Soil infiltrability in the territory of Rennes Métropole

As part of its Local Intercommunal Urban Plan (PLUi), Rennes Métropole wants recommendations to be given to promote the development of stormwater infiltration techniques. These actions aim to limit the impacts related to the waterproofing of surfaces and the associated flood risks. The PLUI will have to lay down urban planning rules to promote the infiltration of rainwater. Stormwater management is indeed one of the major challenges in the implementation of the Management of Aquatic Environments and Flood Prevention (GEMAPI). The impact of waterproofing surfaces must be minimised today and a return of rainwater to the underground environment is necessary to ensure the sustainability of the resource. The implementation of infiltration operations is an important challenge for the surface aquifers present, but also for surface waters linked to these groundwater flows. The layer was designed on the basis of a multi-criteria analysis carried out by the Bureau de Recherche Géologique et Minière (BRGM) as part of the project “Deep of Hydrogeological entities and assessment of constraints on infiltration of urban rainwaters using the territory of Rennes Métropole” (PHOEBUS). The result of the study is based on supposed functional units of homogeneous hydrogeological behavior. Using a precise Digital Land Model, watersheds have been cut to define a right bank, a left bank and an upstream watershed. Among the natural criteria used to determine an infiltration capacity indicator for each functional unit: \- The thickness of the Unsaturated Zone (ZNS); \- Soil permeability; \- The susceptibility index of clays; \- Network Development and Persistence Index (IDPR); \- Soil hydromorphy; \- Slopes; \- The endoreic areas. Each of these criteria was weighted. Favorable and fairly favorable functional units become areas where infiltration is mandatory. Conversely, unfavourable and unfavourable functional units are areas where infiltration is not mandatory. In addition, for functional units where unsaturated areas are less than 1 metre, infiltration is not mandatory. Where a cadastral parcel contains a mandatory area and a non-mandatory zone, as a precaution, infiltration is not mandatory on that parcel. The soil infiltrability layer obtained makes it possible to guide the choices in terms of infiltration projects and to study the possibilities of realisation in sectors considered favourable. It also makes it possible to adapt the rules of the PLUI according to the sectors and their ability to accommodate stormwater infiltration operations. It should be recalled that this layer does not make it possible to dispense with a feasibility study on the plot. To understand the methodology used to design this layer or dispose of the data dictionary, consult the documents attached to this metadata. To identify areas where infiltration is mandatory or not mandatory, use index_11 and note_ZNS. If index_11 is less than or equal to 101 OR note_ZNS is 0, then infiltration is not mandatory. Conversely, if index_11 is greater than 101 AND note_ZNS is not equal to 0, then infiltration is mandatory.