Abstract The frequent sinkholes along the temporary water streams in Saffré basin can be compared with those occurring in Upper Normandy which are commonly referred to as ‘bétoires’. These collapses are not directly linked to karst cavities. They result from a draw-off effect of fine particles contained in the Plio-Quaternary alluvium lying above the carbonate Oligocene series. The draw-off phenomenon appears through fine cracks (1 to a few millimetres) open in soft palustrine limestones broadly comparable with chalk. Based on the knowledge of backfilling work design undertaken at some ‘bétoires’ in Upper Normandy, a new project was undertaken at four sinkhole funnels, which appeared near the surface in the Saffré basin at the end of the winter in 2017. The works consisted of digging the decompressed surface and filling up the excavation pit with materials of decreasing granulometry from bottom to top in order to ease the surface water infiltration through the identified fractured zones to the bottom of the excavation. This design was also complemented by the implementation of a filtering trench in between the watercourse and the filtering bed, laid above the fractured zones, which were enlarged by the dissolution process, on the one hand, and by the construction of a pipe network on top of the filtering bed to allow for pressurized air exhaust towards the surface, on the other hand. The pressurized air comes up from the aquifer as it recharges as soon as the surface water streamflow resumes and takes part in the mechanism, leading to sinkhole collapse, which should be prevented by this backfilling design.

Abstract Chalk groundwater is the main renewable drinking water resource for many cities of the Paris–London Basin. Understanding karst groundwater motion enhancement appears to be a major issue in order to better protect drinking water, to define hydrogeological surveys and to explore the aquifer. In Normandy, the stratigraphy of chalk was investigated in the 1970s and 1980s but this newly developed stratigraphy was not introduced to hydrogeology where chalk aquifers are studied without considering the sequence boundaries and key event surfaces. Upper Normandy is a unique hydrogeological region where both stratigraphy and hydrogeology can be studied together. In this article we focus on field observations and their direct application to scientific theory. Eight hydrogeological surfaces, linked to sequence boundaries or key event surfaces, are identified. They increase porosity and permeability sufficiently to develop karstic features, hereafter called karstogenic horizons. These field observations lead us to propose a new stratified chalk groundwater model. Palaeokarsts and perched springs not aligned to the current base level can be explained from a geodynamic perspective. Global eustatism and regional uplift during the Quaternary Period have to be taken into account with the hydrogeological stratified model, as the controlling factors of the groundwater motion and the karstogenic horizon development. This theory will help hydrogeologists to determine the probability of encountering palaeokarsts above the piezometric level and thereby define well locations with a greater degree of confidence according to the karstogenic horizon drilled. Chemical studies may also be applied to show if this stratified model can enhance water quality by a new well design.

Abstract The small town of Yport in the northern part of the Pays de Caux, Normandy, is located on the shore of the English Channel between Étretat and Fécamp, at the outlet of a long dry valley that incises the chalk plateau. Eight hundred metres to the east, at the base of a cliff, is a cluster of springs, the ‘Fontaines d'Yport’, that emerge on the foreshore. Given their high discharge, measured at between 1 and 2.5 m 3 s −1 , the Le Havre authorities have been interested in these springs since the 1960s, in determining their origin, locating the karst conduit, developing them, and investigating the boundaries and vulnerability of the groundwater basin. Today, one third of the drinking water that supplies the Le Havre conurbation comes from the Yport wells, and investigations into their vulnerability and protection of their resource continues.