Resistance Is Not Futile: Grain Resistance Controls on Observed Critical Shields Stress Variations

Elowyn M. Yager, Mark Schmeeckle, Alexandre Badoux

Research output: Contribution to journalArticle

1 Citation (Scopus)

Abstract

Estimates of the onset of sediment motion are integral for flood protection and river management but are often highly inaccurate. The critical shear stress (τ* c) for grain entrainment is often assumed constant, but measured values can vary by almost an order of magnitude between rivers. Such variations are typically explained by differences in measurement methodology, grain size distributions, or flow hydraulics, whereas grain resistance to motion is largely assumed to be constant. We demonstrate that grain resistance varies strongly with the bed structure, which is encapsulated by the particle height above surrounding sediment (protrusion, p) and intergranular friction (ϕf). We incorporate these parameters into a novel theory that correctly predicts resisting forces estimated in the laboratory, field, and a numerical model. Our theory challenges existing models, which significantly overestimate bed mobility. In our theory, small changes in p and ϕf can induce large changes in τ* c without needing to invoke variations in measurement methods or grain size. A data compilation also reveals that scatter in empirical values of τ* c can be partly explained by differences in p between rivers. Therefore, spatial and temporal variations in bed structure can partly explain the deviation of τ* c from an assumed constant value. Given that bed structure is known to vary with applied shear stresses and upstream sediment supply, we conclude that a constant τ* c is unlikely. Values of τ* c are not interchangeable between streams, or even through time in a given stream, because they are encoded with the channel history.

Original languageEnglish (US)
JournalJournal of Geophysical Research: Earth Surface
DOIs
StateAccepted/In press - Jan 1 2018

Fingerprint

beds
shield
Sediments
Rivers
rivers
sediments
shear stress
Shear stress
grain size
sediment
river management
measurement method
river
entrainment
critical loading
Numerical models
temporal variation
friction
spatial variation
Hydraulics

Keywords

  • bed structure
  • onset of motion
  • sediment transport

ASJC Scopus subject areas

  • Geophysics
  • Forestry
  • Oceanography
  • Aquatic Science
  • Ecology
  • Water Science and Technology
  • Soil Science
  • Geochemistry and Petrology
  • Earth-Surface Processes
  • Atmospheric Science
  • Space and Planetary Science
  • Earth and Planetary Sciences (miscellaneous)
  • Palaeontology

Cite this

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title = "Resistance Is Not Futile: Grain Resistance Controls on Observed Critical Shields Stress Variations",
abstract = "Estimates of the onset of sediment motion are integral for flood protection and river management but are often highly inaccurate. The critical shear stress (τ* c) for grain entrainment is often assumed constant, but measured values can vary by almost an order of magnitude between rivers. Such variations are typically explained by differences in measurement methodology, grain size distributions, or flow hydraulics, whereas grain resistance to motion is largely assumed to be constant. We demonstrate that grain resistance varies strongly with the bed structure, which is encapsulated by the particle height above surrounding sediment (protrusion, p) and intergranular friction (ϕf). We incorporate these parameters into a novel theory that correctly predicts resisting forces estimated in the laboratory, field, and a numerical model. Our theory challenges existing models, which significantly overestimate bed mobility. In our theory, small changes in p and ϕf can induce large changes in τ* c without needing to invoke variations in measurement methods or grain size. A data compilation also reveals that scatter in empirical values of τ* c can be partly explained by differences in p between rivers. Therefore, spatial and temporal variations in bed structure can partly explain the deviation of τ* c from an assumed constant value. Given that bed structure is known to vary with applied shear stresses and upstream sediment supply, we conclude that a constant τ* c is unlikely. Values of τ* c are not interchangeable between streams, or even through time in a given stream, because they are encoded with the channel history.",
keywords = "bed structure, onset of motion, sediment transport",
author = "Yager, {Elowyn M.} and Mark Schmeeckle and Alexandre Badoux",
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AU - Schmeeckle, Mark

AU - Badoux, Alexandre

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N2 - Estimates of the onset of sediment motion are integral for flood protection and river management but are often highly inaccurate. The critical shear stress (τ* c) for grain entrainment is often assumed constant, but measured values can vary by almost an order of magnitude between rivers. Such variations are typically explained by differences in measurement methodology, grain size distributions, or flow hydraulics, whereas grain resistance to motion is largely assumed to be constant. We demonstrate that grain resistance varies strongly with the bed structure, which is encapsulated by the particle height above surrounding sediment (protrusion, p) and intergranular friction (ϕf). We incorporate these parameters into a novel theory that correctly predicts resisting forces estimated in the laboratory, field, and a numerical model. Our theory challenges existing models, which significantly overestimate bed mobility. In our theory, small changes in p and ϕf can induce large changes in τ* c without needing to invoke variations in measurement methods or grain size. A data compilation also reveals that scatter in empirical values of τ* c can be partly explained by differences in p between rivers. Therefore, spatial and temporal variations in bed structure can partly explain the deviation of τ* c from an assumed constant value. Given that bed structure is known to vary with applied shear stresses and upstream sediment supply, we conclude that a constant τ* c is unlikely. Values of τ* c are not interchangeable between streams, or even through time in a given stream, because they are encoded with the channel history.

AB - Estimates of the onset of sediment motion are integral for flood protection and river management but are often highly inaccurate. The critical shear stress (τ* c) for grain entrainment is often assumed constant, but measured values can vary by almost an order of magnitude between rivers. Such variations are typically explained by differences in measurement methodology, grain size distributions, or flow hydraulics, whereas grain resistance to motion is largely assumed to be constant. We demonstrate that grain resistance varies strongly with the bed structure, which is encapsulated by the particle height above surrounding sediment (protrusion, p) and intergranular friction (ϕf). We incorporate these parameters into a novel theory that correctly predicts resisting forces estimated in the laboratory, field, and a numerical model. Our theory challenges existing models, which significantly overestimate bed mobility. In our theory, small changes in p and ϕf can induce large changes in τ* c without needing to invoke variations in measurement methods or grain size. A data compilation also reveals that scatter in empirical values of τ* c can be partly explained by differences in p between rivers. Therefore, spatial and temporal variations in bed structure can partly explain the deviation of τ* c from an assumed constant value. Given that bed structure is known to vary with applied shear stresses and upstream sediment supply, we conclude that a constant τ* c is unlikely. Values of τ* c are not interchangeable between streams, or even through time in a given stream, because they are encoded with the channel history.

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