Hydrological-hydraulic modelling to assess Nature-Based Solutions for flood risk mitigation in an urban area of Catania (Sicily, Italy)

Traditional interventions, based on grey infrastructure approaches, have demonstrated ineffectiveness in managing stormwater in urban environments under the pressures of climate change and urbanization. Nature-Based Solutions (NBS) and related measures such as Green Urban Infrastructures (GUIs) have emerged as advantageous solutions for climate resilience that contribute to climate adaptation and mitigation by minimizing water quantity and improving water quality. An appropriate combination of NBS with traditional grey infrastructures should be implemented to retain, decelerate, infiltrate, and gradually release rainwater. This study aims to identify flood risk areas, locate NBS (i.e., green roofs, rain gardens, porous pavements, rain barrels, and infiltration basins), and evaluate the effectiveness of various small-scale NBS types and combinations in terms of flood peak reduction and delay, as well as runoff volume reduction. To achieve the objective, a novel approach incorporating the integrated utilization of both hydraulic (HEC-RAS) and hydrological models (EPA SWMM), as well as images from traffic and mobile phone cameras, is implemented and validated. This approach is applied to a Sicilian hydrological urban catchment where health facilities providing essential services are situated. Specifically, five distinct scenarios are formulated by varying the type of NBS and their implementation percentages, considering both public areas and fiscal measures. Model simulations are conducted with return periods (T) of 10, 50, and 200 years. Real-time and freely available Volunteered Geographic Information (VGI) images have demonstrated to be a viable alternative to traditional monitoring equipment for validating hydraulic models in data-scarce areas. Furthermore, the findings accentuate the suitability of the proposed methodological approach, enabling effective NBS placement that yields significant reductions in peak flow, runoff volume, and flood area extent. Additionally, the model applications facilitate (i) the evaluation of the performance of different NBS types used in combination for the three considered T values and (ii) the identification of the runoff depth at which NBS are most effective in urban Mediterranean areas.