Flooding caused by rising water tables: a hydrogeological study of the Vallée Sud area in Greater Paris

The need

As part of the process of drawing up the Inter-Municipal Local Urban Planning Scheme (PLUi), the VSGP, comprising 11 municipalities to the south of Paris) requested an analysis of the subsoil to provide factual information for its urban planning documents to take into account the natural phenomenon of flooding due to rising water tables. Accordingly, VSGP and BRGM joined forces to carry out a hydrogeological study to map areas susceptible to rising water tables. 

The results

The geological study determined the nature of the various terrains encountered and how they change laterally. The Beynes-Meudon anticline was demarcated in the VSGP area and its effect on the geometry of the sedimentary bodies was characterised. Summary geological sections were drawn up, one running north-south and the other east-west.

The hydrogeological study was based on information from the literature and a survey held among local residents, followed by a field survey to measure groundwater levels in March 2022. On this basis, a piezometric map of the Fontainebleau sands was drawn up. The main conclusions are as follows:

• The piezometric map of the Fontainebleau sands, although imprecise because it is drawn from different periods, gives an idea of the water flows and the likely influence of the Beynes-Meudon anticline;
• The former streams that have now been channelled are drainage routes;
• The Stampian formations between the Lutetian and the Fontainebleau sands form an intricate combination of more or less permeable layers, with thin aquifer levels (from one to several metres thick). These are the main areas of rising groundwater levels.

All this information was used to draw up the map of sensitivity to rising water tables on the basis of geostatistical interpolation using BRGM's GDM software. The map shows the influence of the ancient streams and the Bièvre valley, as well as the water inflow at the base of the Fontainebleau sands. The municipality of Antony in the south of the area is particularly affected. The northern part of the area is less affected, possibly due in part to the presence of quarries that contribute to water drainage.