High-head Spillways Research Articles

Pressure reduction in plunge pools by triangular wedges at high-head overfall spillways

Overfall spillways are important hydraulic structures for the release of overtopping water flows downstream of a dam. To reduce the effects of the impact of falling water in plunge pools, an overfall spillway outlet design that produces a longitudinally spreading jet is required. This study examined the effects of a triangular wedge placed at the end of an overfall spillway to deflect the water flow in the lateral direction, making the jet longitudinally diffuse before reaching the plunge pool. Laboratory data on the mean pressure coefficient were used to analyse the effects of various design parameters. An increase in the deflection ratio and a decrease in the weir crest length reduced the jet impact pressure. Compared with the impact of a rectangular jet falling into a plunge pool, an appropriate triangular wedge structure was found to improve the spreading of the impingement jet and reduce the mean dynamic pressure coefficient. By optimising the geometric design to accomplish jet impact control, the results of the present investigation may help engineers to design overfall spillways in high-head dam projects.

Aeration performance of high-head siphon-shaft spillways by CFD models

Siphon-shaft spillways are constituted by covering above a shaft spillway with a hood that creates siphonic pressure. This study focused on the aeration the flow through the aerator holes placed on the hood to prevent cavitational damage in high-head siphon-shaft spillways. Three-dimensional computational fluid dynamics (CFD) technique using finite-volume method to solve Reynolds-averaged Navier–Stokes (RANS) equations for the incompressible viscous and turbulent fluids motion was performed to analyze the full-scaled two-phase numerical models. The volume of fluid (VOF) scheme was used to simulate two-phase (water–air) flow, by defining the volume fraction for each of the fluids throughout the solution domain. The accuracy of the numerical model was tested using a procedure recommended by American Society of Mechanical Engineers (ASME) for CFD applications. The numerical results showed that the aeration is highly effective in reducing siphon sub-pressures and cavitation. The optimal relative aeration diameter of 0.45 provided sufficient air entrainment to protect from cavitation and did not decrease the discharge performance too much.

A Comparative Assessment of Various Turbulence Models Applied for Simulation of Air-Water Flow Over Chute Spillway

Chute aerators have been largely used to reduce cavitation hazard in high head spillways. There is no definite turbulence model for simulating these devices in smooth spillways in spite of its importance in critical conditions. A simulation in two-phase air-water chute flow and its aerator with five different turbulence models (RNG, Standard and Realizable k–ε Models, SST and Standards k–ω Models) has been numerically investigated by Fluent software. Finite Volume and VOF methods were used for discretization of flow equations and free surface modeling. Flow depth, velocity and bottom pressure comparison were made along with the air cavity length determination by numerical, experimental and reference equations. The best model with the minimum value of error percentage for flow depth and velocity was RNG k–ε turbulence model. The realizable and RNG k–ε turbulence models showed better results for the pressure head at the bottom of the chute. The RNG k–ε model results for the jet length have a very slight difference with the experimental results. The length of the cavity is closely associated with the flow emergence angle θ’ over aerators. The bottom air concentration of spillway chute simulated by all the turbulence models, except for the RNG k–ε model, can be overestimated and therefore may affect the designing of aerator geometry.

Risk Analysis of Cavitation in Hydraulic Structures

Damages as a result of cavitation are ever existing problems in high-head spillways. This article presents a brief introduction to the problems associated with high-head spillways and describes a procedure that takes into account the flow near the chute surface where the phenomenon occurs, instead of the mean flow. Application of risk analysis methodology presented can be used to estimate whether there is a possibility of cavitation or there is necessity to determine where the first aereator should be located. The advantage of the procedure herein presented takes into consideration an important new variable, such as the surface roughness. In addition, emphasis should be placed first on the need to investigate other high-head dams which climate change may have exposed them to this kind of damages. Second, there is a need to make measurement in prototype. Finally, to underscore the importance of these problems, a comment is made concerning the Oroville Dam contingency in the state of California, (United States).

