جستجوی مقالات مرتبط با کلیدواژه « stepped spillway » در نشریات گروه « آب و خاک »

Step spillways are an example of these massive hydraulic structures that, in addition to passing the excess water of the dams, also cause the consumption of flow energy downstream of the dams. Considering the complex hydraulics of the flow on these spillways and the presence of nonlinear limitations, their optimal design is a very difficult problem. In this study, a new framework based on Metaheuristic algorithms, including Harris's hawk Optimization (HHO), gray wolf Optimizer (GWO), invasive weeds Optimization (IWO) and water cycle Algorithm (WCA), considering the minimization of the amount of concrete used in spillway and the maximization of energy dissipation in Spillway toe were developed as objective functions to design these spillways. Algorithms' performance was first checked and validated on basic functions. Then, to achieve the objectives of the study, the spillway of the Siah Bisheh dam was selected as the study dam and the efficiency of the developed models based on the four mentioned algorithms was evaluated on it. The results showed that, in addition to improving the current spillway design in terms of construction costs and dissipation energy, the HHO-based model has good accuracy and convergence compared to other Metaheuristic algorithms. As the comparison of the design obtained from HHO with the current spillway design showed, in addition to a 35% reduction in the volume of concrete consumed, the amount of energy dissipation increased by 15%, which indicates the success of the design model developed in a multi-objective manner using HHO.

Stepped spillways are convenient and economical options for high dams compared to other types of spillways because of their special abilities such as construction procedure compatibility with the Roller Compacted Concrete Dams (RCC Dam) technology, the ability of self-aeration of flow and reduction the stilling basin dimensions at the downstream dam toe due to significant energy dissipation. Depending on the discharge rate, step height and slope, three flow regimes can be identified on stepped spillways: nappe, transition, and skimming flow. Nappe flow occurs for relatively low discharge rates and lower slopes. Transition flow with a range of intermediate flow rates has a chaotic flow motion with intense splashing. Skimming flow occurs for relatively higher discharge rates and steeper slopes, is characterized by a flow over the pseudo-bottom formed by small vortices between steps. On a stepped spillway, the steps act as macro roughness elements, contributing to enhanced energy dissipation and significant aeration. In a skimming flow, the upstream flow motion is nonaerated, and the free surface appears smooth and glossy up to the inception point of free-surface aeration. In this developing flow region, a turbulent boundary layer grows until the outer edge of the boundary layer interacts with the free surface and air entrainment takes place. In recent decades, much research has been done on various flow regimes over the stepped spillway, the way they dissipate energy and the impact of the geometry of the steps on flow structure. In this research, hydraulic performance of skimming flow over Zhaveh stepped spillway has been studied and qualitative and quantitative comparison of flow between stepped and chute spillways has been presented.

Methodology:

 A 2D numerical models of spillway has prepared using FLUENT 6.3.26 software, k-e RNG turbulence model and Mixture multiphase flow method to simulate and calculate the hydraulic characteristics of the energy dissipation of spillways. The softwares to create the spillways geometry and mesh were SOLIDWORKS and GAMBIT respectively. The meshes in the tank section were quadrilateral and due to irregular geometry and the presence of stairs, meshes in the spillway and chute section were tri / pave. The boundary conditions were velocity inlet for inlet flow, free pressure inlet for free surface flow, outlet pressure for outflow and wall for floor and stairs. The numerical model has been calibrated applying experimental data extracted from the physical model of the Zhaveh spillway. It is stepped spillway with a height of 85 m and located on the Zaveh river at the junction of the Gavrood and Gheshlagh rivers at 35 km south of Sanandaj city. The spillway with a crest length of 55 m and a side slope of 1.2V:1H (50.19 degrees) located on the body of the Zhaveh dam.

