Abstract
AbstractDunes are the most prevalent bedform present in sand‐bedded rivers and their morphology typically comprises multiple scales of three‐dimensional topography. However, our understanding of flow over dunes is predicated largely on two‐dimensional models, a condition which is rare in nature. Here, we present results of Large Eddy Simulations over a static, three‐dimensional dune field, using a two‐ and three‐ dimensional topographic realisation, to investigate the interaction between bed topography and turbulent flow structures. We show that flow over two‐dimensional bedforms increases the velocity over the stoss slope and reduces the size of the leeside separation zone as compared to 3D topography. Flow over three‐dimensional bedforms generates twice as many vortices as over two‐dimensional bedforms, and these vortices are longer, wider and taller than flow over their two‐dimensional counterparts. Turbulence is dominated by hairpin‐shaped vortices and Kelvin‐Helmholtz instabilities that interact with the bed in the brink point region of the dune crest and down the lee slope, and generate high shear stresses for long durations. These results are used to propose a new conceptual model showing the differences between flow over two‐ and three‐dimensional bedforms. The findings highlight how the size, morphology and stacking of coherent flow structures into larger flow superstructures may be critical in sediment entrainment, and may dictate the relationship between event duration and magnitude that drives sediment impulses at the bed. This will ultimately lead to an increased in the three‐dimensionality of bedform morphology.
Highlights
As water flows over a mobile sediment bed, sediment transport occurs when the bed shear stress exceeds a critical threshold of entrainment and generates bed topography
Once dunes were formed and in equilibrium with the flow, the flow was stopped without change to the bed, the flume was drained carefully to avoid any disturbance to the bed, and the three-dimensional topography mapped with a Leica Scan Station 2 terrestrial LiDAR
The difference map between flows shows that the largest differences are in the near-bed region (Figure 2c, label i) where flow over the 2D Digital Elevation Model (DEM) is greater in magnitude (>125%) than over the 3D DEM
Summary
As water flows over a mobile sediment bed, sediment transport occurs when the bed shear stress exceeds a critical threshold of entrainment and generates bed topography. Dunes are perhaps the most common bedform generated in sand-bedded rivers, they are found in sediment sizes from silt to gravel (Best, 1996, 2005a; Bradley et al, 2013). The presence of alluvial dunes significantly influences the mean and turbulent flow, which act as a first order control on flow resistance and sediment dynamics (Best, 2005a; Naqshband et al, 2014; Unsworth et al, 2020; van der Mark, 2008). A prime control upon the fluid and sediment dynamics of bedforms is their morphological geometry (Bradley et al, 2013). In sand-bedded rivers, dependent on particle size and flow velocity, a continuum in morphology HARDY ET AL.
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