Bedload transport variability for alluvial dunes

Abstract

Rivers convey water and sediment, often resulting in dunes covering riverbeds. Yet our understanding on how individual sediment particles contribute to the formation of various dunes remains rudimentary. This major knowledge gap is mainly due to a complete lack of direct particle measurements over dunes. Here, novel particle-scale bedload measurements are presented, obtained with high-frequency imaging of particle velocity and activity over various bedform migration stages. Bedload transport quickly responds to the unsteady non-uniform flow over the bed. Particle velocities linearly increase over the dune slopes towards the dune crest. Particle activity increases faster (power-law) over the dune slopes with particle activity in the top layer eventually saturating for saltation-dominated motion. This observation is important, as it quantitively assesses the complexity of sediment transport under unsteady states and has been eluding researchers for a long time. The spatial and temporal variability of particle motion over dune slopes is high and trends are related to the mean flow strength and varying dune topography. The intrinsic variability at the particle scale may be simplified using double averaging, to obtain general trends over the dune slopes. To bridge the gap between particle motion and bedform migration, a new dataset of migrating laboratory dunes, obtained without draining the flume, is presented. Three-dimensional dune sediment flux is quantified, whilst characterising dune translation and interaction. Patterns in dune deformation flux are related to the dune interaction processes. Splitting, defect creation and defect repulsion cause a peak in the deformation flux, whilst merging onsets an increase in the flux in time. Dune fluxes vary and dune height, dune length and other shape factors describing a dune are changing as well. Changes in the water surface slope and the dune morphology of a train of dunes are mimicked in the flux variations, albeit with a time lag. Highest overall sediment flux in the flume was achieved by a larger number of shorter dunes, whilst excessively long and high dunes are unstable, cause lower overall transport. The process of break up into shorter dunes facilitated by dune interaction processes is being described. Subaqueous dunes are often used as an example to self-organisation of sediment. The interactions between the bed morphology and the flow are slower than the particle-flow interactions, therefore the bed morphology is the controlling component and determines the characteristics and behaviour of the dunes.