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a) Study site with the location of the video monitoring area. Photography of the video monitoring system deployed at the beach of Sète. Geometry parameters (tilt, roll and azimuth) required for image rectification. Rectified plan view timex images (23 Oct 2011) with indication of the camera fields.
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Ces dernières années, de nouvelles stratégies ciblant un accompagnement de la mobilité du trait de côte plutôt que sa fixation ont vu le jour. Parmi celles-ci, les ouvrages atténuateur de houle visent à protéger la côte en dissipant l’énergie des vagues par déferlement bathymétrique, tout en restant invisibles depuis la plage. Leur utilisation a to...
Citations
... La présence de tels courants, dirigés globalement du Nord vers le Sud, est attestée par les différentes flèches littorales allongées vers le Sud [3]. Les littoraux sableux font partie des zones les plus exposées et très vulnérables au risque d'érosion car elles sont composées essentiellement de sédiments meubles [4]. L'évolution du littoral est régie par la dynamique sédimentaire le long du rivage. ...
Cette contribution a pour but d'analyser la dynamique sédimentaire, les caractéristiques granulométriques et sédimentologiques, le mode de transport et les environnements de dépôt des sédiments dans les littoraux sédimentaires de la partie septentrionale des Rivières du Sud. La méthodologie adoptée repose sur des analyses granulométriques dont le prélèvement des échantillons a été effectué suivant deux radiales notamment à Kafountine (K), à Diembéring (D). Sur chaque radiale, six échantillons sont prélevés suivant cinq unités géomorphologiques pour un total de 12 échantillons. Les analyses granulométriques sont effectuées par un granulomètre Laser de type Mastersizer 3000. L'établissement des courbes cumulatives, ainsi que le calcul des différents paramètres granulométriques ont été réalisé à l'aide d'un programme sous Excel (GRADISTAT). Les résultats montrent que les courbes cumulatives des échantillons analysés sont en forme de S, ce qui indique que les sables sont transportés dans un milieu plus au moins agité avec évacuation des particules vers le large. Dans les radiales de Kafountine et de Diembéring, le tri granulométrique s'effectue à partir du cordon dunaire vers la berme. L'indice de classement varie entre 0.1 F et 1.4 F cela indique quatre types de classement, allant de mal classé à très bien classé. Les faciès sont presque symétriques et très asymétriques vers les fins avec une granulométrie uniforme. Les plages étudiées sont caractérisées essentiellement par du sable fin et du sable moyen. Ces sédiments proviennent des dunes côtières et sont transportés généralement par saltation. Mots-clés : dynamique sédimentaire, plage, granulométrie, rivières du Sud, Basse Casamance. 158 Afrique SCIENCE 21(3) (2022) 157-173 Dome TINE et al. Abstract Granulometric characterisation and sedimentary dynamics in the northern beaches of southern rivers : the case of the Lower Casamance The aim of this contribution is to analyse the sedimentary dynamics, the granulometric and sedimentological characteristics, the mode of transport and the depositional environments of the sediments in the sedimentary littorals of the northern part of the Rivières du Sud. The methodology adopted is based on granulometric analyses, with samples being taken along two radial lines, namely at Kafountine (K) and Diembéring (D). On each radial, six samples were taken from five geomorphological units for a total of 12 samples. The granulometric analyses were carried out by a Mastersizer 3000 laser granulometer. The establishment of cumulative curves, as well as the calculation of the various granulometric parameters were carried out using an Excel program (GRADISTAT). The results show that the cumulative curves of the analysed samples are S-shaped, which indicates that the sands are transported in a agitated environment with evacuation of the particles towards the open sea. In the Kafountine and Diembéring radials, the granulometric sorting is carried out from the dune strip towards the berm. The grading index varies between 0.1 F and 1.4 F, indicating four types of grading, from poorly graded to very well graded. The facies are almost symmetrical and very asymmetrical towards the fines with a uniform grain size. The beaches studied are characterised mainly by fine and medium sand. These sediments originate from the coastal dunes and are generally transported by saltation.
... On the other hand, watercourses drained towards the sea have undergone major developments which have reduced sediment transport seaward [7,9]. One of the consequences of these changes is the gradual modification of the nearshore hydrodynamics, which is reflected by an evolution of the coastline [1,6,10]. In the particular case of the Alpes-Maritimes in France, these challenges motivated local authorities to commission a study in 2006 on the evolution of its coastline. ...
