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

Abstract

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.

Why and How Do We Study Sediment Transport? Focus on Coastal Zones and Ongoing Methods

“…Scientific issues are common, and digital communication has allowed researchers to multiply worldwide collaborations, disrupting the production of knowledge. This special issue thus reflects the ongoing international cooperation between northern and southern countries on sensitive topics of common interest, thanks to the support of scientific research organizations such as the Institute of Research for Development in France, or through dedicated projects [57,351,[353][354][355][356].…”

“…Apart from net subsidence and sand mining, the shoreline evolution reacts to the cross-shore redistribution of sands between the beach and the littoral zone (down-sizing beaches in winter, reconstitution in summer, see [54]), and longshore transport induced by oblique swells. Longshore net transport following an oblique wave can be very important; along the coast of Benin, it is estimated between 1.5 × 10 6 m 3 yr −1 [55] and 500,000 m 3 yr −1 [56] of sand per year; along the Senegal coast near Saint-Louis at 600-700,000 m 3 yr −1 [57]. Accretion and erosion on tidal flats are strongly dynamic as well.…”

“…This special issue also contains works whose purpose is to quantify volumes of sediments transiting between morphological domains (estuary, beaches, spit, shelf) or which are redistributed between adjacent compartments, and which shape the coast at seasonal and interannual scales. Several illustrations concern sandy coasts in Senegal [57], Benin [351], Australia [352] and Vietnam [353], and one illustration concerns a muddy environment along the Mekong Delta [354]. Volumes of transported sediments serve as a basis for studying processes and for sensitivity analysis of shoreline changes under different forcing.…”

“…Abundant sand supply and strong wave-induced longshore drift have favoured the construction of a sand spit, which buffers wave energy and protect the back-barrier wetlands and lagoons, and is important in the regulation of the freshwater-saltwater balance and ecology of these areas. Sadio et al [57] examined the spit dynamics from aerial photographs and satellite data between 1954 and 2015 and, using the longshore sediment transport rates they calculated from 1984 to 2015 via the re-analysis hindcast wave data, they analysed the mechanisms and processes behind these changes. Their study typically integrate field surveys, time-series of aerial and satellite data, and products of reanalysis, to enable us to better understand the impact of human action in this sand spit dynamics.…”

On the key influence of remote climate variability from Tropical Cyclones, North and South Atlantic mid-latitude storms on the Senegalese coast (West Africa)

“…The West African coast (5°N-20°N, 17°W-9°E) is generally considered a storm-free environment. Nonetheless, the open coast is dominantly exposed to North Atlantic swells (Sadio et al 2017) and can also be under the influence of long travelling swell generated in the southern hemisphere as well as by tropical storms. Wave regime along the South coast of West Africa (i.e.…”