Chute channel formation is a key process in the transition from a single-thread meandering to a braided channel pattern, but the physical mechanisms driving the process remain unclear. This research combines GIS and spatial statistical analyses, field survey, Delft3D hydrodynamic and morphodynamic modelling, and Pb-210 alpha-geochronology, to investigate controls on chute initiation and stability, and the role of chute channels in the planform dynamics of large, sand-bed meandering rivers. Sand-bed reaches of four large, tropical rivers form the focus of detailed investigations; the Strickland and Ok Tedi in Papua New Guinea, the Beni in Bolivia, and the lower Paraguay on the Paraguay/Argentina border. Binary logistic regression analysis identifies bend migration style as a key control on chute channel initiation, with most chute channels forming at bends that are subject to a rapid rate of extension (elongation in a direction perpendicular to the valley axis). Bend extension rates are shown to track variation in potential specific stream power, such that reaches and sub-reaches of the rivers studied fit within a planform continuum expressed though increasing bend extension rates and chute initiation frequency, and driven by increasing stream power relative to bedload calibre. Field observations of point bar geomorphology and vegetation dynamics illustrate the importance of rapid bend extension in forming wide sloughs between scroll bars that are aligned with the direction of over-bar flow, and in breaking the continuity of vegetation encroachment on point bars. Bathymetric surveys and Delft3D simulations for the Strickland River provide insight into flow and sediment division at bifurcate meander bends. Coupled with GIS analyses, these simulations show that stable chute channels have higher gradient advantages than chute channels subject to infill, but that upstream and downstream changes in bend orientation can also influence chute stability. The process of bend extension is typically associated with an increase in the chute gradient advantage, further elucidating the role of bend migration style in chute stability. At the reach scale, rivers with higher sediment loads (Qs/Q) are characterised by higher rates of chute infill. Strickland River floodplain sedimentation rates derived through Pb-210 alpha-geochronology are substantially higher adjacent to single-thread bends than adjacent to bifurcate bends, potentially due to an observed increase in channel capacity (and reduction in floodplain inundation frequency) associated with bend bifurcation. Further research is needed to determine whether this observation is significant in light of high uncertainty in the spatial variability of sedimentation rate estimates, but the data presented highlight a need for carefully considered stratified sampling approaches in floodplain coring campaigns, and illustrate the complexity of possible sediment dispersal mechanisms, and associated ecological responses. GIS analysis of the response of the Ok Tedi in Papua New Guinea to direct addition of mine tailings elucidates interplay between channel steepening due to the propagation of a tailings sediment slug, and mid-channel bar formation induced by the increased sediment load, with associated topographic forcing of bend and chute development. A temporal pattern of increased chute initiation frequency on the Ok Tedi mirrors the inter- and intra-reach spatial pattern of chute initiation frequency on the Paraguay, Strickland and Beni Rivers, where increased stream power is associated with increased bend extension and chute initiation rates. The process of chute formation is shown to be rate-dependent, and the threshold value of bend extension for chute initiation is shown to scale with reach-scale stream power, reminiscent of slope-ratio thresholds in river avulsion. However, Delft3D simulations suggest that chute formation can exert negative feedback on shear stress and bank erosion in the adjacent mainstem bifurcate, such that the process of chute formation is also rate-limiting. Chute formation is activated iteratively in space and time in response to changes in river energy, selectively targeting sites of greatest change, and thereby mediating the river response.

This thesis concerns the nature and time-scale of bed morphological adjustments towards equilibrium in alluvial meandering streams. Following the prevailing approach, the stream centrelines are assumed to follow sine-generated curves, the banks are fixed, and the flow is turbulent and sub-critical. The movable bed is flat at time t = 0; at t = Tb, the bed reaches its equilibrium or developed state. The specific objectives of this thesis are: 1) to develop a predictive equation for the bed development time Tb; 2) to determine the rate of growth of pool-bar complexes in meandering streams; and 3) to determine whether or not the large-scale, curvature-induced erosion-deposition zones of developing beds migrate upstream or downstream throughout their development. This thesis builds on the preliminary work and experimental tests by Binns (2006). The duration of bed development is found to be proportional to the square of the flow width B and inversely proportional to the channel-averaged bed-load rate (qsb)av, the proportionality factor being a function of the initial deflection angle (i.e., stream sinuosity). The form of this function is revealed on the basis of an extensive series of experimental runs carried out in meandering channels of varying values of the initial deflection angle (i.e., 20, 45, 70 and 95 degrees). In the present tests, the temporal development of the bed from time t = 0 to t = Tb was monitored by periodically stopping the flow in order to measure changes in bed surface elevation. In all runs the bed was observed to deform rapidly during the early stages of the run and slow down considerably as the bed approached equilibrium conditions. Once formed, the location of the erosion-deposition zones remained invariant in flow plan with the passage of time. Results from the runs are used to provide insight into the nature of the deformed bed in meandering streams. An expression describing the temporal rate of growth of pool-bar complexes in meandering streams of varying sinuosity is also proposed. The practical application of the equation for Tb is illustrated with available field data. / Thesis (Ph.D, Civil Engineering) -- Queen's University, 2012-01-26 16:43:52.424

