Characterisation and mapping of land subsidence based on geodetic observations in Lagos, Nigeria

Abstract

The pervasive seasonal flooding, aquifer contamination, and saline water intrusion in parts of Lagos are some of the long-term effects of land subsidence caused by the excessive groundwater exploitation and land reclamation that has been taking place in the city. Efforts to monitor the extent and pattern of land deformation will help in many ways to mitigate the effects of flooding and other geohazards associated with land subsidence. In this study, we characterised and mapped the land subsidence in the Lagos city based on the analyses of geodetic data, which included Global Positioning Satellite (GPS), Envisat, Sentinel-1, and GRACE data. We applied the SBAS technique to the Envisat and Sentinel-1 datasets acquired from 2004–2011 and 2015–2019, respectively, and to perform multi-temporal analyses and produce corresponding subsidence maps over the Lagos city. The GRACE data were used to infer the extent and trend of groundwater changes at the GPS location. The results indicate that the subsidence is a widespread phenomenon in Lagos city, with subsidence rates varying between −2 mm/year and −87 mm/year. The highest subsidence rate was observed around the coastal zones and areas where heavy structures are built on landfills. With the highest rate at present, subsidence has gradually increased in the last 15 years. With the development of indiscriminate groundwater exploitation, urbanization, and rapid population growth, the subsidence rate in Lagos city is likely to rise significantly in the coming years, which, in turn, may further escalate the flood rate and other associated geohazards.