... Imitating natural reefs, the installation of wave dissipation structures a few tens or hundreds of meters from the coast is one of the "hard solutions" observed in many artificialized coastal areas. Breakwaters are sometimes recommended when sediment transport occurs mainly perpendicular to sea fronts [10]. Rockfill breakwaters with a low crown slope are designed to combat erosion and flooding. ...
... Boudouresque et al. [21] found erosion in 7 out of 10 cases in their inventory of studies that focused on the installation of artificial reefs. It can be inferred that wave dissipation does not directly control the shoreline response mode, and that complex processes affect transport of large quantities of sediment [10]. ...
Erosion, coastal submersion, or storms are issues to which many communities that have a seafront are exposed to. This study analyzes the response of an artificialized coastal area to several types of swells to assess the particularity of its response to the development of an underwater dike. The approach is based on diachronic cartographic analysis using the DSAS tool and modeling with TELEMAC. The choice of this method is supported by an existing physical modeling of the study area, which serves as field data for the validation of the results of the numerical modeling. Our study reveals the sensitivity of the study area to erosion, accretion, and marine submersion. The results show an erosion of −0.3 m/year and an accretion of +0.2 m/year, depending on the location. The dissipation of swells is effective for certain developments and leads to a reduction in the risk of degradation of infrastructures. A reduction in sediment transport is observed especially when, in addition to the construction of a dike, a beach nourishment between the structure and the waterfront is implemented. Erosion and accretion have been reduced by more than 50%. A significant reduction in costal submersion flowrates was observed. The results show that overtopping decreased by more than 90% for the type of event modelled. However, a loss of sediment towards the open sea is noted, which could lead to long-term sedimentation in the habitat of seagrasses and lead to their depletion.
... Another advantage of the spectral method over the temporal method is that a larger amount of signals in space and time are used for each point where the bathymetry is intended to be estimated. For weakly monochromatic noisy wave conditions, the spectral method is then often more robust than the temporal method (Bouvier, 2019). ...
Understanding the surf zone circulation and the morphological changes within the nearshore is essential for both scientific and societal interests. However, direct measurements with in-situ instruments are logistically challenging and expensive. The development of optical remote sensing techniques in combination with low-cost image platforms and open-source algorithms offers the possibility of collecting large amounts of information at a reasonable instrumental and computational cost. This work builds on existing and new video monitoring techniques to remotely sense the nearshore bathymetry as well as the surf zone circulation in a high-energy meso-macro tidal beach environment, including storm events. The methods are validated against a dense data set acquired during an intensive field campaign conducted at Anglet beach, SW France. For the first time the temporal and spatial variability of concurrent nearshore bathymetry and surface currents are addressed under high-energy wave forcing.
... Les littoraux doivent s'adapter à ces changements, pour ralentir l'érosion, les scientifiques préconisent de plus en plus l'utilisation de techniques "douces" (Temmerman et al., 2013). Certaines de ces solutions vont se concentrer à ralentir, dissiper l'énergie côtière tels que les atténuateurs de houle (Bouvier, 2019;Ranasinghe et al., 2006), le drainage des plages (Damiani et al., 2011) ou la pose d'herbiers artificiels (Leredde et al., 2016). Une autre solution, utilisée dans de nombreux pays (USA, Pays-Bas entre autres), est de tenter d'équilibrer le stock sédimentaire en réalisant des rechargements sableux (Dean, 2003;Hanson et al., 2002;Stive et al., 2013). ...
