The Influence of Three-Dimensional Topography on Turbulent Flow Structures Over Dunes in Unidirectional Flows

Dunes 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. Plain Language Summary Dunes are the most common bedform present in sand-bedded rivers. They form a complex three-dimensional morphology that is composed of several scales of topography, and this morphology influences the mean and turbulent flow which determine both the flow resistance and sediment dynamics. However, our understanding of flow over dunes is limited as it comes from two-dimensional models, a condition which is rare in nature. Here we apply a numerical model to understand how flow evolves in both space and time over three-dimensional dune fields, using a two- and three- dimensional topographic realisation, to investigate the interaction between bed topography and turbulent flow structures. Our results show that flow over three-dimensional bedforms generates twice as many turbulent vortices, 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. These generate high shear stresses for long durations. The results are used to propose a new conceptual diagram showing the differences between flow over two- and three-dimensional bedforms. Key Points Topographic steering of flow due to dune three-dimensionality significantly alters the flow dynamics compared to 2D models of flow The total kinetic energy throughout the boundary layer is between 120% and 150% greater over 3D dunes than over 2D dune bed There is a two-fold increase in the number of vortices detected over 3D dunes than 2D topography, and these vortices possess a more anisotropic shape