Dynamics of Turbid Plumes Generated by Marine Aggregate Dredging: An Example of a Macrotidal Environment (the Bay of Seine, France)

On the French marine shelf, marine aggregates are currently dredged by trailing suction hopper dredgers (TSHD), without screening, with an overflow process, with or against currents. In the framework of the Groupement d'Interet Scientifique (GIS) Suivi des Impacts de l'Extraction de Granulats Marins (SIEGMA), the dynamics of the turbid plume formed by the overflow was studied at an experimental dredging site (0.6 km(2)), located in the Bay of Seine (eastern English Channel), a macrotidal environment where the seabed is composed of sandy gravel. An original field strategy based on Lagrangian monitoring, using the back-scattered signal of an Acoustic Doppler Current Profiler (ADCP), Laser In Situ Scattering and Transmissometry (LISST) measurements, water samples, and aerial photographs, has shown good results in describing suspended sediment concentration (SSC) dynamics from a turbid plume. The behaviour of suspended sediment has been studied, for the first time, to our knowledge, from their release at the dredger overflow to their deposition on the seabed. A fluid mixture, overflowed from the TSHD, with a mean concentration of 6 g L-1 of mineral and organic silty sands, induces the formation of a turbid plume at the rear of the TSHD, with an immediate dilution effect about 10-100 on SSC. Then, the SSC decreases through the process of lateral dispersion and sedimentation until the plume disappear in 2 hours. Evolution of the plume characteristics (SSC, particle size and nature, geometry) makes it possible to quantify the processes involved: dilution, advection, dispersion, and settling. The passive and dynamic behaviours of the plume are analysed. Extensions of deposits are calculated for spring tide conditions: 800 m for sand and 6.5 km for silt. This study has been conducted in macrotidal and specific dredging conditions (orientation of the dredging tracks against currents, dredger speed, loading capacity, overflow type), whose effects on plume dispersion have been quantified. Indicators relative to the effect of the plume on the water column (I/R) and to the plume duration (T1.25SSC) are proposed.