L’impact de l’ouverture de la brèche dans la langue de Barbarie à Saint-Louis du Sénégal en 2003 : un changement de nature de l’aléa inondation ?

Shoreline Changes on the Wave-Influenced Senegal River Delta, West Africa: The Roles of Natural Processes and Human Interventions

“…In consequence, the much wider mouth appears to have become favourable to a larger tidal prism, manifested by an increase in the tidal range in St. Louis, and confirmed by recent studies [33,34]. Durand et al [34] showed that the maximum semi-diurnal tidal range downstream of the Diama dam has increased three-fold, from a mean of 0. Figure 6) are presumably a function of the balance between the river's hydraulic efficiency, including the tidal discharge, and incident wave energy and sediment bypassing [11].…”

“…Gac et al [27] showed that the farthest downdrift position of the mouth of the river, which corresponds to the maximal distal spit extension, did not exceed 30 km over the 80-year period covered by their observations, which is close to a value of 28 km reported in an earlier study [32]. The successive locations of the mouth of the Senegal River since 1973, which also correspond to those of the distal tip of the southward-extending spit, are shown in Figure 5 (Figure 7) that prevented further flooding [34]. Following this opening, the trench widened rapidly (Figure 8) and became the new river mouth, a case of inadvertent delta-mouth diversion generated by humans.…”

“…However, the problem seems to have more to do with accelerated urbanisation of St. Louis over the last few decades, bringing new populations to encroach on areas of the delta that are susceptible to flooding during exceptionally wet years. Durand et al [34] have highlighted the potential vulnerability of the city and the surrounding low delta plain to sea-level rise. Their model simulating flood propagation in the city, and based on various sea-level scenarios, shows the susceptibility of St. Louis to flooding during the highest annual water levels in the course of the 21st century.…”

“…Although much of the lower delta is characterised by a population density of only about ten inhabitants/km 2 , there are zones of very high population concentrations, as in St. Louis and certain areas of the Langue de Barbarie such as Guet-Ndar (Figure 9) where the 2013 census shows densities exceeding 80,000 inhabitants/km 2 [6]. The artificial breach annihilated the risk of flooding of St. Louis in 2003 and in the following years by enabling more rapid seaward drainage of river water during the high-flow season [34].…”

“…Much of the remaining spit between this new opening and the mouth was eroded through several other washovers that tended to coalesce, widening the mouth and sea-intrusion pathways, as sand was transported southward by longshore drift. (Figure 7) that prevented further flooding [34]. Following this opening, the trench widened rapidly (Figure 8) and became the new river mouth, a case of inadvertent delta-mouth diversion generated by humans.…”