La compréhension de la morphologie est limitée par le manque de continuité entre l'échelle locale (vidéo, drone), et l'échelle régionale : les nouvelles technologies peuventelles répondre à ce besoin ?La ville de Saint Louis, située à l'embouchure du Fleuve Sénégal, et séparée de l'Atlantique par la flèche sableuse de la Langue de Barbarie, représente un lieu phare de la vulnérabilité littorale liée aux changements climatiques, et aux enjeux socio-économiques sur la scène internationale.Les satellites Pléïades (CNES/Airbus) ont une résolution de 1 m rééchantillonnée à 0.5 m, et peuvent couvrir une zone de 350 km*20 km.Une reconstruction topographique à partir de ces clichés est donc réalisable par méthode de tri-stéréogrammétrie, avec le logiciel Ames Stéréo Pipeline (ASP) développé par la NASA.Des séquences rafales (vidéo) de trois à dix images de la Langue de Barbarie (dT ~ 8s) sont acquises tous les 6 mois, et nous offrent une vision inégalée de la dynamique sédimentaire de la zone.Cette méthodologie a déjà montré des résultats prometteurs (ALMEIDA et al., 2019) lors de son application sur la côte Aquitaine.
Les flèches sableuses de Joal et de Djiffère, situées sur la Petite Côte du Sénégal, présentent des évolutions différentes. La cinématique du trait de côte est assez complexe à Joal contrairement à Palmarin-Djiffère où le taux de recul est graduel, en particulier aux extrémités nord et sud de la flèche. La tendance moyenne de recul des traits de côte digitalisés après correction géométrique (1989 à 2013) donne un taux de recul insignifiant de – 0,82 m/an (20,4 m) à Joal et important à Palmarin-Djiffère ( -3,83 m/an soit 91,92 m). Les bilans sédimentaires trimestriels obtenus par suivi des profils de plage de janvier 2012 à septembre 2013 indiquent des échanges sédimentaires permettant d'identifier les mois de janvier, février, juillet et août comme des périodes d'érosion, et mars et septembre de reconstitution des plages. La dynamique de ces littoraux est en cohérence avec le régime des houles de la côte sud de Dakar et de la situation d'ouverture de la brèche de Sangomar.
Ndour, A.; Ba, K.; Almar, A.; Almeida, P.; Sall, M.; Diedhiou; P.M., Floc'h, F.; Daly, C.; Grandjean, P.; Boivin, J-P; Castelle, B.; Marieu, V.; Biausque, M.; Detandt, G.; Tomety Folly, S.; Bonou, F.; Capet, X.; Garlan, T., Marchesiello, P.; Benshila, R.; Diaz, H.; Bergsma, E.; Sadio, M.; Sakho, I., and Sy, B.A., 2020. On the natural and anthropogenic drivers of the Senegalese (West Africa) low coast evolution: Saint Louis Beach 2016 COASTVAR experiment and 3D modeling of short term coastal protection measures. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 583-587. Coconut Creek (Florida), ISSN 0749-0208.West Africa's low and densely populated coasts crystallize most of the environmental and societal problems and resulting vulnerability. It is becoming urgent to document this coast and the natural and anthropogenic forces to understand its evolution. Saint Louis is a historic (World Heritage) city located on the Langue de Barbarie, a 10 km sandspit at the mouth of the Senegal River. Because of its location, it is vulnerable (erosion, flooding) to river and ocean variability. This intermediate barred microtidal beach is located in a storm-free intertropical environment, but is exposed to distant oblique energetic waves from high latitudes, causing one of the highest coastal drifts in the world (∼800,000 m3/year). As part of the COASTVAR project, an intensive international field experiment was conducted in Saint Louis from 4 to 13 December 2016 to quantify the natural protective role played by the sandbar, coastal currents and transient exchanges with the inner shelf. Many instruments have been deployed to measure waves, currents, bathymetry and topography. This article provides an overview on the objectives of the experiment, the deployment and the first results of the modeling of coastal protection strategy.
