Résultats de recherche

Dans le cadre d'une collaboration avec l'intercommunalité Golfe du Morbihan - Vannes Agglomération, une équipe du Shom s'est rendue à Larmor-Baden et à Vannes. Leur but ? Préparer l'installation de deux nouveaux observatoires marégraphiques. Le premier sera déployé à l'entrée du golfe du Morbihan (ouest), à Larmor-Baden, tandis que le second sera installé à Vannes, au fond du golfe (nord-est).

Le marégraphe de Marseille est à la fois le bâtiment et l'appareil d'observation du niveau de la mer qu'il abrite. Créé en 1885 pour déterminer le niveau moyen de la mer formant l'origine des altitudes continentales françaises, il observe le niveau de la mer depuis cette époque. Classé monument historique et désormais complété d'instruments géophysiques modernes, il forme un observatoire dédié à la géodésie et aux sciences du changement climatique. Sa longue série d'observations témoigne de l'élévation du niveau de la mer dans l'ouest de la Méditerranée, avec une tendance de 1,4 mm/an depuis 1885. Les projections climatiques indiquent que le niveau de la Méditerranée devrait s'élever de plusieurs dizaines de centimètres à plusieurs mètres dans les siècles à venir. 

La carte ci-dessous présente les hauteurs de surcotes de pleines mers maximales enregistrées. Les résultats sont issus de calculs automatiques non expertisés à partir de données d'observations marégraphiques non validées.

La carte ci-dessous présente les hauteurs de surcotes de pleines mers maximales enregistrées. Les résultats sont issus de calculs automatiques non expertisés à partir de données d'observations marégraphiques non validées.

La réunion plénière du groupe de travail Tempêtes et Submersions Historique a lieu une fois par an. Elle permet aux membres du Groupe de travail tempêtes et submersions historiques de se réunir et de dresser le bilan des travaux réalisés au cours de l'année écoulée. Que vous soyez membre du GT ou sensibilisé / intéressé par les travaux du GT-TSH, les réunions plénières sont ouvertes à tous.  Le Groupe de Travail Tempêtes et Submersions Historiques (GT-TSH) organise sa plénière les 11 & 12 mars prochain. 

Cuspate shoreline or megacusps, having alongshore lengths of 100-1000 m, are features widely described, and several theories were proposed for their formation and evolution (edge waves, self-organization). Their dynamics is often related with rip-currents migration, crescentic nearshore bars evolution, even if the relationship between shoreline rhythms and inner-bar pattern appears to be extremely variable. An Argus video monitoring system was deployed in 2011 at Sète beach (French Mediterranean). This microtidal wave-dominated environment is characterized by the presence of a double crescentic nearshore bar and a cuspate shoreline with a 400 m wavelength. Wave climate is moderate at this site and most of the significant morphological evolution is observed during storm events. Monitoring of bar and shoreline evolution during two years (from March 2011 to March 2013) permitted to evidence very different behavior in the coupling between bar and shoreline rhythms. Usually, a phase coupling is observed between bar shoals and a seaward bulge in the shoreline. However, during and just after an event, evolution and its timescale is variable for both morphologies, resulting in an apparent out-of-phase relationship. Most of the time, both morphologies are in phase and their evolution on the medium-term is very comparable, and consists in a Net Longshore Migration, quite similar to the well-known Net Offshore Migration. The cusps migrate alongshore, followed by the seaward bulges in the shoreline, and when reaching a certain position, a new cusp is formed downstream and reinitiates the cycle. However, during oblique storms, the cuspate inner bar migrates very rapidly (reaching 200 m in a few hours), while the shoreline oscillations remain quite stable or show low migration due to the erosion of the flank facing incident waves. This results in an out-of-phase position of cusp and shoreline bulges. During the falling storm conditions and just after, the evolution seems to be driven mostly by the remaining wave energy. If the significant wave height decreases to rapidly, the morphologies are “frozen” and remain out-of-phase. If the wave climate remains moderate after an event, keeping the same incidence, a longshore migration of the shoreline is observed, and the seaward bulges recover a phased position with the bar horns after 10 to 20 days. These observations indicate that the coupling between crescentic nearshore bars and shoreline rhythms is timedependent, and a given period with significant energy on the falling storm is needed to recover a phased position of both morphologies.

A detection algorithm aimed at analysing localized intermittent oscillations in sea level time series and identifying meteotsunami events is presented. The algorithm is based on wavelet analysis of high frequency sea level oscillations recorded by tide gauges. Detection thresholds are fixed both for oscillation amplitude and averaged power spectral density for each tide-gauge station. This method is tested on different historic meteotsunami events and during the large event of 26 July 2012. This method is also used to characterize harbour seiche oscillation periods in the NorthWestern Mediterranean Sea.

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.