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Estuaries play a major role in the transfer of sediments from the continents to the shelves and deep ocean basins. Their position at the interface between land and sea promotes them as a key area for the understanding of ocean sediment supply, but yet long-term evolution remains poorly understood. The main reasons of the lack of knowledge about estuaries filling are the lack of hydrodynamic data in the past and the temporal application of numerical models. Oceanographers and geologists have developed numerical models to simulate currents and sedimentation. On one hand, hydro-sediment models allow a good physical representation of estuarine hydrodynamic processes and their impact on sedimentation, but only over time-scale spanning years to decades. On the other hand, stratigraphic diffusive models aim to study the impact of various geological processes on sedimentary basins over millions of years, but they are unable to describe in detail the tidal hydrodynamic processes that govern estuaries. In response to this timescale issue, this study presents a first step attempt to explore the evolution of tidal current distribution in relation with Holocene eustatic variations and seafloor evolution. Here we focus on a macro-tidal estuary, the bay of Brest, where tidal processes dominate, as the estuary is naturally protected from ocean swells. This paper aims to set up a methodology to simulate the (past) tidal currents over a long time period and correlate them with sedimentary data. Major changes in deposit dynamics are first identified from cores and seismic data, and the corresponding paleo-topographies and paleo-sea-levels are rebuilt. A process-based hydrodynamic model (MARS3D) is then used to test the impacts of these paleo-bathymetries on hydrodynamics over a 1-year time span. Four scenarii have been considered, representing four key stages of the Holocene transgression in the Bay of Brest. The simulated barotropic currents distributions were analysed and bottom cur

Storm surge modeling and forecast are the key issues for coastal risk early warning systems. As a general objective, this study aims at improving high-frequency storm surge variations modeling within the PREVIMER system (www.previmer.org), along the French Atlantic and English Channel coasts. The paper focuses on (1) sea surface drag parameterization and (2) uncertainties induced by the meteorological data quality. The modeling is based on the shallow-water version of the model for applications at regional scale (MARS), with a 2-km spatial resolution. The model computes together tide and surge, allowing properly taking into account tide-surge interactions. To select the most appropriate parameterization for the study area, a sensitivity analysis on sea surface drag parameterizations is done, based on comparisons of modeled storm surges (extracted with a tidal component analysis) with four tidal gauges, during four storm events, and over about 7.5 years, where the observed water level is processed in the same way as the modeling results. The tested drag parameterizations are a constant one, as reported by Moon et al. (J Atmos Sci 61: 2321–2333, 2007), Makin (Bound-Layer Meteorol 115: 169–176, 2005), and Charnock (J Roy Meteor Soc 81: 639–640, 1955). Charnock’s parameterization, either constant with high value (0.022) or relying on a full statistical description of the sea state, enables to improve storm surges forecast with peak errors 10 cm smaller than those computed with the other drag coefficient formulations. The impact of the meteorological forcing quality is evaluated over January 2012 from the comparison between surges modeled with different meteorological data (ARPEGE, ARPEGE High Resolution and AROME) and observations. For event time scale, storm surge computation is highly improved with ARPEGE High Resolution data. For month time scale, statistics of model accuracy are less sensitive to the choice of meteorological forcing. As a conclusion, the Charnock’s

The English Channel is characterised by strong tidal currents and a wide tidal range, such that their influence on surges is expected to be non-negligible. In order to better assess storm surges in this zone, tide-surge interactions are investigated. A preliminary data analysis on hourly surges indicates some preferential times of occurrence of large storm surges at rising tide, especially in Dunkerque. To examine this further, a numerical modelling approach is chosen, based on the 2DH shallow-water model (MARS). The surges are computed both with and without tide interaction. For the two selected events (the November 2007 North Sea and March 2008 Atlantic storms), it appears that the instantaneous tide-surge interaction is seen to be non-negligible in the eastern half of the English Channel, reaching values of 74 cm (i.e. 50% of the same event maximal storm surge) in the Dover Strait for the studied cases. This interaction decreases in westerly direction. In the risk-analysis community in France, extreme water levels have been determined assuming skew surges and tide as independent. The same hydrodynamic model is used to investigate this dependence in the English Channel. Simple computations are performed with the same meteorological forcing, while varying the tidal amplitude, and the skew surge differences D-SS are analysed. Skew surges appear to be tide-dependent, with negligible values of DSS (<0.05 m) over a large portion of the English Channel, although reaching several tens of centimetres in some locations (e. g. the Isle of Wight and Dover Strait).

