Dynamical equilibrium state, a key to highlight an anthropogenic disturbance: application to the Octeville dredge disposal sediment site (Le Havre harbour, Eastern Bay of Seine)
Résumé
In the context of the increasing pressure of human activities on marine coastal ecosystems, the study of sedimentary disturbances remains a challenge. The eastern part of the Bay of Seine is very anthropized by the presence of harbour activities (heavy shipping, dredging and sediment dumped deposit …), fishing, sediment extraction… The dredging sediments of the Grand Port Maritime du Havre (GPMH) are deposited on the Octeville site, in the northeastern part of the Seine estuary mouth. The high hydrodynamics (swell and tide) associating with the constant supply of fines particles from both the Seine estuary and the English Channel combined with dumped sediment deposition create a mixed sedimentary zone. To understand sedimentary mechanism in this area, a geochemical and sedimentological study was conducted at the beginning of 2016. A sampling of 179 stations (distance of 500m between stations) was conducted between cape of La Hève and cape of Antifer. In addition, sampling in the harbour basins and in the dredge itself (13 samples in basins strongly or very weakly dredged) and in the Seine estuary (one station in the brackish zone and two stations in freshwater) were also studied. Geochemical elemental composition was obtained through X-Ray Fluorescence spectrometry (SPECTRO xSORT, 43 elements) and infrared spectrometry (LECO CS 744, Total Organic Carbon: TOC). Multiple PCA (Principal Component Analysis) had been conducted to reduce the large set of variables into a more concise one that retains most of the existing variations. Only seven elements had been conserved in the last PCA (Si, As, S, Pb, Rb, organic Br and TOC).
Deposited sediments in the studied area have the same geochemical pattern irrespective of whether their originate from natural or dredged sources. The only difference is that dredged sediments are finer and have undergone changes under the influence of diagenetic processes which are characteristic of harbour environment. As a result, they are enriched in S, TOC, Rb and Pb, which allows to highlight in-situ the dredged deposits. To determine the area under dumped sediments influence and sediments transport directions, we sought if deposits were in a " dynamical equilibrium state ". To do that, TOC is the more accurate parameter. Area under dynamical equilibrium state display, a gradient perpendicular to the shoreline tied up to the granulometrical refinement gradients due to the swell and tide combined actions.
Dumped sediments and area under their influence are clearly highlighted as they disrupt the natural dynamical equilibrium state.