Prioritising Urban Residual Spaces for Nature-Based Solutions through Proximity Analysis: Evidence from Chennai, India

Urban residual spaces fragmented, underutilised land parcels generated as by-products of infrastructure-led development represent a significant and largely overlooked land resource in Indian metropolitan cities. While prior research has addressed the spatial identification and NBS suitability assessment of such spaces, the question of how prioritisation decisions among an inventory of residual spaces should be made and specifically, whether proximity to green-space-deficient, high-density residential populations should govern that prioritisation has not been systematically examined. This study addresses that gap through the development and empirical application of a proximity-based spatial prioritisation framework, tested in the Chennai metropolitan area. A four-phase methodology was employed: the spatial extent and typological distribution of 360 residual spaces covering 556,981 square metres was first established from prior GIS-based mapping; ward-level green space deficit and population density assessments were then conducted across all 200 Greater Chennai Corporation wards; a 300-metre walkability buffer analysis was subsequently applied to rank residual spaces according to their accessibility to underserved residential populations; and a typology-NBS matching process was used to translate prioritisation outcomes into context-appropriate NBS function designations. Results demonstrate that 245 of the 360 identified residual spaces representing 68 per cent of the total inventory fall within walkable distance of residential areas recording acute green space deficiency, establishing that Chennai's residual land resource is strategically positioned to serve the communities most in need of accessible green space. The typology-NBS matching matrix further demonstrates that different residual space categories metro viaducts, highway edges, railway corridors, informal green pockets, and in-between spaces are suited to distinct NBS functions, arguing against generic greening strategies. The proximity-based prioritisation framework developed in this study provides a transparent, reproducible, and equity-driven planning instrument applicable to other Indian metropolitan cities sharing analogous urbanisation conditions.