Effects of Mixing on Hopper Sedimentation in Clearing Mixtures
Publication: Journal of Waterway, Port, Coastal, and Ocean Engineering
Volume 141, Issue 2
Abstract
Hopper sedimentation is the result of precipitation of typically fine sediment from a homogenous, high-concentration mixture, which is not completely deficient of turbulence. If hopper sedimentation or loading is accomplished through a single-inflow system, or if the irregularity of the inflow concentrations is pronounced or simply terminated, then the hopper mixture will clear. Whereas turbulent mixing is redundant, when the mixture is homogeneous, it may take an active role when the mixture is clearing. The role of turbulence on hopper sedimentation has been the focus of several studies, and a common perception of turbulence (or at least of mixing) is that it delays sedimentation. Existing measurements of sedimentation rates in a closed-flume experiment, engineered to provide input to a hopper sedimentation model, revealed that turbulence in a clearing mixture is not necessarily associated with a delay in sedimentation. The experiment showed that sedimentation was boosted by adding a current to a clearing mixture, which infers that turbulence, under certain conditions, may act as a sedimentation agent on the excess sediment in suspension. Therefore, the interactions between turbulent mixing and settling in high-concentration mixtures were examined theoretically. Analytical solutions for clearance of excess concentrations were derived for the limiting cases of (1) still-water clearance and (2) clearance when the amount of turbulence is abundant. When examining these analytical solutions, a potential for enhanced sedimentation was revealed. It was found that mixing-induced dilution of concentration weakens the hindrance in settling to a degree that enhances sedimentation. The analytical findings prompted a more elaborate analysis of the mechanism using a numerical model, which encompassed time- and depth-varying turbulence. This allowed the experimental setup and the observed settling effects to be simulated. The potential of the enhancing sedimentation mechanism was analyzed under more general conditions with the numerical model.
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Acknowledgments
This work was supported by the Danish Ministry of Science, Technology and Innovation, through the GTS grant Marine Structures of the Future. The authors wish to recognize the reviewer who suggested to include turbulence damping in the analysis.
References
Camp, T. R. (1946). “Sedimentation and the design of settling tanks.” Trans. Am. Soc. Civ. Eng., 111(1), 895–936.
Dobbins, W. E. (1944). “Effect of turbulence on sedimentation.” Trans. Am. Soc. Civ. Eng., 109(1), 629–656.
Garside, J., and Al-Dibouni, M. R. (1977). “Velocity-voidage relationships for fluidization and sedimentation in solid-liquid systems.” Ind. Eng. Chem. Process Des. Dev., 16(2), 206–214.
Hjelmfelt, A. T., and Lenau, C. W. (1970). “Nonequilibrium transport of suspended sediment.” J. Hydr. Div., 96(7), 1567–1586.
Jensen, J. H., and Saremi, S. (2014). “Overflow concentration and sedimentation in hoppers.” J. Waterway, Port, Coastal, Ocean Eng., 04014023.
Kynch, G. J. (1952). “A theory of sedimentation.” Trans. Faraday Soc., 48, 166–176.
Miedema, S. A. (2009). “The effect of the bed rise velocity on the sedimentation process in hopper dredges.” J. Dredging Eng., 10(1), 10–31.
OpenFOAM 1.5 [Computer software]. Bracknell, U.K., OpenCFD.
Richardson, J. F., and Zaki, W. N. (1954). “Sedimentation and fluidisation: Part I.” Trans. Inst. Chem. Eng., 32, 35–53.
van Rhee, C. (2002a). “Numerical modelling of the flow and settling in a trailing suction hopper dredge.” Proc., 11th Int. Symp. on Transport and Sedimentation of Solid Particles, BHR Group, Bedford, U.K.
van Rhee, C. (2002b). “On the sedimentation process in a trailing suction hopper dredger.” Ph.D. thesis, Delft Univ. of Technology, Delft, Netherlands.
van Rhee, C. (2002c). “The influence of the bed shear stress on the sedimentation of sand.” Proc., 15th Int. Conf. on Hydrotransport, Agricultural Univ. of Wroclaw, Wroclaw, Poland.
van Rhee, C., and Talmon, A. M. (2000). “Entrainment of sediment (or reduction of sedimentation) at high concentration.” Proc., 10th Int. Symp. on Transport and Sedimentation of Solid Particles, Agricultural Univ. of Wroclaw, Wroclaw, Poland.
Wang, L.-P., and Maxey, M. R. (1993). “Settling velocity and concentration distribution of heavy particles in homogeneous isotropic turbulence.” J. Fluid Mech., 256(Nov), 27–68.
Wang, Z. B., and Ribberink, J. S. (1986). “The validity of a depth-integrated model for suspended sediment transport.” J. Hydraul. Res., 24(1), 53–67.
Winterwerp, J. C. (2001). “Stratification effects by cohesive and noncohesive sediment.” J. Geophys. Res., 106(C10), 22559–22574.
Winterwerp, J. C., de Groot, M. B., Mastbergen, D. R., and Verwoert, H. (1990). “Hyperconcentrated sand-water mixture flows over a flat bed.” J. Hydraul. Eng., 36–54.
Zyserman, J. A., and Fredsøe, J. (1994). “Data analysis of bed concentration of suspended sediment.” J. Hydraul. Eng., 1021–1042.
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Published In
Journal of Waterway, Port, Coastal, and Ocean Engineering
Volume 141 • Issue 2 • March 2015
Copyright
© 2014 American Society of Civil Engineers.
History
Received: Jan 23, 2014
Accepted: Oct 9, 2014
Published online: Nov 5, 2014
Published in print: Mar 1, 2015
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