Scour around various structures obstructing flow in an open channel is a common problem; therefore a better understanding of how turbulent flow affects sediment transport is needed. Additionally, is it the mean flow or the turbulence properties that are more important in contributing to bed shear stress? To this end, an experimental study was conducted in a fixed-bed flume containing a series of dikes. Turbulence intensities and Reynold's stresses were calculated from 3-D velocity measurements gathered with a microADV. Results showed that the maximum shear stress was nearly 12-20 times that of the approach flow, while maximum turbulence intensities were about 3-5 times those of the incoming flow. Highest magnitudes of both were seen at the tip of the second dike in the three-dike series. The mean velocity appeared to have no relation to the formation of scour near the tips of the dikes but the turbulence intensities did.

The intensive research on sediment transport indicates a need of an appropriate equation for predicting the total sediment load in rivers to manage reservoirs, operate dam and design in-stream hydraulic structures. None of the available equations in sediment transport has gained universal acceptance for predicting the total sediment transport rate. These facts indicate the need of a general formula to represent all these formula for predicting the sediment transport rate. The first goal of this dissertation is to find a unified total sediment transport equation for all rivers. On the other hand, scour around hydraulic structures such as spur dikes and bridge piers can be a serious problem that weakens structural stability. An investigation on the turbulent flow field and turbulence distribution around such hydraulic structures is essential to understand the mechanism of local scour and to determine which turbulence properties affect the local sediment transport. In addition, a universal turbulent model that is valid for all cases of turbulent flow in open channels does not exist. This dissertation thoroughly examined the turbulent flow field and turbulence distribution around a series of three dikes. The goal is to determine the significant turbulent properties for predicting the local sediment transport rate and to identify the appropriate turbulence model for simulating turbulent flow field around the dikes.To develop a general unified total load equation, this study evaluates 31 commonly used formulae for predicting the total sediment load. This study attributes the deviations of calculated results from different formulae to the stochastic properties of bed shear stresses and assumes that the bed shear stress satisfies the log- normal distribution. At any given bed shear stress, Monte Carlo simulation is applied to each equation, and a set of bed shear stresses are randomly generated. Total sediment load generated from each Monte Carlo realization of all the equations are assembled to represent the samples of total sediment load predicted from all the equations. The statistical properties of the resultant total sediment loads (e.g. standard deviation, mean) at each given bed shear stress are calculated. Then, a unified total sediment load equation is obtained based on the mean value from all the equations. The results showed the mean of all the equations is a power function of dimensionless bed shear stress. Reasonable agreements with measurements demonstrate that the unified equation is more accurate than any individual equation for predicting the total sediment load.An experimental study and numerical simulation of the flow field and local scour around a series of spur dikes is performed in a fixed flat bed and scoured bed condition. A micro-Acoustic Doppler Velocimeter (ADV) is used to measure the instantaneous velocity field in all the three spatial directions and the measured velocity profiles are used to calculate the turbulence properties. Results show that the local scour develops around the first dike. Turbulence intensity together with the mean velocity in the vertical direction measured at the flat bed closely correlates to the scour depth. In addition, the maximum bed shear stress, occurring at the tip of the second dike in the three-dike series, does not correspond to the maximum scour. Large bed load transport due to bed shear stress may not initiate bed scouring, but turbulence bursts (e.g. sweeps and ejections) will entrain sediment from bed surface and develop the local scour.A three-dimensional numerical model FLOW-3D is used to simulate the turbulent flow field around a series of spur dikes in flat and scoured bed. This study examines Prandtl's mixing length model, one equation model, standard two-equation model, Renormalization-Group (RNG) model, and Large Eddy Simulations (LES) turbulence model. The Prandtl's mixing length model and one equation model are not applicable to flow field around dikes. Results of mean flow field by using the standard two-equation model, and RNG turbulence model are close to the experimental data, however the simulated turbulence properties from different turbulent model deviate considerably. The calculated results from different turbulence models show that the RNG model best predicts the mean flow field for this series of spur dikes. None of the turbulence closure models can predict accurate results of turbulence properties, such as turbulence kinetic energy. Based on those results, this study recommends the use of RNG model for simulating mean flow field around dikes. Further improvements of FLOW-3D model is needed for predicting turbulence properties near this series of spur dikes under various flow conditions.

