[1] Dunes are present in nearly all fluvial channels and are vital in predicting flow resistance, sediment transport, and deposition within many rivers. Progress in understanding the fluid dynamics associated with alluvial dunes has been significant in the last 15 years and has witnessed huge advances in field, laboratory, and numerical investigations. Progress has been made in detailing the principal features of mean and turbulent flow over asymmetric dunes that possess flow separation in their leesides and how these forms affect both downstream boundary layer structure and stress partitioning over the dune. Additionally, the links between sediment transport over dunes and instantaneous coherent flow structures are being increasingly understood, with the feedback of dune three-dimensionality upon flow and sediment dynamics over these bed forms beginning to be recognized as vital. Such research now provides an outstanding background for beginning to address areas of greater complexity that will enable a fuller understanding of these important natural features. This review paper summarizes the principal features of mean and turbulent flow over alluvial sand dunes. Five areas are then highlighted and discussed as a possible focus for future research: (1) the influence of dune leeside angle upon flow processes in the dune wake and downstream flow field, (2) the influence of three-dimensionality in dune shape upon the generation of turbulence and distribution of bed shear stress, (3) flow field modification resulting from bed form superimposition and amalgamation, (4) the scale and topology of dune-related turbulence and its interactions with sediment transport and the flow surface, and (5) the influence of suspended sediment on the dune flow field and dune morphology.

/pdf/the-fluid-dynamics-of-river-dunes-a-review-and-some-future-2nrjtixo1r.pdf

The fluid dynamics of river dunes: A review and some future research directions

[1] Dunes are present in nearly all fluvial channels and are vital in predicting flow resistance, sediment transport, and deposition within many rivers. Progress in understanding the fluid dynamics associated with alluvial dunes has been significant in the last 15 years and has witnessed huge advances in field, laboratory, and numerical investigations. Progress has been made in detailing the principal features of mean and turbulent flow over asymmetric dunes that possess flow separation in their leesides and how these forms affect both downstream boundary layer structure and stress partitioning over the dune. Additionally, the links between sediment transport over dunes and instantaneous coherent flow structures are being increasingly understood, with the feedback of dune three-dimensionality upon flow and sediment dynamics over these bed forms beginning to be recognized as vital. Such research now provides an outstanding background for beginning to address areas of greater complexity that will enable a fuller understanding of these important natural features. This review paper summarizes the principal features of mean and turbulent flow over alluvial sand dunes. Five areas are then highlighted and discussed as a possible focus for future research: (1) the influence of dune leeside angle upon flow processes in the dune wake and downstream flow field, (2) the influence of three-dimensionality in dune shape upon the generation of turbulence and distribution of bed shear stress, (3) flow field modification resulting from bed form superimposition and amalgamation, (4) the scale and topology of dune-related turbulence and its interactions with sediment transport and the flow surface, and (5) the influence of suspended sediment on the dune flow field and dune morphology.

The fluid dynamics of river dunes : A review and some future research directions : Marine sandware and river dune dynamics

ASCE Manual 54 , Sedimentation Engineering, prepared under the leadership of Professor Vito A. Vanoni, has provided guidance to theoreticians and practitioners’ world wide on the primary topic of sediment problems involved in the development, use, and conservation of water and land resources. First published in 1975, Manual 54 gives an understanding of the nature and scope of sedimentation problems, of the methods for their investigation, and of practical approaches to their solution. It is essentially a textbook on sedimentation engineering, as its title accurately refl ects. Manual 5 4 was the fi rst and most comprehensive text of its kind and has been circulated throughout the world for the past 30 years as the most complete reference on sedimentation engineering in the world. It has recently been published again as the Classic Edition (Vanoni 2006). In the spirit of its predecessor, this chapter of Manual of Practice 110 , Sedimentation Engineering, aims at presenting the state of the art concerning the hydraulics of sediment transport in fl uvial systems based on the knowledge gained in the last three decades. A concerted effort is made to relate the mechanics of sediment transport in rivers and by turbidity currents to the morphodynamics of lake and reservoir sedimentation, including the formation of fl uvial deltas.

Sediment Transport and Morphodynamics

[1] A river and its surrounding riparian vegetation are two dynamical systems that interact through several hydrological, geomorphological, and ecological processes. This work focuses on the role played by vegetation on meandering river morphodynamics: River planform evolution forces the riparian vegetation dynamics, which, in turn, affect the mechanical characteristics of the river banks and influence the meandering dynamics of the river itself. It follows that despite the fact that a traditional engineering approach considers vegetation as a static element the study of river morphodynamics should be coupled with the riparian vegetation evolution. To this end, a fluid dynamic model of meandering rivers is here coupled with a process-based model for the riparian biomass dynamics. The feedback of vegetation on river morphology is provided by a relation that links the biomass density to the bank erodibility. The numerical results highlight (1) the remarkable effects of the vegetation dynamics on meander evolution and (2) the role of the temporal scales of vegetation growth and decay in relation to typical morphodynamic scales. In particular, the differences with respect to the constant erodibility case can be of the order of tens or hundreds of meters (10–20% of the meander wavelength), and peculiar meander shapes that do not show the usual marked upstream skewness emerge.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2006WR005234

Significance of the riparian vegetation dynamics on meandering river morphodynamics

Confluence–diffluence units are key elements within many river networks, having a major impact upon the routing of flow and sediment, and hence upon channel change. Although much progress has been made in understanding river confluences, and increasing attention is being paid to bifurcations and the important role of bifurcation asymmetry, most studies have been conducted in laboratory flumes or within small rivers with width:depth (aspect) ratios less than 50. This paper presents results of a field-based study that details the bed morphology and 3D flow structure within a very large confluence–diffluence in the Rio Parana, Argentina, with a width:depth ratio of approximately 200. Flow within the confluence–diffluence is dominated largely by the bed roughness, in the form of sand dunes; coherent, channel-scale, secondary flow cells, that have been identified as important aspects of the flow field within smaller channels, and assumed to be present within large rivers, are generally absent in this reach. This finding has profound implications for flow mixing rates, sediment transport rates and pathways, and thus the interpretation of confluence–diffluence morphology and sedimentology.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/esp.1457

Form roughness and the absence of secondary flow in a large confluence–diffluence, Rio Paraná, Argentina

Detailed field measurements during the 1998 flood of the Rhine River in The Netherlands show that both Manning \in and Darcy—Weisbach friction factor \if increase with discharge. The changes in bedform roughness height and friction factors are attributed to the increased dune height during floods. There is a near-peak hysteresis in the dune height measurements. At a given discharge, dunes are significantly larger after than before the peak discharge. The trend is most apparent for the Bovenrijn with weaker variations for the Waal. The methods of Engelund and Vanoni—Hwang provide similar estimates of form drag. When combined with van Rijn’s method to estimate grain resistance, both methods tend to overpredict the measured bed friction factor after the peak discharge. These methods perform best when field bedform measurements are available to estimate form drag. The composite effect of primary and secondary dunes should be considered in the analysis of resistance to flow.

https://www.engr.colostate.edu/%7Epierre/ce_old/Projects/Paperspdf/Julien-Klaassenet%20alASCE2002.pdf

W. B. M. Ten Brinke

Papers

Case Study: Bed Resistance of Rhine River during 1998 Flood