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

Abstract

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 × 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.

The morphology of fluvial‐tidal dunes: Lower Columbia River, Oregon/Washington, USA

“…However, during the following low‐river stage, tidal‐energy (especially flood‐tidal energy and saltwater intrusion) penetrates further upstream causing the ‘fluvial‐tidal’ dunes between ~ rkm 12–21 to reverse their orientation by 180° (downstream to upstream) and to extend their development upstream to ~ rkm 32 via (Figure 16A,B): (i) cannibalization of previously existing high‐river stage ‘fluvial’ dunes between ~ rkm 27–32, and/or (ii) fining of channel bed alluvium below the possible grain‐size limit necessary to form the coarser ‘fluvial’ dunes. These morphological fluctuations will inevitably generate variations in the magnitude of form drag within the ‘dynamic morphology reach’, which should thus be parameterized for incorporation into hydraulic and morphodynamic simulations (Unsworth et al, 2020). Moreover, relative to those dunes positioned farther downstream or upstream, the style, thickness, and orientation of the deposits of primary dunes within the ‘dynamic morphology reach’ likely show the greatest variance, switching between upstream and downstream oriented, simple to compound, cross‐bedding whose cross‐sets will vary from thinner to thicker.…”

“…Gómez et al, 2010; Knox & Latrubesse, 2016; Nittrouer et al, 2008; Schippa et al, 2016), is vital towards building more robust hydraulic and morphodynamic models (cf. Sandbach et al, 2018; Unsworth et al, 2020; van de Lageweg et al, 2018; van de Lageweg & Feldman, 2018).…”

Morphological Model for Erosion Prediction of India’s Largest Braided River Using MIKE 21C Model

“…The Brahmaputra River features a highly braided channel pattern with dynamic river bars (chars) that change in size and shape seasonally and annually (Bristow, 1987;Best et al, 2007). These bedforms, including large-scale features like bars and islands, and smallscale features such as ripples and dunes, significantly affect flow resistance and bed shear stress (Unsworth et al, 2020;Flemming, 2022;Jan et al, 2022). These bedforms indirectly influence flood and erosion patterns along the river by impacting water levels and flow velocity (Cilli et al, 2021).…”

Plastic pollution in riverbeds fundamentally affects natural sand transport processes

“…1a ). The downstream migration of these riverbed dunes can influence the shape of the wider river channel 28 , and they are important in controlling and maintaining sediment flux that is crucial for a suite of ecosystem services 29 , 30 .…”