Flood risks to urban development on a coastal barrier landscape of Lekki Peninsula in Lagos, Nigeria

Lekki Peninsula, which is home to the nouveau riche, is one of the barrier islands of the barrier-lagoon system of the Lagos coastline. The peninsula has been undergoing rapid urbanization since 1980 in total disregard of its physical characteristics. This study assessed landscape dynamics in the peninsula from 1984 to 2014 and evaluated risks to development from potential storm surge flooding. The datasets included baseline data on geographical indicators/island characteristics, satellite imageries and ancillary data. Storm surge flooding hazards and areas at risk were assessed in a Planar GIS environment and with two-dimensional (2D) hydrodynamic simulation using the Cellular Automaton Evolutionary Slope and River (CAESAR) model. Results reveal that the Lekki Peninsula which was a green zone had grown from about 0.5% built-up area in 1984 to about 18% built-up area in 2014. This growth was due to urban expansion and occurred mostly in areas of ecological assets including mangroves, swamps and vegetation. The highest growth in built-up area occurred in Eti-Osa Local Government Area (LGA). Generally, the peninsula has a low-lying topography with 37% of its area lying between 0.5 m and 3 m while 63% is between 3 m and 5 m above mean sea level. Potential inundation simulation with the CAESAR model for selected storm surge levels reveal that at surge heights of 4 m and 5 m, 25% (mostly in Eti-Osa) and 37% respectively of the peninsula are flooded with the latter choking off Epe Expressway on both ends. Initial ingress of inundation occurred in Eti-Osa LGA. In comparison to Planar GIS water levels, the CAESAR model reflected more realistic flood extents. The base flood elevation (BFE) and design flood elevation (DFE) are recommended to improve the sustainability and resilience of future developments along with the recognition of natural processes in future policy and development of the peninsula.