Pluvial flood risk assessment and nature-based mitigation in urban environments: a pragmatic Curve Number and Initial Abstraction approach

Urban pluvial flooding represents a growing threat to contemporary cities, driven by the compounding effects of climate change, rapid urbanization, and inadequate stormwater infrastructure, prompting the study of nature-based solutions (NBS) as potential interventions for mitigating the detrimental effects of urban flooding. Two-dimensional (2D) hydrodynamic assessments of pluvial flood risk and NBS mitigation potential require a detailed description of the urban drainage system that is not always available. The present study investigates a pragmatic approach for flood risk evaluation in data-scarce environments that surrogates the effects of both the drainage system and NBS through an appropriate choice of Curve Number and Initial Abstraction values within the Soil Conservation Service Curve Number method. The 2D integrated hydrologic–hydrodynamic model, HEC-RAS version 6.7, is employed via the Rain-on-Grid approach to simulate surface flood dynamics and produce pluvial flood hazard and risk maps at the urban scale. Simulations are performed for synthetic and real rainfall events with short return periods that are compatible with frequent but non-negligible damage and widespread flooding. With a focus on green roofs, the methodology is applied to a case study, the city of Naples, Italy, by considering a realistic deployment of the NBS that takes into account structural and architectural constraints. The beneficial effects of the green roofs are quantified in terms of reductions in both the extent of the inundated area and the maximum water depths across the urban fabric. The findings demonstrate that the integration of sustainable urban design practices can influence, albeit moderately, pluvial flood risk profiles, contributing to enhanced urban resilience. This work advances the understanding of multiscale NBS benefits, from the building to the catchment scale, and provides a screening strategy for the development of flood mitigation interventions in complex and densely urbanized areas.