Coastal boulder deposits (CBD) provide relevant information on coast-erosion wave events, -defined here as extreme events combining energetic storm wave and high spring tide level-, and are crucial for understanding giant storm wave impacts on coastal geomorphology. Banneg Island, located off the coast of Brittany (France), faces Atlantic storm waves that are not attenuated in this highly exposed area. This island is characterized by CBD resulting from the erosion of western cliffs, that have accumulated at the top of the island as a result of storm wave/inudation processes. These accumulations have been subjected to topo-morphological survey for the last two decades (using GNSS measurements from 2005 to 2012, and using UAV-based measurements since 2012). Hydrodynamic conditions (wave and water levels) were also analyzed. We used these data to analyse the frequency of extreme events over the last 20 years. The results show a good relationship between boulder transport (measured as volume of boulder displacement) and the frequency of extreme events inundating the island. Over the survey period, we found significant interannual variability in the occurrence of these extreme events, depending on winter weather conditions. This storminess variability is significantly correlated with the WEPA climatic index, calculated on the 3 winter months (December to February). Finally, the results from Banneg Island were compared with long-term monitoring carried out in other sectors along the coast of Brittany, to build a comprehensive picture of storminess and its impact on coastal change in Brittany over the last 20 years.

Abstract This article presents a curated database of the sea‐level measurements acquired by the network of the five geodetic tide gauges managed over French Polynesia by the Geodesy Observatory of Tahiti from 13 June 2009 to 28 January 2021. A unique feature of this database, with respect to previous databases that host the same raw data, like the Intergovernmental Oceanographic Commission database (IOC, www.ioc‐sealevelmonitoring.org) and the database of ‘Réseaux de référence des observations marégraphiques’ (REFMAR, http://refmar.shom.fr) is that all the time‐tags of the raw measurements (1‐ or 2‐min sampling) have been validated and, if necessary, corrected with a precision of 2 min (time shifts of up to 1 hr can be present in the raw data). Possible outliers have also been flagged, but not removed. In addition, smoothed hourly data are also provided, along with tidal analysis results and estimations of the sea‐level trends for the five tide gauges, with respect to their local geodetic datum. The database, entitled ‘SEA LEVEL collected from TIDE STATIONS in South Pacific Ocean from 2009‐06‐13 to 2021‐01‐28’, can be accessed on the NOAA data servers as ‘NCEI Accession 0244182’ and contains two subsets: The first one is relative to the original sampling rate and the second one is relative to an hourly re‐sampling with harmonic tide models for each tide gauge station.

The study of storm surges and more generally of extreme events is a subject of growing interest in the current context of coastal risk management in a changing climate. Recent studies show that the consideration of past events improves statistical models. In this context, important investigations have been carried out in the last decade to retrieve and quantify water levels and skew surges associated with historical events for the 18th or 19th century. The estimation of the historical surges depends on the calculated tidal prediction, and therefore directly on the tidal components and the mean sea level of the period under consideration. Still, as tidal constituents and mean sea level change over time, it is difficult to estimate these parameters for historical events when sea level data are missing or incomplete. Two cases are considered for calculating tidal predictions. Either short time series are available contemporary to the event of interest, allowing a harmonic analysis and subsequent calculation of tidal constituents and mean sea levels. Or there are no or insufficient contemporary data available for the event of interest and current tidal characteristics are used in combination with a mean sea level adjustment. The present study aims to answer the following questions: depending on the data available for a historical event, which tidal constituents and mean sea level should be used when considering events that occurred several decades or even centuries ago? To what extent does the length of the observation series used in the harmonic analysis influence the tidal predictions? How does a mean sea level correction affect tidal predictions made with current tidal components? Finally, what are the uncertainties associated with the estimation of historical extreme skew surges, when there are not enough contemporary observations to estimate tidal components? Two methods are considered to estimate the uncertainties associated with the calculation of tidal predictions. Either the use of contemporary tidal constituents and mean sea level or the use of current tidal constituents corrected for mean sea level. In this study, 14 sea level records of 100 years or more provided by the Global Extreme Sea Level Analysis (GESLA version 3) were analysed. First, the uncertainties of the astronomical tides are assessed. For this purpose, different lengths of observations are implemented in a harmonic analysis and the resulting tidal constituents and tidal predictions are compared. In a second step, current tidal constituents are used for past tidal predictions and a mean sea level trend is inferred to cope with sea level variations. Different time periods are used to estimate the mean sea level trend and their influence on the tidal predictions is analysed. Furthermore, these two methods are combined to study the effects on the estimated skew surges and to assess the associated uncertainties. This study shows that uncertainties of less than 10 cm can be obtained for skew surges by using contemporary tidal components calculated from at least two years of observations. Larger differences are obtained for microtidal or river-influenced sites. When using current tidal constituents and adjusting a mean sea level trend, linear trend correction gives reliable results when more than 60 years of data are used for the trend estimation. In this case, the extreme skew surges obtained are within a range of more or less 5 cm. Finally, on the basis of the results presented, some suggestions are made on the possible applications of these methods according to the availability of data, stressing the importance of site-specific studies.

