Flood Event Escalation and Urban Drainage Design Implications Under Nonstationary Rainfall in São Paulo State, Brazil

Reliable urban stormwater design under nonstationary rainfall is becoming increasingly important, yet quantitative links between floods occurrence, updated design requirements, and associated costs remain limited. This study (i) characterizes the evolution and impacts of flood-related events in São Paulo State, Brazil (1991–2024), and (ii) quantifies how nonstationary rainfall projections (CMIP6 SSP2-4.5 and SSP5-8.5) affect culvert sizing and construction costs across municipalities for standardized hypothetical catchments, across multiple return periods and future horizons. Observations indicate an increase in flood occurrence, from an average of 7.8 events per year in the 1990s to 72.9 events per year in the 2010s. In the immediate future (2015–2055), SSP2-4.5 projected costs remain close to the baseline for most municipalities (for return periods ≤ 25 years, 68% show increases up to 10%), whereas for the distant future (2056–2100) 86% exceed 10% under SSP5-8.5. However, under SSP5-8.5 in the immediate future (2015–2055), and for RP > 25 years, approximately 46% of municipalities exceed 10% additional costs. Design discharges generally rise by 9–43% in the immediate future, with stronger increases under SSP5-8.5 toward the late century. Mapping required hydraulic area to commercially available nominal sizes discretizes upgrades and creates threshold behavior, with larger basins crossing size classes more often. These findings challenge the assumption of stationary design and support the adoption of nonstationary adaptation strategies to reduce the long-term probability of structural failure.