Structural properties of mobile armors formed at different flow strengths in gravel‐bed rivers

Abstract

Differences in the structure of mobile armors formed at three different flow strengths have been investigated in a laboratory flume. The temporal evolution of the bed surfaces and the properties of the final beds were compared using metrics of surface grain size, microtopography, and bed organization at both grain and mesoscales. Measurements of the bed condition were obtained on nine occasions during each experiment to describe the temporal evolution of the beds. Structured mobile armors formed quickly in each experiment. At the grain scale (1-45 mm; 9 ≤ D s50 ≤ 17 mm where D s50 is the median surface particle size), surface complexity decreased and bed roughness increased in response to surface coarsening and the development of the mobile armor. Particles comprising the armor also became flow aligned and developed imbrication. At a larger scale (100-200 mm), the surface developed a mesoscale topography through the development of bed patches with lower and higher elevations. Metrics of mobile armor structure showed remarkable consistency over prolonged periods of near-constant transport, demonstrating for the first time that actively transporting surfaces maintain an equilibrium bed structure. Bed structuring was least developed in the experiments conducted at the lowest flow strength. However, little difference was observed in the structural metrics of the mobile armors generated at higher flows. Although the range of transport rates studied was limited, the results suggest that the structure of mobile armors is insensitive to the formative transport rate except when rates are low (τ * ≈ 0.03 where τ * is the dimensionless shear stress).

Numerical Study of Alternate Bars in Alluvial Channels With Nonuniform Sediment

“…It is of key interest to accurately estimate the sediment transport in natural rivers, since bar evolution (i.e., armor formation and breakup ) depends on fractional transport rates estimation (Orrú et al, ; Parker, ; Powell et al, ). Therefore, the authors proposed to use the model of Wilcock and Crowe () (WC‐2003), which is interesting in the way that (i) it is based on surface investigations and is particularly adapted for the prediction of transient conditions of bed armoring and scenarios of bed aggradation/degradation, (ii) it considers the full size distribution of the bed surface (from finest sands to coarsest gravels), (iii) it was calibrated under a wide range of water discharges and sediment mixtures, (iv) the hiding function has been designed to resolve discrepancies observed from previous experiments (Parker, ; Proffitt & Sutherland, ) including the hiding‐exposure effect of sand content on gravel transport for weak to high values of sand contents in the bulk, and (v) it has already shown efficiency when applied for morphodynamics modeling (An et al, ).…”

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

“…; Powell et al. ; Bertin and Friedrich, ). DEMs representing the grain, bed structure and large bedforms scale were added.…”

“…Indeed, the roughness of the river bed results from both the grain roughness and the larger bed surface variations. The bed roughness adjusts and evolves constantly, influencing near-bed hydraulics and sediment transport (Hardy et al, 2000;Powell et al, 2016;Berni et al, 2018). Complex patterns can develop at different scales, such as specific grain arrangements, structures and large bedforms (Brayshaw et al, 1983;Curran, 2007;Cooper and Tait, 2009;Qin and Leung Ng, 2012).…”

“…Techniques to measure bed surface topography precisely have been developed over the past few years and provide opportunities for a more detailed description of river bed topography (Bertin and Friedrich, 2014;Perret et al, 2016). These methods were found to be helpful for characterizing geometrical bed roughness at different scales based on surface elevations (Butler et al, 2001;Aberle and Nikora, 2006;Hodge et al, 2009;Powell et al, 2016;Groom et al, 2018b).…”

Co‐evolution of coarse grain structuring and bed roughness in response to episodic sediment supply in an experimental aggrading channel

“…In gravel-bed rivers, large grains tend to be relatively over-exposed while the smaller grains are sheltered on the bed (e.g. Kirchner et al, 1990;Hodge et al, 2013;Powell et al, 2016). 'Hiding functions' model how surface GSD feedbacks enhance the mobility of coarser grains relative to finer (e.g.…”