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22049号 / 工博第4630号 / 新制||工||1722(附属図書館) / 京都大学大学院工学研究科社会基盤工学専攻 / (主査)教授 中川 一, 教授 藤田 正治, 准教授 川池 健司 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

No description available.

Yes / Large amount of sediment deposits in the reservoir area can cause dam break, which not only leads to an immeasurable loss to the society, but also the sediments from the reservoir can be transported to generate further problems in the downstream catchment. This study aims to investigate the short-to-long term sediment transport and channel meandering process under such a situation. A coupled explicit-implicit technique based on the Euler-Lagrangian method (ELM) is used to solve the hydrodynamic equations, in which both the small and large time steps are used separately for the fluid and sediment marching. The main feature of the model is the use of the Characteristic-Based Split (CBS) method for the local time step iteration to correct the ELM traced lines. Based on the solved flow field, a standard Total Variation Diminishing (TVD) finite volume scheme is applied to solve the sediment transportation equation. The proposed model is first validated by a benchmark dambreak water flow experiment to validate the efficiency and accuracy of ELM modelling capability. Then an idealized engineering dambreak flow is used to investigate the long-term downstream channel meandering process with nonuniform sediment transport. The results showed that both the hydrodynamic and morphologic features have been well predicted by the proposed coupled model. / This research work is supported by Sichuan Science and Technology Support Plan (2014SZ0163), Start-up Grant for the Young Teachers of Sichuan University (2014SCU11056), and Open Research Fund of the State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University (SKLH 1409; 1512).

L'objet de ce travail de thèse est l'étude en soufflerie du sillage lointain des éoliennes à axe horizontal. La complexité phénoménologique du sillage des éoliennes fait que les mécanismes le régissant sont généralement traités d'un point de vue stationnaire : les principaux paramètres (déficit de vitesse, production de turbulence...) sont alors moyennés dans le temps. Néanmoins, considérer les instationnarités du sillage d'une éolienne placée en écoulement atmosphérique permet d'observer un phénomène appelé meandering, traduisant un battement aléatoire du sillage. Ce travail est construit en deux grandes parties : la vision stationnaire et la vision instationnaire du sillage d'un modèle simplifié d'éolienne, basé sur le principe du disque de Froude, placé dans une couche limite atmosphérique (CLA) modélisée en soufflerie à l'échelle 1/400. La première partie est composée de plusieurs études paramétriques sur l'influence du point de fonctionnement d'une éolienne, sur sa hauteur de mât ou encore sur le type de CLA. Une éolienne en situation de dérapage fait également l'objet d'études paramétriques. Celles-ci visent à étudier le comportement du sillage dans différentes situations et il est alors montré les limites des lois empiriques présentes dans la littérature, concernant notamment l'influence de la turbulence ambiante. La seconde partie se focalise sur la vision instationnaire, jusqu'ici rarement considérée. En utilisant l'anémométrie par fil chaud, les résultats montrent le rôle des grandes échelles de la turbulence atmosphérique sur l'apparition du meandering. L'amplitude du battement et les dimensions du sillage instantané sont ensuite appréhendées de façon quantitative par mesures PIV, montrant l'influence du point de fonctionnement de l'éolienne et de l'intensité de turbulence ambiante.

