Thesis (PhD)--Stellenbosch University, 2002. / ENGLISH ABSTRACT: Local scour is the erosion of a riverbed resulting from the flow of the river around an obstacle. It is a principal cause of failure of bridges and other hydraulic structures. Current design practice relies on the use of empirical formulae that are often extremely inaccurate, or on the use of physical models that are very expensive. Recent advances in the power of microcomputers have however made numerical simulation increasingly attractive. One obstacle to numerical simulation though is that there is no general agreement on the concept of incipient motion, that critical point at which motion - and hence scour - begins. In this dissertation, the unit stream power model developed by Rooseboom (1992) is extended to handle the complex three-dimensional flow conditions that pertain close to the riverbed in the vicinity of an obstacle. The relationship between unit stream power (the dissipation function) and the Movability Number (the ratio of the shear velocity to the terminal settling velocity of the critical sediment particles) is clearly indicated. Since incipient motion is probabilistic in nature, a relationship was established between the Movability Number and the intensity of motion with allowance for bed-slope and relative depth. An extension of this work resulted in a new bed-load transportation equation that could be used to determine the rate of scour development. Physical modelling in a laboratory flume aided the selection of suitable critical conditions for the onset of scour. The usefulness of the above-mentioned relationships was then demonstrated through the construction of a simple mathematical model of scour and deposition around a structure. This model was used in conjunction with commercially available computational fluid dynamics (CFD) software to predict the scour potential around typical engineering structures. Physical model data was obtained for four situations, and the measured scour was compared with that predicted by the numerical model. There was reasonable agreement between the different models and such differences as there were could be readily attributed to constraints on the numerical model, in particular the lack of a free-surface routine and the coarseness of the grid. This dissertation has opened up a new method for the prediction of local scour that could be readily extended to include all types of scour. With the advent of increasingly fast computers, it could become a useful engineering tool that would assist engineers in the design of safe and cost-effective foundations for hydraulic structures. / AFRIKAANSE OPSOMMING: Plaaslike uitskuring is die erosie van 'n rivierbed as gevolg van vloei verby 'n obstruksie. Dit is 'n belangrike oorsaak van die swigting van brfïe en ander hidrouliese strukture. Bestaande ontwerppraktyk berus op empiriese vergelykings wat dikwels hoogs onakkuraat is, of op fisiese modelle, wat baie duur is. Numeriese simulasie het die afgelope tyd 'n al hoe meer aantreklike opsie geword danksy die snelle toename in die kapasiteit van mikro-rekenaars. 'n Struikelblok met numeriese simulasies is die gebrek aan konsensus oor die konsep van begin-van-beweging, daardie kritieke toestand waarby beweging en derhalwe uitskuring begin. In hierdie proefskrif is die eenheidstroomdrywing model, ontwikkel deur Rooseboom (1992), uitgebrei om die komplekse drie-dimensionele vloeitoestande, wat teenaan die rivierbodem verby 'n obstruksie heers,te hanteer. Die verwantskap tussen Eenheid Stroomdrywing (Dissipasiefunksie) en die Beweeglikheidsgetal (verhouding tussen sleursnelheid en die ewewigvalsnelheid van die kritieke sedimentpartikels ) is duidelik uitgewys. Aangesien begin van beweging probabilisties van aard is, is die verwantskap bepaal tussen die Beweeglikheidsgetal en die Intensiteit van Beweging, met voorsiening vir bodernhelling en relatiewe diepte. Verdere uitbreiding het gelei tot 'n nuwe bedvrag vervoervergelyking wat gebruik kan word om die tempo van uitskuring te bepaal. Kritieke toestande, waarby uitskuring begin, is met fisiese modelle in die laboratorium gekwantifiseer. Die bruikbaarheid van bogenoemde verbande is gedemonstreer deur die ontwikkeling van 'n eenvoudige wiskundige model van uitskuring en afsetting rondom 'n struktuur. Hierdie model is saam met bestaande kommersiële sagteware vir vloeidinamika berekenings (CFD) ingespan om uitskuringspotensiaal rondom tipiese ingenieurstrukture te voorspel. Fisiese modelmetings van uitskuring vanaf vier uitlegte is vergelyk met die numeries voorspelde waardes. Bevredigende ooreenkoms is gevind en verskille kon geredelik gewyt word aan beperkings van die numeriese model, veral die gebrek aan' n vryvlakroetine en die growwe maas. Die proefskrif stel 'n nuwe metode vir die voorspelling van uitskuring daar wat geredelik uitgebrei kan word na ander vorms van uitskuring. Met die ontwikkeling van al vinniger rekenaars kan dit 'n nuttige hulpmiddel vir ingenieurs word om veilige en koste-doeltreffende fondamente in waterlope te ontwerp.

