Littoral Drift Impoundment at a Sandbar Breakwater: Two Case Studies along the Bight of Benin Coast (Gulf of Guinea, West Africa)

Abstract

This study assessed the deposition of sediment and shoreline evolution at two newly constructed port facilities in the Bight of Benin, West Africa. Based on the Building with Nature approach, the concept of a sandbar breakwater was implemented at the study sites. The coastal system of the bight is characterized by a sand barrier-lagoon system and a uniform prevailing wave climate, making it a favorable location for this innovative port solution. The case studies were undertaken at the Port of Lomé, Togo, and the Lekki Deep Sea Port (Dangote Sea Port), Nigeria, using remotely sensed shoreline positions and the one-line coastline change model for different periods. After construction of the breakwater, we estimated that the updrift coastline at the two locations accreted in the range of 10–23 m/year and the rates of sediment deposition were estimated to be in the magnitude of 1.0–7.0 × 105 m3/year. The comparative study conducted also showed that these rates could further reach a magnitude of 106 m3/year at other sediment-accreting landforms within the bight. We found that these large magnitudes of longshore sediment transport generated from very oblique incident waves (10°–20°) and sediment input from rivers (in orders of 106 m3/year) have enabled the realization of expected morphodynamic changes on the updrift shoreline of the ports. From these results, downdrift morphological changes should not be underestimated due to potential imbalances induced in the sedimentary budget along the coastline. Future developmental plans within the bight should also continuously aim to adopt nature-based solutions to protect the ecosystem while mitigating unforeseen implications.