Blue-Green Infrastructure as a Territorial Planning Instrument: Quantifying the Contributions of Land Use Change and Climate Forcing to 2050 Flood Risk in a Peri-Urban Coastal Watershed, Rio de Janeiro, Brazil

Urban flood risk is shaped simultaneously by land use and land cover (LULC) change and by climate change, yet their relative weight in peri-urban watersheds of the Global South remains poorly quantified. This study compares both drivers in the Piraquê-Cabuçu River Basin, a 109.7 km2 coastal watershed in western Rio de Janeiro, Brazil. A quasi-2D hydrodynamic model (MODCEL) on a hexagonal grid (60 m side, 11,977 cells), representing open channels but not the storm sewer network, was calibrated against the April 2010 flood (Percent Correct 0.87; Probability of Detection 0.63, with observations concentrated in one district). Twelve scenarios crossed three 2050 urban trajectories (Uncontrolled, Controlled and Desirable, the latter structured by blue-green infrastructure and including relocation from restricted zones) with rainfall intensification of 1–5 mm/decade and sea-level increments of +0.17 m (SSP1-2.6) and +0.24 m (SSP5-8.5), under a 25-year design storm and a spring tide. Hazard and exposure were combined in a cell-scale Flood Risk Index (RI). Under the adopted 2050 scenario assumptions, the Uncontrolled trajectory increased simulated depths in cells covering 50.2% of the basin, against 20.3% for the pessimistic climate scenario alone, and raised the share of cells with RI > 0.4 from 6.5% to 13.1% (baseline inundated area: 3812 ha, 34.7%). Only the Desirable trajectory raised the share of risk-free cells above the baseline, from 70% to 73%, through simulated depth reductions of up to 0.20 m combined with the removal of exposure by relocation. These results are conditional on this watershed, this scenario set and the 2050 horizon.