Understanding drivers of tidal change is a key challenge in predicting coastal floods in the next century. Whilst interactions between tides and atmospheric surges have been studied, the effects of wind and pressure on tides on an annual scale over the Northwestern European shelf have not been investigated. Here, a modelling approach using the shallow water MARS model is carried out to understand and quantify meteorological effects on tidal characteristics. The model setup is validated against the GESLA 3 tide gauge database. Combined and relative influences of wind and pressure are investigated using four modelling scenarios: tide only; tide, wind, and pressure; tide and wind; and tide and pressure. Influences are investigated using a single year of tidal forcing, and across multiple years of meteorological data to examine the sensitivity to temporally changing meteorological conditions. It is found that meteorology influences tidal constituent amplitudes by +/−1 cm, yielding changes that may locally reach 15 cm in the predicted highest tide. Analysis of the shallow water equations show three non-linear interaction terms between tide, wind, and pressure (advective effects, quadratic parameterization of bottom friction, and shallow water effect). Part of the observed changes is shown to arise from meteorologically induced mean sea-level changes.

In this article, we investigate the dependence of extreme surges on the North Atlantic weather regime variability across different timescales using the North Atlantic Oscillation (NAO) and Scandinavian blocking (SCAND) indices. The analysis was done using time series of surges along the North French Coast, covering long time periods (43 to 172 years of data). Time series that exhibited gaps were filled using linear interpolation to allow spectral analyses to be conducted. First, a continuous wavelet analysis on monthly maxima surges in the North French Coast was conducted to identify the multi-timescale variability. Second, a wavelet coherence analysis and maximum overlap discrete wavelet transform (MODWT) were used to study the timescale-dependent relationships between maxima surges and NAO or SCAND. Finally, NAO and SCAND were tested as physical covariates for a nonstationary generalized extreme value (GEV) distribution to fit monthly maxima surge series. Specific low-frequency variabilities characterizing these indices (extracted using MODWT) were also used as covariates to determine whether such specific variabilities would allow for even better GEV fitting. The results reveal common multi-annual timescales of variability between monthly maxima surge time series along the North French coasts: ~2–3 years, ~5–7 years, and ~12–17 years. These modes of variability were found to be mainly induced by the NAO and the SCAND. We identified a greater influence of the NAO on the monthly maxima surges of the westernmost stations (Brest, Cherbourg, Le Havre), while the SCAND showed a greater influence on the northernmost station (Dunkirk). This shows that the physical climate effects at multi-annual scales are manifested differently between the Atlantic/English Channel and the North Sea regions influenced by NAO and SCAND, respectively. Finally, the introduction of these two climate indices was found to clearly enhance GEV models as well as a few timescales of these indices.

