3D CFD Modelling of Barge Filling and Overflow Dredging Plume to Determine the Far-Field Source Term
WODCON XXIII - Dredging is changing - The Practice. The Science. The Business.
L. de Wit, R. van Weerdenburg, T. van der Biezen, C.A. Mosca
"The spill of the overflow plume that is available for far field dispersion is depending on the dredging method and local conditions like the soil characteristics and hydrodynamics. These effects are incorporated in CFD (Computational Fluid Dynamics) simulations that represent the filling of a barge by a cutter and near-field mixing of the overflow plume from the barge. A first series of CFD simulations covers the inside of the barge and simulates the morphological bed update during loading for three different loading characteristics covering the expected variation in inflow particle size distribution (PSD). The overflow losses from the first series are used in a second series of CFD simulations for the near-field mixing of the overflow plume from the barge. Based on the near-field CFD simulations covering the first 15 minutes of plume mixing and deposition a physics-based curve fit method as described in De Wit et al. (2014c) is used to provide an extrapolation to 30 and 60 minutes of plume mixing and deposition to arrive at far-field source terms at 15, 30 and 60 minutes from the source. The far-field source term at 15 minutes from the overflow is 6.2% of the barge inflow for the expected PSD. At 30 minutes it is nearly 2x lower (3.7%) and at 60 minutes it is nearly 4x lower (1.6% of the barge inflow). In case of poor representation of near-bed plume dynamics in a far-field model, e.g. because of a coarse resolution or because a 2D far-field model is used, a far-field source term defined at a larger distance is more accurate. The selected variations in PSD and ambient current conditions highlight that the far-field source term depends strongly on those aspects. The results of CFD modelling are compared with the Becker et al. (2015) approach, which is a simplified approach that is often used in dredging practice. The insights from this study can be used to improve the quality of the estimations of source term fractions which will benefit the determination of the potential environmental impact of a dredging project involving overflow plumes. "
Keywords: Computational Fluid Dynamics, hopper/barge sedimentation, overflow plume, near-field plume dispersion, far-field source term