Rennes Métropole is completing an underground buffer basin in the Saint-Hélier area of south-east Rennes, built to store excess water during heavy rain. The facility is expected to cut the volume of water discharged into the natural environment and the Vilaine by more than 90%. It is part of the metropolitan authority’s investments to modernise its sewer network, adapt it to heavy rain and improve the water quality of the Vilaine.

How the basin works

The south-east sector of Rennes has a combined sewer network, in which wastewater and rainwater are collected in the same pipes. During heavy rain, the basin temporarily stores the surplus water to prevent the network from becoming saturated and overflowing, or even flooding. Its storage capacity is 1,750 cubic metres, equivalent to twice the Villejean swimming pool. Once the rain has stopped, the stored water is gradually pumped to the Beaurade wastewater treatment plant, where it is treated before being released into the Vilaine.

Cost, timeline and contractors

  • Total cost: 5.3 million euros, funded by Rennes Métropole
  • Contribution from the Agence de l’Eau Loire-Bretagne: 1,697,500 euros
  • Construction: December 2024 to October 2026

Project management was provided by the engineering firm ARTELIA with the architect KASO. The works were carried out by a group of companies: CNR CONSTRUCTION (civil engineering), NGE FONDATIONS (special foundations), JOUSSE (equipment), SADE CGTH (networks) and SOTRAV SAS (earthworks and roads).

An underground structure in Saint-Hélier

Land constraints in the area led Rennes Métropole to choose a fully underground structure, which limits its visual impact. A landscaped green space open to all is to be created above the basin by 2029.

The wider water strategy

The basin falls under Rennes Métropole’s biodiversity and water strategy. Its priorities include reducing water use, improving water quality across the Vilaine catchment with Eaux-et-Vilaine, and encouraging water infiltration into soils by fighting soil sealing. The authority says sewerage plays an important role in these efforts by collecting and treating effluent from residents and businesses. Its actions are developed with partners including the Collectivité Eau du Bassin Rennais and the Établissement Public Territorial de Bassin Eaux-et-Vilaine.