Cohesive/cohesionless sediment transition diameter from settling velocity data.

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Bibliographic Details
Title:	Cohesive/cohesionless sediment transition diameter from settling velocity data.
Authors:	Mehta, Ashish¹ mehtanutechinc@gmail.com, Letter, Joseph²
Source:	Ocean Dynamics. Sep2015, Vol. 65 Issue 9/10, p1213-1219. 7p.
Subjects:	Mathematical models of oceanography, Sediment transport, Flocculation, Velocity distribution (Statistical mechanics), Electrochemical analysis, Shearing force
Abstract:	Mathematical models designed to simulate the movement of cohesive and cohesionless particles require as input the diameter d specifying the transition between these two transport modes. As an effort to identify this diameter, Migniot (La Houille Blanche, 7, 591-620, 1968) measured in a water-filled column the settling velocities of flocs and respective deflocculated particles of mainly mineral cohesive sediments. The data were plotted as the ratio of the floc settling velocity to the particle velocity, called the flocculation factor F, against particle diameter d. The trend line was found to approximately follow an empirical power-law such that F increased rapidly as d decreased below d estimated to be about 30 μm at F = 1. Assuming fractal self-similarity among falling flocs, the power-law exponent of 5/3 is shown to correspond to a fractal dimension of 2.65 implying that the flocs were densely packed. The diameter d depends on the electrochemical properties of the suspended particles as well as the kinetics of floc growth and breakup, hence to an extent on the method of determination of d. Its value deduced more directly from measurement of the critical shear stress for erosion of flocs at the surface of cohesive sediment beds has been reported to be about 10 μm, which is lower than 30 μm. Among other reasons, it is likely that the difference is rooted in the limited experimental information available as well as difficulty in characterizing the effect of highly graded distributions of the particle settling velocity. [ABSTRACT FROM AUTHOR]
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Database:	Engineering Source

Description
Abstract:	Mathematical models designed to simulate the movement of cohesive and cohesionless particles require as input the diameter d specifying the transition between these two transport modes. As an effort to identify this diameter, Migniot (La Houille Blanche, 7, 591-620, 1968) measured in a water-filled column the settling velocities of flocs and respective deflocculated particles of mainly mineral cohesive sediments. The data were plotted as the ratio of the floc settling velocity to the particle velocity, called the flocculation factor F, against particle diameter d. The trend line was found to approximately follow an empirical power-law such that F increased rapidly as d decreased below d estimated to be about 30 μm at F = 1. Assuming fractal self-similarity among falling flocs, the power-law exponent of 5/3 is shown to correspond to a fractal dimension of 2.65 implying that the flocs were densely packed. The diameter d depends on the electrochemical properties of the suspended particles as well as the kinetics of floc growth and breakup, hence to an extent on the method of determination of d. Its value deduced more directly from measurement of the critical shear stress for erosion of flocs at the surface of cohesive sediment beds has been reported to be about 10 μm, which is lower than 30 μm. Among other reasons, it is likely that the difference is rooted in the limited experimental information available as well as difficulty in characterizing the effect of highly graded distributions of the particle settling velocity. [ABSTRACT FROM AUTHOR]
ISSN:	16167341
DOI:	10.1007/s10236-015-0865-3