Quantifying gravel overlap and dislodgement forces on natural river bars: implications for particle entrainment

Authors

  • Sean Sanguinito,

    1. Department of Geological Sciences, State University of New York Geneseo, Geneseo, NY, USA
    Current affiliation:
    1. Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX, USA
    Search for more papers by this author
  • Joel Johnson

    Corresponding author
    • Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX, USA
    Search for more papers by this author

  • Current address: Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78712, USA

J. Johnson, Email: joelj@jsg.utexas.edu

ABSTRACT

Equations for predicting particle entrainment typically assume that clast weight is the only factor resisting motion in the force balance on individual grains. In this work, increases in the force required to dislodge surface particles due to overlapping by surrounding clasts is quantified. Field data were collected at two subaerially-exposed bars along the Colorado River in central Texas, USA, with median gravel diameters (D50) of 37 and 64 mm. Clast size, shape, weight, the force required for vertical removal, and the fraction of clast area covered by surrounding grains were measured. Small hooks were glued to individual clasts without disrupting their positions and quasi-static peak forces required to vertically dislodge each clast were measured using a force gauge. Clasts were also colored with dye before being dislodged, and image analysis was used to calculate the fraction of clast surface area covered by surrounding grains. The effect of overlap on the particle force balance is quantified by defining the ‘excess force ratio’ as the dislodgement force divided by the weight of the clast. Excess force ratio varies weakly but systematically with clast size: lifting larger clasts can require forces up to two times the clast's weight, while smaller clasts can require forces up to seven times their weight. The fraction of clast surface area covered by surrounding particles is also weakly correlated with excess force ratio. By assuming that critical shear stresses are proportional to the excess force ratio, the effect that overlap alone may have on particle entrainment is calculated. For a given size class, the most mobile grains should have critical shear stresses controlled only by their weight. However, clast overlap also causes broad distributions of critical stresses for partially-exposed surface grains. The data quantify the significant fraction of bed area that should be less mobile than grain size alone would predict. Copyright © 2011 John Wiley & Sons, Ltd.

Ancillary