The Lagos coast - Investigation of the long-term morphological impact of the Eko Atlantic City project

摘要

The Lagos coast has been suffering high rates of erosion since the construction of three harbour moles, i.e. the West Mole, East Mole and the Training Mole, at the tidal inlet connecting the Lagos Lagoon to the South Atlantic Ocean. To provide for a permanent erosion mitigation measure and to create residential and commercial area for circa 400,000 people, the Eko Atlantic City project has been initiated in 2008. In front of the eroded coast, approximately 9 km² of land will be reclaimed and protected by a revetment. In this study the long-term and large-scale morphological behaviour of the Lagos coast is investigated and subsequently the long-term morphological impact of the Eko Atlantic City project is assessed. First, a conceptual model is created, in which the historical development of the coast is discussed. The long-term morphological behaviour of the coast downstream of the inlet is determined by two main factors: sediment accumulation at the West Mole and sediment import into the tidal inlet and the lagoon, induced by disturbance of the morphological equilibrium by sea level rise and dredging activities. Using the numerical simulation model Unibest, the long-term impact of Eko Atlantic City is assessed. It is concluded that the construction of Eko Atlantic City will not change the total erosion volumes occurring downstream of the inlet. However, as the revetment of the project retains the coast, the erosion will be shifted towards downstream. Downdrift of the project, the erosion rates are locally relatively high. The shape of the sea defence has been designed to minimize the local erosion effect. A monitoring and mitigation strategy has been recommended to monitor this effect and to instruct coastal protection management actions to be implemented if required.