Vertedouros em degraus têm se tornado uma boa opção em barragens pela facilidade de construção e, principalmente, por apresentar uma dissipação significativa de energia ao longo de sua calha, o que faz com que a parcela de energia a ser dissipada por ressalto hidráulico a jusante da barragem diminua, ocasionando uma significativa redução nas dimensões da estrutura da bacia de dissipação. A aplicação de soleira terminal em bacias de dissipação provoca uma melhor distribuição das velocidades e um melhor comportamento da flutuação de pressões ao longo da bacia e a jusante, aumentando assim seu desempenho na dissipação de energia. A combinação destas duas formas de dissipação (vertedouro em degraus e bacia de dissipação com soleira terminal) pode ser utilizada como uma solução econômica e segura, entretanto, o dimensionamento dessas estruturas esbarra na falta de critérios e informações, principalmente quanto ao tamanho e posição ideal para o projeto de uma soleira terminal. No presente trabalho foram analisados os dados de pressões médias e flutuações de pressões no interior da bacia e a jusante da soleira terminal, sendo apresentadas metodologias para dimensionamento de tamanho e para posicionamento de uma soleira em função do número de Froude da entrada do ressalto hidráulico. Também está sendo apresentada uma metodologia para análise das pressões médias em bacia do tipo I (sem soleira), além de uma comparação do comportamento dos esforços em bacia com soleira e sem soleira, com vertedouro em degraus e vertedouro de calha lisa. Os dados utilizados foram obtidos em modelo experimental, construído no Laboratório de Obras Hidráulicas (LOH) do Instituto de Pesquisas Hidráulicas (IPH-UFRGS), através de transdutores de pressões fixados junto ao fundo do canal de ensaios para diversas vazões, considerando, além de bacia tipo I, diferentes tamanhos de soleira terminal, situadas em diferentes posições do canal. / Stepped spillways have become a good option in dams for ease of construction and especially to present a significant dissipation of energy throughout the spillway chute, which causes that the parcel of energy to be dissipated by hydraulic jump downstream of dam decreases, causing a significant reduction in the dimensions of the stilling basin structure. The application of end sill in stilling basins causes a better distribution of speeds and better behavior of the fluctuation of pressure along the basin and downstream, thus increasing their performance in energy dissipation. The combination of these two forms of dissipation (stepped spillway and stilling basin with end sill) can be used as an economical and safe solution, however, the sizing of these structures hindered by a lack of criteria and information, especially regarding the ideal size and position for design of an end sill. In this study the data mean pressures and pressure fluctuations within the basin and downstream of the end sill were analyzed, being presented methodologies for design the size and position of a sill as a function of the Froude number of the input of the hydraulic jump. A methodology for analysis of mean pressure in type I basin (without sill) is also being presented, and a comparison of the behavior of efforts in basin with and without end sill, with stepped spillway and flat spillway chute. The data were obtained in an experimental model built at the Laboratory of Hydraulic Works (LOH) at the Institute of Hydraulic Research (IPH-UFRGS), through pressure transducers attached at the bottom of the test channel for various flow rates, considering, in addition of type I basin, different sizes of end sill at different positions of the channel.

Neste trabalho apresenta-se uma avaliação do estado da arte de aspectos hidráulicos relacionados aos vertedores em degraus submetidos aos diferentes regimes de escoamento. Em uma segunda parte, é sugerida uma metodologia adimensional e simplificada para o pré-dimensionamento do comprimento de bacias de dissipação por ressalto hidráulico, além de uma abordagem conceitual voltada ao escoamento sobre um degrau. Entre os tópicos tratados na avaliação do estado da arte, pode-se citar, por exemplo, a dissipação de energia promovida pelos degraus, o risco de cavitação, a aeração do escoamento, o uso de aeradores de fundo e geometrias não convencionais. Quanto à metodologia desenvolvida, fundamentada na segunda lei do movimento de Newton associada à equação de Darcy-Weisbach, apresenta-se algumas comparações com dados experimentais de diferentes pesquisadores, além de exemplos de aplicação. Foi possível concluir, com a avaliação do estado da arte, que há um interesse crescente pelo conhecimento das características hidráulicas de vertedores em degraus. Através da metodologia desenvolvida, graças às comparações com dados experimentais de diferentes pesquisadores, foi possível concluir que não há um consenso absoluto sobre a magnitude do fator de resistência de Darcy-Weisbach. Com os exemplos de aplicação apresentados, notou-se que para um mesmo problema, o uso de diferentes métodos pode conduzir a projetos significativamente diferentes. / This work presents a state-of-the-art evaluation of aspects related hydraulic to stepped spillways submitted to the different flow regimes. In a second part, it is suggested a dimensionless and simplified methodology for preliminary design of the stilling basin length, besides a conceptual approach related to the free fall hydraulics. Among topics treaties in the state-of-the-art evaluation, it can cite, for example, the energy dissipation promoted by the steps, incipient cavitation, the air entrainment, the use of bottom aerator and unconventional geometries. Regarding the developed methodology, based in the Newton\'s law of motion associate to Darcy-Weisbach equation, it presents some comparisons with experimental data of different researchers, besides application examples. It was possible to conclude, with the state-of-the-art evaluation, that there is an increasing interest for hydraulics characteristic of stepped spillways knowledge. Through the developed methodology, after comparisons with experimental data of different researchers, was possible to conclude that there is not an absolute consensus about the Darcy-Weisbach friction factor magnitude. With the application examples, it noticed that for a same problem, the different methods use can lead for significantly different designs.

