Bedform related macroturbulence: topology and kinematics

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All rivers transport sediment both along their beds and suspended within the water column, with the material rolling along the bed often been sculpted into a range of forms by the water: one of the most common of these forms are dunes that may be generated in both sandy and gravelly sediment. Such dunes are nearly always present in alluvial channels and are one of the main ways in which the coarser sediment is transported by the flow. Many of the world's small and large rivers possess such dunes, which in big rivers may reach up to 6m high and tens or even hundreds of metres in length. Such dunes are generated by the turbulent flow but, once formed, begin to greatly disrupt the flow that is moving over them, thereby extracting energy from the flow in the resistance they offer to the passage of water, and creating large-scale turbulent eddies, or 'macroturbulence', within the flow. These large-scale eddies are very often visible as upwellings or 'boils' of fluid on the water surface of rivers, and are familiar to canoeists and boatmen. This turbulence itself can be responsible for suspending and transporting appreciable quantities of sediment and forms part of a complex series of 'feedback' processes between fluid flow, sediment movement and the development of the dunes. Indeed, linkage between turbulent structure and interaction with the water surface is theorised as the mechanism that limits dune height and thus plays a significant role in bedform phase control. Furthermore, as dunes are the principal element of resistance to flow in many channels they have a fundamental control on stage-discharge relationships and hence flooding levels for a given flow magnitude. Thus, an improved knowledge of these feedback processes is vital if we are better to model and manage many natural waterways and better predict how and where they both transport and deposit sediment. Whilst recent years have seen many advances in some aspects of mathematical modelling of macroturbulence generated by dunes, and have begun to elucidate some aspects of its shape and structure, this has not been matched by sufficient progress in measuring and quantifying such turbulence. This project proposes to use a combined laboratory investigation and numerical modelling approach to determine the topology and kinematics of large scale turbulence generated by dune bedforms. State-of-the-art laboratory technology, in the form of Particle Image Veocimetry, Laser Induced Fluorescence and High-Resolution Thermal Imaging, will enable us to quantify flows over fixed dune forms for a range of flow conditions. The results will allow us to answer important questions concerning the generation, form and dynamics of dune related turbulent structure, investigate how this is advected and interacts with the water surface and will also provide a benchmark dataset for improvements in numerical modelling codes. Such objectives are particularly needed in the future management of river channels, particularly in assessment flow resistance for flooding calculations, determination of sediment transport capacity and linking to an improved understanding of larger scale channel morphodynamics.