Ngom, H.; Ndour, A., and Niang, I., 2018. Impacts of protective structures on sandy beaches: Example of the Saly balnear station, Petite Côte, Senegal. In: Almar, R.; Almeida, L.P.; Trung Viet, N., and Sall, M. (eds.), Tropical Coastal and Estuarine Dynamics. Journal ofCoastal Research, Special Issue No. 81, pp. 114–121. Coconut Creek (Florida), ISSN 0749-0208.Several protective structures have been built in recent years to protect sandy beaches of the seaside resort of Saly threatened by erosion processes. However, the protective seawalls, groynes and breakwaters have not produced the expected results. Erosion processes continue on the sandy coast of Saly. The shoreline evolution was estimated by two methods: a study of the evolution of the shoreline between 1989 and 2016 by the DSAS and a monthly monitoring in 2014 of a series of 8 beach profiles. The results show a considerable sedimentary imbalance between a northwestern sector generally in accumulation and a southeastern sector under erosion separated by the groyne of the Safary residence. The combination of different protective structures inhibits the manifestation of positive effects of some of them. In the southeastern sector, breakwaters slightly reduce the erosion rates of the beach which are still affected by the return flows associated with the groynes of northwestern sector. Riprap walls occupy recreational beaches and are subject to scour and submersion that make them unstable. All beaches just upstream of the groynes are generally in accretion. Beaches protected by newly constructed groynes are experiencing a remarkable accretion. All this shows that groynes are much more favorable than the breakwaters or seawalls for accretion on the sandy coast of Saly. A well-sized and structured groynes field that takes into account the physical and climatic characteristics of the coast could in the long term stabilize the sandy beaches of Saly.
Abstract. The historical city of Saint Louis in Senegal is situated on the Langue de Barbarie sand spit and is particularly prone to erosion: buildings have collapsed and population relocated due to shoreline retreat. At Saint Louis understanding the beach-morphodynamics is essential, and relies on the monitoring of nearshore topography and bathymetry. Remote sensing techniques relying on very high resolution (sub metric) satellites such as the Pleiades constellation and Planet now offer new perspectives in coastal monitoring and engineering. Digital Elevation Models (DEMs) of the emerged part of the beach, topography, and the submerged nearshore bathymetry can be obtained using respectively (tri-)stereogrammetry, and depth inversion through wave kinematics or/and colour based methods. These methods offered promising results (Almeida et al., 2019, Taveneau et al., 2021, Almar et al., 2019a) in previous studies when applied to Pleiades satellite. This work showcases a DEM derived from a 3-images (dT ≃ 9.5s) Pleiades acquisition in March 2020 and covering the key topography/bathymetry morphological continuum. Root mean squared errors of about 5 m for bathymetry and 0.9 m for topography are obtained. To limit erosion at Saint Louis urban beaches, the construction of a protection structure has begun in late 2020. Here, satellite DEM is used to monitor the efficiency of the structure and its downstream impact along the sand spit.
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.
This current study aims to quantify Goree island littoral evolution from 1942 to 2011 in the context of sea level rise. Digital Shoreline Analysis System (DSAS), in ArcGIS software was used to extract shoreline positions based on satellites and aerials photography (1942, 1966 and 2011) and estimate historic rate of change. Rates of change statistics were calculated using End Point Rate (EPR) and Net Shoreline Movement (NMS). In this study, EPR was calculated by dividing NMS by elapsed time between earliest and latest measurements for each transect. Results obtained show that shoreline between 1942 and 1966 retreated (-4m) at an average rate of -0.16m /year, compared an average of -0.06m / year between 1966 and 2011. This dynamic can be interpreted as result of project to preserve and restore Goree island (1979-1981). Slave house, mosque of island, Mariama BA School, and historical museum are the most sector affected by erosion. Analysis of results showed that causes are more natural than anthropic. These results provide coastal erosion specialists with valuable information for warning, prevention and adaptation measures against natural disasters such as flooding.
'%3e%3cg id='b'%3e%3cpath id='a' d='M0-100V0h50z' transform='rotate(18 0 -100)'/%3e%3cuse xlink:href='%23a' transform='scale(-1 1)'/%3e%3c/g%3e%3cuse xlink:href='%23b' transform='rotate(72)'/%3e%3cuse xlink:href='%23b' transform='rotate(144)'/%3e%3cuse xlink:href='%23b' transform='rotate(216)'/%3e%3cuse xlink:href='%23b' transform='rotate(288)'/%3e%3c/g%3e%3c/svg%3e)
