Experimental Evidence for Autosuspension

  1. William McCaffrey,
  2. Ben Kneller and
  3. Jeff Peakall
  1. H. M. Pantin

Published Online: 17 MAR 2009

DOI: 10.1002/9781444304275.ch14

Particulate Gravity Currents

Particulate Gravity Currents

How to Cite

Pantin, H. M. (2001) Experimental Evidence for Autosuspension, in Particulate Gravity Currents (eds W. McCaffrey, B. Kneller and J. Peakall), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304275.ch14

Editor Information

  1. School of Earth Sciences, University of Leeds, Leeds, LS2 9JT, West Yorkshire, UK

Author Information

  1. School of Earth Sciences, University of Leeds, Leeds, LS2 9JT, West Yorkshire, UK

Publication History

  1. Published Online: 17 MAR 2009
  2. Published Print: 24 APR 2001

ISBN Information

Print ISBN: 9780632059218

Online ISBN: 9781444304275

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Keywords:

  • experimental evidence for autosuspension;
  • autosuspension current - defined as particle driven gravity flow;
  • testing and modification of experimental rig to generate and monitor autosuspensive currents;
  • apparatus for Phase 1 experiments;
  • roughness elements;
  • sediment entrainment mechanisms;
  • prediction of autosuspension;
  • interpretation of results phase 2 - erosion, sediment entrainment and increased suspended load;
  • interpretation of phase 1 and phase 2 results

Summary

An autosuspension current may be defined as a particle-driven gravity flow, which can persist indefinitely without an external supply of energy. The ultimate criterion for autosuspension must, therefore, be lack of net deposition, a criterion which is reinforced if the current is also capable of erosion.

A series of experiments has been carried out to investigate the physical conditions required for the onset of autosuspension, and to demonstrate autosuspension in the laboratory. Gravity currents were generated in a tubular channel (5.1-cm i.d. Perspex) with a gradient of c. 20°, the dense fluid employed being either salt solution, or a suspension of industrial silica flour with a modal grain size dmode of about 20 µm. Prior to a run, a series of roughness elements were placed in the tube, and a test bed (almost always silica with a dmode of about 20 µm) was introduced as a saline suspension. The dense fluid forming the current was placed in a Perspex header tank, located at the top of the tube, the whole apparatus being immersed in a deep tank of water. The fluid in the header tank was released and allowed to run over the test bed, measurements being made of the amount of sediment taken into suspension, together with the flow velocity.

The most significant results were obtained in a 1.75-m long tube, with a silica flour suspension in the header tank and using a series of deflector plates, constructed to impart a right-handed swirl to the flow in the tube. Four lines of evidence indicate that autosuspension was achieved, at least on a limited scale:

1 after a run, the test bed revealed a sharp erosional upper surface;

2 about 30% of the suspension load of the flow consisted of test-bed sediment, entrained and incorporated into the flow;

3 at its highest level, the proportion of sediment in subsamples from the flow was found to exceed that in the original suspension in the header tank; and

4 current velocity measurements showed evidence of acceleration.