Seabed gouging by ice, also known as ice scouring, is a common feature of the Arctic and sub Arctic regions of the planet as well as in Antarctica. It is a phenomenon which occurs when ice moves while in contact with the seabed. Ice gouging is of economical significance due to the probability of disruption of seabed structures such as subsea pipelines. Small scale laboratory tests were conducted at 1g to investigate the scour force produced when a scaled iceberg model scours a test bed in dry and saturated conditions. The tests were conducted for a range of scour depths, scour widths, frontal rake angles and soil conditions to study the performance of a rigid indenter (iceberg keel) scouring a test bed. Furthermore, the tests were also conducted at various speeds to study the rate effect in sand. The effect of the drifting speed on the drag force was found to be important: a sandy seabed scoured by an iceberg with a mean drifting speed of 0.1 m/s can generate scour loads twice as large as the static loads. The methods used currently to predict ice scour loads consider only the static loads under drained conditions and these should be revised. The PIV (Particle Image Velocimetry) technique was utilized to study the sub-gouge deformation and the soil failure mechanism associated with ice gouging. The soil resistance and the sub gouge deformation results obtained in the laboratory were compared with centrifuge investigations (the PRISE and PIRAM programs) in order to examine the viability of extrapolating the results from the model scale to a prototype scale.

Three-dimensional scour propagation along offshore pipelines is a major reason to pipeline failures in an offshore environment. Although the research on scour in both numerical and experimental aspect has been extensive over the last three decades, the focus of the investigation has been limited to the two-dimensional aspect. The knowledge on three-dimensional scour is still limited. This dissertation presents the results of an experimental investigation on threedimensional scour along offshore pipelines in (1) steady currents (2) waves only and (3) combined waves and current. The major emphasis of the investigation is to investigate the propagation of the scour hole along the pipeline after the initiation of scour. Physical experiments conducted were used to quantify the effects of various parameters on scour propagation velocities along the pipeline. The problem of monitoring real time scour below a pipeline was solved by using specifically developed conductivity scour probes. Effects of various parameters such as pipeline embedment depth, incoming flow Shields parameter, Keuglegan- Carpenter (KC) number and flow incident angle to the pipeline on scour propagation velocities along the pipeline were investigated. The investigations clearly reveal that scour propagation velocities generally increase with the increase of flow but decrease with the increase of the pipeline embedment depth. A general predictive formula for scour propagation velocities is proposed and validated against the experimental results. There are still some common issues related to pipeline scour that is lacking in the literature to date. One of these issues is the effects of Reynolds number on two-dimensional scour beneath pipelines. A numerical approach was adopted to investigate the Reynolds-number dependence of two-dimensional scour beneath offshore pipelines in steady currents. A novel wall function is proposed in calculating the suspended sediment transport rate in the model. The effects of Reynolds number were investigated by simulating the same undisturbed Shields parameters in both model and prototype but with different values of Reynolds number in two separate calculations. The results revealed that scour depths for prototype pipelines are about 10~15% smaller than those for model pipelines. The normalized time scales was found to be approximately the same, and the simulated scour profiles for the model pipelines agree well with the experimental results from an independent study. The backfilling of pipeline trenches is also an important issue to the design and management of offshore pipelines. A numerical model is developed to simulate the self-burial of a pipeline trench. Morphological evolutions of a pipeline trench under steady-current or oscillatory-flow conditions are simulated with/without a pipeline inside the trench. The two-dimensional Reynolds-averaged continuity and Navier-Stokes equations with the standard k-e turbulence closure, as well as the sediment transport equations, are solved using finite difference method in a curvilinear coordinate system. Different time-marching schemes are employed for the morphological computation under unidirectional and oscillatory conditions. It is found that vortex motions within the trench play an important role in the trench development.

