Smooth Spillways Research Articles

Hydraulic characteristics of flow over stepped and chute spillways (case study: Zirdan Dam)

AbstractStepped spillways are suitable and economical options for high-storage dams that require significant energy dissipation, structural adaptation to roller-compacted concrete dams, and spontaneous flow aeration. In this study, a numerical analysis of the hydraulic characteristics for the skimming flow regime of the stepped spillway of Zirdan Dam is carried out. A comparison between the flow characteristics for stepped and smooth spillways is provided. By preparing numerical models of the spillway using the k-ε RNG turbulence model and the multiphase mixture method, a hydraulic analysis of the flow was completed. To verify the performance of the numerical model, field data were collected and used for validation. The results show that the presence of steps along the spillway cause a significant reduction in the length of the boundary layer and faster aeration occurs. In a stepped spillway, the cavitation index is higher than the critical limit along the entire length of the spillway. Thus, the risk of cavitation and destruction is negligible. On the other hand, with a smooth spillway, the possibility of cavitation may occur. This negative effect (negative pressures) occurs 56 m from the crest of the spillway on the downstream side.

Оценка влияния зигзагообразной шероховатости на кинематические параметры потока

The research was carried out in order to assess the energy dissipation efficiency of zigzag elements of artificial roughness installed on the spillway face of medium- and low-pressure dams. The flow characteristics were evaluated downstream on a spillway dam model with a free flow entry to the spillway. The experiments were carried out in two stages. The first stage was to measure the flow parameters in the downstream with a smooth spillway face of the dam. The second stage provided for the placement of zigzag elements of artificial roughness in the form of a double zigzag on the spillway face of the dam. Evaluation of the effectiveness of the artificial roughness was carried out by comparing the controlled flow parameters in the same section of the transit section of the spillway from the downstream side with both considered design schemes. It has been established that the device of artificial roughness in the form of a double zigzag on the spillway face of the dam leads to a decrease in the second conjugate flow depth behind the structures and allows creating additional head losses, thereby facilitating measures to dissipate excess flow energy in the downstream. Evaluation of the effectiveness of the artificial roughness was carried out by comparing the controlled flow parameters in the same section of the transit section of the spillway from the downstream side with both considered design schemes. placement of zigzag elements on the water slope. It has been established that the device of artificial roughness in the form of a double zigzag on the spillway face of the dam leads to a decrease in the second conjugate flow depth behind the structures and allows creating a It is shown that the installation of a zgizag roughness on the spillway face of mediumand low-pressure dams changes the kinematic characteristics of the outlet flow and significantly reduces the second conjugate depth, which also favorably affects the parameters of the channel erosion behind the structure, reducing them.

Оценка нормативных расчетных экстремальных характеристик стока при использовании различных законов распределения

The research was carried out in order to assess the energy dissipation efficiency of zigzag elements of artificial roughness installed on the spillway face of medium- and low-pressure dams. The flow characteristics were evaluated downstream on a spillway dam model with a free flow entry to the spillway. The experiments were carried out in two stages. The first stage was to measure the flow parameters in the downstream with a smooth spillway face of the dam. The second stage provided for the placement of zigzag elements of artificial roughness in the form of a double zigzag on the spillway face of the dam. Evaluation of the effectiveness of the artificial roughness was carried out by comparing the controlled flow parameters in the same section of the transit section of the spillway from the downstream side with both considered design schemes. It has been established that the device of artificial roughness in the form of a double zigzag on the spillway face of the dam leads to a decrease in the second conjugate flow depth behind the structures and allows creating additional head losses, thereby facilitating measures to dissipate excess flow energy in the downstream. Evaluation of the effectiveness of the artificial roughness was carried out by comparing the controlled flow parameters in the same section of the transit section of the spillway from the downstream side with both considered design schemes. placement of zigzag elements on the water slope. It has been established that the device of artificial roughness in the form of a double zigzag on the spillway face of the dam leads to a decrease in the second conjugate flow depth behind the structures and allows creating a It is shown that the installation of a zgizag roughness on the spillway face of mediumand low-pressure dams changes the kinematic characteristics of the outlet flow and significantly reduces the second conjugate depth, which also favorably affects the parameters of the channel erosion behind the structure, reducing them.

