Quantifying the forcing effect of channel width variations on free bars: Morphodynamic modeling based on characteristic dissipative Galerkin scheme

Abstract

[1] The forcing effect of channel width variations on free bars is investigated in this study using a two-dimensional depth-averaged morphodynamic model. The novel feature of the model is the incorporation of a characteristic dissipative Galerkin (CDG) upwinding scheme in the bed evolution module. A correction for the secondary flows induced by streamline curvature is also included, allowing for simulations of bar growth and migration in channels with width variations beyond the small-amplitude regimes. The model is tested against a variety of experimental data ranging from purely forced and free bars to coexisting bed forms in the variable-width channel. The CDG scheme effectively dissipates local bed oscillations, thus sustains numerical stabilities. The results show that the global effect of width variations on bar height is invariably suppressive. Such effect increases with the dimensionless amplitude A C and wave number l C of width variations. For small A C , l C has little effects on bar height; for A C beyond small amplitudes, however, the suppressing effect depends on both A C and l C . The suppressing effect on bar length increases also with both A C and l C , but is much weaker than that on bar height. The global effect of width variations on bar celerity can be suppressive or enhancive, depending on the combination of A C and l C . For smaller l C , the effect on bar celerity is enhancive; for larger l C , bar celerity tends to increase at small A C but decreases for A C beyond small amplitudes. We present herein an unprecedented data set verifying the theoretical prediction on celerity enhancement. Full suppression of bar growth above the theoretically predicted threshold A C was not observed, regardless of the adopted amplitude of initial bed perturbation A. The global effects of width variations on free bars can be quantified using a forcing factor F C that integrates the effects of A C and l C . The suppressing effects on bar height and length are both proportional to F C 2.16; the global effect on bar celerity is, however, a parabolic function of F C .

Physics-based modeling of large braided sand-bed rivers: Bar pattern formation, dynamics, and sensitivity

“…Theoretically, a temporary perturbation at the upstream boundary migrates downstream and out of the model domain. If no new perturbations occur, the model should become static [ Lanzoni and Seminara , ], and this was indeed observed in pilot runs and in other models [e.g., Van der Wegen and Roelvink , ; Wu et al , ]. Within the model, something similar happened: The reduction of sediment pulses due to reduction of upstream channel and bar erosion led to a reduction in local dynamics.…”

Interactions between flow structure and morphodynamic of bars in a channel expansion/contraction, Loire River, France

“…As see in Figure b and observed by Garcia and Nino [], Whiting and Dietrich [], and Wu et al . [], with the decrease of the aspect ratio in the downstream contraction zone (Figure b), the migration speed of the bars slows down until the bed forms cease their longitudinal migration (along the left bank in the study site). Once the migrating bars have reached the contraction zone, they seem to be forced in this channel expansion/contraction.…”

“…The effect of curvature usually leads to the formation of point bars [ Ikeda and Parker , ] which in turn, have an effect on the flow structure and sediment transport [ Bridge and Jarvis , ; Dietrich and Smith , ]. Forced bars that are associated with the convergence and divergence of flow in a channel expansion/contraction appear as midchannel bars or as two symmetrical lateral bars [ Bittner , ; Repetto et al ., ; Wu and Yeh , ; Luchi et al ., ; Wu et al ., ]. Interactions between hydro‐sedimentary processes and bed morphology around midchannel bars have been the subject of numerous field investigations [ Leopold and Wolman , ; Whiting and Dietrich , ; Ashworth et al ., ; Bridge and Gabel , ; Ferguson et al ., ; Richardson et al ., ; Richardson and Thorne , ; McLelland et al ., ] and experimental studies [ Ashmore , ; Ashworth , ; Federici and Paola , ].…”

“…Recently, Wu et al . [] highlighted that the migration of free bars (in alternate configuration) on forced bars (transverse bars or lateral bars) in a channel of variable width could give rise to mixed bars (free + forced). These mixed bars are arranged in alternate or transverse configurations according to the migration of the free bars.…”

Topographic forcing of tidal sandbar patterns for irregular estuary planforms

“…, ; Repetto and Tubino, ; Wu et al. , ) and (ii) the reach scale, an order of magnitude larger than individual bars, due to variations in channel width and discharge (Tubino et al. , ; Seminara, ; Kleinhans and van den Berg, ; Schuurman et al.…”