Beach-ridge development and sediment supply: examples from West Africa

Abstract

Beach ridges form the dominant mode of Holocene coastal progradation in West Africa. They vary from minor accumulations to massively prograded plains over 100 km long. Variations in the patterns and morphological expression of these sandy deposits record spatial and temporal changes in various environmental parameters. Among these, sediment supply stands out. The importance of this factor is illustrated by the morphological patterns and the current sediment dynamics of minor and major beach-ridge plains in this region. The simplest beach ridges are related to just one or two sources of sand supply from a nearby source. In these examples, sand has been supplied by the nearshore zone or by eroding cliffs. In many areas, beach-ridge formation ceased as the sediment supply dwindled, because of cliff stabilization or establishment of an equilibrium nearshore profile. Major beach-ridge plains in southern Sierra Leone and in the Bight of Benin exhibit complex patterns of development related to a bigger sediment supply from nearshore sources and rivers. In southern Sierra Leone, the beach-ridge plains have been constructed essentially from sands derived from the nearshore zone. Locally, as in the proximal part of Sherbro Island, shallow nearshore shoals encroached upon by the beach-ridge front have led to more massive progradation while engendering complex patterns of beach-ridge development. The sediment supply has been locally supplemented by sands drifting alongshore from the Sewa and Moa rivers. The sediment supply involved in beach-ridge development in the Bight of Benin highlights the role of longshore drift of sands from the Volta Delta. Early stages of progradation were characterized by the development of segmented inner beach-ridge barriers separated by still infilling coastal re-entrants. Nearshore sources were particularly important in these early stages of beach-ridge progradation. As coastal re-entrants and their estuaries became infilled following the middle Holocene eustatic highstand, the longshore drift system evolved from a segmented to a unicellular one. The resulting conditions of through-drift east of the Volta Delta allowed for longshore transport of sands from this river for the construction of outer beach-ridge barriers over a distance of over 250 km. While rivers still actively supply sand to the coast, current patterns of beach-ridge development depend essentially on the coastal sand transport system which determines either the confinement of incoming sand to the vicinity of the very source area itself, its regular delivery to the beach-ridge front or its through-drift to the local downdrift terminus.