A Flood Setback Performance Index to Assess the Pluvial Flood Resilience of School Campuses in a Rapidly Urbanizing Coastal City

Pluvial flooding is intensifying in fast-growing coastal cities as rapid urbanization increases impervious surfaces and reduces natural infiltration, challenging planners to identify scalable and site-level mitigation strategies. Although statutory setbacks around institutional buildings are widely mandated for ventilation, safety, and accessibility, their contribution to urban flood resilience remains largely unexplored. This study develops the Flood Setback Performance Index (FSPI), a dimensionless indicator that integrates setback permeability and area-weighted runoff coefficients to quantify the flood-mitigation potential of mandatory setbacks. The framework was demonstrated using eight government school campuses in the Chennai, Chengalpattu, and Thiruvallur regions, which are part of a flood-prone coastal metropolis in India, following the extreme rainfall event of December 2025. Flood conditions were verified through post-event field surveys and visual evidence, while setback characteristics were extracted from satellite imagery and classified into permeable and impervious surfaces. The permeable fraction of setbacks and the FSPI values exhibited a strong inverse relationship with flood severity (Spearman’s ρ = −0.87, exact permutation p = 0.011). Ordinal logistic regression confirmed this gradient (likelihood-ratio χ2 = 9.3–10.3, p ≤ 0.002, McFadden’s R2 = 0.54–0.60), and campus rankings were made under saturated runoff coefficients. The findings demonstrate that statutory setbacks can function as nature-based flood-mitigation infrastructure with measurable hydrological benefits. Beyond the case study, the FSPI provides a simple, transferable, evidence-based tool for screening institutional sites by flood-mitigation performance and prioritizing where adaptation measures, such as increasing permeable-surface cover within statutory setbacks are needed, supporting climate-resilient urban planning and providing a regulatory reform decision-support tool with three applications: prioritizing the campus-level retrofit of permeable surfaces; screening building applications against setback surface standards; and identifying shelter-designated schools that require drainage upgrades. It thereby supports climate-resilient urban planning, regulatory reform, and progress toward Sustainable Development Goals 11 and 13.