Cavitation in swirling flows of hydraulic spillways

During operation of high-head hydraulic spillway systems, cavitation phenomena often occur, leading to destruction of structural elements of their flow conductor portions. The article is devoted to the study of erosion due to cavitation in the circulation flows of eddy hydraulic spillways, including those equipped with counter-vortex flow energy dissipators. Cavitation destructive effects depend on many factors: intensity consisting in the rate of decrease in the volume or mass of a cavitating body per unit of time, the stage of cavitation, geometric configuration of the streamlined body, the content of air in water, the flow rate, the type of material. The objective of the study consisted in determination of cavitation impacts in circulating (swirling) water flows. The studies were conducted by a method of physical modeling using high-head research installations. Distribution of amplitudes of pulses of shock cavitation impact is obtained according to the frequency of their occurrence depending on the flow velocity, the swirl angle, the height of the cavitating drop wall and the stage of cavitation. The impact energy depending on the stage of cavitation and the flow rate is given for different operating modes of the counter-vortex flow energy dissipators of a hydraulic spillway. In the conclusions, it is noted that cavitation impacts in the circulation flows occur mainly inside the flow, which is a fundamental difference from similar processes in axial flows.

Flow kinematics with oppositely rotating coaxial layers

In modern technological processes, liquid and gas flows play an important role. This is especially mostly evident in hydraulic engineering and hydropower construction when designing the highly efficient hydraulic turbines, various spillway systems and other elements of layout patterns of structures. When designing and constructing such facilities, it is necessary to know the characteristics of the flows to be passed through these structures in order to correctly take into account the various types of impacts from the flow: pressure pulsations, cavitation and erosion due to cavitation, wave formation and abrasive wear. When designing high-head spillway structures, special attention is paid to the presence of excess energy from the stream entering the lower pool, which requires special measures to dissipate it. Nowadays, in the practice of construction of hydraulic structures, swirled flows are used including those with oppositely rotating layers. This type of flow creates a number of effects that allow one to radically solve the problems of energy dissipation of high-velocity flows. The report presents the results of studies of a complicated flow formed by oppositely rotating coaxially arranged layers of liquid made by a physical method. The description of the experimental stand for the model studies of such flows is given. The basic parameters of the installation, the principle of operation of the recording equipment and its main characteristics are given. The results of model studies in the form of distribution of the components of the flow velocity along the cross section of the flow conductor and its length are given.

Mitigating cavitation on high head orifice spillways

ABSTRACT A large hydropower potential lies in the Himalayan region due to perennial rivers. North east India, especially Arunachal Pradesh, is blessed with Brahmaputra and its tributaries which makes it a rich hydropower potential state. Heavy sediment laden rivers are the main challenge to tap hydropower in this region. Orifice spillways are the obvious choice as they serve the dual function of flood disposal and flushing of sediment through the reservoir. The crest of the spillway is provided as near to the river bed as possible with high head over spillway crest. This arrangement, due to high head over crest, is susceptible to cavitation damage due to very high velocity flows. Cavitation damage and its prevention are of increasing concern in designing and operating high head spillways. Proper design of spillway leading to acceptable pressures for all operating conditions, improvement of spillway surface, use of cavitation resistant materials and aeration of the flow are few techniques to prevent the cavitation erosion on the spillway surface. Three high head spillway case studies where the head over spillways is more than 50 m and spillway surface is prone to cavitation damage due to high velocity of the order of 30 to 50 m/s are discussed. This paper describes the choice of cavitation prevention method depending upon hydraulic and functional constraints of particular case.

