Migrating bars influence the formation and dynamics of their peers in large sandy-gravel bed rivers

The present study investigates the morphodynamic interactions between migrating bars in a sandy gravel and nearly straight channel of the Loire River (France). From a large dataset collected from field campaigns performed between 2016 and 2020, we analysed the deflection power exerted by bars on flow and sediment transport that influences the dynamics of other bars present in the channel. We also investigated the role of low flows in bar formation. To this aim, the riverbed evolution has been documented using four years of bathymetrical and aDcp surveys. Flow deflection induced by the presence of bars in the channel differs for pre-existing (first-order) bars and bedload sediment accumulations identified as new developing bars (second-order bars). The latter needs to reach an equilibrium size to influence flow direction. During low flows, first-order bars emerge and create contraction/expansion zones in the channel, inducing a temporary local geometrical forcing that leads to sediment deposition at the outlet of these zones. Sediment deposited in these specific areas can constitute a nucleus for second-order bar formation or contribute to first-order bar aggradation. According to the linear free bar theory, the critical discharge that contributes to expressing an alternate bar pattern is exceeded during moderate flood, leading to the observation of a transitional bar pattern between alternate and central bar pattern.