Problems with erosion in unlined spillways of dams, have caused environmental and safety hazards, and necessitate the development of reliable scientific methods to assess the erodibility of the bedrock material. In an attempt to analyze the causes and effects of different geological, geotechnical, hydraulic, hydrological and engineering design parameters on erosion of unlined spillways, a number of dam spillways in the R.S.A. were studied. All the parameters considered important for the erosion resistance of the rock were collected and correlated with hydraulic factors and observed extent of erosion. Rock type, strength, weathering and uniformity of the geological conditions are important geotechnical considerations while velocity and energy of the water flow are the main hydraulic parameters. The main consideration of the study was the effect of the discontinuities. All the properties of the jointing of the rock mass, such as number of joint sets, RQD, joint spacing, joint separation, joint orientation, roughness and filling material, were surveyed. Joints. weaken the rock mass and induce removal of the rock blocks if the joint properties are unfavourable to stability. The jointed bedrock is much less resistant to flowing or falling water, the result being an extensive and quickly formed scour. Various rock mass classification systems have been applied and a reasonable correlation between rock class and extent of erosion was established by means of slightly modified Kirsten (1982) and Weaver (1975) rippability classifications. A number of methods for the prevention and repair of erosion damage have been proposed. / Dissertation (MSc)--University of Pretoria, 1990. / gm2014 / Geology / unrestricted

Thesis (MScEng (Civil Engineering))--University of Stellenbosch, 2006. / Stepped spillways constructed of roller compacted concrete (RCC) is a hydraulic and cost effective measure to dissipate energy of large water flows over the spillway of a dam. Stepped spillways, like other spillway types, have its limitations and a measure to improve the energy dissipation effectiveness is proposed. Two hydraulic models were constructed at the hydraulics laboratory of the Department of Water Affairs and Forestry (DWAF) in Pretoria, South Africa. The scales of the models were 1:20 and 1:30. The study proposes the use of triangular protrusions applied over the spillway surface. The protrusions have the same height as the steps, with the width of the protrusions varying. The triangular protrusions deflect the water sideways resulting in higher energy losses. The results indicate that the protrusions reduce scouring at the toe of the dam, thus increasing the roughness of the steps. It also indicate that aeration occur earlier than with normal stepped spillways. An optimal spacing, lateral and across the steps, are proposed. The construction of the protrusions is also discussed, as well as the cost implications. It is concluded that the protrusions are effective at a unit discharge up to 35 m3/s.m. This value is however dependent on the configuration of the apron downstream of the toe of the dam. It is proposed that protrusions be added on the downstream face of the dam on every second step, with one protrusion and then no protrusion alternating in the flow path. It is recommended that the protrusions be cast in situ.

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.

