Multiscale bedform interactions in a lowland river

Multiscale bedforms exist in diverse environments. Globally, trains of small secondary bedforms have been observed in fluvial systems, where they are superimposed on larger fluvial dunes. Yet, we understand little about the morphodynamics of these superimposed bedforms and their interaction with larger bedform scales. It is unclear what their impact is on the overall system functioning, for example in terms of sediment transport and (near-bed) flow dynamics.

Bed elevation data with a high spatiotemporal resolution, obtained during a dedicated field campaign in the river Waal, a main distributary of the river Rhine, have shed light on the morphodynamics of fluvial dunes and superimposed bedforms. Results from the study indicate that superimposed bedforms persist over low-angled lee sides, whereas they disintegrate over lee side angles steeper than . The transport of bed sediment associated with secondary bedform migration is significant. The small bedforms migrate with a celerity that is an order of magnitude larger, from which a transport rate can be inferred that equals and in some sections even exceeds the transport associated with primary river dunes. Where superimposed bedforms disintegrate at or downstream of the dune lee slope, superimposed bedforms fully contribute to the migration of the primary dune. Where they persist over the dune lee side however, the sediment transport inferred from superimposed bedforms over the dune crest might partly contribute to primary dune migration. A significant portion, however, will also be transported over the dune lee side and trough and form an additional transport component. Both the persistence of the superimposed bedforms on the primary dune lee and their size and shape, appear to depend on the primary dune morphology. This is likely related to the flow structure—i.e. the presence of flow separation and the properties of the downstream, turbulent wake—that depends on the primary lee slope angle and height.

In our current work, we build upon this study, and  analyse the morphodynamics of these two bedform scales across a much larger spatial and temporal scale. Small-scale superimposed bedforms appear to be ubiquitous in the river Waal and can be observed across a range of discharge conditions. Our analysis quantifies and predicts when secondary bedform occur and persist over primary lee sides. We further aim to understand how secondary bedform morphology depends on primary dune characteristics as well as environmental conditions such as (changes in) discharge, and the bed sediment properties. In relation to that, we question to what extent superimposed bedforms in turn affect the primary dune morphology, their migration celerity and associated bedload transport.