Developing a Novel Compound Flood Risk Framework for Nigerian Coastal Cities Under Climate Change: A Lagos Case Study

Compound flooding, the simultaneous or sequential occurrence of extreme rainfall, storm surge, and riverine inundation, represents an understudied but increasingly critical hazard for Nigeria's rapidly urbanizing coastal cities. This study develops a novel compound flood risk framework and applies it to Lagos as a primary case study, delivering original methodological and empirical contributions to Nigerian climate hazard science. The framework integrates extreme value copula modelling of joint rainfall-storm surge probability, a coupled hydrodynamic-storm surge model (Delft3D-FM), and a spatially explicit social vulnerability index calibrated to Lagos conditions. Bivariate extreme value analysis using Clayton and Gumbel copulas is applied to 40 years of concurrent rainfall and sea level records to quantify joint exceedance probabilities under present climate and CMIP6-projected future conditions for 2050 and 2080. Results demonstrate that treating rainfall and storm surge as independent hazards underestimates the compound 100-year flood extent by 34 percent compared to the copula-based joint probability framework, representing a critical bias in current Lagos flood risk assessments. Under SSP2-4.5 projections for 2050, the joint 100-year compound flood is estimated to inundate 312 square kilometres of Lagos, approximately 2.4 times the area inundated under present-day climate. The vulnerability index reveals that Ajegunle and Badagry are among the highest compound risk LGAs due to coincident high hazard and low adaptive capacity. This novel framework constitutes an original and replicable methodological contribution to tropical coastal compound flood science. Keywords: compound flood, Lagos, copula, storm surge, climate change.