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

Flow mechanisms of compound meandering channels are recognised to be far more complicated than compound straight channels. The compound meandering channels are mainly characterised by the continuous variation of mean and turbulent flow parameters along a meander wavelength; the existence of horizontal shear layer at the bankfull level and the presence of strong helical secondary flow circulations in the streamwise direction. The secondary flow circulations are very important as they govern the advection of flow momentum, distort isovels, and influence bed shear stress, thus producing a complicated and fully three-dimensional turbulent flow structures. A great deal of experiments has been conducted in the past, which explains flow mechanisms, mixing patterns and the behaviour of secondary flow circulations. However, a complete understanding of secondary flow structures still remains far from conclusive mainly because the secondary flow structures are influenced by the host of geometrical and flow parameters, which are yet to be investigated in detail. The three-dimensional Reynolds-averaged Navier-Stokes and continuity equations were solved using a standard Computational Fluid Dynamics solver to predict mean velocity, secondary flow and turbulent kinetic energy. Five different flow cases of various model scales and relative depths were considered. Detailed analyses of the measured and predicted flow variables were carried out to understand mean flow mechanisms and turbulent secondary flow structures in compound meandering channels. The streamwise vorticity equation was used to quantify the complex and three-dimensional behaviour of secondary flow circulations in terms of their generation, development and decay along the half-meander wavelength. The turbulent kinetic energy equation was used to understand energy expense mechanisms of secondary flow circulations. The strengths of secondary flow circulations were calculated and compared for different flow cases considered. The main findings from this research are as follows. The shearing of the main channel flow as the floodplain flow plunges into and over the main channel influences the mean and turbulent flow structures particularly in the crossover region. The horizontal shear layer at the inner bankfull level generates secondary flow circulations. As the depth of flow increases, the point of generation of secondary flow circulations moves downstream. The secondary shear stress significantly contributes towards the generation of streamwise vorticity and the production of turbulent kinetic energy. The rate of turbulence kinetic energy production was found to be higher than the rate of its dissipation in the crossover region, which demonstrates that the turbulence extracts more energy from the mean flu\\' than what is actually dissipated. This also implies that, in the crossover region, the turbulence is always advected downstream by the mean and secondary flows, The strength of geometry induced secondary flow circulation increases with the increase in the relative depth.

Laboratory experiments concerning stage-discharge, flow resistance, bedforms, sediment transport and flow structures have been carried out in a meandering channel with simulated non-vegetated and vegetated floodplains for overbank flow. The effect of placing solid blocks in different arrangements as a model of rigid, unsubmerged floodplain vegetation on a floodplain adjacent to a meandering channel is considered. The aim was to investigate how density and arrangements of floodplain vegetation influence stage-discharge, flow resistance, sediment transport and flow behaviours. Stage-discharge curves, Manning's n and drag force FD are determined over 165 test runs. The results from the laboratory model tests show that the placing of solid blocks along some part of the bend sections has a significant effect on stage-discharge characteristics. The change in stage-discharge by the blocks is compared using different arrangements, including the non-vegetated floodplains case. The experimental results show that the presence of energy losses due to momentum exchange between the main channel and the floodplain as well as the different densities of the blocks on a floodplain induce additional flow resistance to the main channel flow, particularly for shallow overbank flows. In general, the results show that the density and arrangement of blocks on the floodplains are very important for stage-discharge determination and, in some cases, for sediment transport rates, especially for a mobile main channel. Also, the correction parameter, a is introduced in order to understand the effects of blocks and bedforms on the force balance equation. By applied the correction factor c; a stagedischarge rating curve can be estimated when the avalue is calibrated well. Telemac 2D and 3D were applied to predict mean velocity, secondary flow and turbulent kinetic energy. Telemac computations for non-vegetated and vegetated floodplain cases in a meandering channel generally give reasonably good predictions when compared with the measured data for both velocity and boundary shear stress in the main channel. Detailed analyses of the. predicted flow variables were therefore carried out in order to understand mean flow mechanisms and secondary flow structures in compound meandering channels. The non-vegetated and two different cases of vegetated floodplain for different relative depths were considered. For the arrangement on a non-vegetated floodplain shows how the shearing of the main channel flow as the floodplain flow plunges into and over the main channel influences the mean and turbulent flow structures, particularly in the cross-over region. While applying vegetated floodplain along a cross-over section confirmed that the minimum/reduction shearing of the main channel flow by the floodplain flow plunging into and over the main channel is observed from the cross-sectional distributions of the streamwise velocity (U), lateral velocity (V), and secondary flow vectors. In addition to that, the vegetated floödplain along the apex bend region shows a small velocity gradient within the bend apex region. However, strong secondary flow in the cross-over section suggested that the flow interaction was quite similar to the non vegetation case in the cross-over section region.