Coastal erosion at Saint Louis in Senegal is affecting the local population that consists of primarily fishermen communities in their housing and their access to the sea. This paper aims at quantifying urban beach erosion at Saint Louis, Senegal, West Africa which is located on the northern end of the 13 km long Langue de Barbarie sand spit. The coastal evolution is examined quantitatively over a yearly period using Pleiades sub-metric satellite imagery that allows for stereogrammetry to derive Digital Elevation Models (DEMs). The comparison with ground truth data shows sub-metric differences to the satellite DEMs. Despite its interest in remote areas and developing countries that cannot count on regular surveys, the accuracy of the satellite-derived topography is in the same order as the coastal change itself, which emphasizes its current limitations. These 3D data are combined with decades-long regular Landsat and Sentinel-2 imagery derived shorelines. These observations reveal that the sand spit is stretching, narrowing at its Northern part while it is lengthening downdrift Southward, independently from climatological changes in the wave regime. A parametric model based on a stochastic cyclic sand spit behaviour allows for predicting the next northern opening of a breach and the urban erosion at Saint Louis.

Increasingly, it is reported that the coastline of the Dakar region is affected by coastal flooding due to extreme water levels during wave events. Here, we quantify the extreme coastal water levels as well as the different factors contributing to coastal flooding during the period 1994–2015. Severe water levels reach values of 1.78 m and increase by 8.4 mm/year. The time spent above this threshold has already increased by 1.7 over the study period and will increase by 2100 to 8 times with 0.4 m mean sea level rise and up to 20 times with 0.8 m in the IPCC low and high greenhouse gas emission scenarios, respectively. Tide is the main contributor to the extremes when combined with large wave runup, due to wave breaking which contributes to 38% of the increase in extreme events while sea level rises to 44%. Our results show that because of its prominent location, Dakar region is affected by waves coming from the Northern and Southern Hemispheres with contrasted evolutions: wave runup events increase faster (7 mm/year) during austral winter due to a maximum of the South Atlantic storm activity, and have a decreasing trend (−3 mm/year) during boreal winter (December, January, February) driven by the evolution of corresponding climate modes.

Understanding beach dynamics, both in time and in space, is paramount to better understand how and when to intervene to improve coastal management strategies. Beach morphodynamics is expressed in a variety of ways. As indicators of beach change, we can measure the shoreline, the beach topography, and the bathymetry; e.g., in situ measurements rarely cover large extents, are often collected on a local scale (beach), and rarely cover a sufficient time span with a sufficient surveying frequency or a simultaneous measurement of the beach and bar system. Regular-revisit satellites, such as the ESA’s Sentinel-2 mission, provide the opportunity to regularly monitor both shoreline and sandbar dynamics, and the time span is increasing and likely to continue for the decades to come. Using the satellite-derived shoreline and bar position, here, we show that the shoreline and bar are intrinsically coupled. Using Sentinel-2 satellite imagery, we show that the actual erosion/accretion status of the beach at Saint Louis (Senegal) is strongly influenced by the sandbar dynamics. There is a coupled behavior in their seasonal evolution and trend. Our results show that a very large accretive wave of about 50 m observed on the beach is driven by a local welding of the inner sandbar to the beach. Finally, we conclude that this type of event could be anticipated by an analysis of the sandbar.

Télédétection multi-capteurs du littoral à Saint Louis (Sénégal): La ville historique de Saint Louis au Sénégal est située sur la flèche sableuse de la Langue de Barbarie et est particulièrement exposée à l'érosion et à la submersion marine : des bâtiments se sont effondrés et la population a été relogée. A Saint Louis, la compréhension de la morphodynamique de la plage est plus qu'ailleurs d'une importance capitale pour améliorer les décisions de gestion basées sur la science. Ce doctorat teste la faisabilité de l'utilisation de la télédétection par satellite dans le suivi de la morphologie côtière, à la fois la topographie et la bathymétrie. Les techniques de télédétection s'appuyant sur des satellites à la demande à très haute résolution (sub-métrique) tels que la constellation Pléiades offrent désormais de nouvelles perspectives permettant l'estimation de la topographie de la partie émergée de la plage par (tri-)stéréogrammétrie, et l'inversion de la bathymétrie par la cinématique des vagues. Les missions régulières telles que Landsat et Sentinel-2 sont plus adaptées pour capturer à l'échelle régionale l'évolution de la flèche littorale sur un jeu de données de 37 ans, avec le littoral et les barres de sable. Ici, ces observations ont permis de comprendre le lien étroit entre la plage urbaine locale et l'évolution intrinsèque de la flèche sableuse régionale, ainsi qu'avec la plage subaquatique. Malgré les limites actuelles, cette thèse montre l'intérêt (croissant) de l'utilisation de la surveillance par satellite pour comprendre la dynamique des sablières avec des besoins pratiques en ingénierie côtière.