Numerical Study of Alternate Bars in Alluvial Channels With Nonuniform Sediment

Abstract

Rivers typically present heterogeneous bed material, but the effects of sediment nonuniformity on river bar characteristics are still unclear. This work investigates the impact of sediment size heterogeneity on alternate bars with a morphodynamic numerical model. The model is first used to reproduce a laboratory experiment showing alternate bar formation with nonuniform bed material. Subsequently, the influence of sediment size heterogeneity on alternate bars is investigated distinguishing hybrid from free bars, definition based on the presence/absence of morphodynamic forcing, considering the results of nine scenarios. In four of them, a transverse obstacle is used to generate forcing. The computations are carried out with the Telemac‐Mascaret system solving the two‐dimensional shallow‐water equations with a finite element approach, accounting for horizontal and vertical sediment sorting processes. The results show that sediment heterogeneity affects free migrating and hybrid bars in a different way. The difference lies in the presence/absence of a migration front, so that distinct relations between bed topography, bed shear stress, and sediment sorting are obtained. Sediment sorting and associated planform redistribution of bed roughness only slightly modify free migrating bar morphodynamics, whereas hybrid bars are greatly impacted, with decreased amplitude and increased wavelength. Increased sediment size heterogeneity increases the degree of sediment sorting, while the sorting pattern remains the same for both free and hybrid bars. Moreover, it produces averagely higher, longer, and faster free bars, while in the case of hybrid bars their wavelength is increased but no general trend can be determined for their amplitude.

Morphodynamics of alternate bars in the presence of riparian vegetation

“…In addition, simulations with vegetation exhibit a similar hydraulic distribution to that obtained by Cordier et al. () in a channel with non‐vegetated hybrid bars, with velocities and shear stresses maximal in pools and minimal on bar tops. Yet the wavelength of the modeled vegetated bars are in the range predicted for free bars by linear bar theory, and not in the range predicted for steady hybrid bars (Figure and Table ).…”

“…This morphodynamic model has been able to reproduce bar dynamics accurately in comparison to the laboratory experiments of Lanzoni () and Cordier et al. ().…”

“…) 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. ; Perret et al. ; Wintenberger et al.…”

“…Computational time step is set to 0.5 s in order to keep a Courant number around 0.2 (Cordier et al. ). Bed roughness is parametrized through a Strickler coefficient of 40 m s , a value typically obtained for the calibration of 2D hydraulic models on the Isère River (Jourdain, ).…”

“…; Zolezzi and Seminara, ), whereas hybrid bars develop in the presence of a hydraulic forcing that fixes their location (Cordier et al. ). Like free bars, hybrid bars result from a morphodynamic instability of the bed but, in contrast to free bars, they are steady, and they are characterized by a wavelength two to three times greater than that of free bars for similar flow conditions (Crosato et al.…”