Interannual Variability of Rainfall and Its Influence on Coastal Flooding in Lagos Megacity

Lagos, the most populous city in Africa, is situated on a complex lagoonal coastline characterised by extreme sensitivity to interannual rainfall variability, sea level fluctuation, and inadequate stormwater infrastructure, making it one of the most flood-prone coastal megacities in sub-Saharan Africa. This study analyses interannual variability in seasonal rainfall totals and intense rainfall event frequency over Lagos using a 35-year record from 1989 to 2024 from the Lagos Airport weather station and four rain gauge stations, complemented by satellite-derived CHIRPS gridded rainfall data. The relationship between interannual rainfall variability and coastal flood event frequency and severity is established using a flood event database compiled from Lagos State Emergency Management Agency records, newspaper archives, and satellite flood inundation extents derived from Sentinel-1 SAR imagery. Statistical analysis includes wavelet decomposition to identify dominant oscillatory modes in rainfall, correlation with large-scale climate indices (Atlantic Multidecadal Oscillation, El Nino-Southern Oscillation, Indian Ocean Dipole), and generalised extreme value distribution fitting to annual maximum rainfall events. A linear regression model integrates rainfall anomaly, antecedent soil moisture, and tidal level to predict flood severity indices for historic events. Findings are expected to reveal that ENSO-related dry year anomalies actually increase flood risk by concentrating rainfall in fewer, more intense events. Recommendations address the urgent case for upgrading the Lagos drainage master plan and investing in coastal early warning systems. Keywords: interannual rainfall variability, coastal flooding, Lagos megacity, ENSO, urban hydrology