Performance-Based Site Selection of Nature-Based Solutions: Applying the Curve Number Model to High-Resolution Layers to Steer Better Greening Strategies

Urbanisation and consequential soil sealing are primary sources of urban vulnerability towards extreme water-related events, which will be even more frequent with climate change. The high rates of imperviousness characterising contemporary cities generate large volumes of stormwater runoff that traditional drainage systems may not be able to manage, even for small-intensity events. Several planning experiences have proven effective in designing green strategies and implementing nature-based solutions (NBS) to reduce stormwater runoff produced in highly impervious areas. Nevertheless, these experiences usually consist of local-scale plans necessary to evaluate NBS but incapable of providing practical methodologies to scale up NBS performance-based planning. Hence, the paper aims to develop a spatial indicator to identify where a specific type of NBS, namely green roofs, could provide the highest benefits by combining an index related to the need for runoff reduction with an index related to the suitability of green roof interventions. For assessing runoff reduction needs, the method applies the Curve Number model, developed by the Soil Conservation Service, to the Geo-Topographic Database of the Lombardy Region, a well know instrument used by urban planners. The same database is used to identify where green roofs could be implemented. This application allows us to obtain, even for dense cities, a site-specific assessment of good potential green roof locations, i.e., where green roofs are needed and where they can be built. The paper will test the proposed methodology to the city of Como, Lombardy (IT).