Finite elements modelling of a large water table aquifer in transient conditions

Abstract A chalky aquifer located near Liege (Belgium), is recharged by infiltration through the overlying loess. Wells and collecting tunnels produce a daily flow of 60 000 m3 out of this aquifer when hydrogeological balances have shown that an average yield of 100 000 m3/day should be possible. Finite element modelling has been developed to foresee the evolution of the water table to get some additional information especially about the main drainage axis. The transient flow constitutive laws are recalled in confined and unconfined aquifers. A new law is proposed to model the water table surface in transient conditions and with a fixed meshing network. Using the local flow equilibrium and the virtual power principle, the FEM formulation is set up. The time integration and the iteration technique are shortly discussed. The 3D discretization and the modelling of the entire aquifer has been realized. The problem requires about 3600 DOF and 2670 8-nodes isoparametric brick finite elements. The modelling has been quite delicate because of the geometric complexity of the different geological layers. This complexity justifies fully the use of the finite element method; there is indeed a great diversity of geological characteristics and the range of the different permeabilities is very wide. Sensitivity of the model to permeabilities and storage coefficient variations has been studied and various numerical problems have been notified. The calibration procedure is described in its main steps, and the most significant results are presented.