Finite element model of a regional water-table aquifer in cretaceous chalk (Belgium)
The Hesbaye aquifer located in Cretaceous chalks near Liège (Belgium), is recharged by infiltration through the overlying loess and conglomerate. Wells and collecting adits produce a daily flow of 60 000 m3 out of this aquifer, to provide the alimentation of the area of Liège and its suburbs. More Man 500 boreholes, plezometers and wells give many informations for the idealization and the calibration procedures. Variation ranges of permeability and effective porosity are determined from 150 pumping tests. The main drainance axis of the aquifer (called « dry valleys ») are located and characterized by higher permeabilities. Finite element modelling has been developed to foresee the evolution of the water table and to get some additional informations especially about these main drainance axis. The modelling method is based on the LAGAMINE code (from Liège University). The LAGATHER code is one of the module of the LAGAMINE which is initially adapted to treat non-linear thermic problems. Here, it has been developed to compute flows in porous media in transient conditions. The water table is calculated with a fixed mesh structure using a new method developed by (CHARLIER et al., 1988). Constitutive laws of the flow are changed to take into account the state of saturation or no-saturation of the media. Then the flow equation (the Darcy's law) becomes non-linear. The transition from unsaturated to saturated field is treated at a constant zero pressure, according to the analogy with thermodynamical phase changes at constant temperature. The method has been checked on uniaxial and two dimensionnal sections. 3D discretisation and modelling of the whole aquifer has been realized. The problem requires about 2 670 8-nodes isoparametric brick finite elements. The complexity of the geological conditions justifies fully the use of finite element method by the diversity and the wide range of the hydrogeological parameters to introduce in the model. The discretisation is described and some of the must significant results are presented.