Multi-Hazard Risk Modelling and Systemic Vulnerability Assessment for Coastal Megacities: A Theoretical and Empirical Framework Applied to Lagos

Coastal megacities like Lagos in Nigeria's South West geopolitical zone face multi-hazard environments where the simultaneous or sequential occurrence of flooding, storm surge, subsidence, contamination, and infrastructure failure generates systemic risks that exceed the analytical capacity of single-hazard assessment frameworks, necessitating theoretical and methodological advances in integrated multi-hazard risk modelling for complex urban systems. This study develops a theoretical framework for multi-hazard systemic risk assessment in coastal megacities and applies it empirically to Lagos, producing the most comprehensive multi-hazard risk atlas yet produced for any Nigerian city. The theoretical framework integrates disaster risk science, urban complexity theory, and systems thinking, conceptualising multi-hazard risk as an emergent property of the interactions among hazard intensities, exposure patterns, vulnerability conditions, and institutional response capacity. Empirical methods include coastal flooding and storm surge modelling using DELFT-3D hydrodynamic simulations, subsidence mapping from InSAR satellite geodesy using Sentinel-1 data across 2018 to 2023, contamination exposure assessment for groundwater and air quality, and critical infrastructure fragility analysis for road, electricity, and water networks. Vulnerability assessment integrates household survey data from 1,200 respondents across 24 wards with physical exposure metrics. Systemic risk is modelled using Bayesian network analysis to propagate multi-hazard interactions through interconnected exposure and vulnerability layers. Results identify seven spatial clusters in Lagos where multi-hazard interactions produce systemic risk levels substantially higher than any single hazard assessment would project, with the greatest systemic risk concentrations in Ajeromi-Ifelodun and Kosofe LGAs. The study advances multi-hazard risk theory and demonstrates the feasibility of operationalised systemic risk assessment for African coastal megacities. Keywords: multi-hazard risk, systemic vulnerability, Lagos, coastal megacity, South West Nigeria.