Stepped Spillways Research Articles

Numerical simulation to evaluate the effect of the stepped chute on abrasion erosion of a stilling basin type III

Hundreds of concrete cubic meters may be ruined due to the abrasion erosion in several strategic hydraulic structures. Such unfavorable action can cause serious dangers, impose high maintenance costs, and reduce these structures’ life span. The high-velocity water flow, which carries large amounts of sediments, is the main cause of this phenomenon. This study was directed to evaluate the possibility of minimizing the abrasion action by changing the hydraulic structure design criteria; a numerical simulation was conducted depending on the FLOW3D program. It aimed to evaluate the effect of using a stepped spillway chute on the applied water pressure and the flow velocity along the type III stilling basin. The numerical simulation results showed that using a stepped chute instead of the traditional smooth chute provided a better water pressure distribution and reduced the maximum applied pressure by approximately 28.8 to 68.2%. Moreover, it also reduced the flow velocity along the stilling basin. Thus, the stepped chute use can be considered a possible solution to reduce abrasion erosion hazards in hydraulic structures.

Comparative Study of Various Methods for Computation of Energy Dissipation over Stepped Spillway.(Dept.C)

Stepped spillways have become popular with the method of construction adopted for roller Compacted concrete dams. The steps provided on the surface of open spillways help to increase the rate of energy dissipation taking place on the spillway face to reduce the size of stilling basin generally provided at the toe of spillway. The regime of flow over stepped spillway can be either skimming or nappe type. This paper describes the various methods available for computation of energy dissipation over stepped spillway for both the regimes. The results obtained by these methods are compared with the available experimental data, which would help in selecting more accurate method for computation of energy dissipation.

Nappe Flow Characteristics over Stepped Spill Ways.(Dept.C)

Stepped spillways are structurally stable, resistance to water loads, and efficient. energy dissipators, but there are no clear design rules for such structures to date. In the present study, the flow characteristics over stepped spillways were investigated for three different step configurations and for different flow parameters, using physical models. Nappe flow regime over the stepped spillways was studied. A definite equation representing the onset of the nappe flow regime was developed using the experimental observations and experimental data of other researchers. In addition, the energy dissipation for the napp e flow regime was extensively investigated for different flow hydraulic parameters. The experimental observations were compared to empirical and analytical formulas of other researches. Finally, using the experimental data in addition to that of other researchers, analytical equation: have been developed and it can be used for computation purposes of the energy dissipation of nappe flow regime over stepped spillways.

Energy dissipation in stepped spillways using baffled stilling basins

ABSTRACT Stepped spillways have become increasingly popular due to the low-cost of construction and high efficiency. A stepped spillway is a hydraulic structure that is an integral part of the dam that allows the safe passage of overtopping flows. In this paper, an experimental study was employed to investigate the stepped spillway with four steps of 32 cm height and 54 cm width contracted by wing wall width (C) = 0.2 from the width of the flume. Eighteen runs were carried out considering different positions, heights, and widths of the baffle to increase the energy dissipation resulting from this contraction stepped spillway. The results indicated that the best position of baffles was (a/y2 ) = 1.4, the relative baffles height was (hb/y1 ) = 0.95, and the best relative baffles widths (wb/W) = 0.68, compared with cases of contraction stepped spillway without baffles. The results showed that the baffles on the stilling basin downstream stepped spillway increase the energy dissipation. Simple formulae were also deduced to evaluate the energy parameters.

Influence of aeration induced by piers on the starting position of the flow aeration and extreme pressures in stepped spillways

ABSTRACT The operation of stepped spillways is limited by a range of discharges due to the risk of occurrence of the cavitation phenomenon and erosion on its steps. Since there is a demand for spillways with the possibility of overflow of greater discharges, the designs seek to increase the air concentration of the flow, which can occur through the installation of piers in the spillway in order to protect the structure from the above mentioned damage. The aim of this work is to analyze flow characteristics and extreme minimum and maximum pressures with non-exceedance probability of 0.1% and 99.9% acting next to the step edges of the spillway with aeration induced by piers through an experimental analysis in a physical model. Based on the results obtained, flow behavior was defined and equations for predicting the extreme pressures that occur along the stepped spillway with aeration induced by piers were proposed.

Induced aeration flow over stepped spillways: mean pressures, air entrainment and flow behavior

ABSTRACT Stepped spillways can dissipate a great amount of energy during the flow passage over the chute, however these structures have limited operation due to the risk of cavitation damage. The induced aeration may protect the concrete chute through the air concentration near the channel bottom. Furthermore, some research studies have indicated that the presence of air in flows may reduce the mean pressures. The present research aims to analyze mean pressures, air entrainment coefficient and flow behavior over a stepped spillway with aeration induced by two different deflectors, comparing the results to natural aeration flow. Despite the jet impact influence, the induced aeration does not change significantly the mean pressures compared to natural aeration flow. The air entrainment coefficient, as well as the jet impact position, is higher for the deflector with the longer extension and, although air bubbles can be seen throughout the extension of the chute due to the air entrainment through the inferior flow surface, the induced aeration did not anticipate the boundary layer inception point position.

