Green Infrastructure and Grey Infrastructure. Rehabilitation of Disused Infrastructure Assets as an Opportunity for Green Development for Cities

With climate change and extreme weather events resulting from it, a strong demand for urbanization due to a steadily increasing population, cities face a number of complex challenges in responding to current issues and preparing for future needs. Green Infrastructure (GI) have been identified as a useful tool to cope with the effects of climate change. They are positioned as a possible alternative to grey infrastructure. A GI is able to provide a multiplicity of benefits and functionalities that can be assimilated with what emerges from the definitions of Ecosystem Services (ES). Numerous studies, in an attempt to find an unambiguous definition of GI, have developed the concept of a grey-green continuum that underlines the link between green and grey infrastructures. The aim of this paper is to focus on the concept of continuum in order to identify the correct nuance depending on the required benefit and boundary conditions. Furthermore, it is highlighted that variability does not invalidate sustainability as a goal to be achieved. Based on this assumption, three best projects of disused grey urban infrastructure that are given new value will be investigated - social, economic and environmental. The High Line in New York, Seoullo 7017 Skygarden in Seoul and the Xuhui Runway Park in Shanghai represent the three case studies in which a balance between GI and grey infrastructure is noticeable. For each we highlight which path has been followed for a green reconversion.