Influence of Dunes on Channel-Scale Flow and Sediment Transport in a Sand Bed Braided River

Current understanding of the role that dunes play in controlling bar and channel-scale processes and river morphodynamics is incomplete. We present results from a combined numerical modeling and field monitoring study that isolates the impact of dunes on depth-averaged and near-bed flow structure, with implications for morphodynamic modeling. Numerical simulations were conducted using the three-dimensional computational fluid dynamics code OpenFOAM to quantify the time-averaged flow structure within a 400 m x 100 m channel using digital elevation models (DEMs) for which (i) dunes and bars were present within the model and (ii) only bar-scale topographic features were resolved (dunes were removed). Comparison of these two simulations shows that dunes enhance lateral flows and reduce velocities over bar tops by as much as 30%. Dunes influence the direction of modeled sediment transport at spatial scales larger than individual bedforms due to their effect on topographic steering of the near-bed flow structure. We show that dunes can amplify, dampen, or even reverse the deflection of sediment down lateral bar slopes, and this is closely associated with 3-D and obliquely orientated dunes. Sediment transport patterns calculated using theory implemented in depth-averaged morphodynamic models suggest that gravitational deflection of sediment is still controlled by bar-scale topography, even in the presence of dunes. However, improved parameterizations of flow and sediment transport in depth-averaged morphodynamic models are needed that account for the effects of both dune- and bar-scale morphology on near-bed flow and sediment transport.