Stepped spillways with aerators: hydrodynamic pressures and air entrainment

ABSTRACT The possibility of damage due to the phenomenon of cavitation leads the design of stepped spillways considering maximum specific discharges of 15 to 30 m2/s, a limit considerably lower than that practiced on smooth chutes. Aerators promote the insertion of air in the flow, allowing for the increase of specific flow rates. This work analyzes the pressures on the vertical faces of the steps and the air entrainment coefficient in the flow, through an experimental study in a physical model with a stepped chute angle of 53.13o, considering the installation of aerators in different places of the channel. Comparing the tested conditions with the natural aeration, it is concluded that the installation of the aerator does not change the magnitude of the minimum extreme hydrodynamic pressures, but anticipates the beginning of the flow aeration. A new equation is proposed to estimate the air entrainment coefficient, as well as a methodology for forecasting extreme pressures on the vertical faces of the steps, both valid in the range 3.0 ≤ Fr ≤ 6.0.

Hydraulics of Stepped Spillways Having a Prismatic Rectangular Profile and Horizontal Steps

Hydraulics of Stepped Spillways Having a Prismatic Rectangular Profile and Horizontal Steps

Numerical Study to Evaluate the Performance of Nonuniform Stepped Spillway Using ANSYS-CFX

The main features that attract hydraulic engineers for designing stepped spillways are their ability to lose a large portion of the flow energy and add or increase aeration to the flow naturally. Hence, smaller size stilling basin and no aeration device may require. This study aims to find the amount of energy dissipation rate and the location of inception point over non-uniform stepped spillway. The numerical 2D ANSYS-CFX code is applied to generate and run thirty-two models of different configurations using two different moderate slopes (1 V:2 H and 1 V:2.5 H) as most of the downstream slopes designed for moderate slope, and two different step heights (hs= 0.08 m and hs= 0.016 m) under skimming flow discharge for different (dc/hs) ranging from dc/hs= 1–2.2, in which dc is the critical flow deptho n the crest. The volume of fluid is implemented and the renormalized group of k-ɛ turbulence model is activated. The computational results demonstrated that the amount of energy dissipation increases with decreasing the flow discharge, chute slope, and step height. In addition, it is observed that the length of the inception point is directly proportional to the discharge and inversely proportional to both the chute slopes and step height. Moreover, for the design point of view, the results revealed that configuration B can be considered as the optimal one amongst the others examined herein.

Simulation of self-aerated flows by switching interface closures

This work presents a numerical study of a skimming flow regime in a stepped spillway structure, which is a complex two-phase flow with self-aeration phenomenon. The numerical models employed consist in a hybrid approach capable of switching between interface resolving and interface modelling closures on a local level, and a state-of-the-art volume-of-fluid (VOF) method. A new switch criterion for the hybrid approach is proposed, allowing calculations on irregular grid cells. A detached eddy simulation (DES) turbulence model is selected in order to directly resolve the turbulent flow structures triggering the onset of self-aeration. The main flow properties computed in the non-aerated and aerated region are confronted with experimental measurements. Compared to the VOF method, the hybrid approach provides an overall better prediction of time-averaged air concentration, highlighting the positive outcome of modelling sub-grid interface structures.

Numerical study of energy dissipation and block barriers in stepped spillways

Abstract In this research, the accuracy of the Flow-3D numerical model in the flow simulation in a stepped spillway was probed using data obtained from the physical model. In addition, the effects of block barriers on the energy dissipation rate were investigated. To adopt a proper turbulent model, Renormalization Group k-ε, RNG k-ε, and standard k-ε models were employed. Then, the Flow-3D was run in five discharges for nine spillways with the ratios of block length to step length (Lb/l) and block height to step height (Hb/h) as 0.3, 0.4, and 0.5. The results indicated that both turbulent models had almost similar outcomes though the run time of the RNG k-ε model was shorter. The blocks with a shorter length in low ratios of Hb/h and the lengthier blocks in high ratios of Hb/h undergo more relative energy dissipation relative to the no-block situation. For Hb/h = 0.3 and Lb/l equal to 0.3, 0.4, and 0.5, the relative energy dissipation climbed on average as 8.5, 6.5, and 4.5% respectively, compared with the no-block case. The most influence exerted on relative energy dissipation was obtained via the blocks with Hb/h = Lb/l equal to 0.3 and 0.5 with respective increases of 8.6 and 8.4%.