A spur dike is a hydraulic structure built on the bank of a river at some angle to the main flow direction. A series of spur dikes in a row may also be placed on one side or both sides of a river to form a spur dike field. Spur dikes are used for two main purposes, namely river training and bank protection. For river training, spur dikes may be used to provide a desirable path for navigation purposes or to direct the flow to a desirable point such as a water intake. For bank protection, spur dikes may be used to deflect flow away from a riverbank and thus protect it from erosion. It has also been observed that spur dikes provide a desirable environment for aquatic habitat. Despite the fact that spur dikes are useful hydraulic structures, they have been found to increase the flow resistance in rivers and hence increase the flow stage. The present study focuses on the quantification of the flow resistance and associated flow stage increase due to a single spur dike and also that of a spur dike field. Increased flow stage is referred to herein as a backwater effect.<p> In the first stage of the study, the flow resistance due to a single spur dike, expressed as a drag force exerted on the flow in an open channel, was studied and quantified. The work was carried out in a rigid bed flume, with the model spur dike being simulated using various sizes of a two-dimensional (2-D) rectangular plate. Several discharge conditions were studied. The drag force exerted by the spur dike for both submerged and unsubmerged flow conditions was determined directly from measurements made using a specially designed apparatus and also by application of the momentum equation to a control volume that included the spur dike. It was found that the unit drag force (i.e., drag force per unit area of dike) of an unsubmerged spur dike increases more rapidly with an increase in the discharge when compared with that of a submerged spur dike. The results also showed that an increase in the blockage of the open channel cross-section due to the spur dike is the main parameter responsible for an increase in the spur dike drag coefficient, hence the associated flow resistance and backwater effect. Based on these findings, relationships were developed for estimating the backwater effect due to a single spur dike in an open channel.<p> In the second stage of the study, the flow resistance due to a spur dike field expressed as a drag force exerted on the flow was quantified and subsequently related to the backwater effect. The work was carried out in a rigid bed flume, with the model spur dikes simulated using 2-D, rectangular plates placed along one side of the flume. For various discharges, the drag force of each individual spur dike in the spur dike field was measured directly using a specially-designed apparatus. For these tests, both submerged and unsubmerged conditions were evaluated along with various numbers of spur dikes and various relative spacings between the spur dikes throughout the field. It was concluded that the configuration of a spur dike field in terms of the number of spur dikes and relative spacing between the spur dikes has a substantial impact on the drag force and hence the flow resistance and backwater effect of a spur dike field. The most upstream spur dike had the highest drag force amongst the spur dikes in the field, and it acted as a shield to decrease the drag force exerted by the downstream spur dikes. From the experiments on the submerged spur dikes, it was observed that the jet flow over the spur dikes has an important effect on the flow structure and hence the flow resistance.<p> In the third stage of the study, the flow field within the vicinity of a single submerged spur dike was modeled using the three-dimensional (3-D) computational fluid dynamic (CFD) software FLUENT. Application of the software required solution of the 3-D Reynolds-averaged Navier-Stokes equations wherein the Reynolds stresses were resolved using the RNG ê-å turbulence model. One discharge condition was evaluated in a smooth, rectangular channel for two conditions, including uniform flow conditions without the spur dike in place and one with the spur dike in place. The CFD model was evaluated based on some experimental data acquired from the physical model. It was found that the CFD model could satisfactorily predict the flow resistance and water surface profile adjacent to the spur dike, including the resulting backwater effect. Furthermore, the CFD model gave a good prediction of the velocity field except for the area behind the spur dike where the effects of diving jet flow over the spur dike was not properly modeled.

A spur dike is a hydraulic structure built on the bank of a river at some angle to the main flow direction. A series of spur dikes in a row may also be placed on one side or both sides of a river to form a spur dike field. Spur dikes are used for two main purposes, namely river training and bank protection. For river training, spur dikes may be used to provide a desirable path for navigation purposes or to direct the flow to a desirable point such as a water intake. For bank protection, spur dikes may be used to deflect flow away from a riverbank and thus protect it from erosion. It has also been observed that spur dikes provide a desirable environment for aquatic habitat. Despite the fact that spur dikes are useful hydraulic structures, they have been found to increase the flow resistance in rivers and hence increase the flow stage. The present study focuses on the quantification of the flow resistance and associated flow stage increase due to a single spur dike and also that of a spur dike field. Increased flow stage is referred to herein as a backwater effect.<p> In the first stage of the study, the flow resistance due to a single spur dike, expressed as a drag force exerted on the flow in an open channel, was studied and quantified. The work was carried out in a rigid bed flume, with the model spur dike being simulated using various sizes of a two-dimensional (2-D) rectangular plate. Several discharge conditions were studied. The drag force exerted by the spur dike for both submerged and unsubmerged flow conditions was determined directly from measurements made using a specially designed apparatus and also by application of the momentum equation to a control volume that included the spur dike. It was found that the unit drag force (i.e., drag force per unit area of dike) of an unsubmerged spur dike increases more rapidly with an increase in the discharge when compared with that of a submerged spur dike. The results also showed that an increase in the blockage of the open channel cross-section due to the spur dike is the main parameter responsible for an increase in the spur dike drag coefficient, hence the associated flow resistance and backwater effect. Based on these findings, relationships were developed for estimating the backwater effect due to a single spur dike in an open channel.<p> In the second stage of the study, the flow resistance due to a spur dike field expressed as a drag force exerted on the flow was quantified and subsequently related to the backwater effect. The work was carried out in a rigid bed flume, with the model spur dikes simulated using 2-D, rectangular plates placed along one side of the flume. For various discharges, the drag force of each individual spur dike in the spur dike field was measured directly using a specially-designed apparatus. For these tests, both submerged and unsubmerged conditions were evaluated along with various numbers of spur dikes and various relative spacings between the spur dikes throughout the field. It was concluded that the configuration of a spur dike field in terms of the number of spur dikes and relative spacing between the spur dikes has a substantial impact on the drag force and hence the flow resistance and backwater effect of a spur dike field. The most upstream spur dike had the highest drag force amongst the spur dikes in the field, and it acted as a shield to decrease the drag force exerted by the downstream spur dikes. From the experiments on the submerged spur dikes, it was observed that the jet flow over the spur dikes has an important effect on the flow structure and hence the flow resistance.<p> In the third stage of the study, the flow field within the vicinity of a single submerged spur dike was modeled using the three-dimensional (3-D) computational fluid dynamic (CFD) software FLUENT. Application of the software required solution of the 3-D Reynolds-averaged Navier-Stokes equations wherein the Reynolds stresses were resolved using the RNG ê-å turbulence model. One discharge condition was evaluated in a smooth, rectangular channel for two conditions, including uniform flow conditions without the spur dike in place and one with the spur dike in place. The CFD model was evaluated based on some experimental data acquired from the physical model. It was found that the CFD model could satisfactorily predict the flow resistance and water surface profile adjacent to the spur dike, including the resulting backwater effect. Furthermore, the CFD model gave a good prediction of the velocity field except for the area behind the spur dike where the effects of diving jet flow over the spur dike was not properly modeled.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18560号 / 工博第3921号 / 新制||工||1602(附属図書館) / 31460 / 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