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

A chalky aquifer located near Liege (Belgium), is recharged by infiltration through the overlying
loess. Wells and collecting tunnels produce a daily flow of 60OO0m a out of this aquifer when
hydrogeological balances have shown that an average yield of 100 0OO m a/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.