THE LAGOS COAST Investigation of the long-term morphological impact of the Eko Atlantic City project MSc Thesis K.M. van Bentum January 2012 The Lagos coast Investigation of the long-term impact of the Eko Atlantic City project MSc Thesis K.M. (Karen) van Bentum Delft University of Technology Faculty of Civil Engineering and Geosciences Department of Hydraulic Engineering Section of Coastal Engineering A thesis submitted in partial fulfilment of the requirements for the degree of Master of Science in Civil Engineering at the Delft University of Technology Thesis committee Prof. dr. ir. M.J.F. Stive Delft University of Technology – Hydraulic Engineering Dr. ir. M. van Ledden Royal Haskoning and Delft University of Technology Dr. J.E.A. Storms Delft University of Technology – Geotechnology Ir. C. Hoyng Royal Haskoning Ir. A.P. Luijendijk Deltares and Delft University of Technology PREFACE Barely one week after my first acquaintance with the Royal Haskoning department of River and Coastal Management, I found myself on a plane towards Lagos, Nigeria together with four Royal Haskoning employees. This was about the best start of a MSc study a graduate can think of. There I was, right in the middle of the immense city Lagos, the subject of my thesis. This thesis concludes my Master of Science program in Civil Engineering, specialization in Hydraulic Engineering at the Delft University of Technology. The study is initiated by Royal Haskoning, to complement its previous studies on the Lagos coast. Deltares is involved in the modelling part of the study. I would like to express my gratitude to my graduation committee for guiding me through the Master study. I would like to thank prof. dr. ir. M.J.F. Stive for his interest, enthusiasm and motivating discussions; dr. ir. M. van Ledden for his very constructive feedback and for taking a lot of time to aid me during my thesis work; dr. J.E.A. Storms for sharing his knowledge in the geological part of this thesis; ir. A.P. Luijendijk for always triggering me to get the most out of my study and making me reflect critically on the findings and ir. C. Hoyng for her enthusiasm for my study and for all the advices and information she provided me with. I would like to thank Royal Haskoning, and especially, D. Heijboer, M. van Ledden and C. Hoyng, for providing the interesting and challenging thesis subject combined with the opportunity to take part on the trip to Lagos and to make such a great start with my study. Also, the Nedeco fund is thanked for the financial contribution to the trip. Besides the fact that staying two weeks in Lagos was very educational and instructive, it was also a great possibility to get to know the Royal Haskoning employees and the on-going projects in Lagos. Moreover, I owe many thanks to ir. B. Huisman and ir. H. de Vroeg for all their help during the modelling phase and for providing me with lots of useful information and enthusiasm regarding the Unibest software. Finally, especially, I would like to thank my family and close friends for their unconditional support, love and care and of course, for the fantastic years I had during my study. Most of all, I would like to thank Luuk for his never-ending support, encouragement and help when I needed it the most. Karen van Bentum - Delft, January 2012 The Lagos coast | v vi | MSc Thesis K.M. van Bentum ABSTRACT This study is performed to assess the long-term morphological impact of the Eko Atlantic City project on the adjacent Lagos coast. Lagos is situated at the western part of the Nigerian coast around a tidal inlet which leads to the Lagos Lagoon. In 1908 man started to interfere with the natural coastal situation by the construction of three stone moles at the tidal inlet. These moles are the West Mole, the Training Mole and the East Mole. Nowadays, the tidal inlet is called the Commodore Channel and it constitutes the seaside entrance to the Lagos Harbour. As of the completion of the moles, the Lighthouse Beach, situated updrift of the inlet, expanded about 800 m over circa hundred years due to sediment trapping at the West Mole. Consequently, the Bar Beach, located downdrift of the inlet, suffered enormous rates of erosion. Over roughly hundred years the total retreat of the coast has reached values of more than one km. To counteract the large loss of beach width due to the erosion, man started to apply nourishments around 1960. Enormous volumes of sediment have been supplied to the Bar Beach, but the beach kept on eroding anyhow. An additional problem at Lagos is the growing population, entailing large space demands for residential, commercial and recreational activities. To cope with this space deficit and the land loss due to the erosion of the Bar Beach, the private project developer South Energyx Nigeria Ltd. has initiated