Genetic algorithms for optimizing stepped spillways to maximize energy dissipation

Abstract Stepped spillways are important water-management structures that are used for energy dissipation. Use of these spillways has increased in recent decades, they can reduce construction time and they are effective for reducing the flow's downstream kinetic energy. In this study, the width and height of the steps as well as the slope and height of the overflow spillway were considered as variables. Due to the large number of variables, non-linearity of the objective function and constraints, and the lack of an explicit relationship between decision variables, a genetic algorithm (GA) was used. A stepped spillway with optimal dimensions was proposed as a replacement of the smooth spillway of Sarogh Dam located in West Azerbaijan province, Iran. The proposed steps increase energy dissipation; for constant discharge and varying slopes, the changes in the optimal height of the steps were insignificant. Sensitivity analysis using the objective function showed that the relative energy dissipation for a constant discharge is independent of the optimal height of the steps and decreases with increasing spillway slope. In addition, for fixed slopes, increasing the flow rate leads to a decrease in relative energy dissipation and an increase in the optimal height of the steps.

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.

Discussion of “Energy Dissipation of a Type III Basin under Design and Adverse Conditions for Stepped and Smooth Spillways” by D. Valero, D. B. Bung, and B. M. Crookston

Discussion of “Energy Dissipation of a Type III Basin under Design and Adverse Conditions for Stepped and Smooth Spillways” by D. Valero, D. B. Bung, and B. M. Crookston

Closure to “Energy Dissipation of a Type III Basin under Design and Adverse Conditions for Stepped and Smooth Spillways” by D. Valero, D. B. Bung, and B. M. Crookston

Closure to “Energy Dissipation of a Type III Basin under Design and Adverse Conditions for Stepped and Smooth Spillways” by D. Valero, D. B. Bung, and B. M. Crookston

Experimental Investigation of Artificial Aeration on a Smooth Spillway with a Crest Pier

Crest piers placed on overflow spillways induce standing waves at the downstream end of them and the supercritical flow expands after flowing past the rear of the pier. The expanding flow from each side of a pier will intersect and form disturbances or shock waves that travel laterally as they move downstream and eventually reach the chute sidewalls. Recently, investigations regarding crest piers are related with artificial aeration on stepped spillways to eliminate the risk of cavitation damage. However, there is a lack of studies on standing and shock waves in smooth spillways concerning the air entrainment into the flow in presence of crest piers. This paper presents the study of the combined effect on air entrainment of a crest pier and an aerator on the bottom of a smooth spillway (configuration 1). For comparison, experimental tests were developed in the spillway without pier, that is in presence of aerator only (configuration 2). The configuration 1 results show that the air concentration distribution on the spillway bottom across the width and length of the chute increases in comparison with configuration 2, reducing even more the risk of cavitation damage and enhancing the safety of the hydraulic structure.

Mortality and injury rates for small fish passing over three diversion dam spillway models

