Influence of important near field processes on the source term of suspended sediments from a dredging plume caused by a trailing suction hopper dredger: the effect of dredging speed, propeller, overflow location and pulsing

Abstract

Detailed Large Eddy Simulation (LES) results are presented on near field mixing of overflow dredging plumes generated at a Trailing Suction Hopper Dredger (TSHD). Special attention is given to the generation of a surface plume. A surface plume is that part of the dredging plume which separates from the main plume body and ends up near the free surface. A surface plume can stay suspended for hours and during this time it can be transported to ecological sensitive areas around a dredging site. Hence, for a correct environmental impact assessment of a dredging project with TSHD’s it is important to understand near field mixing and thus obtain a correct estimate for the source term of suspended sediments formed by the surface plume. The detailed LES are used to investigate systematically the influence of dredging speed, overflow location, propeller and pulsing frequency (caused by ship motions) on near field mixing of dredging plumes and the generation of a surface plume. LES results are validated with experimental results. The investigated variations have significant influence on the development of the dredging overflow plume in general and surface plume in particular. With normal dredging speed the surface plume varied between 0 and 2 % of the overflow flux with maximum time averaged prototype-SSC (suspended sediment concentration) levels of 1–31 mg/l at the free surface at a horizontal distance $$x/D\,=\,100$$ ( $$D$$ is initial plume diameter). With high dredging speed the surface plume varied between 0.2 and 18 % with maximum time averaged prototype-SSC levels of 10–352 mg/l at the free surface. The large range in surface plume percentage indicates the importance of detailed near field modelling including all significant processes for a correct estimate of the source term of suspended sediment from a dredging plume. All results are obtained without influence of a bed, hence in shallow areas the amount of surface plume could be larger.