Laboratory Investigations into the Threshold of Movement of Sand-sized Sediments
Laboratory Investigations into the Threshold of Movement of Sand-sized Sediments
The present study consists of a collection of theoretical and experimental investigations into the threshold of movement of sand-size sediment. The sediment beds investigated include: cohesionless quartz sand (2.65g/cm3); oolitic sands (2.80g/cm3); Cerastoderma edule (2.80g/cm3) and Mytilus edulis fragments (2.72g/cm3); and naturally-graded quartz sands. A selection of threshold data sets from previous investigations is used to re-examine, selectively, some of the empirical curves used most commonly for the prediction of sediment threshold. Simple analytical formulae are derived, describing: (a) 'single line' curves for mean threshold values; and (b) envelopes for the initial movement of discrete particles and the commencement of mass sediment transport phases of the critical condition.
Investigations into the effects of the 'stress history', leading up to the critical condition for sediment movement, show that if the exposure duration to pre-threshold velocities remains constant, then the critical shear velocity increases with increasing pre-threshold velocity. Likewise, if the pre-threshold velocity remains constant, then the critical shear velocity increases with increasing exposure duration. A set of analytical formulae are proposed to account for the exposure correction to be applied to the critical shear velocities, for an improved description of the magnitude and nature of transport.
Within naturally-occurring ooid sediment samples, incorporated biogenic debris appear to cause higher threshold and lower settling velocities, than in sediments consisting completely of ooid grains. For such sediments the Shields empirical threshold curve, as applied to quartz sediments, is found to describe the results somewhat inadequately. The use of settling velocity, which incorporates some of the effects of grain shape and compositional variations, in the Movability diagram, is proved to be a better predictor of the threshold velocity. Differences in the measured settling and threshold properties of biogenic sedimentary particles (Cerastoderma edule and Mytilus edulis) are found to relate primarily to shape differences.
University of Southampton
Paphitis, Doros
9317ca14-79fc-478e-87b2-056c93d4570f
2001
Paphitis, Doros
9317ca14-79fc-478e-87b2-056c93d4570f
Paphitis, Doros
(2001)
Laboratory Investigations into the Threshold of Movement of Sand-sized Sediments.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
The present study consists of a collection of theoretical and experimental investigations into the threshold of movement of sand-size sediment. The sediment beds investigated include: cohesionless quartz sand (2.65g/cm3); oolitic sands (2.80g/cm3); Cerastoderma edule (2.80g/cm3) and Mytilus edulis fragments (2.72g/cm3); and naturally-graded quartz sands. A selection of threshold data sets from previous investigations is used to re-examine, selectively, some of the empirical curves used most commonly for the prediction of sediment threshold. Simple analytical formulae are derived, describing: (a) 'single line' curves for mean threshold values; and (b) envelopes for the initial movement of discrete particles and the commencement of mass sediment transport phases of the critical condition.
Investigations into the effects of the 'stress history', leading up to the critical condition for sediment movement, show that if the exposure duration to pre-threshold velocities remains constant, then the critical shear velocity increases with increasing pre-threshold velocity. Likewise, if the pre-threshold velocity remains constant, then the critical shear velocity increases with increasing exposure duration. A set of analytical formulae are proposed to account for the exposure correction to be applied to the critical shear velocities, for an improved description of the magnitude and nature of transport.
Within naturally-occurring ooid sediment samples, incorporated biogenic debris appear to cause higher threshold and lower settling velocities, than in sediments consisting completely of ooid grains. For such sediments the Shields empirical threshold curve, as applied to quartz sediments, is found to describe the results somewhat inadequately. The use of settling velocity, which incorporates some of the effects of grain shape and compositional variations, in the Movability diagram, is proved to be a better predictor of the threshold velocity. Differences in the measured settling and threshold properties of biogenic sedimentary particles (Cerastoderma edule and Mytilus edulis) are found to relate primarily to shape differences.
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Published date: 2001
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Local EPrints ID: 464467
URI: http://eprints.soton.ac.uk/id/eprint/464467
PURE UUID: fab18c66-c912-46f1-9cb0-8288888da675
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Date deposited: 04 Jul 2022 23:39
Last modified: 16 Mar 2024 19:32
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Author:
Doros Paphitis
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