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

Abstract

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.

Linking the spatial distribution of bed load transport to morphological change during high‐flow events in a shallow braided river

“…Morphological activity on the Waimakariri River thus continues during low flows although at a lower rate and across a much narrower area. In contrast, in a more confined setting, Claude et al [] note that during periods of low flow, the hydrodynamic processes that drive morphological evolution at high flows continue, albeit at a lower rate, because bar configuration and flow structure do not alter much with discharge. Across the shallow Rees River, the extent of braidplain that experiences morphological change is drastically reduced during low flows.…”

Imaging scours in straightened and braided gravel‐bed rivers with ground‐penetrating radar

“…Furthermore, there is little knowledge on the sedimentary processes related to the formation and migration of scours at high discharge (Claude et al . ) and on the scour footprints left in the sedimentary records (Ashmore and Gardener ). The relationship between bars, scour formation and scour fills is still almost unexplored (Huber and Huggenberger ).…”

Fluvial islands: First stage of development from nonmigrating (forced) bars and woody-vegetation interactions

“…Nonmigrating bars (e.g., point bars, mid-channel bars) correspond to sediment accumulation developing due to a discontinuity in the channel geometry (Olesen, 1984;Struiksma et al, 1985;Parker and Johanneson, 1989;Crosato et al, 2011), while free bars arise because of morphodynamic instability (Tubino, 1991;Tubino et al, 1999). Since the latter are rather unstable and rapidly migrating units (Claude et al, 2014), the development of vegetation and its resistance to physical stresses on these macroforms is rare. Contrarily, because of their conditions of formation, nonmigrating bars are relatively stable units on which woody vegetation can recruit and survive even in morphologically very active rivers (Nakayama et al, 2002;Charron et al, 2011;Crouzy et al, 2013).…”