How does the bed surface impact low‐magnitude bedload transport rates over gravel‐bed rivers?

Abstract

Bedload transport is a complex phenomenon that is not well understood, especially for poorly sorted sediment and low transport rates, which is what is typically found in alpine gravel‐bed rivers. In this paper, the interaction between bedload rate, bed stability and flow is investigated using flume experiments. Significant differences in bedload rates were observed for experiments conducted on beds formed with the same gravel material but presenting diverse arrangements and bedforms. Tests were performed under regimes of low transport rate, which are mainly controlled by gravel‐bed roughness. Different scales of roughness were identified using the statistical characteristics of detailed bed elevation measurements: grain, structure and large bedform scales. The role played by these different roughness scales in bedload dynamics was examined. For quasi‐flat beds, bed stability was quantified using a combination of bed surface criteria describing grain and structure scales. It was found that bed stability affects the bedload rate directly and not only through its influence on the flow or on the incipient motion. For beds with large bedforms, the analysis of bedload dynamics also showed the importance of accounting for effective bed shear stress distributions. An empirical bedload model for low transport regimes was suggested. Compared with previous formulae developed for alpine rivers, this model accounts for bed stability and distribution of effective bed shear stress. It significantly improves the understanding of gravel dynamics over complex beds such as arranged beds or those with large bedforms. However, further tests are needed to use the model outside the range of conditions of this study. © 2019 John Wiley & Sons, Ltd.

Experimental Insights Into the Threshold of Motion in Alluvial Channels: Sediment Supply and Streambed State

“…Haynes, 2013), Perret et al (2020) quantified how the statistics of bed surface roughness, over a range of spatial scales from individual grains to bedforms, could improve predictions of bedload transport rate at low Shields stresses. They proposed an empirical relation between spatial statistics and the scaling exponent on shear stress in a bedload transport relation similar to Recking (2012).…”

“…They found a much weaker dependence of reference stress (their model's threshold parameter) on bed roughness statistics. Nonetheless, the analysis of Perret et al (2020) suggests that roughness statistics that account for multiple lateral scales of grain interactions may be able to account for grain interlocking and other stabilizing effects. We note that their experiments used a very narrow gravel GSD, such that armoring could not form in their experiments, which may have also inhibited other stabilizing effects related to spatial sorting of grains.…”

“…We note that their experiments used a very narrow gravel GSD, such that armoring could not form in their experiments, which may have also inhibited other stabilizing effects related to spatial sorting of grains. In any case, our study explores a complementary parameter space in that we used a constant discharge with sediment supply pulses, while Perret et al (2020) used a variable discharge hydrograph with no upstream sediment supply.…”

“…Coarse grains on the surface are a key control on sediment mobility, hence on channel morphology and stability (Church & Ferguson, 2015;Harrison, 1950;Mackenzie & Eaton, 2017;Parker, 1978). A relatively small number of studies have looked at interactions among coarsening, surface topography, and sedimentary structures, and there is evidence that the temporal evolution and relative importance of these factors in controlling bed stability vary with initial conditions and the history of flow and transport conditions (e.g., Johnson et al, 2015;Johnson, 2017, Masteller & Finnegan, 2017Perret et al, 2018Perret et al, , 2020.…”

“…Others have modified the reference shear stress threshold parameter in the Wilcock and Crowe (2003) transport model to evolve through time as a function of sediment supply, erosion, and deposition (e.g., Johnson, 2016). Recently, Perret et al (2018Perret et al ( , 2020 conducted novel experiments exploring bed stability as a function of surface roughness and grain packing, and found a 5%-19% reduction in critical threshold stress (a reference stress) could be attributed to increased grain packing from different initial conditions. Interestingly, they found a relatively weak dependence of their reference stress on bed surface roughness statistics, although surface statistics did more strongly correlate with transport statistics through a shear stress exponent.…”