The spillway design for the dam’s height over 300 meters

According the current hydropower development plan in China, numbers of hydraulic power plants with height over 300 meters will be built in the western region of China. These hydraulic power plants would be in crucial situation with the problems of high water head, huge discharge and narrow riverbed. Spillway is the most common structure in power plant which is used to drainage and flood release. According to the previous research, the velocity would be reaching to 55 m/s and the discharge can reach to 300 m3/s.m during spillway operation in the dam height over 300 m. The high velocity and discharge in the spillway may have the problems such as atomization nearby, slides on the side slope and river bank, Vibration on the pier, hydraulic jump, cavitation and the negative pressure on the spill way surface. All these problems may cause great disasters for both project and society. This paper proposes a novel method for flood release on high water head spillway which is named Rumei hydropower spillway located in the western region of China. This paper has following components: Determining the type and the layout of the spillway through the perspectives of air entrainment, flow jet angle and water energy dissipation in flushing pond structure. The physical hydraulic model has been used to evaluate spillway design effects.

Computer program for hydraulic calculation of structures for controlling aeration of the flow on spillways

1. An algorithm and computer program were developed for hydraulic calculation of the designs of aerators for controlling aeration of flows on a spillway. 2. A method of engineering calculation is given which makes it possible to optimize the design parameters of a system for anticavitation protection of a high-head spillway consisting of a sequence of aerators. 3. The use of the computer program increases the validity of design decisions being made about aerating anticavitation devices.

A model of a high-head bottom spillway with interaction of concentric swirled flows

1. Eddy spillways compared with, traditional schemes have a number of advantages: possibility of re-equipping temporary diversion spillways into service spillways and eliminating the construction of multilevel systems, which can effect a considerable reduction of the volumes of works and capital investments on spillways of high-head hydro developments. The need to construct large energy-dissipating structures at the tunnel outlet — stilling basins, walls, dissipators, ski-jumps, etc. — is eliminated. An improvement of the cavitation conditions in the waterway as a consequence of reducing the flow velocity can effect in some cases a saving due to a reduction of the requirements imposed on the quality of making the linings and in others can eliminate the danger of erosion of the linings and thereby reduce operating expenses. 2. The investigated spillway is intended for using conventional service vertical-lift and radial gates operating in normal free-flow regimes with a high capacity and possibility of passing trash. Its construction does not cause technological difficulties. 3. The investigated spillway system based on the effect of the interaction of concentric, oppositely swirled flows satisfies the requirements of passage and regulation of the discharge and necessary dissipation of the excess kinetic energy of the flow at a low level of fluctuation loads on the conduit walls. 4. Dissipation of the bulk of the excess energy is accomplished on a short section with a length of 1.5–2 diameters of the exit conduit, which permits making it short. This leads to convenient layouts under mountain conditions and In the case of small spillway lengths, for example, in the lower parts of gravity and arch dams. 5. The layout of the investigated spillway system can be different depending on the local conditions, restrictions are not imposed on it with respect to the design and route of the entrance and exit conduits and location of the intakes, gate chambers, hydromechanical equipment, and transition of the swirled flows. The exit conduit can be made both in pressure and free-flow variants. This permits wide variation during designing.

Use of structural anchors in underground hydraulic structures

1. The experience of using structural anchors in underground hydraulic structures shows that there are still large reserves for creating lightweight economical structural elements of underground structures. 2. A thin-walled drained lining anchored to rock is an effective and reliable structure recommended for wide use in workings being constructed in rocks with a strength coefficient fst⩾4. 3. When pressure tunnels are located near the surface (including near a slope) a passive resistance of the rock can be provided by using support bars drawing a large volume of rock into work. 4. Concrete linings with a steel lining on downstream transition sections of gate chambers of high-head spillways experiencing considerable loads from pressure fluctuations in the flow and groundwater pressure should be made with a minimum thickness (from the condition of installing the lining) and anchored to rock. 5. Steel branches (forks) of high-head underground conduits in rocks can be lightened by transmitting part of the unbalanced forces through a system of support rods to the rock mass and encasing concrete. 6. Operating experience showed that the protective anchor-grout cutoff in the foundation of the end structure of underground spillways is an economical, technological, and reliable structure. 7. In large tunnels and chambers experiencing groundwater pressure it is recommended to support the walls by a thin smoothing, drained lining and anchors installed according to two systems: deep for creating the rock walls; ordinary for fastening the smoothing lining to the rock walls. 8. For accommodating crane tracks in chambers various supports can be used depending on the characteristics of the rocks, geometry of the working, and load capacity of the crane: bar, cantilever-bar, beam-bar, and arch-bar.

