Hydrology–Health Nexus in a Changing Climate: Multi-Hazard Modelling of Cascading Flood–Health Risks in Urban Megacities

Urban flooding, increasingly aggravated by climate change and unplanned urban expansion, poses multifaceted risks to infrastructure and heightens public-health vulnerability by amplifying infectious-disease transmission. Beyond physical damage, floodwaters transport untreated sewage, industrial effluents, and microbial contaminants, substantially increasing human exposure and accelerating waterborne disease spread. These cascading exposure pathways remain insufficiently quantified in rapidly urbanizing and climate-vulnerable settings, underscoring the need for an integrated assessment of the hydrology–health nexus. In this study, we examine the convergence of flood hazards and human-health risks along the Yamuna River corridor in Delhi, a megacity where extreme rainfall, recurrent urban flooding, informal settlements, and stressed sanitation systems collectively heighten vulnerability, conditions expected to intensify under future climate and socio-economic scenarios. We develop an integrated modelling chain that links climate-forced hydrological simulations, coupled urban flood modelling, contaminant transport, and Quantitative Microbial Risk Assessment (QMRA). A semi-distributed SWAT model, driven by grid-wise selected and DQM bias-corrected NEX-GDDP-CMIP6 forcings, simulates future streamflow for the Upper Yamuna Basin under SSP2-4.5 and SSP5-8.5 scenarios. Design discharges are extracted using a Peaks-Over-Threshold framework with Generalized Pareto modelling, while climate-adjusted design rainfall is generated through a copula-based Depth–Duration–Frequency framework integrating historical statistics with CMIP6 projections. These scenario-specific hydrometeorological forcings drive a fully coupled process-based MIKE+ hydrodynamic model to simulate future changes in flood extent, depth, and flow pathways across Delhi’s complex urban terrain. Hydrodynamic outputs feed into the MIKE ECO Lab module to simulate the transport and fate of faecal indicator bacteria (E. coli), and infection risks are quantified using a β-Poisson dose–response model. By integrating hydrological extremes, contaminant transport, climate projections, and exposure pathways, this study provides new insight into cascading flood–disease interactions in urban environments. The results show that the climate-driven increases in extreme rainfall and flood magnitude may exacerbate public-health risks and spatial inequities, challenging emergency response and risk-reduction capacities. The framework is transferable to other hazard-prone settings and offers a basis for developing integrated multi-hazard risk-reduction strategies.

Keywords: Hydrology–health nexus; multi-hazard modelling; urban flooding; climate change; human-health risk