High dam scour hole geometry prediction for fully developed jets plunging into shallow pools on bedrock

Bosman, Adele (2021-03)

Thesis (PhD)--Stellenbosch University, 2021.

Thesis

ENGLISH ABSTRACT: The safety of dams is often endangered by rock scour formation near the dam’s foundation due to a high-velocity free falling jet from the spillway plunging into the plunge pool. The prediction of the scour hole geometry (ultimate depth and maximum extent) is essential in the hydraulic design of high head dams to ensure the stability of the dam. Rock scour is a complex physical phenomenon that is normally predicted by analytical-empirical formulae and methods, which combine laboratory and prototype observations. The empirical formulae however, cannot describe all the hydrodynamic and geo-mechanic effects involved in rock scour. Despite extensive research since the 1950s, presently there is no universally agreed method to accurately predict the equilibrium scour hole dimensions caused by plunging jets at dams. The main purpose of the research is to contribute to the knowledge on the prediction of the equilibrium scour hole geometry downstream of a high head dam. The rock scour process was investigated via physical and numerical models, as well as by evaluation of scour prediction methods from literature. The physical model investigated the hydrodynamic effects of a plunging jet and the subsequent scour of a rectangular, open-ended jointed, movable rock bed. Equilibrium scour hole geometries for various fissured dimensions (simulated with tightly hand-packed rectangular concrete blocks), for a range of flow rates, dam heights, plunge pool depths, rock sizes, and joint structure orientation scenarios were experimentally established with 31 model tests. The experimental results indicated that greater scour occurs if the deposited rocks downstream of the scour hole are removed by floodwaters. For the first time the transient pressures at the joint opening due to a high-velocity plunging jet were measured for a movable bed with a complex joint structure. Additionally, the research studied the scour hole geometry formed by a low-frequency turbulent jet issuing from a rectangular horizontal canal and not discharging from a nozzle. The dynamic pressures at the water-rock interface and the velocity distribution of the wall jet in the plunge pool were recorded and evaluated. Non-dimensional formulae were developed using ordinary least squares regression analysis on the physical model results. The experimental scour results from this study were compared to various analytical methods found in literature. The scour prediction methods yielded a wide range of scour depths for the same input conditions. No single analytical method is superior, but the equilibrium scour hole depth established in this study best agrees with that predicted by the Critical Pressure method, followed by the Erodibility Index Method and Mason and Arumugam’s empirical formula. A three-dimensional, multi-phase, transient numerical model, in combination with the developed scour depth regression formula, was used to simulate the equilibrium scour hole geometry. The numerical model was calibrated against the physical model results. The numerical simulation results were satisfactory and representative of the model pressures in the open-ended joints. The proposed three-dimensional numerical model, in conjunction with the scour depth regression formula developed in this study, is capable of simulating the scour hole geometry, which includes both hydrodynamic and geo-mechanical rock scour aspects, if the diffusion of the jet through the air and plunge pool is modelled accurately. Further research is required to improve the numerical simulations to automatically calculate the deformation of a movable bed using the approach developed by this research.

AFRIKAANSE OPSOMMING: Die veiligheid van damme word dikwels in gedrang gebring deur die rotsuitskuring naby die dam se fondament as gevolg van ʼn hoë-snelheid vryvallende waterstraal vanaf die damoorloop tot in die plonspoel. Die voorspelling van die uitskuurgatgeometrie (maksimum diepte en dimensies) is belangrik vir die hidrouliese ontwerp van hoë damme om die stabiliteit van die dam te verseker. Rotsuitskuring is ʼn komplekse fisiese proses wat normaalweg voorspel word deur analitiese-empiriese formules en metodes, wat laboratorium- en prototipe-waarnemings kombineer. Die empiriese formules is nie in staat om al die hidrodinamiese en geo-meganiese effekte wat rotsuitkuring veroorsaak aan te spreek nie. Ondanks vele navorsing sedert die 1950s, is daar tans geen universele metode om die ewewig uitskuurgatdimensies deur ʼn vryvallende waterstraal vanaf ʼn damoorloop te voorspel nie. Die hoofdoel van die navorsing is om ʼn bydrae te lewer tot die beskikbare kennis om die ewewig uitskuurgatgeometrie stroomaf van ʼn hoë dam te voorspel. Die rotsuitskuringproses is ondersoek deur fisiese en numeriese modelle, sowel as die evaluasie van rotsuitskuring voorspelling metodes ontwikkel in voorafgaande studies. Die fisiese model het die hidrodinamiese eienskappe van ʼn vryvallende waterstraal en die uitskuring van ʼn reghoekige, oop-eindigende nate, beweegbare rotsbed ondersoek. Ewewig uitskuringsgatvorms vir verskillende naat dimensies (gemodelleer met reghoekige betonblokke styf teen mekaar gepak met die hand) is eksperimenteel bepaal met 31 modeltoetse vir ʼn reeks vloeitempos, damhoogtes, plonspoeldieptes, rotsblokgroottes, en naat oriëntasies. Die eksperimentele resultate het aangedui dat ʼn groter uitskurigsgat sal vorm indien die gedeponeerde stene stroomaf van die gat deur vloedwaters verwyder word. Die dinamiese druk ondervind by die opening van die nate as gevolg van die vryvallende waterstraal is gemeet vir ʼn beweegbare bed met ʼn komplekse naatstruktuur. Verder het die navorsing bygedra deur die uitskuurgatgeometrie gevorm deur ʼn lae-frekwensie turbulente waterstraal vanaf ʼn horisontale kanaal en nie vanaf ʼn ronde spuitstuk nie te bestudeer. Die dinamiese druk by die water-rots-oppervlak en die snelheidsverspreiding van die grensstraal in die plonspoel is aangeteken en ondersoek. Dimensielose formules is ontwikkel met die hulp van liniêre regressie analise gebaseer op die fisiese model resultate. v Die eksperimentele uitskuurresultate van die studie is met verskillende analitiese metodes van voorafgaande studies vergelyk. Die uitskuurresultate van die voorafgaande studies het ʼnverskeidenheid van dieptes vir dieselfde inset parameters gelewer. Geen enkele analitiese metode is by uitstek die beste nie, maar die uitskuringdiepte resultate van die huidige studie stem die beste ooreen met diè van die Kritiese Durk-metode, gevolg deur die Erodeerbaarheidsindeks-metode en die empiriese formule van Mason en Arumugam. ʼn Drie-dimensionele, multi-fase, numeriese model vir onbestendige vloeitoestande, in kombinasie met die ontwikkelde regressieformule vir diepteuitskuring, is gebruik om die uitskuring van die ewewigsgat na te boots. Die numeriese model is gekalibreer teen die resultate van die fisiese model. Die numeriese simulasieresultate was bevredigend en verteenwoordigend van die dinamiese druk ondervind by die oop-eindigende rotsnate in die fisiese model. Die voorgestelde drie-dimensionele numeriese model in kombinasie met die diepte-regressie-formule wat ontwikkel is in hierdie studie, is in staat om die uitskuurgatvorm te simuleer, wat beide die hidrodinamiese en geo-meganiese eienskappe van rotsuitskuring insluit, mits die diffusie van die waterstraal deur die lug en plonspoel akkuraat gemodeleer word. Verdere navorsing is nodig om die numeriese simulasies te verbeter deur die vervorming van die bed outomaties te bereken deur middel van ʼn beweegbare bed volgens die benadering wat in hierdie navorsing ontwikkel is.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/109793
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