CNEXO experimental SandPit in Seine Estuary
Contents
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).
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.
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
Three different bathymetries were collected 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 not more than 20 cm for the altimetric coordinate. 1981 bathymetric data are more hazardous and was found by digitalizing an old map. Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.
Morphodynamic evolution of Cnexo Sandpit since 1981
Cross-sections analysis
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.
Differential bathymetric map
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.
Volume analysis of the Cnexo Pit
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).
