One of the largest and busiest ports in Africa, Lagos is the commercial capital of Nigeria and the second fastest growing city on this continent. The location of Victoria Island, the central area of the capital, at the edge of the Atlantic Ocean leaves little room for expansion, and lagoons cover over a fifth of the land in Lagos state.
The coastline averages only around two metres above sea level and has always suffered long term coastal erosion. Historical maps show that the shoreline once extended almost three km into the Atlantic, and continuing erosion has caused parts of the coastline to collapse into the sea, increasing the risk of flooding. The Lagos State Government (LASG) decided to find a sustainable and permanent solution which would also resolve the chronic land shortage in the city and enable further expansion.
The LASG and South Energyx Nigeria Limited (SENL) began feasibility studies in 2004, with Royal HaskoningDHV providing consultancy for the marine works. A visionary solution was proposed to resolve both issues – a new city, with effective flood defences, to be built on land reclaimed from the sea. With an area of approximately 10km2, Eko Atlantic would have a waterfront, an efficient transport system, high quality infrastructure with commercial and residential accommodation, and leisure facilities.
Royal HaskoningDHV’s multidisciplinary experts were responsible for the conceptual design, detailed marine designs, numerous studies related to the hydrology and hydraulics in and around the development, the environmental impact assessment of the dredging and reclamation works and the initial master plan. They were also responsible for the design and construction supervision of the sea defence revetment and supervision of the dredging and reclamation works.
“After extensive discussions with SENL, we decided it would be more appropriate to construct a sea defence structure on the line of the original coastline of 1905 rather than where the sea had advanced”, explains Colin Secklington, Royal HaskoningDHV’s project manager.
“We proposed a natural layout of the revetment and sand filling using the Dutch principle of ‘building with nature’, a modern design philosophy for sustainable coastal rehabilitation which has already been used successfully around the globe. Changes to the coastline follow the historical trends while taking into account the current man-made situation, in this particular case the Lagos harbour breakwaters, or ‘moles’. The revetment has been designed as a great amenity for the new city, providing an esplanade and a view across the Atlantic Ocean, as well as giving coastal protection.”
Called the ‘Great Wall of Lagos’, the 8.4km long and 18m high sea defence revetment in the Atlantic ocean has to withstand the treacherous sea conditions caused by the long swell waves generated from storms in the southern part of the Atlantic. It also has to retain more than 90 million m3 of reclamation sand fill which will form the foundation for the new city.
“There are many technical and logistical challenges with constructing this revetment,” continues Colin. “The sea conditions are hazardous and we had to tailor the design so that rock placement uses land based equipment only. Strict criteria were set to protect the city from coastal flooding and provide a high comfort and security level, and also for the hydraulic stability of the revetment. The designs were modelled and tested in both 2D flumes and 3D basins at the Danish Hydraulic Institute in Denmark.”
The revetment needs to be constructed as fast as possible to protect the sand fill area, while the actual works depend on the sand filling to provide access to construction vehicles. This delicate balancing act was resolved by using a small strip of sand filling on the landward side of the newly constructed section of the revetment, and at the same time protecting the temporary works with rock armour to form the permanent revetment. The temporary works proceed ahead of the permanent construction, with major sand filling operations continuing in the fully sheltered environment on the landward side of the completed sea defence revetment.
The halfway point was reached in February 2013 with over 46 million m3 of material reclaimed and deposited in the area of the new Eko Atlantic city, giving a land area of over 5 million m2 and a sea defence revetment 3.8km long. At the celebrations to mark the occasion, the former US President Bill Clinton commended the innovative and visionary development, and predicted that Eko Atlantic City would draw tourists from all over the world.