Salt water intrusion in the breakthrough valley of the river Aa between the Flemish coastal plain and the Saint Omer basin (France)

Luc Lebbe,Gert Jan Devriese,Devlin Depret,Jasper Claus,A Szymkiewicz,B Jaworska-Szulc,A Sadurski

doi:10.1051/e3sconf/20185400017

Abstract

The salinity distribution in a breakthrough valley has been studied based on field observations and using a numerical model. The studied valley is located in Northern France between the Saint Omer basin and the Flemish coastal plain. The field campaign starts with the installation of observation wells. In these observations wells the electrical conductivity is logged versus depth with an electro-magnetic device. These wells also allow the necessary measurements to deduce the fresh water heads. Some of the wells are used for the performance of a pumping test. The lateral variation of the electrical conductivity of the water in the drainage canals is measured. With a numerical model the evolution of the flow and distribution of the fresh and salt water are simulated. Two simulations are made with the same schematization of the groundwater reservoir consisting of quaternary marine sediments, silt and clay and with the same hydraulic and solute parameters and boundary conditions but with two different initial salinity distributions. The results of both simulations show that the salinity distribution in the quaternary marine sediments depends not only on the lateral variation of the watertable but also on the shape of the incision of the palaeo-valley into the clay of Ypresian age. The salinity distribution in the lower part of the quaternary marine sediments depends first and foremost on the shape of the incision of the palaeo-valley into the clay of Ypresian age whereas the salinity distribution in the upper part of the quaternary marine sediments depends first and foremost on the lateral variation of the watertable.

Highlights

On the edges of coastal plains, and even beyond, salt water intrusions can occur in valleys which are surrounded or flanked by hills
In the breakthrough valley of the river Aa between the Saint Omer Basin and the Flemish coastal plain occurs a salt water intrusion. This salt water intrusion was studied by the aid of field observations and numerical model simulations
A limited number of observations wells allow the collection of diverse data to compare with the results of the numerical model such as the fresh water head, the variation of electrical conductivity of the deposits around the wells and the execution of a pumping test

Summary

Introduction

On the edges of coastal plains, and even beyond, salt water intrusions can occur in valleys which are surrounded or flanked by hills. The initial salinity distribution in the QMS is based on the estimated areal variation of the level of the interface between fresh and salt water. When the finite-difference cells with QMS are situated completely above the interface the initial salt water percentage (Ps) is set equal to zero.

Results

Conclusion