Although flow deflectors are routinely placed in streams to create fish pool habitat, there is dissent in the literature as to what constitutes for them an appropriate design height to ensure their success. Our objectives were to (1) examine the temporal evolution of bed topography around submerged deflectors in a laboratory flume, for different deflector heights and discharge; and (2) quantify the flow dynamics in their vicinity. Results indicate that, all else being equal, higher deflectors (characterized by less overtopping) produce a greater volume of fish pools. In contrast, over a fixed deflector design, a higher discharge (synonymous with greater overtopping) results in larger pools, because flow velocities are then faster. We also observed differences in the temporal behaviour of scour: that next to high and medium height deflectors is self-similar, but not that for the shorter model. Our results challenge the notion that low deflectors are ineffective, provide information on the development of scour next to submerged structures, and reveal key differences between the geometry of the scour holes engendered by the different deflector height models.

A three-dimensional numerical model (PHOENICS) was used to investigate the role of stream deflector angle and length on the flow field in a rectangular laboratory flume. Subsequent bed topography surveys were performed to examine the role of obstruction angle on scour hole development over time. The model was capable of predicting laboratory velocity and turbulent kinetic energy measurements, performing better for flow over a flat stable bed than over a deformed sand bed. A new method of incorporating complex bed topography into a structured Cartesian mesh was developed in the process. Flow field properties such as dynamic pressure, velocity amplification, separation zone length and width, and downwelling extent and magnitude were found to be strongly dependent on deflector geometry. Equilibrium scour hole depths and geometry are also angle-dependent. A predictive equation was produced explaining the rate at which scour holes reach equilibrium, and compared well with existing literature. Finally, a method was developed whereby characteristics of the flow field over a flat, stable bed could be used to predict equilibrium scour hole geometry.

Local scour around bridge piers can cause serious damages and structural failure to the bridge. Correct prediction of the scour is an important criterion for the engineering design. Though the subject has been investigated for many decades, the theoretical developments have been very limited due to the complicated interaction of three-dimensional flow and the sediment transport. This thesis concerns the flow around a vertical bottom mounted cylinder exposed to currents and is considered as the initial phase of a study towards modeling local scour around vertical bottom-mounted structures. The aim of the present study is to obtain a better understanding of the complex three-dimensional flow and the mechanisms related to the scouring. The study started with the development of a three-dimensional numerical model to simulate flow around cylindrical structures. After validation of the model, the model was applied to investigate flow around an isolated rectangular cylinder. Unsteady flows around cylinders of a square cross section (A/D = 1) and a rectangular cross section (A/D = 2) were simulated to understand the flow properties around a cylinder other than a circular cross section. Three-dimensional flow patterns, pressure distribution, forces on the cylinder and vortex-shedding frequencies were discussed. It was found that the present numerical results generally agree well with the experimental data. Flow around a vertical cylinder mounted on a rigid bed was then investigated by the numerical model. A circular and a square cross sections were considered respectively. Flow structures of horseshoe vortex and the wake vortex which are the major mechanisms leading to the scouring around the base of the vertical cylinder were explored. The bed shear stress distributions that directly affect the scour processes were discussed. Finally the numerical model was applied to study the flow around a submerged square cylinder mounted on a bed, which has significant engineering relevance to the local scour around structures, such as bridge pier foundation itself, or a caisson placed underwater around the pier bottom for scour protection. The numerical calculations were carried out at different cylinder heights to investigate the effect of the cylinder height on the flow properties. It was found that for submerged cylinders with the height of less than one and a half of the cylinder side width the maximum bed shear stress amplification is about 60% of the value of an infinite long cylinder. The quantification of the shear stress reduction is important for scour protection design.

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 202-207).

