The engineering design of a hydraulic structure such as a river bridge requires consideration of the factors that affect the safety of the structure. Among them, one of the most important variables is bridge foundation scour. However, engineering experience seems to indicate that computation of scour depth using current scour formulas tends to overpredict scour in comparison to field measurements. The result can be an overdesigned bridge foundation that increases the cost of the bridge. One possible reason for the overprediction is the current practice of adding separate estimates of contraction scour and pier scour when in fact these processes occur simultaneously and interact. During the occurrence of a flood, velocities and depths increase but they are affected by changes in the distribution of discharge between the main channel and floodplain. In addition, the time history or time development of contraction scour and local pier scour is not the same. As a result, the influence of contraction scour on pier scour, for example, is time dependent. Laboratory experiments are proposed using a 1:45 scale hydraulic model of the Ocmulgee River bridge at Macon, Georgia. Initially, the contraction scour will be measured without the bridge piers in place. In this experiment, the time history of the scour and the velocity distributions at the equilibrium state will be measured. Then the piers will be placed at the bridge cross-section in the flume, and the same measurements will be made. The sensitivity of the measurements to small changes in depth at the same discharge will also be determined, and comparisons will be made with field measurements of scour depth. The results will be used to assess the relative contribution of contraction scour and local pier scour to the final design of the bridge foundation depth.

Recent natural disasters, such as major tsunamis, have prompted researchers and practicing engineers to improve their understanding of the impacts of bore-like waves on structures and their foundations. The high velocity and the relatively short duration of hydraulic bores causes local scouring which is different from that generated by river flows and waves. The present study uses an experimental model to simulate the propagation of a hydraulic bore over a movable sediment bed placed around a circular pier-like structure. Measurements of water surface elevation, bore propagation velocity and scour distribution were taken. The linear relationship between reservoir depth and bore depth led to an increase in flow acceleration, and thus to an increase in flow velocity. Final scour bed elevations indicated that scour depth was highly dependent on the bore velocity. The scour depth ratios suggested in current design guidelines were significantly lower than those obtained in this study.

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.

Scour is a phenomenon of soil erosion around foundations under currents and waves. It is a major cause for the disruption to water-borne structures such as bridges and marine structures. Pile foundations supporting these structures are required to be designed against the scour damage. However, at present, there is no accepted method for the design of piles in scoured conditions probably due to an inadequate understanding of scour effects on foundations. Although numerous efforts have been made to evaluate the scour effects on single piles using numerical simulations and centrifuges tests, the scour susceptibility of piles in different soil properties is still not well understood. Furthermore, there is no study concerning scour effects on the lateral responses of pile groups. Therefore, a series of three-dimensional finite element (FE) parametric analyses were conducted to investigate scour effects on lateral behavior of both single piles and free-head pile groups by varying scour-hole dimensions, soil properties, pile properties, and pile group configurations. Moreover, to facilitate the routine design, a modified p-y method that was modified based on the widely used p-y method was proposed for both scoured single piles and pile groups, and was validated against the results from the FE analyses. The results show that scour induced lateral capacity loss to both single piles and pile groups, which was approximately 10% more in dense sands than that in loose sands. Simplification of local scour as a general scour that has been commonly used in general design practice resulted in a maximum of 17% underestimate of lateral capacity of pile foundations. Pile groups were more susceptible to scour than single piles under equivalent scour conditions. A pile group with smaller pile spacing or larger pile numbers tended to experience less lateral capacity loss due to scour. / Graduate / 2020-08-19

