Risk redistribution and inequality under partial flood protection in a core- periphery city

As flood hazards intensify, many cities adopt partial structural protection rather than comprehensive defenses, reshaping how flood risk is distributed in space and time. Although large-scale analyses suggest that partial levee coverage can reduce overall damage, its spatiotemporal effects remain understudied, particularly in cities of the Global South. Using a 1D-2D coupled hydrodynamic model forced by extreme discharges (100-year return period flood event), together with depth-damage curves and demographic data, we show that partial levee construction in coastal city Surat, India, lowers citywide flood losses by ₹31.24 billion (US$380 million) in core urban wards and by ₹10.34 billion (US$125 million) in suburban neighborhoods. However, both damage and exposure become more inequitable, with the Gini index (0 = perfect equality, 1 = maximum inequality) rising by 20% for damage (0.55 to 0.66) and by about 26% for exposure (0.31 to 0.39). To capture the underlying spatial and temporal mechanisms driving these patterns, we introduce flood stripes and a protection-induced time shift (PITS), revealing that certain near-river wards remain flood-free for up to 12 hours longer, while some downstream areas flood up to 7 hours earlier under partial levee coverage. When ranked by marginal worker share, inequality further intensifies, with the Gini index increasing from 0.19 to 0.23 for damage and from 0.02 to 0.06 for exposure, and the most vulnerable 50% of wards absorbing 60.7% of losses and 56.4% of at-risk residents. Together, these results highlight the importance of evaluating both spatial and temporal consequences of partial flood protection when designing equitable urban adaptation strategies.