Formation et évolution fini-holocènes et dynamique actuelle du Delta Saloum - Gambie (Sénégal - Afrique de l'Ouest) : géomorphologie, stratigraphie, sédimentologie et dynamique sédimentaire

L'étude porte sur la mise en place fini-holocène des unités constituant le delta Saloum-Gambie. L'analyse morphosédimentaire, stratigraphique et chronologique des dépôts des chenaux, des cordons, des vasières et des tannes est associée à l'étude de la dynamique sédimentaire actuelle pour en déterminer les différentes étapes. Le travail s'est, en conséquence, appuyé sur la caractérisation des formations internes relativement stabilisées du delta et de celles du littoral. La méthodologie de terrain utilisée dans le delta interne s'appuie sur des sondages avec des prélèvements à différentes profondeurs tandis que les variations du littoral très instable sont analysées à partir de profils topobathymétriques associés à un échantillonage sédimentaire de surface. Ce travail de terrain a été complété par l'utilisation de la télédétection sur photos aériennes, en raison des difficultés majeures d'accés sur certaines îles du delta méridional. L'analyse texturale, minéralogique et exoscopique conduit à définir des populations sédimentaires en relation avec les provinces géologiques et les réseaux hydrographiques. Les facteurs responsables des changements morphodynamiques actuels sont étudiés et les stocks sédimentaires déplacés sont évalués en fonction du temps. La position du delta Saloum-Gambie dans un domaine intertropical aux influences contrastées, sahéliennes sèches au nord et tropicales humides au sud, lui confére une bipolarité climatique. Celle-ci se manifeste dans le régime des apports et dans le type et le mode d'évolution des unités sédimentaires qui caractérisent, finalement, un découpage du delta en deux parties distinctes séparées par un espace non encore comblé, le Diomboss. Le delta septentrional, bien que mis en place par le fleuve Saloum, évolue en fonction d'apports éoliens récents et de la dérive littorale du nord... (D'après résumé d'auteur

Clay Fraction Distribution in the Sediments of the Senegal River Estuary after the Diama (Senegal) Dam Construction

International audienceClay fraction contents distribution in the Senegal River estuary after the building of the anti-salt Diama dam has shown that upstream sediments are very muddy. Clay fraction content is as high as 60 % of sediments in the middle of the channel with a pronounced downstream decreasing trend on the banks comparing to the contents in the middle of the channel. Towards the river mouth, the fine fraction tends to disappear leading to a shelly sand sediment type essentially. During low water period, clay fraction contents increase particularly at the vicinity of the dam reservoir. In the estuarine reach, downstream the dam, this increasing trend during the low water stage is particularly marked in the middle of the channel. The clay mineral assemblage shows that kaolinite and smectite are the main components, whereas interlayered illite-smectite and illite are minor constituents. The upstream-downstream profile shows a decrease in kaolinite content although it remains the dominant constituent. Floating clay minerals (smectite and illite-smectite) concentrate rather in the middle of the channel and on the right bank where the water depth is higher. According to seasonal variations, the distribution of these clay minerals often follows the concentration of the entire clay fraction; their contents increase during low water stage period. This hydrodynamics sorting of floating minerals, evident in the up-dam zone, tends to disappear in the eastern reach between Diama dam and the mouth. Clay minerals assemblage shows, through the time, a rather homogeneous composition which does not seem to be altered by the seasonal fluctuations

Rapport II.2 Les mécanismes d'envasement de certaines zones littorales

Silting mechanics of certain littoral zones. The paper analyses the mechanics of mud sedimentation specific to three tidal seashore sites : littoral mud-banks and marshes, channels adjacent to estuaries, and ostreicultural areas. They share numerous analogies in the factors that goven their silt-up : -the settling-lag effect (Van Straaten) , difference between the floating silt and the carrying-back in suspension by the ebb : phenomenon that is normal in marshy areas but amplified in estuary-neighbouring channels ; -the biological factors : diatoms or reeds on sea-shore marshes, molluscs for shellfish culture ; -human activities : part played by the sluices and the conchological installations.On analyse les mécanismes de la sédimentation des vases propres à trois milieux littoraux particuliers situés dans les mers à marée : les vasières littorales et les roselières, les étiers adjacents aux estuaires, et les zones ostréicoles. Elles présentent de nombreuses analogies dans les facteurs qui régissent leur envasement : -le settling lag effect (Van Straaten), différence entre les apports en flot et la remise en suspension par le jusant, phénomène normal sur les vasières, amplifiés dans les étiers, -les facteurs biologiques : diatomées ou roseaux sur les vasières ou étiers, mollusques pour la conchyliculture, -activités humaines : rôle des écluses et des installations conchylicoles.Diara Mariline, Sornin Jean-Marc, Ottmann François, Nikodic Jean, Gouleau Dominique. Rapport II.2 Les mécanismes d'envasement de certaines zones littorales. In: L'hydraulique et la maitrise du littoral. Problèmes côtiers posés par le mouvement des sédiments et la pollution. Dix-huitièmes journées de l'hydraulique. Marseille, 11-13 septembre 1984. Tome 2, 1984