High-head spillway with swirling of the flow in the outlet section

1. A high-head spillway with swirling of the flow in the outlet section has economic, layout, and operational advantages. The cost of the proposed spillway — its main route, length, and cross-sectional area — is the same as for an ordinary spillway, since it is determined by the requirement of diverting the flow at low heads around the retaining structures of the hydropower development under construction. However, the cost of the entire development with the proposed spillways is less, since it is not required to construct special energy dissipators of the flow at the spillway exist, which are necessary for ordinary spillways. Energy dissipation is accomplished inside the proposed spillway. 2. For the spillway it is not required to develop and master new complex flow-swirling hydromechanical equipment. It is specially intended to use ordinary, simple vertical-lift and radial gates whose manufacture, assembly, and operation have long been mastered, operating in a free regime normal for them with regulation of the discharge and passage of trash. 3. The spillway does not require a long shutdown for converting from a diversion regime to a service regime, since the gates can be installed during construction or discharge of the construction flow through the spillway as a consequence of locating the gates on branches at a certain distance from its route. 4. In the case when the gate chambers of the spillway (usually two) are located on different branches with the provision of counterswirling of the flow, the length of the outlet section of the spillway can be reduced to 6 diameters, which leads to convenient layouts under mountainous conditions and in the case of small lengths of the spillways, for example, in the lower parts of gravity and arch dams.

Forecast and provision of cavitation-erosion safety in high-head spillways

1. A cavitation-safe layout (design) is one for which the depth of the expected erosion is technically admissible and sufficiently clearly forecastable. In this case we mean by technical admissibility such a situation when for the worst forecast of erosion its extent does not threaten the integrity or bearing capacity of the structural elements and the damaged places can be repaired. 2. The occurrence of a vacuum and cavitation upon separation of the flow from the gate, its sill, and grooves is not admissible in a cavitation-safe layout; all such separation zones should be reliably aerated. In the case of attachment of the through-going flow to the walls (bottom) of the gate chamber below the main gate, all excess energy of the wall (bottom) part of the flow which can cause cavitation erosion on “intrinsic” cavitation sources of the linings (joints, welds, waviness and roughness of the surface) should be absorbed by a cavitation-resistant lining and dissipated by it.

Design of models for investigating dynamic phenomena in mechanical equipment of high-head spillways

A model for hydrodynamic investigations should in all details be geometrically, kinematically, and dynamically similar to the prototype structure. This means that the design of the model should duplicate the calculation and design of the prototype structure. Since a model with its gauges serves for measuring various mechanical quantities characterizing the hydrodynamic processes occurring in it, when designing and fabricating the model it is necessary to treat it as a measuring instrument, assessing and establishing the possibilities and errors of operation of each component.

Improvement of the mechanical equipment of high-head spillways

1. The wide range of computational, development, and experimental model investigations of the designs of high-pressure emergency-guard gates and main regulating gates, their movable parts and seals, hydraulic lifting gear of large capacity, gate chambers, and schemes of surplusing works performed in recent years showed the real possibility and economic expediency of manufacturing mechanical equipment for regulating large waste discharges at a head of 200 m. 2. Studies showed also the effectiveness of using the effect of swirling of the discharging flow in spillway systems. 3. Further perfection of the mechanical equipment of high-head spillways should improve the layouts, increase the precision and quality of manufacturing gates and drives, operating reliability of the seals, vibration resistance, and protection from cavitation.