Thesis (MSc)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: A trial closure of the emergency gate of the Berg River Dam was undertaken by the Trans- Caledon Tunnel Authority (TCTA) on 12 June 2008. The air vent downstream of the emergency gate was designed to introduce air to mitigate the negative pressures that were expected in the conduit during emergency gate operations. The emergency gate has to close when the radial gate at the downstream end of the outlet conduit fails. Contrary to the theoretical design, the measured air vent velocities in the field indicated that, while the emergency gate was closing, very large volumes of air were apparently continuously being released from the air vent, commencing when the gate was about 30% closed (i.e. 70% open). This is in contrast to what the design intended, namely that air should have been drawn into the vent. This thesis is concerned with the testing of a 1:14.066 physical model representing the outlet works and air vent of the Berg River Dam as a means to determine the reasons for the release of large volumes of air from the air vent during the trial closure in 2008. It also seeks solutions to mitigate the excessive airflow from the air vent. It was concluded that the air velocity in the air vent was independent of the rate of closure of the emergency gate, but to increase with increasing water head. The problem at the Berg River Dam was determined to be one of air blowback. Modifications were made to the configuration of the model in order to determine whether the configuration of the outlet works caused air to be released from the air vent. It was determined that the downward sloping roof at the outlet of the conduit, used to accommodate the radial gate chamber, was the cause of the air blowback phenomenon. An additional air vent was fitted directly onto the conduit at the constriction was found to be ineffective in reducing the air blowback. It was concluded that there are no rational structural change that can prevent or inhibit a recurrence of the blowback phenomenon in the Berg River Dam outlet conduit. The recommendation follows that the outlet conduit should not be constricted by any structural or mechanism further downstream in the conduit. / AFRIKAANSE OPSOMMING: ʼn Toetssluiting van die noodsluis van die Bergrivierdam is op 12 Junie 2008 deur die TCTA (Trans-Caledon Tunnel Authority) uitgevoer. Die lugskag stroomaf van die noodsluis is ontwerp om lug in te voer om die verwagte negatiewe drukke tydens die noodsluissluiting te beperk. Die noodsluis moet sluit indien die radiaalsluis aan die einde van die uitlaatpyp sou faal. In teenstelling met die teoretiese ontwerp, het die gemete lugsnelhede in die lugskag in die veld aangedui dat groot volumes lug voortdurend uit die lugskag vrygelaat word wanneer die noodsluis ongeveer 30% toe is (dit wil sê 70% oop). Dit is in teenstelling met die ontwerp, want die lugskag is ontwerp vir die insuig van lug. Hierdie tesis het ten doel om die redes vir die vrylating van groot volumes lug uit die lugskag vas te stel met behulp van ʼn 1:14.066 fisiese skaalmodel van die uitlaatwerke en lugskag van die Bergrivierdam soos getoets tydens die inwydingstoetssluiting in 2008. Die toetse op die model het getoon dat die lugsnelheid in die lugskag onafhankik van die sluistoemaak tyd is, maar verhoog met die toename in die watervlak. Die Bergrivier dam probleem was bepaal as die van lug terugslag. Die model is gewysig ten einde te bepaal of die spesifieke samestelling van die uitlaatwerke die oorsaak van die vrystelling van lug uit die lugskag is. Die analises en verandering aan die uitleg toon aan dat die skuins afwaartse dak van die uitlaattonnel om die radiaalsluiskamer te huisves die rede was vir die vrylating van die lug uit die lugskag. ‘n Addisionele lugskag was gebou in die dak van die uitlaattonnel reg bo die sametrekking, maar was oneffektief om die terug vloei van lug te verminder. Die gevolgtrekking is dat daar geen rasionele strukturele verandering aangebring kan word aan die Bergrivier dam om die vrystelling van lug uit die lugskag te verhoed of te verminder nie. ’n Aanbeveling vir toekomstige ontwerpe is dus dat die uitlaattonnel nie beperkend by die uitlaatend moet wees nie.

This study reviews the current knowledge and state of the practice for handling floating debris for dam safety. This report is a guidance to improve the understanding of risks of floating debris for dam safety and of countermeasures for risk reduction. The strengths and limitations of current approaches related to dam vulnerability assessment and handling strategies are discussed and recommendations are provided. Several countries experienced issues with floating debris which often led to severe damages to  dam spillways. High and extreme flood events could be responsible for similar incidents in Sweden which requires to systematically assess dam vulnerability to floating debris and examine suitable countermeasures. A vulnerability assessment involves investigating the potential for debris production, for debris transport to the dam facility and for blockage and drawdown at spillways. Various concepts for reducing the vulnerability of dam spillways to floating debris were presented in the technical literature. Countermeasures for floating debris management based on a river perspective approach involve controlling the debris yield produced in a catchment, the interception of floating debris in tributaries and reservoirs and measures taken at dam spillways to facilitate floating debris passage. Floating debris management often requires opting for a combination of suitable and cost-efficient measures rather than a single line of defense.

