Rapidly migrating secondary bedforms persist in the lee of slowly migrating primary dunes in a sand-bed river

Abstract:

Trains of secondary bedforms, superimposed on primary dunes, have been observed in rivers worldwide. To date, it has remained unclear how these secondary bedforms affect the sediment transport dynamics in a fluvial system. In this study, a field campaign was conducted to acquire a dataset with a high spatial and temporal resolution, enabling to track secondary bedform migration. Presented results show that two distinct scales of bedforms are actively migrating. In parts of the studied area, superimposed bedforms fully dissipate above the lee slope of the underlying primary dune, suggesting that the secondary bedforms contribute to the migration of the primary dune. In other parts, however, superimposed bedform persist on the lee slope and trough of the primary dune. The steepness of the primary lee slope seems to be a controlling factor in this. Sediment transport rates associated with primary and secondary bedform migration have been calculated based on dune tracking. The sediment transport associated with secondary bedform migration is of the same order of magnitude and even exceeds the transport associated with primary dune migration. This implies that secondary bedforms have a crucial role in the transport of bedload sediment, and thus they should also be considered in the quantification of sediment transport based on dune tracking.