A field study was carried out to investigate the development of alternate bars in a secondary channel of the Loire River (France) as a function of discharge variations. We combined frequent bathymetric surveys, scour chains and stratigraphical analysis of deposits with measurements and modelling of flow dynamics. The channel exhibited migrating bars, non-migrating bars and superimposed dunes. Possible mechanisms of bar initiation were found to be chutes associated with changes of bank direction and instability resulting from interactions between existing bars during the fall in water level after floods. We propose that the reworking of bar sediments during low flows (high width-to-depth ratio β), reinforced by high values of the Shields mobility parameter, can explain the formation or re-generation of new alternate migrating bars during a subsequent flood. The migration pattern of the bars was found to be cyclic and to depend mainly on (i) channel layout and (ii) the dynamics of superimposed dunes with heights and lengths depending on location and discharge value. For instance, the hysteresis affecting the steepness of dunes influences the flow resistance of the dunes as well as the celerity of migrating bars during flood events. We compare the findings from the field with results from theoretical studies on alternate bars. This gives insight in the phenomena occurring in the complex setting of real rivers, but it also sheds light on the extent to which bar theories based on idealized cases can predict those phenomena.
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.
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