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What causes sediment pulses in mountain rivers?

© 2018 EPFL Bob de Graffenried. Blaise Dhont working on the Armfield flume

© 2018 EPFL Bob de Graffenried. Blaise Dhont working on the Armfield flume

One might think that pulses of sediment transport are due to a sudden change in the water discharge or sediment input. Recent experiments carried out by Blaise Dhont and Christophe Ancey show that in the laboratory, even with constant water discharge and a constant supply of gravel upstream of an inclined flume, there are significant variations in the sediment transport rates, with fluctuations as large as 10 times the mean value. The work jas just been published in Geophycal Research Letters (https://doi.org/10.1029/2018GL077792, 2018) and is featured in EOS, the AGU magazine (Eos, 99, https: / /doi.org/10.1029/2018EO102311).

As part of his thesis on sediment transport funded by SNSF (https://infoscience.epfl.ch/record/232681?ln=en), Blaise Dhont was interested in the dynamics of fluctuations of sediment transport rates. An originali feature was to design an experimental setup that made it possible to monitor the morphodynamic evolution of a torrential bed over long durations (up to a month of continuous experience!). He observed that even under constant water-inflow and sediment-supply conditions, the initially uniform bed did not remain so very long. Quickly it formed morphological structures (in our case, an alternation of bars and pools), and the sediment transport rate measured at the flume outlet varied significantly (with fluctuations that can exceed 10 times the average flow).

The phenomenon is well-known, and for a long time it had beenconsidered that the sediment pulses resulted from the downstream migration of the bars. Blaise Dhont's experiments show a wide range of fluctuations, and if indeed the main pulses are due to bar migration, the intermediate-sized pulses are caused by erosion cycles of the pools. In contrast with the idea expressed, bar migration does not occur continously over time, as can be seen in the desert with sand dunes, but in a jerky way (a displacement called stick-slip motion). Thus for an experiment of 600 hours, we never observed a stable and perennial bed, but a succession of equilibrium states, which were brutally broken and gave rise to new equilibria.

With his thesis director Christophe Ancey, Blaise Dhont wrote an article summarizing his reseach in the prestigious Geophysical Research Letters (https://doi.org/10.1029/2018GL077792, 2018 open access). The paper has been featured by the American Geophysical Union magazin (Eos, 99, https://doi.org/10.1029/2018EO102311).

Funding

SFNS, grant 200021_129538

References

Blaise Dhont and Christophe Ancey,  Are Bedload Transport Pulses in Gravel Bed Rivers Created by Bar Migration or Sediment Waves? Geophysical Research Letters 45, 5501–5508, 2018. https://doi.org/10.1029/2018GL077792, Published on 9 June 2018.

Cook, T. (2018), How are sediment pulses generated?, Eos, 99, https://doi.org/10.1029/2018EO102311. Published on 24 July 2018