Kinematics, Topology and Significance of Dune-Related Macroturbulence: Some Observations from the Laboratory and Field
- Michael D. Blum2,
- Susan B. Marriott3 and
- Suzanne F. Leclair4
Published Online: 17 MAR 2009
Copyright © 2005 International Association of Sedimentologists
Fluvial Sedimentology VII
How to Cite
Best, J. (2005) Kinematics, Topology and Significance of Dune-Related Macroturbulence: Some Observations from the Laboratory and Field, in Fluvial Sedimentology VII (eds M. D. Blum, S. B. Marriott and S. F. Leclair), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304350.ch3
Baton Rouge, Louisiana, USA
School of Geography and Environmental Management, University of the West of England, Bristol BS16 1QY, UK
Department of Earth and Environmental Sciences, Tulane University, Dimwiddie Hall, New Orleans, LA 70118, USA
- Published Online: 17 MAR 2009
- Published Print: 15 FEB 2005
Book Series Editors:
- Ian Jarvis
Series Editor Information
School of Earth Sciences and Geography, Centre for Earth and Environmental Science Research, Kingston University, Penrhyn Road, Kingston-upon-Thames KT1 2EE, UK
Print ISBN: 9781405126519
Online ISBN: 9781444304350
- dune-related macroturbulence - kinematics, topology and significance;
- dunes - most common alluvial bedforms;
- laboratory studies of dune macroturbulence;
- field observations of water surface structure;
- topology of dune-related macroturbulence;
- DANTEC particle imaging velocimetry (PIV) system
Macroturbulence, which may advect through the entire water depth, dominates the flow field associated with alluvial sand dunes and has long been regarded as the principal mechanism for suspending bedload sediment over dunes. The origin of this macroturbulence has been linked to shear layer development in the dune lee, often associated with flow separation, and the form of these coherent flow structures has been noted as ‘boils’ that erupt onto the water surface. Although past work has quantified the mean and turbulent flow characteristics of flow over dunes using at-a-point measurements, these studies have not been able to trace the evolution of such macroturbulent events over a dune-covered bed. Additionally, the topology of the dune-related macroturbulence has not been explained in relation to the structure of the developing surface boils. This paper tackles both of these issues using a twofold approach: (i) use of whole flow field quantification using particle imaging velocimetry (PIV) over a series of fixed laboratory dunes; (ii) observations of the water surface over large sand dunes in the Jamuna River, Bangladesh.
Particle imaging velocimetry results are in good agreement with past work detailing the mean flow field over sand dunes. The PIV images over the lee and stoss sides of an experimental dune, and observation of the water surface above natural dunes, reveal four key dynamic attributes to flow.
1 The shear layer and separation zone associated with dunes are spatially and temporally dynamic, and ‘flapping’ of the shear layer may be modulated by turbulent coherent flow structures generated upstream.
2 Reynolds stresses in the lee side are dominated by the free shear layer associated with the separation zone.
3 Ejections of low downstream momentum fluid away from the bed dominate the instantaneous flow field over the crestal regions of the dune. These ejections, in turn, however, create return flows towards the bed both in front of, and behind, the ejection. The highest instantaneous Reynolds stresses are associated with these ejections and inrushes.
4 ‘Boils’ on the water surface over a natural dune field often consist, firstly, of a central upwelling that has a spanwise axis of rotation and, secondly, later secondary vortices that possess a vertical axis of rotation. This pattern of flow can be explained by the interaction of a vortex loop with the free surface.
These results provide a mechanism that links the flow fields of adjacent dunes and highlight how dune-related macroturbulence may dominate the entrainment of sediment into both suspended and bedload transport. Additionally, the mechanism identified here may also be applied to explain the sequence of turbulent events present over other large grain and form roughness in depth-limited alluvial channels.