La façade nord des Hauts-de-France correspond à une frange côtière de 60 km orientée OSO-ENE ouverte sur la mer du Nord. De nombreux bancs sableux sont présents le long de ce littoral où ils forment des corps sédimentaires massifs parallèles à obliques au trait de côte, localisés sur l'avant-côte jusqu'à plusieurs dizaines de mètres de profondeur. Des levés bathymétriques ont été exécutés depuis les années 1830 dans cette région. Les résultats de ces levés hydrographiques sont conservés au format papier dans les archives du Shom. Un premier travail d'inventaire, de scannage, de numérisation et de calages (géographiques et vertical) de ces documents a été réalisé. L'analyse des évolutions bathymétriques révèle une forte variabilité de la morphologie de l'avant-côte depuis le 19 ème siècle, notamment en raison de la mobilité des bancs prélittoraux. Nous présentons les résultats obtenus sur deux sites, à l'Est de Dunkerque et au niveau de Calais. Les bancs sableux sur ces sites (Riden de la Rade à Calais et Banc Hills à Dunkerque) tendent à se déplacer longitudinalement et à se rapprocher de la côte, mais avec des vitesses variables. Deux périodes se démarquent en terme d'évolution pour les deux sites, fin 19 ème avec un fort exhaussement des bancs et mi-20 ème siècle avec une accélération de la migration des corps sableux. La mise en place d'une modélisation hydrodynamique (houle et courants) sur les différents lots de bathymétries disponibles permettra d'apprécier les interactions entre l'évolution morphologique de l'avant-côte et l'hydrodynamique littorale. Mots-clés : Dynamique du trait de côte, Morphologie des petits-fonds, Bancs sableux, Hauts-de-France, Mer du Nord.
Tidal sand banks are common along the coast of northern France facing the North Sea, where they form linear shore-parallel or slightly oblique sand bodies from shallow coastal areas to depths of tens of meters. Hydrographic surveys have been carried out since the 1830s for mapping the seabed of the coastal zone. An analysis of the bathymetry evolution shows significant morphological changes have occurred across the shoreface since the early 19th century, largely due to cross-shore and longshore sand bank migration. Our results show that nearshore sand banks mainly migrated onshore and gained sediment, especially during the 20th century; acting as temporary sediment sinks, which can in turn serve as sand sources for providing sediment to the coast. Alongshore, the migration and elongation of sand banks can be related to tidal asymmetry that is mostly directed to the east-north-east in the region. Shore-perpendicular movement can likely be explained by the action of shore-normal storm-waves in the nearshore zone after their refraction over shallow offshore sand banks. A seaward displacement of sand banks was also observed. This may be related to the combined action of waves and tidal currents which can induce erosion on one side of the bank, decreasing its width, and eventually leading to its seaward migration. Our observations point out that some nearshore sand banks respond to the action of currents and waves, and interact between each other via feedback morphodynamic processes induced by sand bank morphological changes. The substantial morphologic changes that affected the nearshore zone of northern France during the last centuries probably had large impacts on coastal hydrodynamics and associated shoreline evolution.
The shoreface off the coast of northern France is characterized by the presence of numerous tidal sand banks, forming linear shore-parallel or slightly oblique massive sand bodies. Digitization of historic hydrographic field sheets from the French Hydrographic Service (Shom) archives enabled to produce Digital Elevation Models (DEMs) of the seabed morphology at different dates since the early 19th century. Our analyses of bathymetry changes based on differential DEMs showed significant morphological variations across the shoreface near Calais during the 19th and 20th centuries, which are largely due to changes in nearshore sand bank morphology and position. Sand bank mobility can be partially explained by tidal current asymmetry, inducing an elongation and an alongshore migration of this sand body in the direction of the net residual tidal current. Observed changes in shoreface and nearshore morphology attempt to be related with possible variations in hydrodynamics, this paper presenting the results obtained on tidal currents. TELEMAC-2D model was used to simulate 2D velocity field and water depth during a spring and neap tidal cycle over bathymetry grids based on historical bathymetric soundings. Depending on the bathymetric setting, modeling of tidal residual currents revealed notable increases or decreases in residual current velocity during distinct time periods, particularly in nearshore areas, which may lead to seabed erosion where current strengthening is observed through time or conversely to sediment accumulation in areas of decreasing current velocity. These results are consistent with the shoreline changes that occurred during the 20th century, with a correspondence between nearshore areas of decreasing (or increasing) residual current velocity and adjacent shorelines that were affected by seaward progradation (or erosion), suggesting strong links between changes in seabed morphology over the shoreface and shoreline evolution.
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.
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