Transport of moderately sorted gravel at low bed shear stresses: The role of fine sediment infiltration

Abstract

A reliable estimation of sediment transport in gravel‐bed streams is important for various practical engineering and biological studies (e.g., channel stability design, bed degradation/aggradation, restoration of spawning habitat). In the present work, we report original laboratory experiments investigating the transport of gravel particles at low bed shear stresses. The laboratory tests were conducted under unsteady flow conditions inducing low bed shear stresses, with detailed monitoring of the bed topography using a laser scanner. Effects of bed surface arrangements were documented by testing loose and packed bed configurations. Effects of fine sediments were examined by testing beds with sand, artificial fine sand or cohesive silt infiltrated in the gravel matrix. Analysis of the experimental data revealed that the transport of gravel particles depends upon the bed arrangement, the bed material properties (e.g., size and shape, consolidation index, permeability) and the concentration of fine sediments within the surface layer of moving grains. This concentration is directly related to the distribution of fine particles within the gravel matrix (i.e., bottom‐up infiltration or bridging) and their transport mode (i.e., bedload or suspended load). Compared to loose beds, the mobility of gravel is reduced for packed beds and for beds clogged from the bottom up with cohesive fine sediments; in both cases, the bed shear stress for gravel entrainment increases by about 12%. On the other hand, the mobility of gravel increases significantly (bed shear stress for particle motion decreasing up to 40%) for beds clogged at the surface by non‐cohesive sand particles. Copyright © 2017 John Wiley & Sons, Ltd.

Morphodynamics of alternate bars in the presence of riparian vegetation

“…In order to confirm the trends demonstrated in this study, a first step would be to test numerically other configurations, varying slope, confinement width, sediment size and vegetation properties. Moreover, considering transport mechanisms associated with fine sediments would provide a more realistic representation of the accretion of vegetated bars (Allain, 2002;Kleinhans et al, 2018) and may provide insights concerning processes and feedbacks associated with fine sediment deposits, grain size sorting and transport processes in the presence of vegetation (Cordier et al, 2019;Perret et al, 2018;Wintenberger et al, 2015). Confronting modeled behaviors with physical model experiments and with additional field cases is necessary to generalize the results of this study.…”

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

“…Three areas were identified (Figure ): one regrouping data from L beds, one gathering data from arranged beds (H, P and HP beds) and one formed with data from the WW bed only (Perret et al. ). Concerning the quasi‐flat beds, is always lower for loose bed configurations than for arranged bed configurations.…”

“…Tests were performed in the tilting laboratory flume of the hydraulic and hydromorphology laboratory (HHLab) of INRAE Lyon-Villeurbanne (18 m 1 m 0.85 m). Figure 1 presents the experimental procedure (bed formations and sediment transport experiments) and details the diverse measurements carried out during one experiment (see Perret et al, 2018, for more details). During the experiments, channel longitudinal slope was set to 1%.…”

“…A total of eight experi-ments were performed using the same procedure (Figure 1a), which can be divided into 18 tests conducted over different bed configurations where the flow, bed topography and bedload rate were characterized. For further details on the experimental procedure and conditions, refer to Perret et al (2018) and to the supporting information, Section 3, where raw data are provided for each test. Bed configurations and experimental conditions were chosen to represent alpine rivers and their natural gravel-bed arrangements: (i) the sediments were moderately sorted; (ii) the channel slope was inclined (1%); (iii) the relative roughness, namely the ratio between the median grain size and the water depth, varied between 0.08 and 0.14; and (iv) the dimensionless bed shear stresses (or Shields parameter) ranged between 0.05 and 0.08.…”

Effects of Sand Addition and Bed Flushing on Gravel Bed Surface Microtopography and Roughness

“…In this context, previous research interested in sand transport over a coarse bed explored the relations between sand supply limitation (e.g., due to the presence of an armor layer or limited supply from upstream) and the sand bedforms that can develop (Grams & Wilcock, 2014;Kleinhans et al, 2002;McLean, 1981;Tuijnder et al, 2009). Concerning the transport of the coarse bed fractions, the nature of the fine sediment is an important consideration as different sediments have been shown to affect the bed stability differently (Diplas & Parker, 1992;Perret et al, 2018). On one hand, silt and clay size material inhibits sediment transport by adding cohesion to the substrate (Barzilai et al, 2013).…”