The morphodynamic functioning of the Sillon de Talbert gravel barrier spit is analysed using a high-frequency survey carried out between September 2012 and December 2019. It is based on beach profile measurements along two transects, modelling offshore wave data (WW3), tide gauge records, and shallow waves and water levels recorded in the intertidal zone. A barrier retreat of -23 to -30 m over the 7-year survey (i.e. -3.3 to -4.3 m yr(-1)) is measured. This retreat is not related to long-term sea level rise (macroscale of 10(2)-10(3) yr), but to mesoscale (10(0)-10(2) yr) morphogenic events combining storm wave and high spring tide. Over 87-90% of the barrier retreat is due to three significant events (1-2 February 2014, 9 February 2016, and 3 January 2018). The storm impact scale model of Orford and Carter is tested. The estimation of the wave runup for the calculation of extreme water levels [i.e. peak overflow elevation (O-e) component] is based on the calibration of an equation performed from in-situ measurements of the swash elevation. The flow depth (O-d,O-q) overtopping the crest of the barrier (B-h) is thresholded by taking into account the morphological response of the barrier in order to define regimes corresponding to overtopping, discrete overwash, and sluicing overwash. While the Orford and Carter model is generally successful in reproducing the morphodynamic evolution of Sillon de Talbert, the wave energy flux (F) must be considered as an additional parameter in order to improve the fit of the model, so far as it contributes in some cases to change the morphodynamic regime. Thus, the wave energy flux constitutes a key component in the quantification of the water flow across the barrier (O-d,O-q) corresponding to the hydrodynamic forcing of the model, which becomes (O-d,O-F).

Long-period waves propagating inside harbours can lead to the generation of seiche that can affect and significantly disrupt port operations. This study is based on the analysis of a multi-years tide gauge database provided by the sea-level observation network (RONIM). It aims to propose simple models to compute seiche amplitudes at a series of French harbours. The study sites, located along the Atlantic coast, English Channel and North Sea, are representative of the diversity of French harbour configurations and are exposed to different wave regimes. First, the resonant periods of each site are determined by combining analysis of measured water levels and solutions of a linearised mild-slope equation model. Then, the role of incoming bound waves on the generation of seiches is investigated using the solutions of the WAVEWATCH III spectral wave model to compute low frequency spectra using the (Hasselmann in J Fluid Mech 12(04): 481–500, 1962) nonlinear theory. The study shows strong correlation between incoming bound waves and low frequency harbour oscillations for small ports and inside harbour waterway. For a series of sites, empirical formulations based on offshore bulk wave parameters (Bowers in Edge (ed) Proceedings of 23rd international conference on Coastal Engineering, Venice, Italy, 1992) are proposed to predict the long wave significant heights inside the harbour basins generated by short waves. Comparisons with observations show a fairly good agreement. This result suggests that seiche events can be reasonably predicted using such formulations forced by a spectral wave model as WAVEWATCH III. In addition, this study highlights the value of maintaining long-term water-level observation systems to develop operational forecasting system and improve harbour operation management.

Infragravity (hereafter IG) waves are surface ocean waves with frequencies below those of wind-generated “short waves” (typically below 0.04 Hz). Here we focus on the most common type of IG waves, those induced by the presence of groups in incident short waves. Three related mechanisms explain their generation: (1) the development, shoaling and release of waves bound to the short-wave group envelopes (2) the modulation by these envelopes of the location where short waves break, and (3) the merging of bores (breaking wave front, resembling to a hydraulic jump) inside the surfzone. When reaching shallow water (O(1–10 m)), IG waves can transfer part of their energy back to higher frequencies, a process which is highly dependent on beach slope. On gently sloping beaches, IG waves can dissipate a substantial amount of energy through depth-limited breaking. When the bottom is very rough, such as in coral reef environments, a substantial amount of energy can be dissipated through bottom friction. IG wave energy that is not dissipated is reflected seaward, predominantly for the lowest IG frequencies and on steep bottom slopes. This reflection of the lowest IG frequencies can result in the development of standing (also known as stationary) waves. Reflected IG waves can be refractively trapped so that quasi-periodic along-shore patterns, also referred to as edge waves, can develop. IG waves have a large range of implications in the hydro-sedimentary dynamics of coastal zones. For example, they can modulate current velocities in rip channels and strongly influence cross-shore and longshore mixing. On sandy beaches, IG waves can strongly impact the water table and associated groundwater flows. On gently sloping beaches and especially under storm conditions, IG waves can dominate cross-shore sediment transport, generally promoting offshore transport inside the surfzone. Under storm conditions, IG waves can also induce overwash and eventually promote dune erosion and barrier breaching. In tidal inlets, IG waves can propagate into the back-barrier lagoon during the flood phase and induce large modulations of currents and sediment transport. Their effect appears to be smaller during the ebb phase, due to blocking by countercurrents, particularly in shallow systems. On coral and rocky reefs, IG waves can dominate over short-waves and control the hydro-sedimentary dynamics over the reef flat and in the lagoon. In harbors and semi-enclosed basins, free IG waves can be amplified by resonance and induce large seiches (resonant oscillations). Lastly, free IG waves that are generated in the nearshore can cross oceans and they can also explain the development of the Earth's “hum” (background free oscillations of the solid earth).

