Laboratory determination of the settling, threshold and transport velocities of unusually shaped grains (Foraminifera) : the effect of shape on grain hydrodynamics
Laboratory determination of the settling, threshold and transport velocities of unusually shaped grains (Foraminifera) : the effect of shape on grain hydrodynamics
Laboratory experiments are undertaken in which the hydrodynamic properties of empty foraminifera tests (and other sediments) are examined. Measurements are made of the settling and threshold velocities of individual grains, as well as the speed at which they move in a transporting flow. A final experiment studies the mass redistribution of a mixed sediment sample, containing foraminifera, sands and shell fragments of mixed size and shape, under a sustained transporting flow.
The foraminifera have similar settling velocity ranges (around 0.05ms-1), despite shapes ranging from large flat discs (diameters 0.5-1mm) to (near) perfect smaller (diameter 0.1-0.5mm) spheres (along with irregular examples of these shapes). Mean densities of 1.62g/cm3 (Orbulina universa) and 1.19g/cm3 (Sorites orbiculus), which compare favourably with estimates by previous investigators. These values are applied as 'best guesses' for the density of similarly-shaped species. Settling equivalent diameter Dq is taken as the grain size in all models of grain behaviour, removing most of the effects of grain shape from the data.
Sediment threshold is found to conform closely to Shields curve, allowing for the effects of grain protrusion (p). The large flat grains rest across the tops of all except very coarse sand beds, thus they appear to rest on beds of uniform size (p=1) and their threshold is close to that predicted by Shields curve. Spherical grains are displaced below the curve when threshold is measured on beds of sand finer than the diameter of the tests.
Spheres are found to roll continuously and begin to saltate in flows only a little above threshold velocity. The corresponding motions of flaky grains are first intermittent, then continuous movements, as flow speed increases and grains are rolled on edge (rather than end-over-end). The threshold of first grain movement is found to be different to the minimum conditions for grain rolling, in the case of non-spherical grains of low rollability. Threshold for grain rolling conditions is defined by the mixed bed model of Wiberg and Smith (1987). However, motions of salting grains above a well-defined velocity (or flow stage) rapidly become independent of the bed and, hence, threshold conditions for saltation change again. Rather than depending upon a constant (experimentally measured) threshold condition, the relative ease of transporting different shapes changes with the mode of transport and rate of flow.
The lower settling velocity and rollability of a flake-shaped test means that, for a small increase in flow speed, the flake may suddenly become more mobile than an easily-rolled sphere. Many small-scale shape sorting effects may possibly be explained with reference to a simple model.
University of Southampton
1998
Wallbridge, Steven
(1998)
Laboratory determination of the settling, threshold and transport velocities of unusually shaped grains (Foraminifera) : the effect of shape on grain hydrodynamics.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
Laboratory experiments are undertaken in which the hydrodynamic properties of empty foraminifera tests (and other sediments) are examined. Measurements are made of the settling and threshold velocities of individual grains, as well as the speed at which they move in a transporting flow. A final experiment studies the mass redistribution of a mixed sediment sample, containing foraminifera, sands and shell fragments of mixed size and shape, under a sustained transporting flow.
The foraminifera have similar settling velocity ranges (around 0.05ms-1), despite shapes ranging from large flat discs (diameters 0.5-1mm) to (near) perfect smaller (diameter 0.1-0.5mm) spheres (along with irregular examples of these shapes). Mean densities of 1.62g/cm3 (Orbulina universa) and 1.19g/cm3 (Sorites orbiculus), which compare favourably with estimates by previous investigators. These values are applied as 'best guesses' for the density of similarly-shaped species. Settling equivalent diameter Dq is taken as the grain size in all models of grain behaviour, removing most of the effects of grain shape from the data.
Sediment threshold is found to conform closely to Shields curve, allowing for the effects of grain protrusion (p). The large flat grains rest across the tops of all except very coarse sand beds, thus they appear to rest on beds of uniform size (p=1) and their threshold is close to that predicted by Shields curve. Spherical grains are displaced below the curve when threshold is measured on beds of sand finer than the diameter of the tests.
Spheres are found to roll continuously and begin to saltate in flows only a little above threshold velocity. The corresponding motions of flaky grains are first intermittent, then continuous movements, as flow speed increases and grains are rolled on edge (rather than end-over-end). The threshold of first grain movement is found to be different to the minimum conditions for grain rolling, in the case of non-spherical grains of low rollability. Threshold for grain rolling conditions is defined by the mixed bed model of Wiberg and Smith (1987). However, motions of salting grains above a well-defined velocity (or flow stage) rapidly become independent of the bed and, hence, threshold conditions for saltation change again. Rather than depending upon a constant (experimentally measured) threshold condition, the relative ease of transporting different shapes changes with the mode of transport and rate of flow.
The lower settling velocity and rollability of a flake-shaped test means that, for a small increase in flow speed, the flake may suddenly become more mobile than an easily-rolled sphere. Many small-scale shape sorting effects may possibly be explained with reference to a simple model.
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Published date: 1998
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Local EPrints ID: 463221
URI: http://eprints.soton.ac.uk/id/eprint/463221
PURE UUID: 94a4794c-9588-4512-b137-b1ea5910425e
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Date deposited: 04 Jul 2022 20:47
Last modified: 04 Jul 2022 20:47
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Author:
Steven Wallbridge
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