Findings:

 The results indicated in addition to agreement between laboratory and numerical data, having steps alongside the chute spillway can reduce significantly the length of the boundary layer which is developed from the spillway crest and encountered with the flow surface from where the flow air entrainment is initiated. So the inception point related to air entrainment is located further upstream. Analysing the flow pattern indicated that due to aeration after the inception point, the flow depth and velocity in stepped spillway increases and decreases respectively compared to a chute spillway. Flow aeration causes the cavitation index to become much higher than the critical value (0.2) in the entire stepped spillway thus the risk of cavitation occurrence and the relevant damages are reduced considerably. While it was observed that the possibility of cavitation occurrence was high on the chute spillway (without steps) starting from 56 m downstream of the spillway crest. As a result, cavitation erosion was more likely expected on the chute spillway surface. Also for design discharge, the flow energy was effectively dissipated alongside the stepped spillway in comparison to chute spillway with a discrepancy of 46%.

Conclusion:

 The application of the stepped spillway would be more appropriate and economical than the chute spillway due to the various advantages mentioned above and also there is no risk of cavitation on the Zaveh stepped spillway, while in the end of the chute spillway Corresponding to it, we may encounter cavitation erosion.

The cost of implementing spillways includes a large part of the dam construction budget. Optimizing these structures can significantly reduce the cost of building dams. So far, many studies have been done on optimizing the dimensions of spillways in order to reduce the amount of materials used. Among the used optimization methods, the methods based on meta-heuristic algorithms have had stunning results. The review of the studies conducted in the field of stepped and labyrinth spillways shows that the goals of all studies in stepped spillways have been to provide an acceptable plan for energy dissipation and in labyrinth spillways to reach a the largest amount of water passing. The purpose of this study is to combine stepped spillways with labyrinth spillways and optimization in order to increase the water passage coefficient and energy consumption at the same time. For this purpose, the multi-objective optimization approach has been used.

In the present study, the redesign of the spillway of the Sarouk rockfill dam located in West Azarbaijan province has been studied. In the case of labyrinth-stepped spillway, the decision variables, constraints and objective functions of the two spillways are combined and analyzed as a multi-objective optimization problem. In this study, a dynamic coefficient is used to improve the performance of the dragonfly algorithm in the search and exploitation stage. After determining the optimal values of the regulatory parameters of the MODA, NSGA-II, MOPSO and MOIDA algorithms, four benchmark examples were solved with each of them 20 times with an initial population of 50 and a maximum number of iterations equal to 100.

After validating the improved dragonfly algorithm on benchmark functions and evaluating its performance in solving multi-objective problems, this algorithm was used to design a combined spillway on the introduced dam. In multi-objective problems, there is a set of optimal solutions rather than a single optimal solution, which appear as a Pareto front. The implementation of the improved dragonfly algorithm with 200 initial population and 500 repetitions has resulted in providing 200 optimal answers on the Pareto chart. After performing the calculations related to the fuzzy decision-making method, a design with the values of the objective functions of concreting volume: 59795.54 cubic meters, dissipated energy: 75.18%, flow rate: 1255.19 cubic meters per second was selected as the optimal answer. In addition, the design with the highest and lowest cost of concrete consumption was selected as two other answers. The flow rate passing over the spillway has increased by 120% in plans A and B, and by 25% in plan C compared to the initial design flow rate. The amount of energy dissipation of plans A and B is about 76 and 75%, respectively. In plan C, where the flow rate has not increased significantly (25%), energy dissipation has reached about 81%, which is more than the other two cases.

Among the available answers, according to the priority of the cost, the plans with the lowest (C) and the highest (A) implementation cost based on the volume of concrete were selected. Also, by using the fuzzy decision-making approach, an optimal plan (B) that has a suitable balance between the three objectives was selected. Based on the results, it was found that each of the plans A, B and C can be used based on the needs of the employer. From the economic point of view, according to the project conditions and the needs of the employer, all the proposed plans are suitable options for replacing the actual spillway and reduce the implementation costs. Also, the comparison of plan C with the stepped spillway plan presented by previous researchers showed that with almost the same volume of concrete, using a multi-objective approach in comparison with a single-objective optimization approach, in addition to increasing flow energy consumption, also led to an increase in flow through the spillway.

In the present study, we investigated the effect of the location and height of a three-dimensional porous obstacle (with porosity in all three dimensions) which is located on the bottom of the steps in the width of the flume and the number of steps with this type of obstacle has been investigated on a stepped spillway.