The Sillon de Talbert is situated on the Northern coast of Brittany; it is a large 3.5 km-long swash-aligned gravel spit barrier comprising a volume of sediment of 1.23x10(6) m(3). Since 2002 a morphodynamic survey was carried out. It is based on annual DEMs and monthly beach profile measurements. Waves and water levels are also surveyed using modelling data and field measurements. The 15-years (2002-2017) sediment budget calculation shows that cross-shore transfers are dominant and represent a total volume of 370,000 m(3), while the longshore sediment transfer is estimated at about 50,000 m(3). However, considering the volume of sediment eroded from the beachface (as much as -411,000 m(3)), and the cross-shore and longshore sediment accumulation (reaching +420,000 m(3)), the global sediment budget is balanced for the whole survey period. The landward displacement of the spit due to rollover processes has increased almost doubled during the last 15 years with rates of retreat of 2 m.yr(1) between 2002 and 2017, vs 1.2 m.yr(1) between 1930 and 2010. Storm events control more than 95% of this retreat due to catastrophic overwash/inundation processes. The spit retreat has led to the opening of a breach in the proximal section at the beginning of March 2018, which has experienced a rapid enlargement (35 vs 15 m) and deepening (3.4 vs 1.25 m) over the last seven months. The morphological evolution of the Sillon de Talbert is driven by anthropogenic forcing (i.e., impact of coastal defence structures, cutting off of longshore sediment transport), and natural forcing such as the depletion of the supply of sediment from the platform, and the increase of extreme storm events over the last few years.

The Sillon de Talbert is the largest gravel barrier spit in Brittany and forms a swash-aligned formation exposed to swell that enters the English Channel from the west. It partially protects the islands of the Brehat Archipelago against coastal erosion and marine flooding of low-lying land. The main morphological changes to the Sillon de Talbert have been studied since the 17th century, and were accurately quantified between 1930 and 2010, based on early marine maps, aerial photographs and topographic surveys recently conducted by DGPS. The examination of early maps shows that this barrier was attached to the Olone Islands until the end of the 17th century. Towards the mid-18th century, a breach that had formed in the north of the barrier resulted in its transformation into a trailing spit. A gradual change in its general direction towards a more drift-aligned position promoted longshore sediment drift towards the distal end. A slow cannibalisation processes began, dividing the spit into a source area at the proximal section, a transit zone in the median section and an accumulation zone at the distal section of the spit. Long-term analysis of shoreline changes over the past decades highlights the mobility of the Sillon de Talbert, which is characterized by rapid landward retreat by rollover. This trend is mainly due to a sediment supply deficit, which can be explained above all by sediment depletion on the continental shelf and, to a lesser extent, by anthropogenic activities particularly gravel extraction. The results obtained show that for the entire period (1930-2010), the mean migration rate was 1.1 m/yr. Upon closer investigation, the rate of retreat and the main morphological changes proves to differ between the morphosedimentary units of the spit and illustrates cannibalisation processes. Thus, the proximal section shows very high retreat rates (1.35 m/yr) due to a sediment budget deficit, which can be explained by gravel migration towards the north-east section of the spit. For these sections, this evolution results in greater sensitivity to erosion and breaching during severe storms, as was the case in April 1962 and during winter 1989-90. Its median section corresponding to the transit zone, shows lower retreat rates than the proximal section (1.05 m/yr) and a relatively balanced sediment budget, as illustrated by the high resilience of this section of the spit submitted to sluicing overwash. Finally, the distal section has undergone retreat by rollover of its exposed outer face and progradation of its inner face which benefited from sediment supply from proximal section. From the 1970s, several human interventions based on coastal defense strategy attempted to slow the spit's retreat due to rollover. The failure of these interventions led to the definition of a new coastal management policy in 2004.