Wind tunnel studies of the wake behind model wind turbines with one, two and three blades have been made in order to get a better understanding of wake development as well as the possibility to predict the power output from downstream turbines working in the wake of an upstream one. Both two-component hot-wire anemometry and particle image velocimetry (PIV) have been used to map the flow field downstream as well as upstream the turbine. All three velocity components were measured both for the turbine rotor normal to the oncoming flow as well as with the turbine inclined to the free stream direction (the yaw angle was varied from 0 to 30 degrees). The measurements showed, as expected, a wake rotation in the opposite direction to that of the turbine. A yawed turbine is found to clearly deflect the wake flow to the side showing the potential of controlling the wake position by yawing the turbine. The power output of a yawed turbine was found to depend strongly on the rotor. The possibility to use active wake control by yawing an upstream turbine was evaluated and was shown to have a potential to increase the power output significantly for certain configurations. An unexpected feature of the flow was that spectra from the time signals showed the appearance of a low frequency fluctuation both in the wake and in the flow outside. This fluctuation was found both with and without free stream turbulence and also with a yawed turbine. The non-dimensional frequency (Strouhal number) was independent of the freestream velocity and turbulence level but increases with the yaw angle. However the low frequency fluctuations were only observed when the tip speed ratio was high. Porous discs have been used to compare the meandering frequencies and the cause in wind turbines seems to be related to the blade rotational frequency. It is hypothesized that the observed meandering of wakes in field measurements is due to this shedding. / QC 20101018

Laboratory experiments of rip current systems are performed in a wave basin with a bar and rip channel geometry at the Ocean Engineering Laboratory at the University of Delaware. The experiments include both in situ water level and velocity measurements and optical visualization of the flow field under a variety of normal-incident wave conditions. Digital video is used to record surface drifters moving through a rip current system. A method is presented that tracks these digitally-recorded drifters into long Lagrangian sequences. The laboratory measurements capture both an Eulerian and Lagrangian description of the rip current system. Time-averaged rip current properties are calculated and analyzed using both in situ and video measurements. From the video, Lagrangian velocities are computed with forward differencing of the low-pass filtered drifter tracks. Wave properties are also estimated using the orbital drifter motions and linear (Airy) wave theory. The effects of various wave conditions on the time-averaged rip current systems are investigated to show that wave height is a critical parameter. Measurements of circulation cells are obtained by spatially averaging the drifter track velocity measurements into a polar grid ranging from 0.25 m to 3.25 m from the center of the cell. Circulation cell features, such as the center of circulation and cell width, are calculated to characterize their response to various wave conditions. Spectral analyses are used to characterize the rip current pulsations in the experimental measurements. Three frequencies are found to be energetic in several of the experiments in the low frequency band: the wave group frequency, a lower frequency, and the interaction of the wave group and lower frequencies. Some experiments have significant energy at each of the three peaks, where others have only one or none. The lower frequency motions have also been found in the video measurements and attributed to rip meandering. Possible causes for the low-frequency pulsations, including wave basin seiching, circulation cell instabilities, and wave-current interaction, are discussed. This thesis adds to previous rip current studies by providing a spatially-large and time-varying perspective of rip current systems as a whole.

<p>Wind tunnel studies of the wake behind model wind turbines with one, two and three blades have been made in order to get a better understanding of wake development as well as the possibility to predict the power output from downstream turbines working in the wake of an upstream one. Both two-component hot-wire anemometry and particle image velocimetry (PIV) have been used to map the flow field downstream as well as upstream the turbine. All three velocity components were measured both for the turbine rotor normal to the oncoming flow as well as with the turbine inclined to the free stream direction (the yaw angle was varied from 0 to 30 degrees). The measurements showed, as expected, a wake rotation in the opposite direction to that of the turbine. A yawed turbine is found to clearly deflect the wake flow to the side showing the potential of controlling the wake position by yawing the turbine. The power output of a yawed turbine was found to depend strongly on the rotor. The possibility to use active wake control by yawing an upstream turbine was evaluated and was shown to have a potential to increase the power output significantly for certain configurations. An unexpected feature of the flow was that spectra from the time signals showed the appearance of a low frequency fluctuation both in the wake and in the flow outside. This fluctuation was found both with and without free stream turbulence and also with a yawed turbine. The non-dimensional frequency (Strouhal number) was independent of the freestream velocity and turbulence level but increases with the yaw angle. However the low frequency fluctuations were only observed when the tip speed ratio was high. Porous discs have been used to compare the meandering frequencies and the cause in wind turbines seems to be related to the blade rotational frequency. It is hypothesized that the observed meandering of wakes in field measurements is due to this shedding.</p>

No description available.