Dans les environnements côtiers, les interactions entre les processus morphodynamiques, océanographiques et anthropiques (agissant sur différentes échelles de temps) contrôlent l'évolution des systèmes littoraux. Cette thèse est axée sur une échelle de temps séculaire afin de déterminer l'influence non seulement des activités humaines, mais aussi l'impact du changement climatique. Le long de la côte nord de la France, des mesures de niveau d'eau ont été effectuées depuis le début du 19ème siècle et conservées dans les archives du Service Hydrographique et Océanographique de la Marine (Shom). Parrallèlement à ces mesures tidales, des levés hydrographiques ont été effectués pour cartographier les fonds marins de cette zone côtière, caractérisée par la présence de nombreux bancs sableux, formant des corps sédimentaires massifs parallèles à subparallèles au rivage. Pour déterminer les tendances à long terme, la numérisation et l'analyse des documents historiques ont été réalisées. Ceci a permis de reconstituer les variations du niveau de la mer à Dunkerque et Calais, et d'évaluer les changements de la morphologie et de la position des bancs sableux. A partir de ces séries inédites, l'étude de l'évolution des niveaux marins moyens et des composantes de marée a révélé des changements significatifs. De plus, l'évolution bathymétrique a montré des variations morphologiques importantes depuis le 19ème siècle, qui sont en grande partie dues à la mobilité des bancs sableux. Afin d'étudier l'influence des changements morphologiques sur les processus hydrodynamiques, une modélisation numérique de la propagation de la houle et de la circulation tidale a été réalisée à l'aide de module de la chaîne de calcul TELEMAC. Dans un contexte d'érosion sédimentaire, une accélération des courants et une augmentation de la hauteur des vagues le long du rivage sont détectées. Inversement, dans un contexte d'accumulation sédimentaire, nos résultats mettent en évidence une diminution de la vitesse des courants et une plus grande dissipation de l'énergie des vagues dans les petits-fonds. Cette étude souligne le rôle des rétroactions morphologiques entre l'hydrodynamique côtière et la morphologie du littoral. L'identification de ces mécanismes à une échelle de temps séculaire est essentielle pour évaluer les facteurs potentiels des changements côtiers.
Satellite monitoring of coastal dynamics: case study of beach management effects at the ''Petite Côte'' (Senegal, West Africa). The aim of this study is to show the evolution of the coastline on the beaches of Rufisque, Ndayane and Saly, located on the ''Petite Côte'' of the Senegalese coast. The methodological approach includes a multitemporal analysis of Landsat 5, 7 and 8 and Sentinel 2 satellite images, using the ''CoastSat'' tool, over a 38-year time sequence (1984-2022). The first order linear regression applied to these data reveals that the accretion trend predominates the erosion trend between 1984 and 2022. The study highlights a seasonal cycle of the coastline: coastline advances during the dry season, and retreats during the rainy season-a period when the swell is more energetic. Our results also illustrate the impact of protection structures on coastal morphodynamics. The site where the impact of coastal protection structures is most visible by detecting changes in the position of the coastline is the city of Saly. Thus, our results underline that the breakwaters have allowed a lateral extension of the beaches at this site. On the other hand, on the beaches where the presence of a groin is observed , it is possible to see the impact of the groin on the blocking of longitudinal sediment transport: the beach undergoes a significant accretion upstream of the structure, while the sections located downstream undergo erosion. This situation is observed at Rufisque and Ndayane. According to our results, the construction of protection structures has caused disparate and even asymmetrical evolution dynamics of the ''Petite Côte'', marked by sectors well served in sediment giving rise to morphotypes such as tombolos in Saly, and others that are under fed with the consequence of a retreat of the coastline (Ndayane). In the medium term, the use of soft management measures, in particular sediment cell nourishment, appears highly desirable.
The EU Water Framework Directive (WFD) ecological status classification of surface water bodies (SWBs) provides a synthetic assessment of how anthropogenic pressures impact on aquatic environments. In the evaluation of River Basins Management Plans of the second cycle (2015–2021), the EU commission reported the need for Spain: (i) to improve apportionment of pressures among individual sectors to be able to target appropriate measures, (ii) to take operational measures to mitigate climate change effects on the different sources of pressure affecting surface water quality status. Urban wastewater treatment plants (WWTPs) discharges to river SWBs represent an impactful point source of pressure. This study postulates that the dilution factor (DF) of WWTPs discharged to river flow can be estimated for EU Member states and can be used as suitable proxy to predict the EU SWBs ecological status. The implemented approach uses the available datasets centralized by the Environmental European Agency, and a focus on Spain’s situation is provided. Climate change effects on SWBs river flows are considered using a set of scenarios based on IPCC regionalized estimates. Results confirm that cumulative WWTP discharge and DF can be estimated and mapped for EU member states. The study confirms that it is statistically sound to use DF to assess the probability for a SWB to reach or not a good ecological status. 21% and 33.9% of the SWBs in Spain show a decreasing DF for the projections based on the high representative concentration pathway emission scenarios (RCPs 4.5; 8.5), respectively for the early (2011–2040) and midcentury (2041–2070) periods. Using the DF as an indicator, results show that in the span of these two periods, 55% of the 1046 SWBs with a current good ecological status have a higher than 70% probability to lose their good ecological status classification.KeywordsWaterbase databaseRCPsLogistic regressionWFDSurface water bodies