Effect of 30-Degree Sloping Smooth and Stepped Chute Approach Flow on the Performance of a Classical Stilling Basin

Advances in dam construction techniques have significantly increased the number of stepped spillways implemented worldwide. Although stepped chutes provide enhanced energy dissipation along the chute, as compared to smooth chutes, an adequate energy dissipater is usually needed at their toe to govern the remaining energy. Stilling basins downstream of stepped spillways are currently designed using the approaches developed for smooth chutes. As a stepped surface alters the structure of the approaching flow, such practice is questionable. This paper reports a study on the effect of stepped chute approach flows on the performance of a classical stilling basin. Physical modeling was conducted using a large-scale facility of a smooth and stepped spillway with a 30° sloping chute. Experiments were performed under different discharges, two step heights, and variable approach flow aeration. The characteristics of the hydraulic jump were described, focusing mainly on flow depth, bottom pressure, and length. The results indicated a significant effect of stepped chute approach flows on bottom pressure and length of the hydraulic jump. Near the jump toe, pronounced fluctuating and extreme pressures were observed after stepped chutes and attributed to the higher turbulence level of the incoming flow. The normalized hydraulic jump lengths were found to be about 17% longer downstream of stepped chutes as compared to smooth chutes.

Numerical Simulation of Flow over Stepped Spillways with varying Step-Angle

In the present study, the flow over the stepped spillway was ‎ numerically investigated by using Flow3D model. The effect of step angle on different properties of Nappe flow regime such as the water surface profile, location of free-surface‎ aeration inception, Froude number at the ‎spillway’s toe, and pressure, flow velocity, air concentration and cavitation index were evaluated. The realizable k–ε was applied as the turbulence ‎model, and Volume of Fluid (VOF) model was used to determine the free surface flow profiles of the spillway. ‎The model was verified using experimental data. In order to investigate the different ‎characteristics of Nappe flow regime, 17 numerical runs was designed, in which, ‎four step angles, four flow discharge were considered to investigate the flow characteristics over the stepped spillway. The results indicated that the numerical model is well suited with the experimental data over the stepped spillway (RMSE = 0.147 and ARE = 6.9%). In addition, with increasing the step angles, the aeration inception point is generally moved ‎downstream. By increasing the step angles from zero to 10 degrees, the Froude number does not change ‎significantly, however, at the angle of 15 degrees, the Froude number decreases by about 42 ‎percent.

Numerical investigation of flow characteristics over stepped spillways

Abstract Spillways are constructed to evacuate flood discharge safely so that a flood wave does not overtop the dam body. There are different types of spillways, with the ogee type being the conventional one. A stepped spillway is an example of a nonconventional spillway. The turbulent flow over a stepped spillway was studied numerically by using the Flow-3D package. Different fluid flow characteristics such as longitudinal flow velocity, temperature distribution, density and chemical concentration can be well simulated by Flow-3D. In this study, the influence of slope changes on flow characteristics such as air entrainment, velocity distribution and dynamic pressures distribution over a stepped spillway was modelled by Flow-3D. The results from the numerical model were compared with an experimental study done by others in the literature. Two models of a stepped spillway with different discharge for each model were simulated. The turbulent flow in the experimental model was simulated by the Renormalized Group (RNG) turbulence scheme in the numerical model. A good agreement was achieved between the numerical results and the observed ones, which are exhibited in terms of graphics and statistical tables.

Robust estimators for free surface turbulence characterization: a stepped spillway application

Robust estimators are parameters insensitive to the presence of outliers. However, they presume the shape of the variables’ probability density function. This study exemplifies the sensitivity of turbulent quantities to the use of classic and robust estimators and the presence of outliers in turbulent flow depth time series. A wide range of turbulence quantities was analysed based upon a stepped spillway case study, using flow depths sampled with Acoustic Displacement Meters as the flow variable of interest. The studied parameters include: the expected free surface level, the expected fluctuation intensity, the depth skewness, the autocorrelation timescales, the vertical velocity fluctuation intensity, the perturbations celerity and the one-dimensional free surface turbulence spectrum. Three levels of filtering were utilised prior to applying classic and robust estimators, showing that comparable robustness can be obtained either using classic estimators together with an intermediate filtering technique or using robust estimators instead, without any filtering technique.

Effects of Inclination Angles on Stepped Chute Flows

Owing to its effective energy dissipation and aeration, a stepped spillway is commonly used for flood release in hydraulic projects. Its conventional design features horizontal step surfaces. Designed for certain flow rates, it does not function satisfactorily at larger discharges. To improve this, layouts with inclined step surfaces, both downward and upward, are proposed. Computational fluid dynamics (CFD) modelling in 3D is performed to examine and compare their flow properties in the skimming flow. The results suggest that a shift from a downward to an upward layout leads to a gradual decrease in the flow velocity at the chute end; the latter exhibit higher energy dissipation efficiency. Moreover, equations are developed to estimate the velocity and energy loss. The flow velocity in the developing zone, described by a power law, shows a decline with an increase in the angle of inclination. The downward layout is subjected to somewhat higher risk of cavitation if implemented in a prototype. The extreme pressure loads acting upon an upward layout are larger, and a correlation is proposed for its prediction. On an inclined surface, either upward or downward, the pressure demonstrates an S-shaped distribution. On a vertical surface, the flow pressure increases, after an initial decline over a short distance, towards the chute bottom.