The slope of the spillway was 1: 2 and had 8 steps, the total height was 87 cm and the width of the flume was 1.2 m. The measuring instruments in the present study were point gage, image processing technique and observations of the experiment.

In a continuous three-dimensional porous obstacle, according to the relative height, location and number of steps with an obstacle, the starting flow boundaries of the placement change compared to the flat step, so that by increasing the relative distance of the obstacle from the edge of the steps and decreasing the number of steps with continuous porous obstacle. The tendency of the flow to expand in the upper range of the transition flow (neighborhood with the skimming flow regime) increases.

In the nappe flow regime, the placement of a three-dimensional porous obstacle for the variables of the present study increases the energy dissipation by up to 5% more than in the flat step (control). For transitional and skimming flow regimes, in most cases, continuous porous obstacle placement has no positive depreciation effect. In addition, according to the size of the areas formed by BIV, it can be said that in conditions where the mixing area measured in two consecutive steps was more than the flat step, energy dissipation has also increased.

Today, gabion structures and especially gabion stepped spillways have become more popular due to the significant effect of stairs on flow energy dissipation, appropriate stability, being economic, easy implementation, and raising the oxygen level in the water. This type of weirs has more flexibility compared with its impervious type and is resistant to loads due to water pressure. The resistance to water load is likely to be related to flow passing through the porous media, and the gabion stairs can assist with the faster water drainage and reduce the water load behind the structure (Zhang & Chanson, 2016). Extensive studies have been performed on impervious stepped spillways, namely Gonzalez et al. (2016) and Zhang and Chanson (2015).Reeve et al. (2019) used a numerical model to investigate the flow hydraulic properties on gabion stepped spillway. They studied gabion stepped spillways with four different stair geometries under similar conditions. Their results indicate that flat gabion steps can dissipate more energy than overlapping, inclined, and pooled steps.Despite extensive investigations on impervious stepped spillways, there has not been sufficient research on gabion stepped spillways. Hence, the primary purpose of this study is to investigate the gabion stepped weirs features, including energy dissipation and characteristics of downstream hydraulic jump.

One of the issues that researchers have always considered in the design of storage dams is reducing the kinetic energy of flow passing through the spillways to control the velocity, reduce the destructive energy, and scour depth downstream of spillways. Stepped chutes and flip buckets are the most common energy dissipator structures in emergency spillways at dams. Regarding the effect of scour depth at downstream of these structures on their stability and safety, in the present study, the effect of hydraulic and geometric parameters on variations of scour characteristics downstream of smooth and stepped chutes was investigated experimentally. Experiments were conducted at a hydraulic lab at the University of Guilan in 2020-2021 with a ratio of critical flow depth and step height (yc/h) of 0.53-0.97, bucket exit angles of 15° and 30°, the ratio of tailwater and critical flow depth (ht/yc) ranging from 1.5–2, the ratio of height falling and step height (HF/h) varied in the range of 2-4, and a spillway slope of 1:2 (V: H). The comparison of results demonstrated that implementing steps over a smooth chute profile reduced maximum downstream scour depth of the flip bucket by 15° and 30° exit angles in comparison with a smooth chute for the minimum relative critical flow depth by 22 and 41 percent, respectively, and for the maximum relative critical flow depth, by 14 and 20 percent, respectively. Also, with an increase in the exit angle from 15° to 30°, the maximum scour depth decreases for the minimum and maximum tailwater depths by an average of 18 and 11 percent, respectively, for a smooth chute and by an average of 19 and 17 percent, respectively, for a stepped chute. Furthermore, a regression relationship was derived for predicting the scour depth at downstream of stepped chutes with the flip bucket.