Cette thèse porte sur le développement d’un modèle numérique de l’affouillement causée par des obstacles montes sur le lit, combinant les processus hydrodynamiques et morphologiques. Le modèle numérique est basé sur le solveur de champ d’écoulement polyphasique de l’outil CFD open-source OpenFOAMR qui est distribue par OpenCFD Ltd. Le module hydrodynamique du modèle résout les équations de Navier-Stokes avec moyennes de Reynolds (RANS) et les modèles des turbulences k-ε ou k-ω. Il existe deux interfaces dans le domaine de simulation: la surface libre entre l’eau et l’air, qui est suivi par la méthode de Volume de Fluide (VOF); et l’interface entre l’eau et le lit du sédiment, qui est représentée par un maillage de surface finie déformable construit à partir de la limite en bas du maillage de volume fini. En outre, un module morphologique qui a été développé dans le cadre du projet se compose de trois composantes: un modèle de transport de sédiments comprenant la charge suspendue et le charriage; l’équation d’Exner pour mesurer la déformation du lit; et un mécanisme de glissement du sable pour limiter la pente du lit à être plus petite que l’angle de repos du sédiment. Le changement morphologique est incorporé dans le modèle hydrodynamique par la déformation du maillage. Des conditions limites spéciales et des corrections nécessaires pour le calcul en parallèle sont également ajoutées au modèle. Chaque partie du modèle est validée séparément avec les tests préliminaires correspondants, y compris les fonctions de paroi rugueuse, les performances de la méthode VOF, le modèle de transport de charge suspendu et le mécanisme de glissement de sable. Le modèle numérique est ensuite appliqué pour étudier un affouillent bidimensionnelle cause par un jet immerge provenant d’une ouverture sous écluse. Comparaison des résultats de la simulation avec des données expérimentales prouve la capacité du modèle. Et les limites du modèle sont également discutées. Enfin, le modèle est appliqué à l’étude du champ d’écoulement tridimensionnel et de la formation d’affouillement autour d’un obstacle dans l’écoulement. Tout d’abord, la déformation du lit n’est pas activée. Le tourbillon en fer à cheval devant un obstacle et le champ d’écoulement turbulent autour d’un cylindre sur un lit lisse ou rugueux sont simulés. Deux types de simulation pour le module hydrodynamique sont effectués: une simulation qui utilise une surface fixe et rigide pour représenter l’interface air-eau, et une simulation incluant à la fois les domaines de l’eau et de l’air avec la surface libre suivie par la méthode VOF. Les influences de la surface libre sur le champ d’écoulement sont identifiées et discutées. La comparaison avec les données expérimentales confirme l’importance de la déformation de la surface libre sur le champ d’écoulement. Ensuite, le lit est autorisé à se déformer et le développement temporel de l’affouillement tridimensionnelle autour d’un cylindre sur le lit est simule. Le développement temporel d’affouillement et la profondeur maximale du trou calcule devant et derrière le cylindre conviennent assez bien avec les mesures expérimentales. Les influences de l’affouillement sur le champ d’écoulement sont aussi étudiées et la performance du modèle numérique développé est discutée. / This thesis describes the development of a numerical model for local scour caused by bed-mounted obstacles, combining the hydrodynamic and morphological processes. The basis of the numerical model is the multiphase flow field solver in the open-source CFD toolbox OpenFOAMR which is released by OpenCFD Ltd. The hydrodynamic module of the model solves the Reynolds Averaged Navier-Stokes (RANS) equations with either a k-ε or a k-ω model. There are two interfaces in the simulation domain: the free surface between water and air, which is tracked using the Volume of Fluid (VOF) method, and the interface between the water and the sediment, which is represented by a finite area mesh constructed from the bottom boundary of the finite volume mesh. A morphological module which has been developed as part of the project consists of three components: a sediment transport model which includes suspended load and bed load transport; the Exner equation to compute the bed deformation, and a sand-sliding mechanism to restrict the bed slope angle to be smaller than the angle of repose. The morphological changes are incorporated into the hydrodynamic field through deformation of the computational mesh. Additional boundary conditions and parallel computing corrections are also added into the model. Each individual part of the model has been validated separately with corresponding preliminary test cases including the rough wall functions, the performance of the VOF method, the suspended load transport model and the sand-sliding mechanism. The numerical model is then applied to study two-dimensional scour caused by a submerged jet issuing from an opening under sluice gate. Comparison of the simulation results with the experimental measurements proves the ability of the model for conducting two-dimensional simulations and the limitations of the model are also discussed. Finally, the model is applied to study the three-dimensional flow field and scour formation around an obstacle in flow. Initially, the bed deformation is not activated in the model. The horseshoe vortex formed in front of an obstacle in water and the turbulent flow field around a cylinder on smooth and rough beds are simulated. Two types of simulations for the hydrodynamic module are used: a rigid lid simulation with a slip boundary condition to represent the air-water interface, and a free surface simulation including both the water and air domains with the free surface tracked by the VOF method. The influences of the variation of the water depth on the flow field are identified and discussed. Comparison with the experimental data also confirms the importance of the water surface variation on the flow field. Next, the bed is allowed to deform in the model. The temporal development of three-dimensional scour around a cylinder on live-bed in a steady current is simulated. The development of the scour with time and the computed maximum scour depths in front of and behind the cylinder agree quite well with the experimental measurements. The influences of the scour process on the flow field are also studied and the performance of the numerical model is discussed.

