Geodetic Analysis of Vertical Land Motion and Sea Level Rise Vulnerability at Lagos Coastal Zone

Accurate assessment of sea level rise vulnerability at the Lagos coastal zone requires separation of absolute sea level change from vertical land motion, a distinction that is critical for designing appropriate coastal protection and adaptation strategies but has not been adequately resolved for the Nigerian coast. This study conducts a geodetic analysis of vertical land motion at the Lagos coastal zone using a combination of Sentinel-1 DInSAR time-series analysis, GPS benchmarking from the Lagos CORS station, and tide gauge records from the Lagos Harbour hydrometric station. InSAR time-series processed by the SBAS algorithm provides spatially continuous vertical displacement fields at millimetre precision across the study area. GPS-derived absolute vertical velocity at the CORS station is used to calibrate the InSAR relative displacement estimates into an absolute vertical motion reference frame. Corrected vertical land motion rates are combined with satellite altimetry-derived absolute sea level trends from the CCI Sea Level Climate Data Record to produce spatially explicit relative sea level change rates for the coastal zone. Results reveal absolute land subsidence rates of 4 to 22 millimetres per year across the coastal zone, with the highest subsidence concentrated in Victoria Island, Bar Beach, and the Lekki peninsula reclaimed areas, amplifying apparent relative sea level rise rates to nearly double the climate-driven component alone. The study provides critical geodetic inputs to the Lagos State Coastal Zone Management Authority and the Federal Ministry of Environment's National Adaptation Plan. Keywords: vertical land motion, InSAR, sea level rise, Lagos, geodetic analysis.