Thesis (Ph.D.)--University of Iowa, 2006. / Supervisors: George Constantinescu, Larry J. Weber. Includes bibliographical references (leaves 288-296).

The circular wave basin provides a means of physically modeling the nearshore without the typical problems associated with end walls. Three different coastal processes were examined to demonstrate the use of a spiral wavemaker in a circular wave basin. These were longshore currents, shear waves, and groin circulation. A beach was designed and constructed to concentrate breaking in a narrow region and minimize wave reflection. Currents in the longshore direction were generated by both the motion of the wavemaker and oblique wave approach. Two methods for measuring nearshore currents were employed. First, a 3-D acoustic current meter was positioned at various locations in the cross shore and the local radial and tangential velocities were recorded. Second, a video camera was placed approximately 8 meters above the wave basin to record the motion of a ball in the nearshore. The video tape was digitized by an image processor and the motion of the ball was determined. Measurements of nearshore circulation in the circular wave basin were used to investigate longshore currents, shear waves, and groin circulation. Average measured longshore current profiles in the cross shore were compared with numerical model predictions. An analysis of the existence of shear waves in the circular wave basin was performed by calculating longshore and cross shore current spectra. Particular attention was focused on the low frequency end of the spectra where shear waves are most energetic. Model groins were placed in the circular wave basin and measured currents were compared to predicted circulation patterns. All three applications indicated that the circular wave basin is a useful device for simulating coastal processes in a laboratory environment. / Graduation date: 1992

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.

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No description available.

Thesis (MScEng)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: Groynes are hydraulic structures typically constructed from gabions to extend from the outer bank of a river channel into the main flow. The principal uses for groynes are as river training structures, and to protect the riverbank from erosion. This is done by deflecting the current away from the riverbanks, dissipating energy and creating lower flow velocity zones, reducing the effects of erosion. Groynes have been used extensively in the Western Cape since the early 1990s as a means to protect agricultural lands. There is, however an absence of published manuals for the design of groynes around bends as a means of riverbank protection. This study was done to better understand the flow, scour and sediment deposition patterns associated with implementing a series of groynes around a bend. A physical model of a typical sinusoidal South African river was constructed in the Hydraulics Laboratory at the University of Stellenbosch. The model consisted of two succesive 90˚ bends to best simulate erosion patterns. Different layout designs for a series of groynes were tested to determine the optimal design for the given situation in terms of the projection lengths of the groynes, the spacing between the groynes as a factor of the projection length, and the orientation of the groynes with regard to the oncoming flow. An integrated software package, that was developed at the National Centre for Computational Hydroscience and Engineering, at the University of Mississippi, named CCHE2D was used to simulate the physical model numerically. The model was calibrated by adjusting the Manning‟s n value of the sediment, and the normal tailwater depth was calculated and used for the physical and numerical models. The flow rate used during the physical model experiments was also used for the numerical model. The model was validated by comparing scour depths obtained from the physical model to the resulting depth in the hydrodynamic model. From the results obtained from the physical and numerical models, it was found that groynes with a perpendicular orientation to the direction of the oncoming flow were optimal. Groynes with an upstream orientation resulted in excessive scour around the nose of the groynes, while recirculating flow patterns occurred at a velocity which caused bank failure. Groynes with a downstream orientation created very little recirculation of flow, resulting in less scour protection for the outer bank than found for groynes with perpendicular orientation. Finally, the optimal design was identified as a perpendicular groyne orientation with a projection length L and a spacing of 3.5 xL. For the given design, eddy currents covered the entire area between consecutive groynes, causing low flow zones near the outer banks, promoting sediment deposition and decreasing the potential for scour. Recommendations for further studies include the investigation of the effects of different bend radii, the sediment size, as well as various channel widths, flow rates and river slopes. The use of a three-dimensional hydrodynamic model would also be advantageous in order to better understand the flow and scour mechanics associated with different designs. / AFRIKAANSE OPSOMMING: Groynes is hidrouliese strukture, tipies gebou uit skanskorwe in Suid Afrika, wat vanaf die buiteoewer van 'n rivier strek na die hoofkanaal. Groynes word hoofsaaklik aangewend as rigmure in riviere en om rivieroewers te beskerm teen erosie. Erosiebeskerming word geskep deur vloei weg te deflekteer van die rivieroewer en energie te dissipeer. Sodoende vorm areas met lae vloeisnelhede naby die oewer wat die potensiaal vir erosie verminder. Groynes is sedert die vroeë 1990s in die Wes-Kaap gebruik as 'n manier om landbougrond teen erosie te beskerm. Daar is egter 'n tekort aan gepubliseerde ontwerpriglyne vir die gebruik van groynes vir erosiebeskeriming in riviere. Hierdie studie is gedoen om die vloei-, uitskuringsen sediment neersettingspatrone, geassossiëer met die implementering van groynes, te ondersoek. 'n Fisiese model van 'n tipiese kronkelende Suid-Afrikaanse rivier is gebou in die Hidrouliese Laboratorium by die Universiteit van Stellenbosch. Die model bestaan uit twee opeenvolgende 90˚ draaie om die erosie-patrone te simuleer. Verskillende uitlegte vir 'n reeks groynes is getoets om die optimale ontwerp vas te stel in terme die projeksielengte van die groynes, die spasiëring tussen opeenvolgende groynes as 'n faktor van die projeksielengte en die oriëntasie van die groynes ten opsigte van die aankomende vloeirigting. 'n Geïntegreerde sagteware pakket wat ontwikkel is by die “National Centre for Computational Hydroscience and Engineering”, by die Universiteit van Mississippi, genaamd CCHE2D is gebruik om die fisiese eksperimente numeries te simuleer. Die model is gekalibreer deur die Manning n waarde van die sediment aan te pas, die stroomaf waterdiepte wat bereken is, en die vloeitempo wat waargeneem is tydens die fisiese eksperimente. Die model is bekragtig deur uitskuurdieptes van die fisiese- en hidrodinamiese modelle te vergelyk. Vanuit die resultate wat verkry is in die fisiese- en numeriese modelle, is daar gevind dat groynes met 'n loodregte oriëntasie met betrekking tot die aankomende vloei optimaal is. Daar is gevind dat groynes met 'n stroomop oriëntasie oormatige uitskuring rondom die neus van die groynes ondervind, terwyl hersirkulasie van vloei teen 'n hoë snelheid plaasvind, wat erosie van die oewer tot gevolg het. Groynes met 'n oriëntasie na die stroomaf kant het baie min hersirkulasie van vloei tot gevolg gehad, wat tot minder erosiebeskerming van die buitebank gelei het. Die optimale ontwerp is geïdentifiseer as groynes met loodregte oriëntasie, 'n projeksielengte van 0.675 m en 'n spasiëring van 3.5 keer die projeksielengte. Vir die gegewe ontwerp is werwelstrome gevind wat die hele area tussen opeenvolgende groynes dek, wat area van lae vloei sones veroorsaak naby die oewer, wat sediment deponering bevorder en die potensiaal vir uitskuring verminder. Aanbeveling vir verdere studies sluit die studie van die effekte van verskillende draai radiusse en die sediment-grootte in, sowel as verskillende kanaalbreedtes, vloeie en rivierhellings. Die gebruik van 'n drie-dimensionele hidrodinamiese model sal ook voordelig wees om die betrokke vloei- en uitskuringspatrone rakende verskillende ontwerpe beter te verstaan.