Boudewijn Decrop

New methods for ADV measurements of turbulent sediment fluxes – application to a fine sediment plume

ABSTRACT New methods have been developed to extract turbulent fluxes of momentum and fine sediments from Acoustic Doppler Velocimeter (ADV) data. The methods were validated with turbidity plume experiments. The ADVs backscatter amplitude signal was used to determine the sediment concentration and its turbulent fluctuations. However, different kinds of noise are found in the backscatter amplitude and the velocity signals, which are polluting the turbulent fluxes results. Therefore, spectral noise correction methods have been developed, which allow more accurate quantification of turbulent velocity and sediment concentration fluctuations. The techniques are applied to two benchmark cases of a vertical sediment-laden jet. Reynolds stresses, turbulent intensity of velocity and sediment concentration as well as turbulent sediment fluxes are shown to agree well with two-fluid plume measurements reported in the literature. The methods presented in this paper can be applied to the processing of measurements in cohesive sediment plumes, turbidity currents or mixing layers in the absence of, or in stable, flocculation.

Numerical simulation of near-field dredging plumes: efficiency of an environmental valve

AbstractNumerical simulations of the sediment-air-water buoyant jet released through the hopper dredgers’ overflow shaft have been performed. The release of sediments into the marine environment due to skimming the excess water from the dredging vessel’s hopper can lead to increased turbidity and adverse effects on the adjacent environment. Base-case simulations have been validated using in situ field observations. Simulations have been performed using the large-eddy simulation technique, which allows including the effect of large turbulent structures on the sediment dispersion. The complex nature of the flow field poses challenges for numerical simulations, such as the presence of propeller jets and three different phases: water, sediment, and air bubbles. The model has been applied to simulate the effect of a so-called environmental valve, which reduces air inclusion. This valve has been used in the past, but its efficiency as a function of the boundary conditions was never analyzed before. It is shown ...

A parameter model for dredge plume sediment source terms

The presented model allows for fast simulations of the near-field behaviour of overflow dredging plumes. Overflow dredging plumes occur when dredging vessels employ a dropshaft release system to discharge the excess sea water, which is pumped into the trailing suction hopper dredger (TSHD) along with the dredged sediments. The fine sediment fraction in the loaded water-sediment mixture does not fully settle before it reaches the overflow shaft. By consequence, the released water contains a fine sediment fraction of time-varying concentration. The sediment grain size is in the range of clays, silt and fine sand; the sediment concentration varies roughly between 10 and 200 g/l in most cases, peaking at even higher value with short duration. In order to assess the environmental impact of the increased turbidity caused by this release, plume dispersion predictions are often carried out. These predictions are usually executed with a large-scale model covering a complete coastal zone, bay, or estuary. A source term of fine sediments is implemented in the hydrodynamic model to simulate the fine sediment dispersion. The large-scale model mesh resolution and governing equations, however, do not allow to simulate the near-field plume behaviour in the vicinity of the ship hull and propellers. Moreover, in the near-field, these plumes are under influence of buoyancy forces and air bubbles. The initial distribution of sediments is therefore unknown and has to be based on crude assumptions at present. The initial (vertical) distribution of the sediment source is indeed of great influence on the final far-field plume dispersion results. In order to study this near-field behaviour, a highly-detailed computationally fluid dynamics (CFD) model was developed. This model contains a realistic geometry of a dredging vessel, buoyancy effects, air bubbles and propeller action, and was validated earlier by comparing with field measurements. A CFD model requires significant simulation times, which is not available in all situations. For example, to allow correct representation of overflow plume dispersion in a real-time forecasting model, a fast assessment of the near-field behaviour is needed. For this reason, a semi-analytical parameter model has been developed that reproduces the near-field sediment dispersion obtained with the CFD model in a relatively accurate way. In this paper, this so-called grey-box model is presented.

Large-eddy simulations of a sediment plume released by a dredger using overflow

Sediment plume predictions are part of the assessment of environmental impacts of dredging. The main source of turbidity while employing Trailer Suction Hopper Dredgers is the release of excess water through the overflow shaft. The near-field plume dynamics below and directly behind the sailing hopper dredgers are traditionally unknown during predictions of far-field plume dispersion. Indeed, an accurate input of the vertical and horizontal distributions of sediment at the source location is important to obtain reliable results at environmentally sensitive areas further away. In this paper, a computational fluid dynamics model is presented as a tool to determine the three-dimensional flows of water, sediment and air bubbles directly after release from the overflow shaft. The full dredger hull geometry and an actuator disc accounting for propeller action are included. It is shown that the model can reproduce two different cases of overflow plumes measured in the field with fair accuracy.