Sand barrier development in the bight of Benin, West Africa

The morphology, sedimentary facies, stratigraphy and development history of re-entrants and sand barriers in Togo and Benin (West Africa) have been analysed from boreholes and field data, and available radiocarbon dates. Re-entrant deposits range from basal Late Pleistocene fluviatile sands into Holocene estuarine and fluvio-deltaic sands. As sea level stabilized between 6000 and 5500 B.P., the “compartmented” coast associated with these re-entrants favoured a polycellular longshore drift system. As the latter became infilled throughout the bight of Benin between 6000 and 4000 B.P., the littoral drift system evolved into a unicellular system fed by the Volta delta in Ghana, which has been the major sand source for an almost 300 km long outer barrier. In spite of the massive and consistent volume of sand drifting eastward (1.2–1.5×106 m3/yr), progradation slowed down or ceased as equilibrium was achieved between coastal/shoreface geometry and the hydrodynamic regime, resulting in a balance between incoming and outgoing sand, and the downdrift migration of the accumulation sector towards Nigeria. This situation was probably achieved around 1500 to 2500 years ago in Benin, but apparently occurred much earlier in the updrift sector in Togo. This stability has been perturbed by the construction of deepwater ports in 1962 in Cotonou and 1967 in Lomé leading to segmentation of the unidirectional drift and spectacular progradation and erosion, respectively, updrift and downdrift of breakwaters.