Session A3 - Multi-criteria prioritisation methods

Sustainable urban drainage systems (SUDS) improve stormwater management and bring other benefits, such as an improvement for thermal comfort for citizens. However, implementing SUDS on such a scale can be challenging due to the diversity of stakes and constraints. Our works aims to develop a “semi-automatic” methodology for deploying SUDS at the scale of Nantes Métropole. It is based on a review of the scientific literature and the thesis works of Chavez (2024) and Betou (2024). Detailing the studied SUDS configurations improve the potential for implantation, at the metropilitan scale. The first version of a survey, sent to a few experts, gives refinement insights for a larger broadcast.

Nature-based solutions for stormwater management (NBSsw) offer low carbon footprints and substantial ecosystem services, yet they have not become the preferred pathway for adapting cities to more extreme precipitation. Unlike runoff from roads, roof runoff holds potential energy that enables conveyance into vertically configured, aboveground NBSsw elements, which are more space efficient and allows for detention of extreme volumes on double programmed neighboring areas, as exemplified  with the Green Climate Screen in Copenhagen This study uses GIS analysis to examine whether Copenhagen’s 2100 climate adaptation target of increasing the drainage capacity of the combined sewer system by 30 % to maintain a 10-year service level can be met solely by diverting roof runoff into aboveground stormwater elements. The combined sewer catchment of Copenhagen is around 6000 ha, of which 4000 ha are sealed surfaces, distributed with 2500 ha of roads/pavements and 1500 ha of roofs. A target of disconnecting 30 % of sealed surfaces corresponds to 1200 ha, which thus in theory can be achieved through roof disconnections alone. In addition to providing potential energy for gravity-driven conveyance, roof runoff is typically cleaner than road runoff and is often located adjacent to areas that would benefit from increased ecosystem services. The paper discusses the feasibility of this strategy with particular focus on ownership conditions and the local availability of space for implementing vertically oriented NBSsw.

This research proposes a methodology for prioritizing urban areas and implementing nature-based solutions (NbS) aimed at improving and quantifying the thermal regulation ecosystem service and mitigating urban heat islands (UHIs). The methodology combines three components: biophysical variables derived from remote sensing to measure vegetation characteristics; citizen perception of ecosystem service (ES) needs in the territory; and computational tools based on machine learning (ML) models to quantify the relationship between vegetation, urban configuration, and land surface temperature (LST). The urban contexts to be evaluated are the “El Reencuentro” area in Bogotá, Colombia, where no NbS have yet been implemented, and the campus of the Javeriana University in Cali, Colombia, where these interventions are well established. Given this contrast in case studies, methodology offers analytical tools that are applicable in both cases. The results show that low vegetation cover limits thermal regulation capacity and that scenarios with green NbS (i.e. urban forests and green spaces) effectively reduce LST. The magnitude of this reduction will depend on the spatial configuration, the type of intervention, adaptation to the urban environment, and maintenance. In conclusion, the structure, continuity, and health of urban vegetation determine the magnitude of the benefit, providing solid and applicable evidence for different urban contexts seeking to design UHI mitigation strategies through NbS focused on vegetation.

In the years 2024 and 2025, Cerema, in partnership with the watershed syndicate called SMBVAS, conducted a research and development study aimed at proposing a tool to identify the potential for unsealing and renaturation in the territory of Barentin (Normandy, France). This tool provides a possible response to climate change impacts adaptation. The approach is based on a detailed analysis of the green and blue infrastructure at the municipal level, highlighting the framework of ecological networks and prioritizing areas that promote biodiversity. Specific challenges for the territory are analyzed to enhance the method, including limiting runoff, flood and underground related risks, optimising stormwater management, integrating nature into urban environments to improve quality of life, reducing the effects of urban heat islands. The spatialisation of these elements and the projection of their evolution in the context of climate change serve as a basis to target the sectors where unsealing and/or renaturation on the territory would have most effects. The proposed results should primarily be seen as tools for raising awareness and supporting decision-making among the various relevant services and structures, as well as the local population, in order to ultimately facilitate the acceptability of the proposed solutions.