The objective of this study is to document the circulation in the vicinity of La Reunion and Mauritius islands, i.e., within 500 km offshore, on the intraseasonal time scale, using a high-resolution realistic modeling strategy. The simulated sea level anomalies, water mass properties, and large-scale circulation compare favorably with satellite and in situ observations. Our high-resolution simulation suggests that the currents around the islands are maximal locally, oriented southwestward, to the southeast of both islands which is not visible in low-resolution satellite observations. It also highlights the high degree of variability of the circulation, which is dominated by westward propagating features. The predominant time scale of variability is 60 days. This coincides with the period of a barotropic mode of variability confined to the Mascarene Basin. The characteristics of the westward propagating anomalies are related to baroclinic Rossby waves crossing the Indian Ocean but only in the long-wave resting ocean limit. Tracking those anomalies as eddies shows that they also have a meridional tendency in their trajectory, northward for cyclones and southward for anticyclones, which is consistent with previous studies. Sensitivity experiments suggest that they are predominantly advected from the east, but there is also local generation in the lee of the islands, due to interaction between the circulation and topography.

La mesure du niveau de la mer dans un référentiel mondial avec une précision subcentimétrique représente un réel défi dans le contexte actuel du réchauffement climatique et de l'élévation du niveau des mers qui en résulte.Les systèmes mondiaux de navigation par satellite GNSS (Global Navigation Satellite System) permettent la mesure directe du niveau de la mer rapporté à un référentiel géocentrique absolu.Nous présentons ici les résultats d'une expérience multi-instruments avec trois bouées équipées d'un système de positionnement par satellite GPS (Global Positioning System), un marégraphe radar et une échelle de marée.Cette expérience s'est déroulée à l'île d'Aix (côte ouest de la France) les 27 et 28 mars 2012.L'erreur quadratique moyenne (RMSE) calculée à partir de la différence entre les données des bouées GPS et celles du marégraphe radar est comprise entre 1 cm et 2,2 cm, ce qui est satisfaisant pour les applications marégraphiques et offre d'intéressantes perspectives pour les futures études sur l'évolution du niveau de la mer.

Measuring sea-level in a global reference frame with sub-centimeter accuracy is a relevant challenge in the context of current global warming and associated sea-level rise. Global Navigation Satellite Systems (GNSS) can provide sea-level measurements directly referenced in an absolute geocentric frame. We present here the results of a multi-instruments experiment with three buoys equipped with Global Positioning System (GPS), a radar tide gauge and a tide pole. This experiment was carried out at Aix Island (West coast of France) on the 27-28 March 2012. The GPS buoys were evaluated against conventional tide gauge measurements through a Van de Casteele test. The Root Mean Square Error (RMSE) computed from the difference between the GPS-buoys and radar tide gauge data ranges from 1 cm to 2.2 cm, which is suitable for tidal applications and offers interesting perspectives for future sea-level variations studies.

The ECOLOPHY experiments aimed at investigating physical exchanges between coastal and open sea. They were carried out in June and December 2005 over the shelf-break in the North-eastern part of the Gulf of Lions (Northwestern Mediterranean Sea). This area is considered to be the generation zone for the eddy and meandering structures of the Northern Current (NC). The objective of the present work is to examine mesoscale variability of this coastal slope current in the light of available data. Numerical modeling is used to support the field data analysis. ADCP current measurements over a one-year period show that mesoscale activity is maximal in late winter, correlating with the seasonal variability of the NC and, also, partly with local winds. Measured currents exhibit mesoscale fluctuations with periods ranging from 3 to 30 days, in agreement with previous analyses. Fluctuations of periods longer than 10 days are found to be mainly oriented in the direction of the mean current, whereas more frequently observed high frequency fluctuations tend to be oriented cross-slope, suggesting a relationship with the NC mesoscale meandering. Moreover, trajectories of surdrift buoys launched in the NC vein exhibit mesoscale phenomena, such as current meanders or eddies and on-shelf intrusions. Numerical modeling provides a synoptic point of view and is used hereafter to support physical interpretation of punctual eulerian or lagrangian measurements. Therefore, modeled hydrodynamic fields are used to analyze surdrift buoy trajectories and computed vertically averaged current and Ertel potential vorticity provide a better understanding of these behaviors.