It is an important task that design engineers in practice predict the local scour around bridge piers as accurately as possible because excessive local scour around bridge piers unbalance and demolish the bridges. Many equations have been proposed previously by various researchers, based on their experimental findings, but no general method has been developed so far due to the complexity of the topic. In the present study two new bridge pier groups were employed to investigate the inclination effect of the most upstream and downstream piers on the local scours around all piers. Total of 72 experiments have been conducted with 3 inclination angles, one of which representing the vertical case, each experiment lasting 6 hours, under uniform flow and clear-water conditions for a range of water depths and flow velocities on the uniform bed material. It is clearly observed and measured that the amount of local scour reduces substantially by the effect of inclination in the group piers, especially the reduction in the scour around the most upstream pier is found notable. Based on the experimental data, regression analyses are made and an empirical scour depth equation is developed for each individual pier in the pier groups studied. Comparisons with the similar studies performed by other researchers have been made and the results discussed.

Many river bridges fail or are seriously damaged due to excessive local scouring around piers and abutments. To protect a bridge from scour-induced failure, it should be designed properly against excessive scouring and its scour criticality should be checked regularly throughout the service life to take prompt action. The Federal Highway Administration of United States (FHWA) developed a program, HYRISK, as a basis for evaluation of existing scour failure risk of a bridge. It provides implementation of a risk-based model, which is used to calculate the annual risk of scour failure of a bridge or series of bridges in monetary values. A case study is carried out for a bridge crossing Fol Creek in Black Sea Region (close to Vakfikebir), for the illustration of this software. Besides, hydraulic analysis and scour depth computations of the bridge are carried out via HEC-RAS program. Also, a study is carried out to recommend scour countermeasures that can be applied to the aforementioned bridge.

Local scour around bridge foundations has been recognized as one of the main causes of bridge failures. The objective of this study is to investigate the relationships among field, laboratory, and numerical data for the purpose of improving scour prediction methods for complex bridge piers. In this study, three field sites in Georgia were selected for continuous monitoring and associated laboratory models were fabricated with physical scale ratios that modeled the full river and bridge cross sections to consider the effect of river bathymetry and bridge geometry. Three different sizes of sediment and several geometric scales of the bridge pier models were used in this study to investigate the scaling effect of relative sediment size, which is defined as the ratio of the pier width to the median sediment size. The velocity field for each bridge model was measured by the acoustic Doppler velocimeter (ADV) to explain the complicated hydrodynamics of the flow field around bridge piers as guided by the results from a numerical model. In each physical model with river bathymetry, the comparison between the results of laboratory experiments and the measurements of prototype bridge pier scour showed good agreement for the maximum pier scour depth at the nose of the pier as well as for the velocity distribution upstream of each bridge pier bent. Accepted scour prediction formulae were evaluated by comparison with extensive laboratory and field data. The effect of relative sediment size on the local pier scour depth was examined and a modified relationship between the local pier scour depth and the relative sediment size was presented. A useful methodology for designing physical models was developed to reproduce and predict local scour depth around complex piers considering Froude number similarity, flow intensity, and relative sediment size.

A series of relatively recent bridge failures due to pier scour, as reported in literature, has rekindled interest in furthering our understanding of the scour process and for developing improved ways of protecting bridges against scour. Moreover, increased attention is being given to the state of Canadas infrastructure, a major aspect of which is the transportation network. In part, there is concern about both the impact of a failure on the handling of traffic flow while the failure is being remedied and on the cost of replacing the failed system component. As such, attention is being given to the scour design of new bridges and to the inspection, maintenance and management of existing bridge structures. The two major countermeasure techniques employed for preventing or minimising local scour at bridge piers are increased scour resistance and flow alteration. In the former case, the objective is to combat the erosive action of the scour-inducing mechanisms using hard engineering materials or physical barriers such as rock riprap. In the latter case, the objective is to either inhibit the formation of the scour-inducing mechanisms or to cause the scour to be shifted away from the immediate vicinity of the pier. This research focuses on a particular application of the latter technique. <p> In this study, the use of collars for reducing the effects of local scour at a bridge pier is presented together with the time aspect of the scour development. The adoption of a collar is based on the concept that its existence will sufficiently inhibit and/or deflect the local scour mechanisms so as to reduce the local scour immediately adjacent to the pier. The overall objective of the research is to study the temporal development of the scour for a pier fitted with a collar and a pier without a collar. More specifically, the objectives are: i) to evaluate the effectiveness of a pier collar for mitigating the depth of scour that would otherwise occur at a bridge pier; and ii) to assess the occurrence of an equilibrium scour condition, if achieved, or of the implications of not achieving such a condition in respect of interpreting the results obtained from a physical hydraulic model study. <p>The study was conducted using a physical hydraulic model operated under clear-water conditions in cohesionless bed material. Tests were conducted using two different pier diameters so as to determine the effect of pier diameter on the temporal development of scour for a plain pier. Also investigated was the effect of collar size on the time development of scour and its efficacy at preventing scour at a bridge pier. The time development of the scour hole around the model pier with and without a collar installed was compared with similar studies on bridge piers. Several equations for the temporal development of scour depth and those for the prediction of the equilibrium scour depth were tested as part of this study. <p>The results of the model study indicated that the maximum depth of scour is highly dependent on the experimental duration. The depth of the scour hole increases as the duration of the increased flow that initiates the scour increases. The extent of scour observed at the pier also increases as the duration of the tests increases. It was found that the temporal development of the scour hole at the pier was dependent on whether or not the pier was fitted with a collar placed at the bed level. The pathway to an equilibrium scour depth is different depending on whether the pier is fitted with a collar or not. With a collar in place, the development of the scour hole is considerably delayed. A truly equilibrium scour condition is not readily attainable and was not achieved in the work reported herein. It was demonstrated that wrong conclusions may be reached if a test is stopped short of an equilibrium state. As regards the temporal development of scour depth and for the tests in which no collar was fitted to the pier, it was noted that the form of equation that fits the experimental data well was the one given by Franzetti et al. (1982). Furthermore, it is possible to reach a variety of conclusions about the efficacy of using collars as a pier scour countermeasure technique, depending on which definition of time to equilibrium scour is adopted.