Tree-based nonlinear ensemble technique to predict energy dissipation in stepped spillways

In this study, under the skimming flow regime, energy dissipation was investigated with data-driven methods. The data set obtained from the laboratory experiments were modelled by different machine learning (ML) methods, including support vector machine (SVM), K-star (K*) algorithm and artificial neural networks (ANN). Afterwards, for the first time in the literature, linear and nonlinear ensemble models were established in order to improve the accuracy of single models in predicting energy dissipation. Simple average (SA-E) and weighted average (WA-E) were performed as linear ensemble models while M5 Model Tree (M5-MTE) and Random Forest (RF-E) were used to establish non-linear ensemble models. The model results were evaluated according to performance metrics, such as Coefficient of Correlation (CC), Percent Bias (PBIAS), Performance Index (PI), Willmott’s index of agreement (WI) and Nash-Sutcliffe efficiency criteria (NSE). The NSE values are calculated as 0.986, 0.909 and 0.985 for SVM, K* and ANN models, respectively. Moreover, for the ensemble models, higher NSE values were obtained for both linear (NSESA-E = 0.9887, NSEWA-E = 0.9916) and tree-based non-linear (NSEM5 MT-E = 0.9963, NSERF-E = 0.9974) models. Overall, it can be stated that tree-based ensemble models make better predictions for energy dissipation calculation in step spilways compared to single ML methods.

Energy dissipation evaluation for stepped spillway using a fuzzy inference system

In this study, to evaluate the effects of geometry parameters on the dissipation rate in flows over the stepped spillways, a numerical model is provided and validated by using a physical model. A type of Fuzzy Inference System (FIS) is used to study the control of the rates of dissipation. It is used to measure the estimated energy dissipation under different scenarios, and the results are compared with prepared numerical database. An error criterion is used to examine the prediction accuracy of the FIS measurements. The realization of these kinds of considerations may, in turn, lead to a remarkable reduction in the dimensions of stilling basins. The results indicate that proposed FIS can be used as a valuable tool for operational control of dissipater structures with consideration of different geometric parameters. The method is proposed somehow to consider the uncertainty problems with consideration of the benefits, and some shortcomings noted in the literature.

Investigating skimming flow conditions over stepped spillways using particle image velocimetry

Turbulent flow over stepped spillways can be considered the most significant flow that can cause severe problems at the downstream side, near the toe of the structure, such as sediment erosion which normally occurs due to the high amount of water energy. The presence of turbulent flow over the steps can cause cavitation damages due to pressure differences over the steps. The turbulent flow, which is induced at certain times of the year, especially during the flooding seasons, is examined experimentally and numerically in this study. Flow measurements were conducted using Particle Image Velocimetry (PIV) system in a hydraulic flume where the dam break condition is applied in order to achieve the skimming turbulent flow. Two cases of stepped spillways were tested, normal stepped spillways and gabion stepped spillways. For each case, measurements of the instantaneous turbulent velocity field were taken at different locations of the physical models of a slope of (1V:2H). A comparison has been conducted between the gabion and normal steps to assess the required time to attach skimming flow. The results indicated that the presence of the porous media could increase the required time to attach skimming flow.

Computational Study to Predict the Free-Surface Flow over Non-uniform Stepped Spillway Using ANSYS-CFX

Using stepped chutes as a structure for controlling flood discharges is applicable for long time. Measuring the depth of flow over that structure is essential for designing of the side walls. The aim of this paper is to determine the free-surface that flows on spillway equipped with non-uniform step sizes. For that purpose, the two-dimensional software package code of ANSYS-CFX has been utilized to run eight configurations of two moderate slopes (1V:2H and 1V:2.5H) and for four different discharges 1≤dc/hs≤2.2 to determine the effect of flow discharges, chute slopes, and step heights on the position of free surface along the structure over non-uniform stepped cascade. The hexahedral grid size of 0.015 m is selected with inflation technique close to the walls. In addition, the renormalized group of k-ε (RNG) turbulence model is implemented and the numerical volume of fluid software is employed. The results show smoother stream for higher discharges, and the free-surface drops when the slope of chutes increases. Moreover, it is found that the step size has insignificant effect on the depth of water. The results of this study are important because they provide new insight in improving the design of stepped spillways. It is recommended to perform more investigations to evaluate their effectiveness in other flow parameters including pressure distribution and energy dissipation rates.