## 3 Results
Key stakeholders that played roles in the different networks were mostly responsible for creating intra-network connections, with the Community Design Collective members often working as the central figures in establishing connections and the Nature Forum members also playing a key role. In Milan, a polycentric structure was identified in the UIP establishment process right from November 2018. An appointed project manager within the directorate of urban planning of the Municipality of Milan (CDM) led the project activities. The central role of the municipality directorate in this core network is related to the top-down governance structure that comes from the financial resources to be deployed in the implementation of the NBS in the three ULLs. Afterward, each ULL brought together other actors that took on the role of enablers in the process within the local stakeholders' groups, see [42]. Based on each ULL's NBS types, responsibilities, and management processes, a new substructure of collaborative governance models that strengthen intra-networks connections emerged, as seen in Figure 3. In the ULL1 of the green roofs and walls, the main gatekeeper partner is leading the decision-making, especially related to the technical competencies of the co-design phase. Here, the CDM was responsible for the legislative procedures within the public call for building owners to be involved in verifying the building's construction status with respect to the added loads. In ULL2, two appointed actors adopted the role of enablers within the process. The facilitation partner and the local neighborhood association worked together to represent the local community from a bottom-up perspective. In ULL3, an enabler led a partnership in this model, governing the construction management of the NBS projects through two private partners. In all of the three ULLs of Milan, citizen participation was represented through neighborhood associations and local stakeholders that joined the UIP inter-networks, which reinforced the satellite constellation of the polycentric model initially developed from the start of the project in June 2018. In Hamburg, a structure with some similarities to London and Milan was put in place. An appointed department within the District Public Administration (Department for the economy, construction, and environment) was given the lead role for the project activities; their goal was to streamline the activities at a local level in compliance with the project requirements. Other partners in the project were in charge of mediating between the administration and the local activities, such as the community planning agency and the universities [43]. This core group is represented in the middle of Figure 4. Each ULL within the Hamburg case followed a different process according to the steps taken to activate actors and the type of NBS to be co-created. ULL1 focused on a green corridor in which several local groups were coordinated by the UIP to realize small interventions following an overall strategy for implementation. Here, the public administration was responsible for the coordination of all efforts and building permissions. The planning agency activated local groups and brought them on board, while the university was coordinated and provided inputs for local innovative activities. In ULL2.1, focused on a green facade for social housing project, the governance model was laid out in a more top-down fashion. In this case, the housing company held the most power in the decision-making process, but through the mediation of the public administration and planning agency, they put in place a participatory process to determine principal elements of the facade, such as the form and the type of vegetation. The other part of ULL2.2 was concentrated on stormwater management activities. In this case, many small NBS interventions were realized in the area in a process led by the Ministry of the Environment, with the Public Administration intervening for building permits.