Session A10 -  Lessons learned: Stormwater management in constrained sites

The redevelopment project of Place Jean Jaurès in Romans-sur-Isère transforms a vast 2.1-hectare impervious parking lot into an exemplary urban promenade-garden. Facing stormwater management challenges and the need to disconnect from the existing combined sewer system, the project relies on ambitious de-sealing: 73% of the total surface becomes permeable, representing 1.5 hectares divided between 65% green spaces and 35% permeable pavements. Comprehensive stormwater management is achieved at source through infiltration, designed for a 20-year rainfall event, using a diversified system combining swales, drainage trenches, lowered green spaces and permeable concrete. This project demonstrates that a high-quality landscape approach can be combined with excellent technical performance in urban hydraulics, while significantly improving the quality of infiltrated water and restoring the natural water cycle. The operation illustrates a comprehensive approach to climate change adaptation in dense city centers.

The Aubette-Martainville Joint Development Zone (ZAC), located on a heavily polluted former railway site in Rouen, illustrates how combining a well-controlled management plan for contaminated soils with a sustainable, integrated stormwater management approach can remove the technical and economic barriers to an urban redevelopment project. The initial project, based on excavating and treating polluted soil, along with an underground collection system discharging stormwater into the Aubette River-entailed major additional costs that made the operation unfeasible. The strategy proposed by the project management team - in-situ containment of polluted soils within watertight pavement structures and the use of on-site stormwater management at the parcel level, including for a 100-year return period rainfall event - made it possible to significantly reduce investment costs and render the project viable. The savings generated helped finance the reopening and ecological restoration of the Aubette, bringing the watercourse back to the heart of the neighborhood.

The Samoa gave been in charge of developing the urban project of the Île de Nantes since 2003. The project is built upon high ambitions regarding attractivity, urban and social mix, urban comfort, as well as building a city adapted and resilient to climate change, rethinking the place of nature, vegetation and water in urban fabric. Urbanwater has been assisting the Samoa on this subject of water in the city since 2015 : its presence -whether constant (Loire), seasonal (flood), ephemeral (rain), invisible (phreatic tables), twice-daily (tide), or even only regulatory (highest-water mark) must be better revealed in the urban fabric of the island for landscape, urban or environmental reasons. Water is a dynamic expression of nature in urban areas, and therefore is a significant challenge for the urban project of the island. The Samoa, through its urban planning, embodies high environmental ambitions and aims to support reflection on environmental issues. A perfect example is the rainwater management in the République neighborhood and the Jardins de l’Estuaire, in the South-West of the island, with the management of a thirty-year return period rainfall in spite of the contaminated soils.

The urban renewal project for the industrial and military brownfield site of « ZAC La Courrouze » – Rennes began in the early 2000s. It aimed to accommodate 10,000 residents and nearly 5,000 jobs across its 115-hectare perimeter. In addition to urban and landscaping issues, integrated stormwater management principles were adopted. The presence of numerous pollutants in the soil prevented the implementation of stormwater infiltration, but several nature-based techniques created the link betwwenn water and plant. Thanks to the ambition set by the designers at the begining of the studies and the ongoing commitment of the local authority, the « ZAC La Courrouze » now has a stormwater management system that is fully integrated into the neighbourhood, 80% of which has been completed. The associated benefits of this management system are clear: combating urban heat islands, supplying water to plants, landscaping, biodiversity, etc.  Finally, advances in knowledge now make it possible to consider infiltration on this site.