H11D-0783
Coherent flow structures in a depth-limited flow over a gravel surface: the role of near-bed turbulence and influence of Reynolds number
In gravel bed rivers, the ratio between the mean flow depth to roughness height seldom exceeds a value of 10 for normal flow conditions, and thus the detailed microtopography of the bed exerts a significant effect on the generation of the turbulent flow structures. Past field measurements and laboratory visualization have indicated that shallow flows over gravel beds contain coherent macroturbulent structures. However, the origin of these macroturbulent phenomena, and their relationship to the ensemble of individual roughness elements forming the bed, is not quantitatively well understood. Here we report upon a flume experiment in which flow over a measured heterogeneous gravel surface is quantified through the application of 2D digital Particle Imaging Velocimetry. This methodology allows study of the downstream (u-) and vertical (w-) components of velocity over the entire flow field at a spatial resolution of 0.002 meters and a temporal resolution of 15 Hz. These dPIV measurements were linked to high-resolution quantification of the bed surface topography obtained using digital photogrammetry. This experimental set-up enabled study of the generation and evolution of coherent flow structures over a known bed topography that allowed the potential to identify: i) the topographic characteristics required for generation of macro-scale turbulent flow structures; ii) the geometric shape of the flow structures; iii) the temporal length scales of the flow structures, and iv) how these characteristics change with flow Reynolds number.
H11D-0784
The Influence of Large-Scale, Meandering, Shear-Induced Instabilities on Jet Hydrodynamics and Sediment Transport
Sedimentation leading to the development of leveed, prograding floodplain tie channels arises from the interaction of a sediment-laden jet with quiescent lake waters. In the field, a meandering, coherent turbulent instability generated by shear along the margins of the jet is a dominant feature of tie channel jets. Spatially, this instability scales with the width of the jet which is five to ten times the flow depth and appears to be quasi-two dimensional. In the laboratory, we have conducted physical experiments on tie channel morphodynamics that reproduce these quasi-two dimensional, large-scale, coherent turbulent structures. Using acoustic doppler velocimetry (ADV) we collected detailed measurements on these experimental jets and quantified the influence of the meandering instability on the jet hydrodynamics. Based on spectral and autocorrelation analysis of the velocity time series data, we analyzed the velocity data at two timescales; one corresponding to the large-scale, quasi-two dimensional turbulent structures generated by shear at the jet margins, and the second at the shorter timescales of the three-dimensional turbulence generated by bed-induced shear. Our results indicate that the presence of the meandering structure doubles the magnitude of both the streamwise and cross-stream turbulent intensities and boundary shear stresses across the jet. Estimates of lateral momentum diffusivity that include the meandering structure are an order of magnitude greater than diffusivities generated by bed shear alone. Suspended sediment and deposition rate measurements show that the coherent meandering structure dominates the lateral transport of sediment to the jet margins and the developing channel levees. The apparent lateral diffusivity of sediment increases down-jet and reaches a value 20 times the diffusivity of momentum. The settling of sediment particles through the water column leads to an additional timescale (the settling time) that is relevant to the dynamics of lateral transport. By comparing the settling timescale to the local timescale of the jet meander, we can estimate a ratio of sediment diffusivity to momentum diffusivity (beta) and treat lateral sediment transport by the large-scale turbulence as a diffusive transport mechanism.
H11D-0785
Image processing and form recognition applied to the quantitative visualisation of coherent flow structures.
Flow visualisation methods such as dye tracers have long been a core methodology for the analysis of turbulent flows. These methods are ideally suited to qualitative observations of coherent structures and their past usage has yielded important insights into turbulent flows. However, the analysis of flow visualisation data need not be limited to qualitative observations. Digital image processing and basic form recognition methods largely developed in the context of remote sensing and earth observation can be applied to flow visualisation experiments in order to extract quantitative information. This paper will demonstrate how such methods can be used on digital films of dye tracer experiments. Specifically, we will examine naturally occurring flow structures observed during a dye tracer experiment conducted in a gravel bed river in Quebec, Canada. The image analysis will be applied in order to automatically identify individual coherent flow structures, measure their size, their orientation in the flow and finally their mean downstream velocity. This novel application of image processing methods to dye tracer experiments allows for quantitative flow visualisations which in turn yield a much more detailed description of coherent flow structures.
H11D-0786
The Interaction of Bedforms and Changing Discharge
In a fluvial environment, the response of the flow properties such as mean velocity and water depth (stage) to changes in discharge are vital to predict, for example, the extent of flooding. Bedforms such as dunes play a crucial role in determining how changes in discharge are partitioned between changes in mean velocity and stage. Most past flume experiments on bedforms use steady, unidirectional flow in order to establish relations incorporating the interactions between flow, sediment transport and bedforms. Under these conditions the bedforms grow and propagate until they approach equilibrium. Applying these relations to natural flows such as rivers often exposes their inability to predict bedform morphology, sediment transport, and water levels in flows which are constantly changing. Conversely, studies conducted in the field suffer from difficulty in taking measurements, unknown history of bedform development and the lack of ability to control conditions, making untangling the dominant physical processes impossible with current models. To bridge the gap between lab studies of bedforms under steady flow conditions and field studies a series of experiments with changing flow conditions is currently being carried out. This study establishes two equilibrium conditions, low (stage A) and high (stage B) flow, each with corresponding equilibrium dunes. In order to better understand the processes which occur in nature, flow conditions are systematically changed to get a better handle on the key processes controlling the evolution of bedforms. The experiments focus on the response of bedforms to step changes in properties, both from low to high and from high to low discharge, but also more realistic gradual changes from Stage A to Stage B and vice versa. In addition, changing depth while maintaining mean velocity and changing mean velocity while maintaining depth are investigated. Data is collected using a Multi-Transducer Array, an Acoustic Doppler Velocimeter, and Acoustic Doppler Profiler. Relatively two-dimensional conditions are achieved so that the Exner equation can be used to estimate sediment flux as a function of position over the dunes in changing flows. The goal is a robust transfer function between flow and sediment response in a bedform environment.
H11D-0787
Characterizing turbulence in the presence of coherent structures in an estuary
As part of the COHerent STructures in Rivers and Estuaries eXperiment, (COHSTREX), we investigate the effect that vortices and "boils" have on the flow field and turbulent kinetic energy (TKE) immediately downstream of a rocky sill. The field site is located in the Snohomish estuary near Everret, WA, a macrotidal estuary (4 m tidal range) characterized by large, intermittent density stratification over a tidal cycle. Turbulence statistics are estimated using velocity data from a bottom-mounted Acoustic Doppler Velocity Profiler (ADCP) located immediately downstream of the sill and from a vessel-mounted array of acoustic Doppler velocimeters (ADVs) moored within the boil field. These data are compared with data from calibrated thermal (longwave infrared, IR) images of the flow over the sill acquired from an aerial platform. Differences in water temperature measured with the IR platform, as well as data from an echo sounder, are used to determine the presence and intensity of coherent structures. These data are used to determine the boil onset time and are compared with fluctuations in the measured TKE and flow field in order to investigate the modification of the turbulence field by the boils.