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

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 Lynyrd de Wit · Cees van Rhee · Arno Talmon 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 42 Environ Fluid Mech (2015) 15:41-66 (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.

Experiments on and Simulations of Hollow Cylinders Falling through Quiescent Liquids

“…In various branches of engineering, one encounters solid particles suspended in a fluid. In civil and environmental engineering these include dredging [1], sediment transport in rivers and coastal areas [2], and wastewater treatment. In chemical engineering, fluidization and catalytic slurry reactors are used for (reactive) mass transfer between solids and fluids.…”

Modeling of Breaching-Generated Turbidity Currents Using Large Eddy Simulation

“…Therefore, large eddy simulations can provide a wealth of insights into the structure and physical behavior of turbidity currents, in particular into the turbulence structure including turbulent kinetic energy and Reynolds stresses. The numerical model that we use has been applied earlier to gain insights into the complicated dredge plume behavior close to a dredging vessel where density differences, turbulent mixing and sediment settling play an important role [27][28][29]. The model has been validated for a wide range of flow cases relevant to the present study, such as the front speed of density currents radially spreading and density currents running over both flat and sloping beds, the deposition from high-concentration suspended sediment flow at a flat bed [30], and high sediment concentration conditions encountered in hopper sedimentation [31].…”

Estimating source terms for far field dredge plume modelling

“…A dredge plume resulting from a TSHD overflow, for example, has a complicated dynamic phase that is difficult to predict in detail due to several interacting processes. The most significant processes are entrainment of ambient water into the negatively buoyant plume, interaction of the plume with the cross flow due to currents and TSHD trailing speed, entrainment of air in the overflow mixture, mixing by propellers of the TSHD and influences resulting from flow around the hull (Van Eekelen, 2007;De Wit, 2010;De Wit et al, 2014b). The relative importance of these processes may differ further depending on equipment specifications, work methods and environmental conditions (under-keel clearance, currents, project progress etc).…”