Effects of low-head diversion dam spillways on survival and injury of fish passing downstream are poorly understood, especially for fragile, early life stages. The ubiquitous nature of diversion dams in waterways worldwide led us to assess survival and injury rates for small and large size-classes (nominal mean total lengths 25 and 50 mm, respectively) of morphologically general fishes: fathead minnow Pimephales promelas; hybrid rainbow × cutthroat trout (Oncorhynchus mykiss × O. clarkii, hereafter trout); and small razorback sucker Xyrauchen texanus. Fish were released in three laboratory spillway models: a 5.7-m-high, free-impinging overfall spillway (0.023 m3·s−1·m−1) with three stilling basin depths; a 5.4-m-high, smooth-faced ogee-shaped spillway; and the ogee-shaped spillway fitted with energy dissipation steps. Ogee-shaped spillway flows were 0.012–0.24 m3·s−1·m−1. Mean survival proportion, corrected for handling mortality, was high in all models (0.97–1.0) for all species, size-classes, and flow conditions, except in the free-overfall spillway with stilling basin depth of 2.5 cm, where survival was lower (0.78–0.94). Survival rates of fathead minnows and trout were similar in the stepped and smooth spillway experiments, and across all flow levels. Survival of razorback suckers was slightly lower in the stepped than the smooth spillway but differences were likely due to poor fish condition, which may have also reduced their survival at the highest flows in ogee-shaped spillway models. The free-overfall spillway caused negligible injury to small fish if receiving pool depth was 15 cm or deeper. Injury rates in the stepped spillway were slightly higher than in smooth one, but serious injuries were rare. Although mortality and injury rates to fish after passage over the spillway models were typically low, higher cumulative rates from passage over many diversions indicates managers should use structures that reduce injury and mortality rates wherever possible.

Energy Dissipation of a Type III Basin under Design and Adverse Conditions for Stepped and Smooth Spillways

AbstractNew information regarding the influence of a stepped chute on the hydraulic performance of the United States Bureau of Reclamation (Reclamation) Type III hydraulic jump stilling basin is pr...

Investigation and analysis of scour downstream of a spillway

Abstract Local scour downstream of spillways due to hydraulic jump is considered the most complicated and tedious problems which threats the overall stability of the spillway. Throughout this paper, eighty experimental runs were carried out to investigate the effect of using cylinder blocks fixed on the back slope of the spillway on the scour hole dimensions downstream of spillway under different flow conditions. Case of smooth spillway was included to estimate the influence of the considered modifications. Experiments were performed using different diameters, lengths, and arrangements of cylinder blocks with different discharges. Data collected from the experiments were analyzed and graphically introduced. Furthermore, experimental data for smooth spillway were compared with scour formulae for other researchers. Finally simple formulae were predicted to estimate the different scour and deposition parameters. Use of concrete cylinder blocks will assist engineers in the design of spillways.

PREDICTION OF DISSOLVED OXYGEN ON STEPPED SPILLWAY WITH DIFFERENT CONFIGURATION

The increase of water quality is related to the presence of dissolved oxygen. Even, the oxygen concentration in surface waters is a main indicator of the water quality for human use as well as for the aquatic biota. Air entrainment on stepped spillway is also recognised for its contribution to the oxygen transfer. The oxygen transfer on stepped spillways in skimming flow regime is increased due to earlier self-aeration and slower flow velocities in comparison to smooth spillways. This paper presents the results gained on a physical model by using a variety of different configurations of stepped spillway. The slopes of stepped spillway (θ) used are 30˚ and 45˚, the number of step (N) are 40 and 20, and two types of steps are flat steps and pooled steps. The experiments were conducted for ten Froude number (Fr) run ranging from 1.117 to 9.909. This research aimed to investigate the influence of different configuration in stepped spillway for predicting of dissolved oxygen. The results showed that the dissolved oxygen of the stepped spillway increases with an increase in chute of slope, number of step, and surface roughness on steps. The increases of Froude number as a function of discharge will cause turbulence flow becomes decreases, and the concentration of air bubble in the water will be decreased. The decreased value of turbulence flow will make dissolved oxygen level decrease. In skimming flow condition, the dissolved oxygen level decreases with increasing discharge per unit width especially for steep bed slope.

