Prioritising nature-based solutions for urban flood mitigation in public and private spaces: A study in Trento, Italy

Urban flooding is a hydrologic–hydraulic phenomenon influenced by climate change and driven by intense urbanization. Nature‑Based Solutions (NBS) can help mitigate this phenomenon by providing water‑regulating ecosystem services. NBS spatial prioritisation is increasingly used to identify which interventions should be implemented and where. However, most existing approaches are based on simple hydrologic analyses of surface runoff that overlook hydraulic pressure within the sewer system. Moreover, opportunities for NBS implementation are often assessed without distinguishing between public and private spaces, thus missing key information for decision-making. This study develops and tests a spatially explicit method at the subcatchment level that combines three hazard-contributing indicators (Runoff Coefficient, Flooding Ratio and Surcharging Ratio), with suitability analysis for different NBS types in both public and private spaces. The testing application in Trento, Italy, shows that green roofs, rain gardens, and permeable pavements generate distinct priority patterns, and that the three indicators capture complementary and spatially heterogeneous hazard contributions at the subcatchment level. Results also highlight that private areas suitable for NBS implementation recur more often in subcatchments classified as high priority under multiple hazard-contributing indicators, whereas suitable public areas are fewer and predominantly associated with a single indicator. By combining hydrologic and hydraulic hazard contribution with public and private suitability for NBS implementation, the study provides a spatially‑explicit prioritisation tool for urban planning, translating hazard contributions into spatial priorities to identify where different NBS types should be most appropriately targeted within the urban fabric.