Study of the spatial variability of marine pollution around the peninsula of Cape Verde

International audienceMarine pollution, the scourge of modern times, is due to the runoff of domestic and industrial waters as well as to various anthropogenic activities, i.e. products and objects deliberately or accidentally discharged into the sea. The samples taken from 11 sites on the Cap-Vert peninsula in Senegal, indicate the presence of certain polluting substances in varying amounts. The objective of this work is to study the correlations between the physical, microbiological and chemical parameters in order to highlight the similarities between the sites and, if possible, to determine the most relevant parameter(s) to characterize the pollution. PCA results have shown that some sites appear to be less chemically polluted than others that are more polluted with eutrophication and chemicals (e.g., copper, mercury). From a physical point of view, for example, we observe that the characteristics of sediments (large silt, clays, fine silt) are related to certain chemical parameters.The AFC performed between the overall toxicity of the sediments and the microbiological quality of the water shows that the site of Ouakam has a medium toxicity and a good microbiological quality while that of Cambérène and the Vivier are characterized respectively by bad and good quality but also by low toxicity at both sites. The two sites of Hann (Hann1 and Hann2), Soumbédioune, Ngor, Yoff Tonghor and Dakar Le Dantec are characterized by high toxicity and poor microbiological quality. Those in the Madeleine Islands and the Port of Dakar are characterized by high toxicity and bad microbiological quality. Moreover, as expected Soumbédioune appears as the most polluted sites in terms of microbiological load. The interest of the multivariate approach (ACP and AFC) is then discussed in this type of analysis

Study of the spatial variability of marine pollution around the peninsula of Cape Verde

International audienceMarine pollution, the scourge of modern times, is due to the runoff of domestic and industrial waters as well as to various anthropogenic activities, i.e. products and objects deliberately or accidentally discharged into the sea. The samples taken from 11 sites on the Cap-Vert peninsula in Senegal, indicate the presence of certain polluting substances in varying amounts. The objective of this work is to study the correlations between the physical, microbiological and chemical parameters in order to highlight the similarities between the sites and, if possible, to determine the most relevant parameter(s) to characterize the pollution. PCA results have shown that some sites appear to be less chemically polluted than others that are more polluted with eutrophication and chemicals (e.g., copper, mercury). From a physical point of view, for example, we observe that the characteristics of sediments (large silt, clays, fine silt) are related to certain chemical parameters.The AFC performed between the overall toxicity of the sediments and the microbiological quality of the water shows that the site of Ouakam has a medium toxicity and a good microbiological quality while that of Cambérène and the Vivier are characterized respectively by bad and good quality but also by low toxicity at both sites. The two sites of Hann (Hann1 and Hann2), Soumbédioune, Ngor, Yoff Tonghor and Dakar Le Dantec are characterized by high toxicity and poor microbiological quality. Those in the Madeleine Islands and the Port of Dakar are characterized by high toxicity and bad microbiological quality. Moreover, as expected Soumbédioune appears as the most polluted sites in terms of microbiological load. The interest of the multivariate approach (ACP and AFC) is then discussed in this type of analysis

Study of the spatial variability of marine pollution around the peninsula of Cape Verde

International audienceMarine pollution, the scourge of modern times, is due to the runoff of domestic and industrial waters as well as to various anthropogenic activities, i.e. products and objects deliberately or accidentally discharged into the sea. The samples taken from 11 sites on the Cap-Vert peninsula in Senegal, indicate the presence of certain polluting substances in varying amounts. The objective of this work is to study the correlations between the physical, microbiological and chemical parameters in order to highlight the similarities between the sites and, if possible, to determine the most relevant parameter(s) to characterize the pollution. PCA results have shown that some sites appear to be less chemically polluted than others that are more polluted with eutrophication and chemicals (e.g., copper, mercury). From a physical point of view, for example, we observe that the characteristics of sediments (large silt, clays, fine silt) are related to certain chemical parameters.The AFC performed between the overall toxicity of the sediments and the microbiological quality of the water shows that the site of Ouakam has a medium toxicity and a good microbiological quality while that of Cambérène and the Vivier are characterized respectively by bad and good quality but also by low toxicity at both sites. The two sites of Hann (Hann1 and Hann2), Soumbédioune, Ngor, Yoff Tonghor and Dakar Le Dantec are characterized by high toxicity and poor microbiological quality. Those in the Madeleine Islands and the Port of Dakar are characterized by high toxicity and bad microbiological quality. Moreover, as expected Soumbédioune appears as the most polluted sites in terms of microbiological load. The interest of the multivariate approach (ACP and AFC) is then discussed in this type of analysis