Empirical Analysis of the Impact of Climate Change on Flood Frequency in Lagos Metropolitan Area

Lagos metropolitan area, South West Nigeria, is highly vulnerable to urban flooding, and empirical evidence on whether flood frequency has increased due to observed climate changes is critical for urban flood risk management planning. This study empirically analyses the impact of climate change on flood frequency in the Lagos metropolitan area using long-term hydrological and meteorological data. Rainfall records from seven rain gauge stations in Lagos State spanning 1970 to 2023 were subjected to non-stationarity analysis using the Mann-Kendall test and the Pettitt change point detection method. Annual maximum daily rainfall was fitted to GEV distributions under both stationary and non-stationary assumptions. Frequency analysis was performed for both frameworks, and return period rainfall estimates were compared. Urban area expansion was quantified using Landsat land cover analysis at five-year intervals. Stormwater runoff was modelled using the SWMM model calibrated to recorded flood events. Results confirm statistically significant increasing trends in annual maximum daily rainfall at four of seven stations, with mean trend magnitudes of 1.8 mm per year. Non-stationary frequency analysis shows that the current 10-year return period design storm is equivalent to a 6-year event under 2010 stationarity assumptions. Imperviousness increase from 34 to 58 percent since 1990 amplifies rainfall-runoff response independently of rainfall increases. The study fills an empirical gap in Lagos urban flood climate attribution research.

Keywords: climate change, flood frequency, urban flooding, Lagos Metropolitan Area, non-stationarity