Patterns of Sand Spit Development and Their Management Implications on Deltaic, Drift-Aligned Coasts: The Cases of the Senegal and Volta River Delta Spits, West Africa

Abstract

The sand spits associated with the Volta and Senegal River deltas, the two largest river deltas in West Africa, after that of the Niger River, show complex patterns of morphodynamic development while also strongly reflecting the recent impacts of human activities. The large spit of the Volta delta seems to be a direct outgrowth of a natural change in the location of the mouth of the Volta and of a marked reduction in sand supply on the eastern coast of Ghana that largely predated the construction of the Akosombo dam, but which has been strongly aggravated since this dam was completed in 1961. Spit formation has led, in particular, to segmentation of the unique sand drift cell that prevailed on the Bight of Benin coast between the Volta delta mouth and the western confines of the Niger delta. These changes have been associated with strong gradients in longshore drift and fundamental modifications in the dynamics of sand barriers on the Bight of Benin coast and hitherto fed by sand supplied by the Volta River. The spit has prograded massively by the adjunction, in situ, of new individual beach ridges, rather than by undergoing elongation, a pattern of growth that has entailed sand sequestering within the confines of the delta. The distal tip of the spit has recently welded to the shoreline, creating a new barrier-lagoon system, and assuring the resumption of integral sand drift from the mouth of the Volta towards the rest of the hitherto sand-starved Bight of Benin coast. The process should also offer limited respite from erosion to the beleaguered town of Keta, located just downdrift of the former distal tip of the now welded spit. The Langue de Barbarie spit at the mouth of the Senegal River delta is an outgrowth of strong longshore drift affecting one of the finest examples of a wave-dominated delta. The spit developed jointly with the delta and appears to have inexorably extended downdrift in conjunction with river-mouth diversion and migration southwards during the late Holocene, albeit with occasional breaches, the oldest of which are only preserved north of the city of St. Louis. The spit has had a downdrift migration range of about 30 km, beyond which alongshore sand drift and river discharge conditions led to stabilisation of the position of the river mouth. Numerous natural breaches have been identified on the Barbarie spit between 1800 and the present day, no doubt related to years of exceptionally high river discharge in an overall Sahel-influenced climatic context of rather irregular discharge. The recent evolution and potential future demise of the spit reflect the consequences of hasty and short-sighted artificial breaching to solve an impending flooding problem facing the historic city of St. Louis in 2003. An artificial breach through the spit created by engineers in just a few hours to counter a risk of flooding of St. Louis from exceptional river discharge in 2003 has had dramatic consequences on the integrity of the spit, notably by acting as a new river mouth that underwent rapid and significant widening. Part of the spit sand further downdrift of the new mouth is being recycled into river-mouth bars, while the rest of the eroded sand is transported downdrift by longshore currents, including beyond the sealed pre-2003 mouth. This dismantling of the spit has had dramatic consequences on recent settlements and tourist facilities and infrastructure. The erosion is probably a joint result of sequestering of part of the sand load transported by longshore currents in the new widened river mouth and a reinforced tidal prism through this new mouth. A new phase of spit dismantling and the formation of a second mouth a few kilometres downdrift of the new mouth occurred during high river discharge conditions in October 2012, thus illustrating the potential seasonal effect of high river outflow on spit reworking.