Flow processes in the unsaturated Chalk of the Hallue Basin (France)

During the spring of 2001, large floods occurred in the northern part of France (Somme catchment). The duration of this flood was particularly long (more than 3 months) and it resulted in very significant damages. Due to the dual porosity of the Chalk, the possibility is explored that a hydraulic continuity occurs between the unsaturated and saturated zones of the basin under abundant rainfall conditions, leading to a fast rise of the groundwater level. Until now, the role played by the unsaturated Chalk in the groundwater flooding events has not been clearly established and remains poorly understood. In 2005, an experimental site was designed and equipped in the Somme catchment in order to study the flow process in the unsaturated Chalk. The equipment includes 10 "shallow" tensiometers and 12 "jacking tensiometers" (borehole tensiometers) installed for the monitoring of the suction on a whole profile of the unsaturated Chalk; and an Envirosmart probe equipped with 16 sensors for the monitoring of the water content at various depths. The water table level and the rainfall were also monitored at the site. The analysis of the first data collected during one full year since December 2005 shows a very regular evolution of the pressure in the unsaturated Chalk with the same trend at different monitored depths, the negative pressures range from -800 to -70 cm of water. However, this suction variation does not lead to a significant variation of the water content (around 2%); in fact the water content is close to saturation in the whole unsaturated profile. The suction profiles show a slight and regular increase and decrease of suction during 2006; this means that under normal or rather dry climatic condition like in 2006, the water and pressure transfers are mainly controlled by the Chalk matrix. This was corroborated by a slow piezometric variation in 2006.