A inserção de ar em escoamentos de altas velocidades ao longo de estruturas hidráulicas é uma técnica bastante eficiente para prevenir a cavitação. A sua importância é majorada quando se considera os custos econômicos e as questões de segurança que estão associadas à estabilidade de uma barragem. No presente trabalho são apresentados equacionamentos para quantificar a entrada de ar em vertedores através de aeradores de fundo. Essas equações foram obtidas utilizando princípios físicos de conservação de massa, energia e quantidade de movimento nos escoamentos de ar e água que ocorrem no aerador, permitindo organizar informações advindas de cada fase. Ressalta-se que buscou-se tornar o equacionamento independente da subpressão do jato, uma vez que esta subpressão é um parâmetro de difícil determinação a priori pelo projetista. Entretanto, toda formulação é direcionada justamente para melhor representar este parâmetro utilizando os princípios físicos clássicos e as variáveis decorrentes da sua utilização. Buscou-se a validação de modelos teóricos obtidos por meio de formulações baseadas nas leis de conservação de massa, quantidade de movimento e energia para aeradores de fundo. Neste contexto, comparam-se os resultados dos equacionamentos propostos e os dados experimentais encontrados na literatura, tendo se verificado boas correlações. Este tipo de quantificação essencialmente teórica de incorporação do ar em aeradores de fundo ainda é raro e o presente trabalho visa contribuir na validação de modelos com estas características. Adicionalmente, efetua-se a comparação com as equações empíricas e semi-empíricas encontradas na literatura. A experiência adquirida na área mostra que esta é a forma mais adequada de abordar o problema. / The introduction of air in flows around bottom aerators in spillways of dams is an efficient technique to prevent cavitation. Its importance is increased when one considers the costs involved and the safety issues that are associated with the stability of a large dam. Equations are presented in this study to quantify the air inlet through bed aerators in flows along spillways. The equations were obtained using the physical principles of conservation of mass, energy and momentum in both the flows of air and water in the aerator, allowing to organize the information obtained from each phase. It was possible to show the parameters that are relevant for quantifying the induced air flow in bed aerators. In addition, a comparison was conducted between the equations resulting from this analysis and empirical and semi-empirical expressions found in the literature. It is noteworthy to mention that one of the objectives of this study was to obtain a final equation independent of the relative pressure under the jet, since this low pressure is a parameter difficult to determine a priori by the designer. However, the entire formulation was directed precisely to better represent this parameter using the principles of classical physics and the variables arising from their use. The experience acquired in this area shows that this is the most appropriate way to address this problem.

The development flow region of stepped chutes was studied experimentally. Three configuration of chute bed slopes 3.5H:1V, 5H:1V, and 10H:1V were used to study the flow characteristics. Each model had five horizontal steps and with constant step height of 15 cm. Constant temperature anemometry was used to investigate the velocity field characteristics as well as local void fraction. Pressure transducers were used to examine the pressure distribution. The conditions of aerated and non-aerated cavity were studied. It was found that the temperature anemometry is a valuable tool in the study of water flow problems due to its good spatial and temporal resolution. It is recommended that the constant overheat ratio procedure should be used in dealing with non-isothermal water flows. Flow conditions along the development flow region were found to be quite complex with abrupt changes between steps depending whether or not the flow jet has disintegrated. The flow on this region does not resemble a drop structure and after the first step, the step cavity condition does not affect the flow parameters. Pressure distribution was also found to be complex. It was found that there are no conclusive pressure profiles either on the step treads nor on step risers. No correlation was observed with the values of pool depth. The instantaneous characteristics of the velocity field along the jet of a drop structure were also studied. It was concluded that the cavity condition does not affect the velocity field of the sliding jet. The shear stress layer at the jet/pool interface was quantified. / May 2006

The development flow region of stepped chutes was studied experimentally. Three configuration of chute bed slopes 3.5H:1V, 5H:1V, and 10H:1V were used to study the flow characteristics. Each model had five horizontal steps and with constant step height of 15 cm. Constant temperature anemometry was used to investigate the velocity field characteristics as well as local void fraction. Pressure transducers were used to examine the pressure distribution. The conditions of aerated and non-aerated cavity were studied. It was found that the temperature anemometry is a valuable tool in the study of water flow problems due to its good spatial and temporal resolution. It is recommended that the constant overheat ratio procedure should be used in dealing with non-isothermal water flows. Flow conditions along the development flow region were found to be quite complex with abrupt changes between steps depending whether or not the flow jet has disintegrated. The flow on this region does not resemble a drop structure and after the first step, the step cavity condition does not affect the flow parameters. Pressure distribution was also found to be complex. It was found that there are no conclusive pressure profiles either on the step treads nor on step risers. No correlation was observed with the values of pool depth. The instantaneous characteristics of the velocity field along the jet of a drop structure were also studied. It was concluded that the cavity condition does not affect the velocity field of the sliding jet. The shear stress layer at the jet/pool interface was quantified.