The Eastern English Channel is a macrotidal epicontinental sea, subject to an increasing demand for marine aggregate extraction since several decades. Effects on the submarine landscapes and ecosystems are conditioned by the physical impacts generated by extraction. In the objective of the integrated coastal zone management, a good knowledge of impacts, of their forcing parameters and their recovery potential is necessary. Two extraction sites with physical characteristics typical of the Eastern English Channel are studied off Dieppe and in the Bay of Seine. For the last one, extraction is experimental and has been monitored for 3 years, during and after extraction, on a sandy-gravelly substrate. The study focuses on 3 main physical impacts: (i) a direct hydrological impact due to sediment resuspension, forming a turbid plume at the back of the dredger, (ii) a direct morphosedimentary impact generated during sediment excavation in the extraction zone, and (iii) an indirect morphosedimentary impact corresponding to the deposition of the particles from the turbid plume, in and around the extraction zone. The turbid plume, for which a monitoring strategy is proposed, lasts around 2-3h and displays a dynamics which is essentially passive and subject to dispersion, settlement and advection processes. Concentrations, grain-size and nature of the suspended matter composing the turbid plume are mainly controlled by the nature of the extracted sediment and the pump discharge of the dredger. Water level and wave climate control the plume dispersion time and the resuspension of the sediment that settled from previous plumes. The impacts in the extraction zone are concerned with furrows and depressions whose morphological parameters depend on extraction intensity. In excavations sediment nature is function of the vertical sediment variability and the infilling potential (sediment fluxes). In the vicinity of the depositional areas, the impact is essentially concerned with an increase in the proportion of silts and sands. Extension of medium sands is potentially restricted to 250 m around the extraction location, whereas silts may extend up to 6.5 km. Deposit distance depends on turbid plume dynamics, particle settling velocity and current velocity. The nature of deposits is function of the nature of the sediment put in suspension at the surface from the overflow and at the sea bottom by the dredging draghead, so by the nature of substrate and the speed of the TSHD (Trailing Suction Hopper Dredger). Morphological recovery of excavated areas is estimated to reach 10.5 years in the Bay of Seine, as generally reported in the literature for this type of seabed. Recommendations for the monitoring of impacts and management tools (indices and equations) are proposed to characterize and quantify impacts, and compare them form one site to another.
Morphosedimentary impacts of marine aggregate extraction. Eastern English Channel case study