the Eko Atlantic City project. This project comprises a land reclamation of 9 km² in front of the Bar Beach, directly east of the East Mole. The newly reclaimed land is protected against erosion by a revetment, which has a length of roughly 6.5 km. Royal Haskoning provides, among others, the consulting services for the marine works of Eko Atlantic City. One of its main studies concerning the Eko Atlantic City project has been into the short-term impact of the project on the adjacent coast. Preliminary conclusions on the impact of the project are drawn and locations where erosion is expected are pointed out. To provide for a comprehensive approach, the long-term effects of the project on the adjacent coastline should be investigated as well. Therefore, Royal Haskoning initiated this study. The main objective of the study performed in this thesis is: Investigate the long-term morphological behaviour of the coastal stretch around Lagos with the presence of Eko Atlantic City in a qualitative and quantitative way. The Lagos coast | vii To achieve the objective, several research questions need to be answered: 1) What are the governing processes and mechanisms determining the long- term and large-scale morphological behaviour of the Lagos coast? 2) What was the influence of the construction of the Lagos Harbour Moles and other human interferences on the historical coastline development? 3) How does the construction of Eko Atlantic City change the present morphological processes? In the study into these questions, two clear parts can be indicated. In the first part the Lagos coast is analysed by a theoretical approach, which results in a conceptual model of the Lagos coast. And thereafter, the numerical simulation model Unibest is applied to complete the analysis and to assess the impact of Eko Atlantic City. So, the first part of the analysis of the Lagos coast starts with a literature study into the historical development and into the governing processes and mechanisms at the Lagos coast. Using the results of the literature study, preliminary conclusions on the governing processes and mechanisms can be drawn and a conceptual model regarding the long-term and large-scale morphological development of the Lagos coast is constructed. In the conceptual model, the governing sediment fluxes, sources and sinks in the Lagos coastal system are quantified via consideration of the sediment balance at the coast of Lagos. Thereby, the first research question can be answered. Persistent high-energy swell waves, reaching Lagos with a mean direction of 188°, induce a mean eastward longshore sediment transport. In general, the whole Lagos coast suffers erosion due to high rates of relative sea level rise. This high relative sea level rise is induced by the natural dewatering and compaction of the soil, combined with large volumes of water extraction by men and the Eustatic sea level rise. The high rates of erosion downdrift the inlet are not induced by sea level rise alone however. The governing factors are the accumulation of sediment by the West Mole and the import of sediment into the Commodore Channel and the Lagos Lagoon. The sediment import occurs due to sea level rise, dredging of the channel and dredging of the lagoon. These processes disturb the morphodynamic equilibrium of the inlet and the lagoon as they create additional sediment accumulation space. This leads to a so-called sediment demand in the lagoon and the tidal inlet. The result of the sediment trapping at the West Mole and the sediment import into the tidal inlet and the lagoon is that almost no sediment is able to bypass the East Mole. Thereby, a sediment deficit in the longshore current downdrift of the tidal inlet arises. Subsequently, the Bar Beach starts to erode, to provide for a sediment source to the longshore sediment transport. viii | MSc Thesis K.M. van Bentum The first part of the second research question is hereby answered. The construction of the moles led to expansion of the Lighthouse Beach, via sediment accumulation at the West Mole. And, together with the other processes occurring in the tidal inlet and in the lagoon, the construction of the moles resulted in significant erosion of the Bar Beach. To answer the second part of the second research question, the effect of the nourishments performed on the Bar Beach since 1960 is investigated. Although erosion of the Bar Beach was not prevented by the artificial sediment supply, the nourishments clearly have had an alleviating effect on the erosion. If the nourishments had not been conducted, the erosion of the Bar Beach would have been even worse. Subsequently, the conceptual model is used to formulate hypotheses regarding the future morphological behaviour of the Lagos coast with the presence of the Eko Atlantic City project.