THE GREAT WALL OF LAGOS Eko Atlantic City By: Colin Seckington, Project Manager at Royal HaskoningDHV Reclaiming over 10km2 from the Atlantic Ocean to form the foundation upon which the new Eko Atlantic City will be located has presented Royal HaskoningDHV together with their Client South Energyx Nigeria Ltd (SENL) with many technical challenges. This 8 year long reclamation project takes place on the south coast of Nigeria to provide an extension to the former capital and the expanding business district in Victoria Island, Lagos. Initially Royal HaskoningDHV were involved in the feasibility study in 2004 to come up with a sustainable and permanent solution to the strong coastal erosion that had been afflicting the coastal area on the eastern side of the entrance to the Port of Lagos. This had been taking place for over one hundred years since the construction of the Lagos harbour moles and for which no adequate solution had to date been found. Following extensive discussions with SENL it was concluded that it would be more appropriate not to construct a protective sea wall where the sea had advanced to, but rather to construct a sea defence structure on the line of the original coastline, as it was in 1905 prior to the construction of the moles. Royal HaskoningDHV proposed a natural layout of the sea defence revetment and sand filling that would follow the modern design philosophy for sustainable coastal rehabilitation and embrace the Dutch principle of “building with nature”. This design philosophy was developed over the last several decades and has been successfully applied recently on a large scale around the globe. The method implies that coastline corrections or adoptions follow the historical trends as much as possible, taking into account the influence of the present man-made situation, in this particular case the Lagos harbour moles. It was from these humble beginnings that the idea of the Eko Atlantic City was born. The concept would solve a multiplicity of problems not least the chronic land shortage of Victoria Island, Lagos with its ever increasing population but also give a sustainable and permanent solution to coastal erosion. To achieve this would in the end entail having to design and later construct the “Great Wall of Lagos” an 8.4km long sea defence revetment in the Atlantic ocean that would have to withstand the treacherous sea conditions, caused by the long swell waves, generated from storms in the southern part of the Atlantic; this in addition to having to retain in excess of 90 million cubic metres of reclamation sand fill which would form the foundation for the new city. Following the initial feasibility studies, SENL concluded a concession agreement with the Lagos State Government for the execution of the project with Royal HaskoningDHV providing consultancy services for the marine works. The Client was to construct the revetment themselves and employed Dredging International Services Cyprus Ltd (DI) to supply the sand fill. Royal HaskoningDHV were appointed to design the revetment and supervise its construction. The design of the revetment was tailored, due to harsh sea conditions, to allow ease of rock placement using land based equipment only. 1 The Great Wall of Lagos - Eko Atlantic City In order to formulate designs that were appropriate to withstand such harsh conditions, it required imposing strict design criteria. Design profiles of the sea defence revetment cross sections consisting of a rock core, secondary rock armour, primary concrete armour unit, a toe berm and scour protection were made for varying crest heights. The designs underwent rigorous physical model testing in the Danish Hydraulic Institute in Denmark to investigate the stability and overtopping characteristics of the revetment. Tests were undertaken in both 2D flumes and 3D basins. The crest level and crest berm of the sea defence revetment were optimised in satisfying the allowable overtopping criterion. These criteria were set to safeguard the city from coastal flooding and provide a high comfort and security level and also the hydraulic stability of the revetment. The revetment has been designed not only as coastal protection but also as an amenity, providing the foundation for an esplanade and a view across the Atlantic Ocean. Constructing what ostensibly would become a 18m high structure sitting on the bed of the Atlantic ocean in sea conditions that at times would be quite hazardous because of these long swell waves has, during the initial construction stages, presented many technical and logistical challenges and will continue to do so in the future as construction proceeds. One of these major challenges on site was to determine an optimum phasing of construction of the revetment and the sand filling. It became evident at an early stage that delivery of the sand fill should be carried out from sheltered waters, which implied the optimal usage of the sheltered function of any of the revetment that had been completed. This would mean that construction of the revetment should run somewhat ahead of the major sand filling operations, starting from the east harbour mole. However, as construction of the revetment is dependent on the sand filling to provide access to construction vehicles, it was necessary to establish a construction front consisting of a small strip of sand filling on the landward side of the newly constructed section of the revetment. This process would continue over the entire length of a respective sand filling phase. Following this, major sand filling operations would continue in the fully sheltered environment on the landward side of the completed sea defence revetment. Although the above construction methodology is the current modus operandi on site it is a delicate balancing act fraught with many challenges. The sea defence revetment is not an autonomous breakwater structure, as it is necessary for protection of the sand fill slope, which makes both the delivery of the sand filling and construction of the sea defence revetment highly interrelated. This remains one of the biggest challenges on site, to proceed with the construction of the sea defence revetment as fast as possible to protect the sand fill area, but at the same time balance this with protecting the temporary works of the revetment with secondary and primary armour to form the permanent works. The temporary works therefore proceed only 50 m ahead of the construction of the permanent works of the revetment. To date over 46 million m3 of material has now been reclaimed and deposited in the area of the new Eko Atlantic city, resulting in an equivalent area of over 5 million m2 of land 2m above chart datum and a sea defence revetment 3.8km long. 2 The Great Wall of Lagos - Eko Atlantic City Photo 1 - The Great Wall of Lagos at 3km length Photo 2 - April 2012 aerial view of land reclamation Photo 3 - 3D isometric view of The Great Wall of Photo 4 - Components of The Great Wall of Lagos Lagos 3 .