Sediment Transport on a Sand Bed With Dunes: Deformation and Translation Fluxes

Abstract

As dunes move along the riverbed, they change in size, shape, and arrangement. This involves sediment fluxes on top of the net downstream motion of the dune field, but how much dune dynamics affect total sediment flux remains unclear. In this study, we obtain high-resolution and high-frequency digital elevation models of migrating submerged dunes in the laboratory. We use the measurements to identify three-dimensional dune sediment fluxes and characterize dune interaction processes. We show how variations in dune migration lead to flux variations. Varying bed and water surface slopes and time-varying sizes and shapes of dunes are imprinted in the observed flux variations, with a corresponding time lag. Feedback between bed morphology and water surface slope causes (some) dunes with high crests to lengthen into long dunes (of about twice the mean dune length), until a breakup of this dune into shorter dunes occurs, typically facilitated by merging of superposed bedforms until the primary dune splits. A large number of short dunes have high overall transport rates. Dune interaction causes patterns in the dune deformation flux. Dune splitting, defect creation and defect repulsion cause a peak in the deformation flux, whilst merging triggers a temporal increase of the total flux. Estimating the fluxes with spatially varying dune migration, and contributing fluxes to either deformation or translation processes continues to be a challenge, even for the detailed data set we have presented. Our analysis suggests that sediment transport variability over dune-covered riverbeds is best described by considering the topography of multiple dunes.