What causes sediment pulses in mountain rivers?

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).