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SUMMARY:Downstream dispersion of bedload tracers - Eric Lajeunesse\, IPGP
DTSTART:20180308T113000Z
DTEND:20180308T123000Z
UID:TALK97591@talks.cam.ac.uk
CONTACT:Catherine Pearson
DESCRIPTION:In November 1960\, Sayre and Hubbell (1965) monitored the prop
 agation of a plume of radioactive sand in a Nebraskan stream. Using a scin
 tillator detector\, they observed that the plume gradually spread as it wa
 s entrained downstream. Tracking pebbles in gravel-bed rivers\, or glass b
 eads in a laboratory channel\, reveals a similar behavior: sediments dispe
 rse as they propagate downstream. To investigate this process\, we derive 
 the equations governing the evolution of a plume of tracers from the erosi
 on-deposition model introduced by Charru et al. (2004). Neglecting velocit
 y fluctuations\, we find that the exchange of particles between the bedloa
 d layer and the sediment bed disperses the tracers. Furthermore\, we find 
 a transition between two asymptotic regimes. The tracers\, initially at re
 st\, are progressively set into motion by the flow. During this regime\, t
 he plume continuously accelerates\, spreads non-linearly with time and bec
 omes increasingly skewed in the direction of propagation. With time\, the 
 skewness of the plume eventually reaches a maximum before decreasing. This
  marks the transition to an advection-diffusion regime in which the plume 
 advances at constant velocity\, spreads linearly\, and becomes symmetrical
 . We express analytically the position\, the variance and the skewness of 
 the plume\, and investigate their asymptotic regimes. In the field\, bedlo
 ad transport is intermittent. We show that these asymptotic regimes become
  insensitive to this intermittency when expressed in terms of the position
  of the plume\, instead of time. 
LOCATION:Open Plan Area\, BP Institute\, Madingley Rise CB3 0EZ
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