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Thesis (MScEng (Civil Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: The scour process of unlined spillways is an important research topic of value in engineering practice. In South Africa numerous unlined spillway dams have experienced severe erosion. This led, in some cases, to the costly concrete lining of spillways for erosion protection. On the other hand, the erosion of unlined spillways can lead to damage to, and even failure of dams and consequently can affect public safety, properties, infrastructure and the environment. In this regard, methods to predict erosion of unlined spillways are therefore needed as tools in the risk management and design of existing unlined spillways as well as future spillway structures. The prediction of the rock scouring process is challenging and empirical formulas have been established to predict incipient conditions for scour. These empirical methods however do not predict the rate of scour or the ultimate equilibrium rock scour. The key objective in this study was to investigate the applicability of a non-cohesive two-dimensional (2D) sediment transport hydrodynamic mathematical model to simulate unlined spillway scour. A physical model flume test was set up to simulate rock scour represented by uniformly sized polyethylene cubes. The flume slope and discharge were varied in the different test scenarios. The 2D mathematical model correctly predicted the extent and location of scour as observed in the laboratory tests. Temporal changes in the scour formation were also predicted with reliability. This was achieved by only calibrating the hydraulic roughness of the 2D model, and by specifying the ”rock ” particle settling velocity and material density. The simulation results were satisfactory, providing an accurate and detailed erosion prediction. From this, the mathematical modelling was validated by using a field case study. The results obtained with the mathematical model were promising for non-cohesive cases and could be applied to field prototype cases if the rock joint structure is known. This would typically apply in fault zones, where the joints would give an idea of the rock size to be used in the mathematical model. In general, where jointed rock is more massive and acts ”cohesive ”, rock parameters describing critical scour conditions for the rock in terms of stream power are required to be built into the mathematical models. / AFRIKAANSE OPSOMMING: Die uitskuringsproses van onbelynde oorlope is ’n belangrike navorsingsonderwerp in die ingenieurspraktyk. In Suid-Afrika word ernstige erosie in baie onbelynde oorlope van damme ondervind. In sommige gevalle het dit gelei tot die duur belyning van oorlope met beton, om die oorlope te beskerm. Aan die anderkant kan die erosie van onbelynde oorlope lei tot groot skade en selfs tot die faling van ’n dam. Dit kan weer lei tot skade aan eiendom, infrastruktuur en die omgewing, en die publiek in die gevaar stel. Daarom is dit nodig dat daar besin word oor metodes om erosie in onbelynde oorlope te voorspel, sodat die risiko bestuur kan word en om te sorg vir die beter ontwerp van onbelynde oorlope in die toekoms. Dit is moeilik om die uitskuringsproses te voorspel, maar empiriese formules is bestaan om die aanvang van uitskuring te voorspel. Hierdie empiriese metodes voorspel egter nie die snelheid waarteen die uitskuring sal plaasvind of die uiteindelike mate waartoe dit sal gebeur nie. Die hoofdoelwit van hierdie studie was om die toepasbaarheid van ’n nie- kohesie, twee-dimensionele (2D) hidrodinamiese wiskundige model te ondersoek, om sodoende die uitskuring van onbelynde oorlope te simuleer. ’n Fisiese model om die uitskuring van rots te simuleer is ook gebou. Die rots is deur polietileen blokkies van dieselfde grootte gemodelleer. Verskillende kanaalhellings en deurstromings is in verskillende toetse gebruik. Die 2D wiskundige model het volgens die waarnemings in laboratorium toetse, die mate en ligging van die uitskuring korrek voorspel. Veranderinge wat met verloop van tyd in die uitskuring formasie plaasgevind het, is ook betroubaar voorspel. Dit is gedoen deur die hidrouliese ruheid van die 2D model te kalibreer en deur te spesifiseer hoe vinnig die ”rots ” deeljies afsak en wat die digtheid van die materiaal is. Die uitslag van die simulasie was bevredigend en het die erosie akkuraat en in detail voorspel. Die wiskundige modellering is gevalideer deur middel van ’n gevallestudie. Die uitslae wat met die wiskundige model verkry is, is belowend, en geld vir nie-kohesie gevalle. Dit kan op prototipe gevalle in die veld toegepas word as die rots se naatstruktuur bekend is. Dit kan toegepas word in foutsones waar die nate ’n aanduiding sal gee van die grootte van ”rotse ” wat in die wiskundige model gebruik moet word. Maar as die rots baie groot is en die kohesie goed is, is dit nodig om meer parameters betreffende uitskuringstoestande gekoppel aan stroomdrywing, in die wiskundige model te gebruik.