CNEXO experimental SandPit in Seine Estuary

Introduction

Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply. In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary. Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.

Initial state of bathymetry

The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).

Figure 1: Cnexo pit location

A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.

Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.

The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials.

Figure 2: Cnexo dredging

The whole granted domain was not exploited at the same time. From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m. From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).

Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.

Bathymetric data

CETMEF collected 3 different bathymetries for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and no more than 20 cm for the altimetric coordinate. Concerning 1981 bathymetric data, we must be more cautious if we want to use it as a comparison with the two others. 1981 is more hazardous and was made by digitalizing an old map. This leads to a small shifting of the sandpit compared to 1996 and 2002 bathymetries. This shifting was already noticed by Michel Lemoine, which means that this error may be due to shipping uncertainties during field measurements or mistakes during the drawing of this map. Anyway we have shifted the 1981 bathymetry so that it fits the 1996 and 2002 pit's characteristics but these 1981 data must be used cautiously especially if some volume calculations have to be done. Even if the field measurements had been made correctly we should not have expected better precision than 10-15 m for planimetric coordinates and 30cm for the altimetric coordinate.

These 3 bathymetries are clearly written in (x;y;z) text format files and figures 4, 5 and 6 with a common scale give a good idea of their global evolution.