Macro-turbulent characteristcs of pressures in hydraulic jump formed downstream of a stepped spillway

ABSTRACT Stilling basins are structures built at the base of the spillway to dissipate energy, by means of a hydraulic jump. Hydraulic jump is a turbulent phenomenon that causes large pressure fluctuation in the stilling basin bottom, and can damage the sink structure through mechanisms such as fatigue, upflit pressure and cavitation. The use of stepped spillways allows the dissipation of a parcel of the energy while the water falls by the spillway, allowing a reduction in the stilling basin’s dimensions and cost. The present article presents the analysis of the longitudinal distribution of mean pressure, pressure fluctuations, skewness coefficient and kurtosis coefficient, derived from tests on physical hydraulic models. Pressure values measured in a stilling basin downstream of a stepped spillway (for Froude numbers between 5 and 8) were compared with data observed in a stilling basin downstream of a smooth spillway with a radius of concordance between the chute and the basin (for Froude numbers between 4.5 and 10). The results of these studies show that the mean pressures and the pressure fluctuation observed in the stilling basin downstream of stepped spillway present maximum values at the spillway’s closest point, differing, thus, from those at the smooth spillway. The longitudinal distribution of skewness and kurtosis coefficients enabled to define the positions for flow detachment start, roller ending and as well as the ending of the influence of the hydraulic jump over the flow.

Application of Numerical Methods in Design of Hydraulic Structures

This study was aimed to evaluate the use of numerical methods in the design of hydraulic structures and use a numerical model to validate the simulation of flow over three types of spillway which are: smooth spillway, various of step spillway, labyrinth spillway and side spillway. Numerical methods can assist in the design of hydraulic structures in the evaluation of the energy loss, calculated discharge coefficient and cavitation phenomena investigated by examining the pressure field and flow field. Designers are challenged to use numerical methods for the design of hydraulic structures due to the complexity of the specific flow field of hydraulic structures, such as the free surface flow, two-phase and multi-phase flows, turbulent flow and turbulence. In this study provide the best and efficient numerical methods for the numerical modeling of hydraulic structures, which are obtained by various researchers who have conducted research studies on numerical methods. Using the guidelines presented in this study can help designers in numerical modeling of hydraulic structures that are under all the facts and reliable models apply the results of the numerical method.Computational Fluid Dynamics (CFD) is a type of numerical model that can be used to solve problems involving fluid flow. CFD can provide a significant amount of computation time and more economical solution than a physical model. The fundamental principles for all numerical models are similar. Problems can be described, physically, by a set of partial differential equations. Then, a numerical method is used to formulate a set of algebraic equations that represent the partial differential equations.

Discharge coefficient and energy dissipation over stepped spillway under skimming flow regime

Stepped spillways have a wide range of applicability as a hydraulic structure in large dams to dissipate energy of high-velocity flows at a downstream area of dams as well as release overflows into the downstream. In this study, the numerical simulation of the flow over the stepped spillway was investigated by using Flow3D software as an analytic flow field. The RNG k-ε model was applied as the turbulence model, and Volume of Fluid (VOF) model was used to determine the free surface flow profiles. At the first stage, the model was verified by reliable experimental data. Then, in order to investigate the various features of skimming flow regime, 112 numerical spillway models was designed, which 96 models were stepped spillway models and 16 models were smooth spillway models (i.e., WES profile). In these numerical experiment, two step sizes, six configuration, four passing discharge and four profile slopes (15, 30, 45 and 60 degrees) with various relative discharges were considered to investigate the energy dissipation and discharge coefficient rates. The results indicated that discharge coefficient rate and energy dissipation rate have inverse relationship. Also it was observed that when relative discharges increased, energy dissipation rate decreased and discharge coefficient rate increased.

Discussion of “Skimming, Nonaerated Flow on Stepped Spillways over Roller Compacted Concrete Dams” by Inês Meireles, Floriana Renna, Jorge Matos, and Fabián Bombardelli

The nonaerated region may occupy a large portion of the skimming flow in steep, stepped spillways, particularly for relatively high unit flow rates. In spite of the numerous contributions on the hydraulic properties at both the inception point of air entrainment and the aerated region, much less is known regarding the flow in the nonaerated region. In this paper, new empirical evidence, based on an extensive data set obtained during several years in a large-scale facility, sheds light on the features of the nonaerated-flow region. Diverse ways to locate and estimate the main hydraulic properties at the inception point are first discussed and compared. Then, expressions capable of characterizing the main flow variables along the nonaerated region are presented, namely, the boundary-layer development, the velocity distribution, the equivalent clear-water depth, the characteristic depth taking into account the free-surface unsteadiness due to turbulence, and the energy dissipation. The energy dissipation is observed to be larger than that for smooth spillways, although much smaller than values typically reported for aerated flows at the toe of stepped spillways.