Mechanical equipment of gate chambers of high-head spillways

1. For the first time in design practice Gidromontazh developed in the preliminary design stage mechanical equipment of a gate chamber of a deep spillway with vertical lift and radial regulating gates for passing discharges up to 2250 m3/sec at heads up to 200 m with an area of the outlet being closed up to 50 m2. 2. The variant with an emergency-guard slide gate with an upstream π-shaped contour of the seal located on the gate and a main slide gate with an upstream flat closed contour of the seal located on the casing was taken as the main variant when developing the specifications. 3. The high-velocity jet is diverted from elements of the downstream part of the chamber at all openings of the main gate by appropriate selection of the orifice dimensions and shapes of the lower edge of the upstream skin plate of the main gate. 4. The parameters of the developed mechanical equipment are within the limits of the technological possibilities of manufacturing palnts. 5. When designating the number of bays of the gate chamber it is advisable to strive for the creation of standard-size mechanical equipment in order to improve its quality, increase operating reliability, and increase the rate of construction works. 6. The introduction of the developed equipment on the diversion tunnels of the I and II levels of the Rogun hydrostation makes it possible to reduce the cost of the gate chambers by 3.2 million rubles and to unify the mechanical equipment of the spillways of the hydrostation. We consider it expedient to develop further the specifications of the mechanical equipment of the diversion spillways of the Rogun hydrostation in two variants: with vertical lift and radial regulating gates.

High-head spillway system with a countereddy energy dissipator

1. The complexity of creating reliably operating high-head spillways is due to the need to protect structural elements of tunnel linings from the action of cavitation and dynamic loads and to dissipate the excess energy of the flow directly inside the tunnel. In connection with this the problem of developing new efficient designs is especially urgent. 2. The use of swirled flows in spillway systems makes it possible to improve considerably the cavitation and dynamic conditions in the passageway and to create reliably operating high-head spillways. 3. One of the most promising spillway systems with swirling of the flow is the countereddy spillway system proposed and tested at MISI, which permits actually at any head reducing the velocity of the through-going flow to values of 15–20 m/sec, which are safe in a cavitation and dynamic respect. These properties of the countereddy spillways are confirmed by the results of laboratory investigations. 4. Preliminary technicoeconomic calculations show that the introduction of such a system into hydrotechnical construction practice opens fundamentally new ways to design high-head spillways and effects a considerable saving of capital investments.

Concrete for surfaces of high-head spillways

Concrete for surfaces of high-head spillways

Deformations of channels below high-head spillways on rock foundations

The main physic=mechanical properties of rocks which must be taken into account when constructing calculated relations of the maximum scouring depth are joint voids, degree of weathering of the rocks, and crushing strength of the rocks. It is necessary to take into account also the strength of the rock and settling velocity for different values of rock density and coefficients of aeration of the flow. We will take the strength of diabases as equal to unity. Then on the basis of the main physical and mechanical properties of the rocks the coefficient of rock strength K r from sandstones to diabases will vary from 0.54 to 1.0 (Table 3). We will represent the interrelated parameters in the form of curves e, auh , adh as a function of a s (Fig. 2), using for this the available laboratory and field data for spillways (Sayano-Shushenskoe, Krasnoyarsk, Bratsk, Ust-llim, Zeya, Mogilev-Podolsk, Konstantinovka, Irikla, Farkhad, etc.). For the investigated spillways and others a s varies from 16.5 = to 37.5 ~ . For these values 8 varies from 24 = to 46 ~ . With consideration of the threedimensionality of the flow angle 8 is always greater than for a two-dimensional flow and, naturally, greater than a s . It is necessary to know the relationship of a s to 8 when determining the shape and size of the local scour hole. In the prototype the angle of inclination of the upstream slope of the local scour hole for foundation rocks (from standstones to gabbro and diabases) varied from 14 ~ to 22 ~ with change in 8 from 24 = to 46 = . For individual prototype structures a s =24 =, auh =17 ~ for the / o Krasnoyarsk spillway; u s =Ii =, auh =14 = for Dneproges; =s =33~ auh =21 for the Farkhad spillway. We note that for nonrock foundations of low-head spillways auh in the prototype varies from 12 = to 14 =.

Conference on Hydraulics of High-Head Spillway Structures (GVVS-73)

Conference on Hydraulics of High-Head Spillway Structures (GVVS-73)