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| Titel |
Bed load transport above a bimodal sediment bed |
| VerfasserIn |
Eric Lajeunesse, Morgane Houssais |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250074399
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| Zusammenfassung |
| Despite several decades of investigations, accounting for the effect of the wide range of grain
sizes composing the bed of rivers on bedload transport remains a challenging problem. We
investigate this problem by studying experimentally the influence of grain size distribution on
bedload transport in the simple configuration of a bimodal sediment bed composed
of a mixture of 2 populations of quartz grains of sizes D1 = 0.7 ± 0.1mm and
D2 = 2.2 ± 0.4mm, respectively. The experiments are carried out in a tilted rectangular
flume inside which the sediment bed is sheared by a steady and spatially uniform turbulent
flow. Using a high-speed video imaging system, we focus on the measurement of the
average particle velocity and the surface density of moving particles, defined as the
number of moving particles per unit surface of the bed. These two quantities are
measured separately for each population of grains as a function of the dimensionless
shear stress (or Shields number) and the fraction of the bed surface covered with
small grains. We show that the average velocity and the surface density of moving
particles obey the same equations as those reported by Lajeunesse [2010] for a bed
of homogeneous grain size. Once in motion, the grains follow therefore similar
laws whether the bed is made of uniform sediment or of a bimodal mixture. This
suggests that the erosion-deposition model established by Lajeunesse [2010] for a bed
of uniform sediment can be generalized to the case of a bimodal one. The only
difference evidenced by our experiments concerns the critical Shields number for
incipient sediment motion. Above a uniform sediment bed, the latter depends on
the particle Reynolds number through the Shields curve [Shields, 1936]. In the
case of a bimodal bed, our experiments show that the critical Shields numbers of
both populations of grains decrease linearly with the fraction of the bed surface
covered with small grains. We propose a simple model to account for this observation. |
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