Session A2 - Regional strategies for greener and more permeable cities

Act 2 of the Lyon Metropolis' “Permeable City” initiative was launched in 2022 to accelerate and scale up efforts to disconnect stormwater and reduce impermeability. It has benefited from strong political support from the executive branch, with the ambition of making rainwater a resource for the region and a matrix for all our public policies and development projects. After more than three years of coordination, the Lyon Metropolis is conducting an initial assessment of this second phase (progress, ownership, obstacles removed, achievements, etc.) to measure the reality of the change in scale but also the road ahead. It also involves assessing the prospects in terms of governance, coordination, financing, and consistency with GEMAPI and runoff issues in order to continue the momentum that has been built.

Although nature-based solutions are widely recognised for their potential to mitigate flooding, improve water quality, and enhance urban liveability, large-scale implementation remains constrained by governance, funding, and monitoring challenges. NBSsw are defined here as stormwater measures that operate in alignment with ecological processes, provide meaningful biodiversity value, and deliver hydrological, social, and environmental co-benefits with a low life-cycle carbon footprint. This contrasts with heavily engineered, sealed, or underground systems that offer minimal ecological function. This paper presents three main results. First, a comparative analysis of Copenhagen, Paris, and Athens reveals that, while all three cities deploy NBSsw, their strategies differ significantly. Second, an NBSsw assessment framework is operationalised, linking hydrology, biodiversity, and carbon, and its applicability is demonstrated across diverse urban contexts. Third, several cross-cutting institutional levers, including long-term investment pipelines, multi-sector coordination, and performance-based standards, are identified as mechanisms to accelerate transitions toward nature-based stormwater regimes. Collectively, these findings establish a robust foundation for co-developing transition pathways with local partners under future climate scenarios.

This communication presents the profound renewal of the public policy dedicated to rainwater in the Department of Seine-Saint-Denis. This new strategy is based on two guiding documents: the Manifesto ‘la Seine-Saint-Denis, territoire d'eau’ and its action plan. This communication presents these two documents, adopted in April 2024, and highlights the innovative and collaborative nature of the approach through a variety of actions that have been implemented. The aim is to add new dimensions (e.g. ecological and social dimensions) to the consideration of water in development projects, to rethink the tools needed to support local stakeholders and to produce multidisciplinary knowledge. Finally, several areas of work that will contribute to the future deployment of the strategy are presented.