The influence of three-dimensional topography on turbulent flow structures over dunes in unidirectional flows

R.J. Hardy, J.L. Best, T.I. Marjoribanks, D.R. Parsons, Philip Ashworth

Research output: Contribution to journalArticlepeer-review

Abstract

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. 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 diagram 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.
Original languageEnglish
Article numbere2021JF006121
Pages (from-to)1-24
Number of pages24
JournalJournal of Geophysical Research: Earth Surface
Volume126
Issue number12
DOIs
Publication statusPublished - 9 Nov 2021

Bibliographical note

© 2021. The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Natural Environment Research Council (NERC). Grant Number: NE/I01456X/1

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