Bed load and suspended load contributions to migrating sand dunes in equilibrium

Abstract

Dunes dominate the bed of sand rivers and are of central importance in predicting flow roughness and water levels. The present study has focused on the details of flow and sediment dynamics along migrating sand dunes in equilibrium. Using a recently developed acoustic system (Acoustic Concentration and Velocity Profiler), new insights are obtained in the behavior of the bed and the suspended load transport along mobile dunes. Our data have illustrated that, due to the presence of a dense sediment layer close to the bed and migrating secondary bedforms over the stoss side of the dune toward the dune crest, the near-bed flow and sediment processes are significantly different from the near-bed flow and sediment dynamics measured over fixed dunes. It was observed that the shape of the total sediment transport distribution along dunes is mainly dominated by the bed load transport, although the bed load and the suspended load transport are of the same order of magnitude. This means that it was especially the bed load transport that is responsible for the continuous erosion and deposition of sediment along the migrating dunes. Whereas the bed load is entirely captured in the dune with zero transport at the flow reattachment point, a significant part of the suspended load is advected to the downstream dune depending on the flow conditions. For the two flow conditions measured, the bypass fraction was about 10% for flow with a Froude number (Fr) of 0.41 and 27% for flow with Froude number of 0.51. This means that respectively 90% (for the Fr = 0.41 flow) and 73% (for the Fr = 0.51 flow) of the total sediment load that arrived at the dune crests contributed to the migration of the dunes.

Morphodynamics of river dunes : suspended sediment transport along mobile dunes and dune development towards upper stage plane bed

“…This "loss of sediment" (the bypass fraction) contributes to the lowering of the dune heights as also observed in the flume experiments [e.g. Naqshband et al, 2014a;2014b]. Furthermore, for dunes in equilibrium under both experimental conditions (EXP1 and EXP2), the BL-model gives same lee side angles of 30° while the BL+SL-model predicts a lower lee side angle for the suspended load dominant condition (23°) compared to the bed load dominant condition (26°) [see also Smith and McLean, 1977;Bridge and Best, 1988;Amsler and Schreider, 1999;Best, 2005a and references therein].…”

“…Nikora and Goring, 2002]. Over the lee side of the dune and in the dune trough, where turbulent intensities are the highest [see Naqshband et al, 2014b], the contribution of ' ' c u to the total sediment flux is even larger and reaches up to 50%. …”

“…Below the experimental set-up and procedure are outlined briefly followed by the experimental conditions. For more details on the experimental work reference is made to Naqshband et al [2014b].…”

“…Recently, the ACVP has been successfully applied under waves Chassagneux and Hurther, 2014] and in flume experiments above migrating sand dunes, where the distribution and contribution of both bed load and suspended load to dune migration were investigated [Naqshband et al, 2014b]. By deploying the ACVP in the present study, we are able to measure -for the first timethe contribution of the turbulent sediment flux to the total sediment flux along the dune profile and over the entire water column.…”

“…This is shown to be appropriate for dune development in the lower flow regime (bed load dominant transport regime) but insufficient for the transition of dunes to upper stage plane bed in the higher flow regime where suspended sediment transport becomes dominant and plays an important role in changing dune shapes [e.g. Kostaschuk et al, 2009, and Furthermore, detailed acoustic measurements of the sediment transport along migrating dunes by Naqshband et al [2014b] showed that, for the flow conditions measured, bed load fully contributes to dune migration by avalanching on the dune lee side and in the dune trough. However, a significant part of the suspended sediment transport was observed to bypass the flow reattachment point of the dune and therefore did not contribute to the migration of the dune front.…”