Morphology, Bedload, and Sorting Process Variability in Response to Lateral Confinement: Results From Physical Models of Gravel‐bed Rivers

Abstract

This paper uses flume experiments to investigate the influence of lateral width confinement on channel morphology, sediment sorting, and bedload transport. Three runs of about 60 hr were carried out under constant feeding rate equal to 0.6 l/s and 8 g/s, with a bimodal mixture of natural sediments, a fixed flume slope of 3%, and width imposed by lateral walls from 0.12 to 0.50 m in order to model three different flow confinement configurations. Despite the three runs transporting at the same rate on average, they presented different gravel-bed river morphologies and different degrees of bed complexity. The three runs also presented differences in bedload transport rate fluctuations associated with different magnitude and mechanisms of bed storage and release of sediments. The intermediate width configuration (Run 2) was found to have a minimum storage and release of sediments compared with both the wider configuration (Run 1) and the narrower configuration (Run 3). Runs 1 and 3 were characterized by a significant storage and release of sediments resulting in a highly fluctuating bedload transport rate; however, while Run 1 (braiding morphology) stored and released sediments through lateral deposits and bed variations with planimetric sorting, Run 3 (single-thread channel) stored and released sediments through variations in bed elevation accompanied by vertical sorting. We extrapolated this concept to the full set of gravel-bed river morphologies, thus speculating about the channel morphology for optimal sediment transfer.

Process feedbacks that control transport capacity at formative flows in laterally-constrained gravel-bed rivers: a laboratory study

“…With increasing mean shear stress, there was an increase in bar wavelength (and decrease in amplitude) such that channels with the highest unit discharge had a more subdued morphology. The development of alternate bars in wider channels with lower excess shear stresses is supported by several investigations (Fujita, 1989;Garcia Lugo et al, 2015;Carbonari et al, 2020;Rhoads and Welford, 1991). Notably, our experiments conform to the threshold w/d ≈ 10 ( Chang et al, 1971;Ikeda, 1984).…”

Three-Dimensional Hole Size (3DHS) Approach for Water Flow Turbulence Analysis over Emerging Sand Bars: Flume-Scale Experiments

“…In recent hydro-geomorphological numerical and experimental research, the study of the coherent structures corresponding to emerging bars was analyzed for correlating sediment transport to instantaneous bursting events [18][19][20][21][22][23]. In hydrodynamics, the study of the 2D turbulent flow structures in the vicinity of the wall is generally performed by applying the well-known quadrant events analysis [24,25], whilst it is not sufficiently rigorous and satisfactory when analyzing 3D water flows [26][27][28][29]. In these cases, Leary and Schmeeckle [30] and Schobesberger et al [31] have recently applied the so-called analysis of the octagonal bursting events [32][33][34], also known as octant events analysis, for overpassing the classical Reynolds stress approach [35,36].…”

Threshold Conditions for the Shift Between Vegetated and Barebed Rivers

“…We recall that the hypothesis of equilibrium conditions accounts for the constant value of vegetation characteristics over a timescale longer than the total duration T. Yet, it is worth noting that retrieving data for vegetation properties is time-consuming, and it is usually based on a reach-scale approach (e.g., Forzieri et al, 2011;Latella et al, 2020). Similarly, recent investigations recommend to characterize morphodynamic parameters at a reach-scale length (Carbonari et al, 2020;). Nevertheless, an approach based on a shorter length-scale may be adopted as well, provided that the assessment of vegetation parameters is meaningful to understand the entire channel dynamics.…”