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.
The study of the relationship between flow structure and morphodynamic of bars in a channel expansion/contraction is essential to better understand the processes that control the evolution of rivers. Thus, multibeam echosoundings and Acoustic Doppler Profiler (ADP) measurements were performed with a high temporal resolution in an expansion/contraction zone of the Loire River (France) occupied by bars. During the monitoring period, the macroforms presented successively an alternate, a lateral and a transverse configuration. Field data were analyzed to study how the primary and secondary velocities, the flow directions, the bed shear stresses, and the bed roughnesses (associated to dunes) evolve as a function of the water discharge and bars configuration. The bars modify the flow structure imposed by the channel width variations. In fact, the bars induce a topographic forcing which enables the separation and reducing of the mixing of two currents formed in the upstream channel expansion. This forcing is enhanced by the turbulence formed by the large dunes superimposed on the bars. Therefore, the bars promote a nonuniform flow in the channel. In turn, in the channel expansion/contraction, the migrating bars' morphodynamics are affected by the downstream channel narrowing which stops their downstream migration and forced the bars in the system. Then the nonuniformity of the flow encourages the lateral migration of the macroforms until they reach a bank and become nonmigrating. Finally, the nonmigrating bars are eroded by the flow deflected during the migration of a new bar in the channel expansion/contraction.
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