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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.

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.

Anthropogenic factors associated with damming and water abstraction, and the resultant environmental pressures, are reviewed in six African river catchments using records and forecasts of climatic, demographic, and land-use change. Changes in the states of the flow regime through catchment drainage systems to the coastal sea are considered in conjunction with climate change and other human-induced pressures. The impacts of these changes on downstream and coastal environments and their communities are described in past, present, and future perspectives. Linkages between the issues and the pressures of damming and water abstraction are appraised and scientific, policy, and management responses proposed aimed at remedying existing and perceived future negative impacts. The study proposes that there is a need to integrate catchment and coastal management to account for the whole water flow regime together with its human dimensions. Management priorities relating to the operation of existing damming and abstraction schemes and planning of future schemes include the following: consideration of ways in which water discharges could be adjusted to provide improvements in downstream and coastal environmental and socioeconomic conditions; addressing the problem of sediment trapping impacting on the sustainability of dam reservoirs; and assessment of downstream and coastal impacts of future schemes in the light of climate change forecasts.

No abstract

This paper examines the environmental and socio-economic consequences of hydraulic infrastructural changes along the Senegal River valley and estuary. During the 1980s, two dams were built along the valley floor to facilitate hydro-electrical production and regular water supply for crop irrigation. In 2003, a breach was dug across Barbary Spit to alleviate flooding in the nearby Saint-Louis city. Although these structures helped regulate the river flow, they also resulted in a series of unforeseen impacts. The study examines these, after on-site measurements (topometry, water salinity, piezometry), analysis of hydrological data, field observations, and surveys with dam managers, state regional directions and local stakeholders. It stresses the urgency of elaborating models of hydraulic management to limit the negative consequences of the hydraulic constructions. These models should consider the variety of water uses in the middle valley and delta, and also the water safety and quality (salinity) downriver from Diama Dam.

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Climate change is causing wide-ranging effects on ecosystem services critical to coastal communities and livelihoods, creating an urgent need to adapt. Most studies of climate change adaptation consist of narrative descriptions of individual cases or global synthesis, making it difficult to formulate and test locally rooted but generalizable hypotheses about adaptation processes. In contrast, researchers in this study analyzed key points in climate change adaptation derived from coordinated fieldwork in seven coastal communities around the world, including Arctic, temperate, and tropical areas on four continents. Study communities faced multiple challenges from sea level rise and warmer ocean temperatures, including coastal erosion, increasing salinity, and ecological changes. We analyzed how the communities adapted to climate effects and other co-occurring forces for change, focusing on most important changes to local livelihoods and societies, and barriers to and enablers of adaptation. Although many factors contributed to adaptation, communities with strong self-organized local institutions appeared better able to adapt without substantial loss of well-being than communities where these institutions were weak or absent. Key features of these institutions included setting and enforcing rules locally and communication across scales. Self-governing local institutions have been associated with sustainable management of natural resources. In our study communities, analogous institutions played a similar role to moderate adverse effects from climate-driven environmental change. The findings suggest that policies to strengthen, recognize, and accommodate local institutions could improve adaptation outcomes.