AERATION AT OVERFLOW DAMS WITH CURVED SURFACES BY DIFFERENT FLASHBOARD SPILLWAYS

Aeration using overflow dams is an eco-friendly and economical method of improving dissolved oxygen levels in polluted urban streams. Laboratory investigations of aeration performance in smooth spillways, as well as in parallel flashboard and interlaced flashboard spillways with different flashboard intervals, for overflow dams with curved surfaces have been conducted separately. Aeration efficiencies of the different types of spillways, in particular the effects of varying discharge rate, total spillway height, and flashboard interval, have been discussed in detail. The test data illustrate that aeration efficiency in all spillways increases with spillway height and decreases with increasing discharge. Flashboard spillways appear to provide significantly higher aeration efficiency than smooth ones, and aeration efficiency increases with the number of flashboards, but with a continuously declining growth rate. By combining Fds with hd*, a new dimensionless parameter Ψ is created to characterize the comprehensive effects of hydraulic conditions on aeration. Empirical formulas for oxygen-transfer efficiency in smooth, parallel flashboard, and interlaced flashboard spillways have been developed with Ψ and the dimensionless number of flashboards per unit area of spillway N* as independent variables, and the properties of these formulas are discussed.

Closure to “Skimming, Nonaerated Flow on Stepped Spillways over Roller Compacted Concrete Dams” by Inês Meireles, Floriana Renna, Jorge Matos, and Fabián Bombardelli

The nonaerated region may occupy a large portion of the skimming flow in steep, stepped spillways, particularly for relatively high unit flow rates. In spite of the numerous contributions on the hydraulic properties at both the inception point of air entrainment and the aerated region, much less is known regarding the flow in the nonaerated region. In this paper, new empirical evidence, based on an extensive data set obtained during several years in a large-scale facility, sheds light on the features of the nonaerated-flow region. Diverse ways to locate and estimate the main hydraulic properties at the inception point are first discussed and compared. Then, expressions capable of characterizing the main flow variables along the nonaerated region are presented, namely, the boundary-layer development, the velocity distribution, the equivalent clear-water depth, the characteristic depth taking into account the free-surface unsteadiness due to turbulence, and the energy dissipation. The energy dissipation is observed to be larger than that for smooth spillways, although much smaller than values typically reported for aerated flows at the toe of stepped spillways.

Model of flow over spillways by computational fluid dynamics

The objective of the present study was to simulate the flow behaviour through smooth, 25-step and 50-step spillways using a multiphase flow model with the realisable k–ε model. The simulation results were compared with the data from a large-scale physical model. A grid convergence index was used to reduce the discretisation error. The simulation results of flow along stepped spillways and the flow on the step agreed well with experimental data for both smooth and stepped spillways. The flow profiles, velocity profiles, turbulence intensity, and energy dissipation of flow through spillways are illustrated and discussed. Expressions for the turbulence intensity distributions in smooth and stepped spillways are also proposed.

Inception point of air entrainment over stepped spillways

The location of the inception point of the air entrainment directly affects the energy dissipation ratio, the cavitation damage control, and the training wall height designs for a stepped spillway and a stilling basin. In this paper, the boundary layer theory of plates is used to predict the location of the inception point of the air entrainment over the stepped spillways by assuming the steps on the spillways as a kind of roughness. An empirical formula is presented based on the physical model experiments, with the maximum error less than 1% except at one point where the error is 1.6%, as compared to the experimental data. Meanwhile, it is shown that the location of the inception point of the air entrainment for the stepped spillway is much nearer to the top of the spillway than that for a smooth spillways, which explains why the high ratio of the energy dissipation is provided for the stepped spillway.