Shoreline Evolution and Responses to Port Engineering at Lekki Coast, Nigeria

The increasing coastal erosion associated with human activities and resulting negative environmental impacts have necessitated the need for improved understanding of the effects of coastal engineering works on coastal processes. Here, the proposed Port of Lekki, situated in the Gulf of Guinea 60 km east of Lagos, Nigeria, is investigated as a case study. Coastal surveys of cross shore profiles and sediment sampling were conducted. Based on investigations of coastal morphology, present erosion and deposition, longshore sediment transport, wave action, beach profile and sediment grain size, a sediment transport and shoreline evolution model was established to exam shoreline evolution and shoreline responses to the proposed port breakwaters. Results indicate that the Lekki coast is wave dominated. Swell waves from the south-southwest induce a longshore current travelling to the east, and leading to eastward longshore sediment transport. The capacities of eastward and westward sediment transport vary between 602,000 m³ a⁻¹ and 963,000 m³ a⁻¹, and between 1000 m³ a⁻¹ and 18,000 m³ a⁻¹, respectively. The eastward net sediment transport capacities were estimated to be between 584,000 m³ a⁻¹ and 962,000 m³ a⁻¹. Under natural conditions, the shoreline near the proposed port is relatively stable. The model predicted that the shoreline to the west of the port will shift a maximum of 442 m, reaching the bend of the main breakwater in approximately 20 years. However, sediment bypassing will begin by approximately 6 years after construction leading to the shore shifting a maximum of 260 m due to longshore sediment transport in the lower surf zone. The shoreline to the east of the proposed port will retreat a maximum of 220 m by 20 years after construction. The maximum erosion is predicted to occur approximately 500 m to the east of the eastern breakwater, and not directly east, due to the effects of the main breakwater on wave propagation. Effective measures to alleviate erosion include the construction of groins and beach fill through dredged sands from the channel and basin.