This paper presents the main results obtained by a long term experiment using HF radars which was conducted in the eastern part of the Gulf of Lions. The analysis focuses on statistical observations of the North Current, a large scale current vein occurring in the North of the Western Mediterranean, of current motions at diurnal and quasi-inertial periods and of submeso-mesoscale eddies. The study uses high resolution modellings of wind and current fields.

A comparison of chlorophyll data from SeaWiFS imagery and modeling results from a 3D hydrodynamical model was performed over the northwestern Mediterranean for the entire year of 2001. The study aims at investigating the information content brought by satellite-derived chlorophyll concentration ([Chl]) maps concerning surface dynamics in coastal zone. The study is mainly focused on the Gulf of Lions (GoL) and its outer region, which are mainly influenced by the Rhône River, local winds and the Northern Current (NC) flowing from the East along the continental slope. The physical hydrodynamical model was continuously run and 40 SeaWiFS images, presenting a significant coverage of the studied area, were selected. The comparison between [Chl] and sea surface salinity (SSS) fields on a pixel basis showed no definite correlation trends. Three reasons are given in discussion for that result. However, the comparison emphasized areas close to the coasts which were under the influence of different inputs not considered in the model and also of upwellings. A qualitative analysis of the data performed out of these regions exhibited significant similarities between [Chl] and SSS features. The signature of the Rhône ROFI (Region of Fresh Water Influence) and, in some cases, of the NC, was evidenced on [Chl] maps. We found that the intensity of this signature is seasonally modulated, e.g., it is low in open sea during the summer, oligotrophic, season. In addition, the signature of the Rhône ROFI in the western part of the GoL can be only partial due to local chlorophyll deficits. We conclude that, for the regional case studied, chlorophyll imagery can be used as a tracer of surface dynamics through surface salinity but with limitations, especially near the coasts.

A chain of three nested models, based on the MARS 3D code, is used to simulate the North-western Mediterranean Sea circulation with a finest grid of 1.2 km resolution and 30 vertical sigma levels. This modelling system allows to resolve the coastal dynamics taking into account the influence of the general basin circulation. The aim of this study is to assess the ability of the nested MARS-3D models to reproduce most of the circulation features observed in the North-western Mediterranean Basin and in the Gulf of Lions. Comparisons of modelled sea surface temperature and salinity with MEDAR/MEDATLAS climatology and NOAA/AVHRR satellite measurements show that the model accurately reproduces the large and coastal scale variability. Over the Northern Basin, the seasonal changes of the cyclonic gyre extension are correctly simulated, even though in summer, the modelled temperature of the surface layer remains in basin-average 1°C cooler than the satellite measured temperature. As soon as the stratification erodes, modelled and observed temperatures become closer. Over the Gulf of Lions, realistic coastal responses are obtained under different wind conditions. Upwellings are correctly located and their intensity and spatial extension were here improved by the use of Aladin wind fields (10 km spatial resolution) and the introduction of a drag coefficient fitted according to the stability of the planetary boundary layer. The dispersion of fresh Rhone water discharge and the mesoscale circulation simulated by MARS-3D also agree with satellite measurements.

Current meter measurements have been carried out for 3 years on the continental slope of the Gulf of Guinea, near 7.5∘S off the Angola coast. Currents in a water depth of 1300m over the continental slope show a remarkable biweekly oscillation, bottom intensified, and with currents oriented nearly parallel to the isobaths. With a peak-to-peak amplitude reaching 20–30cms-1 at 30m above the bottom, this signal is the most energetic at sub-inertial frequencies. Simultaneous measurements deeper on the continental rise (in a water depth of 4000m) show a more complex signal dominated by lower frequencies, and with less clear polarization. Simple linear topographic wave theories are compared to the observations. A combination of coastal trapped waves with cross-slope mode 3–5 could be consistent with the observed currents. A three-dimensional 1/6∘ model suggests the existence of modes trapped to the slope, although with lower amplitude than observed.