How Divergent Are Flood Risks During Monsoon and Post-Monsoon Seasons? Revealing Contrasting Impacts over Coastal Multi-Hazard Catchments

Global coastal catchments are uniquely vulnerable to flooding due to the interplay of multiple flood drivers, including intense rainfall, storm surges, and tidal influences. These regions face particularly complex challenges because the nature and magnitude of flood risks vary significantly with seasonal changes. During the monsoon season, prolonged and heavy rainfall often leads to widespread inundation, whereas in the post-monsoon period, compounded effects of residual waterlogging, storm-tides, and episodic rainfall events create equally severe but distinctly different flood scenarios. This study, for the first time, develops an integrated framework to quantify and compare flood risks during these seasons, advancing flood management literature with a novel approach. A sophisticated 1D-2D coupled hydrodynamic flood model is employed to generate high-resolution flood hazard maps by simulating the compound interactions of rainfall and storm tides. Simultaneously, flood vulnerability is assessed at the finest administrative scale using a comprehensive suite of physical and socio-economic indicators. A Bivariate Risk Classifier framework is introduced to integrate hazard and vulnerability assessments, enabling nuanced spatial representation of risks through choropleth maps. Two novel indices are developed to enhance the understanding of multi-hazard flood risks: the Area Index, which highlights the spatial extent of risk, and the Multi-Hazard Risk Index, which captures the compound and marginal contributions of hazards and vulnerabilities. These indices provide critical insights into the varying nature and magnitude of flood risks during monsoon and post-monsoon periods. Our findings reveal a significantly higher proportion of villages falling into medium to very high hazard classes during the post-monsoon season, a critical insight that would remain obscured under conventional methodologies. Vulnerability assessments highlight that the majority of coastal villages exhibit severe vulnerability levels, driven largely by dense populations of illiterate and non-working residents. This research demonstrates that flood risks differ markedly between seasons, with varying degrees of impact on infrastructure and human systems. The integrated framework and incisive indices proposed herein offer actionable insights to support tailored, long-term flood management strategies aimed at mitigating risks and enhancing resilience in coastal floodplains.