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The fluid and sediment dynamics of sand dunes under unidirectional water flows: insights from laboratory and field experiments -- 29/10/03
Jim Best, (University of Leeds)
Sand dunes are one of the most common morphological elements of many natural aqueous environments: dunes formed under unidirectional flows are vital agents of sediment transport and deposition in many of the world’s great rivers and also form important elements of flow resistance and bed erosion. Flow over such dunes has long been described as being dominated by the influence of topographic forcing and flow acceleration over the upstream (stoss) side of the dune, the dominant effect of flow separation in the steep dune leeside, and the influence of this separation on the developing boundary layer on the stoss side of the next downstream dune. Many current models of dune dynamics are based on this conceptual framework which, although providing a basis, does not capture either the complexity of dune flow and form in the natural environment or the details of the instantaneous nature of flow. This paper will review the flow associated with alluvial sand dunes and explore several aspects of their fluid and sediment dynamics, including: i) the origin and topology of large-scale turbulence associated with dunes, ii) the influence of flow separation, vortex shedding from the free shear layer, and possible water surface-vortex interactions, in providing inrushes of high-momentum fluid towards the bed at the next downstream dune crest, iii) the morphology of natural sand dunes and influence of leeside angle on the nature of flow separation and turbulence generation, iv) the influence of suspended sediment on the presence of flow separation, and, v) the entrainment and transport of sand at the dune crest and leeside. These aspects will be examined through a combined approach of examination of the characteristics of natural sand dunes in large rivers and detailed laboratory experiments using physical models of fixed dunes. Research in large river channels has included the Fraser River, Canada; Jamuna River, Bangladesh and Paraná River, Argentina. Single and multi-beam echo-sounding records over natural dune fields highlight their morphology complexity, whilst acoustic Doppler current profiling has permitted the major features of flow fields over these dunes to be ascertained, together with indications of the links between turbulence and sediment transport. Research in the laboratory using both laser Doppler anemometry (LDA), ultrasonic Doppler velocity profiling (UDVP) and single and two-phase digital particle imaging velocimetry (DPIV) will be illustrated to examine the nature of flow over low-angle dunes, the evolution of vorticity over the dune lee and stoss sides and new results detailing the links between vortical structures and particulate transport.