is able to improve both complex scenarios’ analysis for experts and dissemination of these future coastal changes to the general public.

International audience; Coastal flooding assessment has considerably improved in recent years, especially thanks to the enhancements in probabilistic approaches and in numerical modeling. This study, conducted on the city of Hyères-les-Palmiers (French Riviera), aimed to evaluate how sea level rise could modify coastal flooding hazards in the urban areas located near small estuaries in a microtidal context. A joint probability approach on 3 variables allowed establishing typical storm parameters associated with specific return periods (30 years, 50 years and 100 years), taking into account both offshore conditions (sea level and significant wave height) and the river level. Storm scenarios have then been established based on these parameters and on the chronology of the most impacting recent storm (rising, peak and decrease of the storm). The sea level rise has been simply integrated, considering a rise of 20 cm for year 2030 and a rise of 60 cm for year 2100. The coastal flooding associated with these different scenarios has been simulated with a non-hydrostatic non-linear shallow-water model: SWASH (Zijlema et al., 2011). The calculations have been realized on high resolution DEM (1 to 5 m mesh size) based on LiDAR data, in order to consider the presence of buildings and coastal protections in the flooding simulation. The simulation approach has been validated by reproducing a recent flooding event in this area. Obtained results show the importance of wave overtopping in current coastal flooding hazard in this area. Nevertheless, if Hyères-les-Palmiers is currently little exposed to coastal flooding, these simulations highlight an increasing role of overflowing due to sea level rise by climate change. Consequently, the results show that some areas should be significantly flooded in 2100, even for quite frequent events. These conclusions should be used by the municipality to guide the future land use planning in the exposed areas. Overview of the overtopping and flooding simulation in Ayguade village: 30 years return period storm in 2030, situation after 8 hours of simulated storm This work has been supported by the Hyères-les-Palmiers municipality and the BRGM.

Resume : Cette etude, realisee sur la ville d’Hyeres-les-Palmiers (Var), vise a evaluer l’impact de l’elevation du niveau de la mer sous l’effet du changement climatique sur les submersions marines susceptibles d’affecter les zones urbanisees en contexte microtidal, notamment a proximite de petits estuaires. Les periodes de retour des evenements (30, 50 et 100 ans) ont ete determinees par analyse multi-variee et calcul de probabilites conjointes des parametres de forcages (vagues, niveau marin, niveau de la riviere). L’elevation du niveau de la mer (echeances 2030 et 2100) a ete fixee conformement aux recommandations utilisees pour l’etablissement des Plans de Prevention des Risques Littoraux (respectivement +20 cm et +60 cm). Le modele implemente (SWAN pour la propagation spectrale des vagues, SWASH pour la submersion a terre par franchissement et/ou debordement) rend compte des processus hydrodynamiques a haute-resolution (de 1 a 5 m), en integrant notamment le bâti et l’occupation du sol. Apres validation sur la submersion causee par la tempete de decembre 2008 (seul evenement recent de submersion suffisamment renseigne sur ce littoral), differents scenarios ont ete simules (periodes de retour 30 ans a echeance 2030 et 2100, et periodes de retour 50 et 100 ans a echeance 2100). Les resultats mettent en evidence la vulnerabilite des zones urbanisees littorales le long de la plaine du Ceinturon, et confirment certains elements de connaissance sur les processus locaux, comme le role des lones sur la propagation des eaux sur la zone arriere-littorale, les roles relatifs des processus de franchissement et de debordement dans la submersion totale et le role des ouvrages. Ces resultats pourront desormais etre utilises par la municipalite pour orienter l’amenagement futur dans les zones exposees. Mots-cles : Submersion marine, Tempete, Vague, Surcote, Franchissement par paquets de mer, Modelisation 2D, Zone urbaine.

National audience; La méthodologie présentée dans le présent article vise à simuler de manière la plus réaliste possible les franchissements par paquets de mer et la submersion qui en résulte en milieu urbain. Elle s'appuie sur des simulations régionales à locales de niveaux d'eau et de vagues pour établir des conditions de forçage pour un modèle à résolution de phase ("vague-à-vague") utilisant des schémas numériques adaptés. Ce modèle est alors utilisé sur un Modèle Numérique d'Elévation (MNE), intégrant les bâtiments, de manière à simuler simultanément le franchissement et la propagation de l'inondation. Cette méthodologie a été appliquée avec le modèle SURF-WB à la submersion survenue à Gâvres (Morbihan) lors de la tempête Johanna (10 mars 2008). Les résultats obtenus s'avèrent tout à fait cohérents avec les observations disponibles (zones de franchissement, étendue de l'inondation, hauteurs d'eau et chronologie). Cette méthode permet d'améliorer la qualité des résultats en milieu urbain par rapport aux approches plus classiques, notamment vis-à-vis de la vitesse des courants à terre. Le développement rapide et récent de ce type de modèle devrait permettre la généralisation de ce type d'approche dans les années à venir. Ce travail a été réalisé dans le cadre du projet JOHANNA (partenariat BRGM-UBO, cofinancé par la Fondation MAIF).

International audience

National audience

International audience; This work has been performed by a French national consortium within the framework of the nationalproject Tandem, with aim to improve knowledge about tsunami risk on the French coasts. Workpackage#1 of this project was the opportunity to build a database of benchmark cases to assess the capabilitiesof 18 codes, solving various set of equations with different numerical methods. 14 test cases were definedfrom the existing literature with validation data from reference simulations, theoretical solutions or lab experiments.They cover the main stages of tsunami life: 1) generation, 2) propagation, 3) run-up and submersion,and 4) impact. For each case several of the numerical codes were compared in order to identify the forces andweaknesses of the models, to quantify the errors that these models may induce, to compare the various modellingmethods, and to provide users with recommendations for practical studies. In this paper, 3 representativecases are selected and presented with an analysis of the results.