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Abstract:The Senegal River delta in West Africa, one of the finest examples of "wave-influenced" deltas, is bounded by a spit periodically breached by waves, each breach then acting as a shifting mouth of the Senegal River. Using European Re-Analysis (ERA) hindcast wave data from 1984 to 2015 generated by the Wave Atmospheric Model (WAM) of the European Centre for Medium-Range Weather Forecasts (ECMWF), we calculated longshore sediment transport rates along the spit. We also analysed spit width, spit migration rates, and changes in the position and width of the river mouth from aerial photographs and satellite images between 1954 and 2015. In 2003, an artificial breach was cut through the spit to prevent river flooding of the historic city of St. Louis. Analysis of past spit growth rates and of the breaching length scale associated with maximum spit elongation, and a reported increase in the frequency of high flood water levels between 1994 and 2003, suggest, together, that an impending natural breach was likely to have occurred close to the time frame of the artificial 2003 breach. Following this breach, the new river mouth was widened rapidly by flood discharge evacuation, but stabilised to its usual hydraulic width of <2 km. In 2012, severe erosion of the residual spit downdrift of the mouth may have been due to a significant drop (~15%) in the longshore sand transport volume and to a lower sediment bypassing fraction across the river mouth. This wave erosion of the residual spit led to rapid exceptional widening of the mouth to~5 km that has not been compensated by updrift spit elongation. This wider mouth may now be acting as a large depocentre for sand transported alongshore from updrift, and has contributed to an increase in the tidal influence affecting the lower delta. Wave erosion of the residual spit has led to the destruction of villages, tourist facilities and infrastructure. This erosion of the spit has also exposed part of the delta plain directly to waves, and reinforced the saline intrusion within the Senegal delta. Understanding the mechanisms and processes behind these changes is important in planning of future shoreline management and decision-making regarding the articulations between coastal protection offered by the wave-built spit and flooding of the lower delta plain of the Senegal River.

Cette étude fournit, à l'échelle mensuelle et annuelle, l'ensemble des informations disponibles sur les écoulements du fleuve Sénégal de 1903 à 1983. Le bilan hydrologique est établi aux deux stations-clefs de Bakel (exutoire amont) et de Dagana (assimilée à l'embouchure du fleuve : les modules interannuels y sont respectivement de 715 et de 641 m3/s. Le caractère exceptionnel de la sécheresse actuelle est présenté au niveau du réseau de surface et de subsurface. Les flux de matières particulaires à l'embouchure du fleuve ont été évalués au cours des cycles déficitaires 1981-1982 (module : 374 m3/s) et 1982-1983 (module : 254 3/s) à 2 800 000 tonnes et 1 200 000 tonnes. Les turbidités moyennes ont été de 252 mg/1 et de 196 mg/1. Les minéraux argileux (kaoliiiite et smectite) prédominent. L'expulsion continentale annuelle de matière organique est faible et ne représente que 1,7 % des transferts solides vers l'océan. En dernier lieu, de nouvelles hypothèses sont formulées pour mieux cerner les mécanismes d'acquisition de la charge solide par les fleuves sahéliens.

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