A series of relatively recent bridge failures due to pier scour, as reported in literature, has rekindled interest in furthering our understanding of the scour process and for developing improved ways of protecting bridges against scour. Moreover, increased attention is being given to the state of Canadas infrastructure, a major aspect of which is the transportation network. In part, there is concern about both the impact of a failure on the handling of traffic flow while the failure is being remedied and on the cost of replacing the failed system component. As such, attention is being given to the scour design of new bridges and to the inspection, maintenance and management of existing bridge structures. The two major countermeasure techniques employed for preventing or minimising local scour at bridge piers are increased scour resistance and flow alteration. In the former case, the objective is to combat the erosive action of the scour-inducing mechanisms using hard engineering materials or physical barriers such as rock riprap. In the latter case, the objective is to either inhibit the formation of the scour-inducing mechanisms or to cause the scour to be shifted away from the immediate vicinity of the pier. This research focuses on a particular application of the latter technique. <p> In this study, the use of collars for reducing the effects of local scour at a bridge pier is presented together with the time aspect of the scour development. The adoption of a collar is based on the concept that its existence will sufficiently inhibit and/or deflect the local scour mechanisms so as to reduce the local scour immediately adjacent to the pier. The overall objective of the research is to study the temporal development of the scour for a pier fitted with a collar and a pier without a collar. More specifically, the objectives are: i) to evaluate the effectiveness of a pier collar for mitigating the depth of scour that would otherwise occur at a bridge pier; and ii) to assess the occurrence of an equilibrium scour condition, if achieved, or of the implications of not achieving such a condition in respect of interpreting the results obtained from a physical hydraulic model study. <p>The study was conducted using a physical hydraulic model operated under clear-water conditions in cohesionless bed material. Tests were conducted using two different pier diameters so as to determine the effect of pier diameter on the temporal development of scour for a plain pier. Also investigated was the effect of collar size on the time development of scour and its efficacy at preventing scour at a bridge pier. The time development of the scour hole around the model pier with and without a collar installed was compared with similar studies on bridge piers. Several equations for the temporal development of scour depth and those for the prediction of the equilibrium scour depth were tested as part of this study. <p>The results of the model study indicated that the maximum depth of scour is highly dependent on the experimental duration. The depth of the scour hole increases as the duration of the increased flow that initiates the scour increases. The extent of scour observed at the pier also increases as the duration of the tests increases. It was found that the temporal development of the scour hole at the pier was dependent on whether or not the pier was fitted with a collar placed at the bed level. The pathway to an equilibrium scour depth is different depending on whether the pier is fitted with a collar or not. With a collar in place, the development of the scour hole is considerably delayed. A truly equilibrium scour condition is not readily attainable and was not achieved in the work reported herein. It was demonstrated that wrong conclusions may be reached if a test is stopped short of an equilibrium state. As regards the temporal development of scour depth and for the tests in which no collar was fitted to the pier, it was noted that the form of equation that fits the experimental data well was the one given by Franzetti et al. (1982). Furthermore, it is possible to reach a variety of conclusions about the efficacy of using collars as a pier scour countermeasure technique, depending on which definition of time to equilibrium scour is adopted.