Non-equilibrium flocculation characteristics of fine-grained sediments in grid-generated turbulent flow

Alan J. S. Cuthbertson, Ping Dong, Peter A. Davies

    Research output: Contribution to journalArticle

    Abstract

    Results are presented from a series of settling column experiments investigating temporal variations in the flocculation characteristics of purely cohesive (kaolin clay) sediment suspensions and cohesive (kaolin) and non-cohesive (fine sand) sediment fraction mixtures. Experimental runs were conducted under controlled hydrodynamic conditions generated by a rigid array of in-phase oscillating grids. The results indicated that rapid initial floc aggregation occurred under low turbulent shear rates, with peak maximal and root-mean-square (r.m.s.) floc sizes (similar to 400 pm and similar to 200 mu m, respectively) attained after relatively short time periods, before reducing with time. By contrast, lower aggregation rates and smaller floc sizes were observed under higher shear conditions, with flocs retaining suspended in the settling column for longer time scales due to the increased turbulence. The mud input concentration displayed some correlation with maximal and r.m.s. floc sizes at higher shear rates but no correlation was apparent at low shear rates. This observed floc behaviour may be attributed to the differences in concentration gradients at high and low shear rates that affect both floc settling rate and time required for flocs to attain equilibrium size. The addition of the fine sand fraction to the kaolin clay suspension reduced both the initial floc formation (i.e. aggregation) rate and the maximal and r.m.s. floc sizes attained throughout the experiments. The reduction in maximal floc sizes appeared to be enhanced by an increase in the ratio of fine sand to kaolin clay content within the mixture. (C) 2009 Elsevier B.V. All rights reserved.

    Original languageEnglish
    Pages (from-to)447-460
    Number of pages14
    JournalCoastal Engineering
    Volume57
    Issue number4
    DOIs
    Publication statusPublished - Apr 2010

    Keywords

    • Flocculation
    • Aggregation
    • Cohesive sediment
    • Oscillating grid turbulence
    • Sand–mud mixtures

    Cite this

    Cuthbertson, Alan J. S. ; Dong, Ping ; Davies, Peter A. / Non-equilibrium flocculation characteristics of fine-grained sediments in grid-generated turbulent flow. In: Coastal Engineering. 2010 ; Vol. 57, No. 4. pp. 447-460.
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    Non-equilibrium flocculation characteristics of fine-grained sediments in grid-generated turbulent flow. / Cuthbertson, Alan J. S.; Dong, Ping; Davies, Peter A.

    In: Coastal Engineering, Vol. 57, No. 4, 04.2010, p. 447-460.

    Research output: Contribution to journalArticle

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    AU - Cuthbertson, Alan J. S.

    AU - Dong, Ping

    AU - Davies, Peter A.

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    AB - Results are presented from a series of settling column experiments investigating temporal variations in the flocculation characteristics of purely cohesive (kaolin clay) sediment suspensions and cohesive (kaolin) and non-cohesive (fine sand) sediment fraction mixtures. Experimental runs were conducted under controlled hydrodynamic conditions generated by a rigid array of in-phase oscillating grids. The results indicated that rapid initial floc aggregation occurred under low turbulent shear rates, with peak maximal and root-mean-square (r.m.s.) floc sizes (similar to 400 pm and similar to 200 mu m, respectively) attained after relatively short time periods, before reducing with time. By contrast, lower aggregation rates and smaller floc sizes were observed under higher shear conditions, with flocs retaining suspended in the settling column for longer time scales due to the increased turbulence. The mud input concentration displayed some correlation with maximal and r.m.s. floc sizes at higher shear rates but no correlation was apparent at low shear rates. This observed floc behaviour may be attributed to the differences in concentration gradients at high and low shear rates that affect both floc settling rate and time required for flocs to attain equilibrium size. The addition of the fine sand fraction to the kaolin clay suspension reduced both the initial floc formation (i.e. aggregation) rate and the maximal and r.m.s. floc sizes attained throughout the experiments. The reduction in maximal floc sizes appeared to be enhanced by an increase in the ratio of fine sand to kaolin clay content within the mixture. (C) 2009 Elsevier B.V. All rights reserved.

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    KW - Cohesive sediment

    KW - Oscillating grid turbulence

    KW - Sand–mud mixtures

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