This paper describes the surface Lagrangian residual circulation (LRC) over 2.5 day intervals in the Iroise Sea, west of France, and evaluates, for operational purposes, the influence of the different physical mechanisms that govern it. The method consists of the calculation of water displacements with a diagnostic Lagrangian tool that computes the trajectories of numerical particles in a given velocity field. The LRC is inferred from trajectories integrated over five M2 tidal cycles. The analysis is applied to both gridded genuine current measurements and ocean model outputs: the sea surface currents are derived from high-frequency (HF) radar measurements and from MARS, a 3-D regional ocean model used here in idealized configurations. To substantiate the analysis, the Lagrangian residual currents are also compared to genuine movements of drifters released in the Iroise Sea in 2005 and 2007. The LRC is mapped for typical scenarios identified from the Lagrangian analysis of HF radar surface currents measured in winter and summer, under weak (<7 m/s) and strong (>10 m/s) wind conditions, and in neap tide and spring tide seasons. Idealized numerical simulations that switch on and off each individual physical process are used to isolate in the LRC the patterns induced by the atmospheric forcing, tides, and density-driven currents.

The aim of this work is to assess the lagrangian residual circulation in the Iroise sea and to evaluate the relative contribution of the different physical processes at play. We calculate water displacements with a diagnostic lagrangian tool that computes the full trajectories of Active particles advected in a given velocity field. The lagrangian residual circulation is inferred from trajectories integrated over an appropriate period. This analysis is applied both to ocean model ouputs and genuine, gridded current measurements. These two sources of data are indeed complementary to study the lagrangian residual circulation according to the impact of the physical processes at play. We used sea surface currents measured by HF radars and surface currents computed with a 3D ocean model, forced by high resolution meteorological data from an atmospheric model. The lagrangian residual currents are compared to genuine movements of drifters. Our analyses and diagnoses provide a realistic description of the lagrangian residual circulation in the Iroise sea and evaluate the influence of the different driving mechanisms that govern it. We determine the contribution of tides, wind and density gradients by analysing HF radars surface currents during winter and summer, weak and strong wind conditions. idealized numerical simulations that switch on and off each individual physical processes help us refine our conclusions. To exploit this process study in terms of operational oceanography, we draw maps of the lagrangian residual circulation over five M2 tidal cycles in the Iroise sea. We provide representative samples of various meteorological and oceanic situations., La mer d'Iroise est une zone particulière qui possède une structure hydrologique et un hydrodynamisme singuliers. Les forts courants de marée dont elle est le siège participent en été a la création de fronts thermiques. Ces zones frontales permettent le développement d'une activité biologique intense. Par ailleurs, la zone Iroise se distingue par un trafic maritime très dense. Que ce soit en termes de prévention ou en temps de crise, il est essentiel de comprendre la circulation résiduelle lagrangienne dans la zone. Cette compréhension repose sur l'évaluation des contributions respectives des différents mécanismes physiques (la marée, le vent et les gradients de densité) à cette circulation résiduelle lagrangienne. La cartographie de la circulation résiduelle lagrangienne de la zone a été étudiée selon trois angles d'approche complémentaires. Les deux premiers consistent à calculer les courants résiduels lagrangiens sur un nombre fini de cycles de l'onde de marée M2 grâce à l'utilisation d'une méthode numérique de suivi de particules advectées dans des champs de courant de surface mesures par radar HF ou simulés avec un modèle d'océan. La troisième approche est une analyse fine des trajectoires suivies par des bouées dérivantes lâchées dans la zone. Afin d'obtenir des résultats pertinents, une modélisation haute résolution de la mer d'Iroise a été mise en place, suivie par le développement d'une méthodologie de calcul des courants résiduels lagrangien de la zone à partir de n'importe quels champs de courant instantanés grilles et enfin par la description de la circulation résiduelle lagrangienne de la zone basée sur l'identification de scénarios types.

This study was aimed at modeling, as realistically as possible, the dynamics and thermodynamics of the Iroise Sea by using the Model for Applications at Regional Scale (MARS), a regional ocean 3D model. The horizontal resolution of the configuration in use is 2 km with 30 vertical levels. The 3D model of the Iroise Sea is embedded in a larger model providing open boundary conditions. As regards the atmospheric forcing, the originality of this study is to force the regional ocean model with the high-resolution (6 km) regional meteorological model, Weather Research and Forecasting (WRF). In addition, as the air surface temperature is highly sensitive to the sea surface temperature (SST), this regional meteorological model is improved by taking into account a regional climatologic SST to compute meteorological parameters. By allowing a better coherence between the SST and the temperature of the atmospheric boundary layer while giving a more realistic representation of heat fluxes exchanged at the air/sea interface, this forcing constitutes a noticeable improvement of the Iroise Sea modeling. The different sensitivity tests discussed here pinpoint the importance of entering, in WRF, SST data of sufficiently high quality before the computation of meteorological forcing when the aim is a study of dynamics and thermodynamics far away from the coast. On the other hand, when the target is the reproduction of coastal small-scale features in Iroise Sea modeling, the resolution of the meteorological forcing and the quality of SST are both paramount. The simulation of reference was carried out throughout the Summer and Autumn of year 2005 to allow comparisons with a campaign of surface current measurements by high-frequency radars conducted at the same period.