Near-Field Resuspension Model for a Cutter Suction Dredge
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abstract
The quantity of resuspended solids (RSS) created during dredging operations is both a production and environmental issue. Numerical models predict the movement of sediment plumes from dredging operations; however, these far-field models need a "source term" as input. The purpose of this work was to generate a "source term" by developing a near-field resuspension model for cutter suction dredging operations. The near-field cutter resuspension model (NFCRM) used a discretized domain to obtain a more accurate velocity structure and incorporated real laboratory data to model the fluid dynamics of the cutter suction dredging environment. A two-dimensional finite-difference method produced temporal evolution of resuspension from the dredging operation. The output of the near-field model clarified dynamics of the cutter-head environment and provided a source term for a far-field model. To develop and verify the near-field model, turbidity and velocity were measured around a 34.3 cm (13.5 in.) diameter cutter head. Turbidity measurements were equated to RSS concentrations. The NFCRM incorporated laboratory data to determine the velocity field and the turbulent diffusion. Settling velocity of sediment was the primary scaling variable. Cutter speed provided a direct relationship to scale the advective field. The Pclet number scaled the diffusive field. Both point-specific and overall source-term resuspension values predicted from the NFCRM were validated with laboratory testing and field data. Conclusions from this research demonstrate that undercutting produced a larger point-specific RSS concentration than overcutting in laboratory testing. RSS concentration was positively correlated with cutter speed and thickness of cut, and inversely correlated with suction flow rate. The NFCRM produced higher point-specific regions of RSS for undercutting but larger mean values of RSS for overcutting. The NFCRM was comparable to all laboratory testing and field data with suitable mean absolute error (MAE) values. Greater accuracy was achieved for overcutting simulations. The NFCRM followed laboratory trends with cutter speed and suction flow rate but did not show a strong correlation with cut thickness. The NFCRM was numerically sensitive to swing speed because of instabilities in the advective field. Future research should investigate different cutter-head designs and three-dimensional modeling. 2012 American Society of Civil Engineers.
