Submarine dune dynamics are controlled by tidal currents and wind forces. According to the relative influence of these forces and the nature of dune sediment, different bedform behaviors can be observed. The footprint of the different hydrodynamic agents is recorded into the internal architecture of dunes. This paper is concerned with bedforms that compose the thick sediment wedge located in the eastern English Channel, off the Bay of Somme. This sedimentary archive constitutes an interesting feature to achieve a better understanding of seabed sediment dynamics and its timeline building stages. The dynamics of large submarine dunes, which are organized in fields, are studied thanks to bathymetric and seismic data over the periods 1937-1993 and 1993-2007. Dune morphology presents low lee and stoss side slopes (on average 8°and 3°, respectively) and dune migration rate is not very high. Dune movements are in the direction of residual tidal currents, i.e. toward the east, with mean migration rates around 0·8 to 5 Ϯ 0·25 m yr -1 and up to 6·6 Ϯ 0·7 m yr -1 , respectively, at multi-decennial and decennial time scales. The dune internal architecture is complex with superimposed eastward prograding units, displaying locally opposite progradation. Second-order discontinuities (dip of 0·5°-4°perpendicular to dune crests) constitute dune master bedding. By counting the number of second-order reflectors between 1937-1993 and 1993-2007, the formation periodicity of these bounding surfaces is estimated to range from 4 to 18 years. These time intervals coincide with the long-term tidal cyclicities and also with the inter-annual to decennial variability of storm activity in northern Europe. Two theories were made to interpret the dune internal structures: the second-order surfaces are interpreted either as the depositional surfaces corresponding to the marks of weak energy periods (weak tidal and storm action), or as erosive surfaces due to an opposite direction of dune migration provoked temporarily by exceptional storms from the northeast.

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Résumé :La hausse du niveau de la mer liée aux changements climatiques globaux demeure l'une des préoccupations sociétales majeures. L'analyse des données marégraphiques, constituant souvent les seules informations disponibles à l'échelle des décennies, voire des siècles passés, se révèle être un moyen idéal pour parvenir à dégager des tendances sur le long terme. Or, malgré un patrimoine historique marégraphique français conséquent, la majorité de ces mesures est encore sous forme papier et n'est donc pas exploitée. Dans ce contexte, un important travail de "Data rescue", ou sauvetage des données historiques, est entrepris au Shom depuis quelques années afin d'inventorier et de valoriser ses archives marégraphiques historiques. L'exemple de la reconstruction de la série marégraphique de Saint-Nazaire est présenté pour illustrer les étapes nécessaires à ce type de travail. Les données du marégraphe ont été numérisées, contrôlées, corrigées et validées, ce qui a abouti à la constitution d'une série de hauteurs d'eau horaires couvrant une période de plus de 150 ans (1863-aujourd'hui). Cette série est globalement de bonne qualité (seules 16 % des données digitalisées qualifiées de "potentiellement de mauvaise qualité") et est caractérisée par une tendance à l'augmentation des niveaux moyens à Saint-Nazaire depuis 1863 de l'ordre de +1.03± 0.05 mm/an.

Au cours de la dernière décennie, la tempête Xynthia et d'autres ont marqué l'actualité de par leur intensité et les dégâts causés lors de leur passage sur le territoire français. Ces événements peuvent donner l'impression qu'ils n'ont été "jamais vu par le passé", mais le recensement des tempêtes plus anciennes (1953, 1987, 1999…) montre que de tels phénomènes surviennent en fait régulièrement sur le littoral métropolitain. Dès lors, la collecte et l'analyse d'informations historiques apparaissent incontournables pour améliorer la prévention des risques littoraux, en particulier liée à la submersion marine. Afin de mutualiser les informations disponibles en France, une base de données relationnelle et spatiale a été développée au sein de l'IRSN. À ce stade, plus de 650 événements de tempête et/ou de submersion sur le littoral français et des pays voisins ont pu être recensés, sur une période s'étendant de 1500 à nos jours. En parallèle à l'élaboration de cette base de données, un groupe de travail interdisciplinaire s'est constitué. Il intègre des ingénieurs, chercheurs, statisticiens et historiens appartenant à différents organismes. Un axe majeur de travail est l'intégration de nouvelles données historiques issues de diverses archives dans la base de données, ainsi que leur interprétation et la quantification des niveaux marins associés.

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Coastal water level measurements represent one of the earliest geophysical measurements and allow an assessment of historical sea level rise and trends in tides, river flow and storm surge. However, recovery and digitization of archival tidal records have been much less widespread and systematic than, for example meteorological records. In this contribution, we discuss data rescue efforts and lessons learned in France, the United States and the United Kingdom, countries with early and extensive tide gauge networks by the mid-19th century. We highlight the importance of (a) cataloguing the historical gauge records, as a first step towards locating them; (b) locating data in archives, and then recovering and saving data by any means necessary, including photographs and scanning; (c) obtaining metadata, including both quantitative survey records, gauge checks and clock data, but also qualitative records such as gauge notes, letters and reports; and (d) quantitative statistical analysis of data and datum quality, using both standard data-entry checks but also tools that leverage the unique predictability of tide measurements. Methods for digitizing original analogue records are also discussed, including semi-automatic, computer-based methods of digitizing tidal charts (marigrams). Although the current best practice is described, future improvements are desirable and needed to make the more than estimated 10,000 station years of unused, undigitized records available to the scientific community.

No abstract

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.