Steps in the stepped spillway by creating an artificial roughening bed, dissipate the flow of energy more than other types of spillways. An increase of roughness leads to a uniform and continuous distribution energy of the flow over the spillway. The present study deals with experimental study regarding the appendance elements on the steps and its impact on the variation of the flow pattern, inception point, energy dissipation and Darcy roughness coefficient. Experiments were performed on a physical model, with appendance elements on the steps in different configurations and type (height and notch), and the results were compared with flat stepped spillway. The results showed that the appendance elements on the steps causes some the instabilities and turbulence on the center axis and sides of the steps, which causes the inception point to be moved upstream of the stepped spillway. A nappe and transition flow regimes on stepped spillways with appendance elements occur at lower discharges than flat stepped spillway. With notched, the change of the flow regime on the stepped spillway occurs later than in the height state. Also, the appendance elements on the steps have no effect on the energy dissipation and Darcy roughness coefficient. However, with notched on the appendance elements, their performance is improves and the amount of energy dissipation and Darcy roughness coefficient increases 7.33% and 8% compared to flat stepped spillway, respectively

Concerning the importance of water saving in Iran, as an arid and semi-arid country, dam construction plays a crucial role in water resources management. Spillways are one of the most important components of a dam. They are different in shape and function. Stepped spillway is one of the most designed and operated ones. Numerical simulation of the stepped spillway of Jare dam using FLOW 3D software and the geometric optimization of the steps' dimension using the multi-objective genetic algorithm is investigated in this research. The idea of using stepped spillways goes back to 3500 years ago (James et al., 2001). The oldest stepped spillway built in Iran has been recorded from 600 years ago. Studying the geometric features of stepped spillways in order to optimize the size and dimension of steps has also been the issue of interest for researchers (Chanson, 1996 and 20021; Pegram et al., 1999; Ferrari, 2010).

An experimental model of Stepped spillway of Jare Dam has been set up first in order to calibrate and verify the numerical model. Flow 3D software is applied for numeric simulation of the spillway and the multi objective genetic algorithm (NSGAII) is implemented to optimize the geometric dimensions. Calibration of the model has done after introducing the experimental models' geometry to FLOW 3D. Comparing the velocity data recorded by the numerical model and the experimental velocity data, the software has been verified.Turbulence modeling is the construction and use of a mathematical model to predict the effects of turbulence. Turbulence models are simplified constitutive equations that predict the statistical evolution of turbulent flows. K-epsilon (k-ε) turbulence model is a practical model to simulate the mean flow characteristics for turbulent flow conditions. It is a two-equation model which gives a general description of turbulence condition of the ambient flow by means of two transport equations (PDEs). The RNG model was developed using Re-Normalisation Group (RNG) methods to renormalize the Navier-Stokes equations, to monitor the effects of smaller scales of motion especially those of vertex movements. In k-ε model the eddy viscosity is determined from a single turbulence length scale, so the diffusion seen in the calculated turbulence is that which occurs only at the specified scale, although in real physical situations, all scales of motion will contribute to the turbulent diffusion especially those with more curvature streams. RNG turbulent model, as mathematical method that can be utilized to extract turbulence similar to the k- ε, results in a modified form of the epsilon equation. We have implemented both methods to simulate the turbulancd in the flow over the stepped spillway and to compare the effectiveness of both models when flow is dealing with a complicated solid as the Jare Dam spillway. Five different types have been considered for the geometry of the stepped spillway. Numbers of steps are designated 3 to 7 steps and are earmarked as the algorithm constrains. The variables are then defined and the fitness function of the algorithm is extracted. The multi objective genetic algorithm is then coded in MATLAB. In optimization procedure the geometric features including width, height and the number of steps in each five discussed type are calculated.

Velocity results using two turbulent models, RNG and K-ε, have been calculated separately. The results of the RNG model depict better match in accordance to the physical model's velocity data with less than 10 percent error. In optimization procedure the stepped spillway with 4 steps, 0.072m width (1:5) and 0.0665m height (1:5), is considered as the most optimum choice regarding the economic and hydraulic concerns.

Flow 3D software simulated the flow over the stepped spillway of Jare Dam quite acceptable. The simulating model depicted the most accuracy using the RNG turbulent model and the multi objective genetic algorithm used (NSGAII) suggested the 4 steps spillway as the most economic and functional choice for Jare stepped spillway.

The purpose of this paper is to investigate the energy dissipation in stepped spillways using numerical method with ANSYS CFX software. In this study, finite volume method based on element was applied for modeling and the SIMPLE iterative algorithm was applied to couple the velocity and pressure terms. Field of flow solution was continued until reaching to remaining with amount of 10-5. The RNG k-ε model was used to evaluate the turbulence using CFX software to model the mixing of two-phase water-air and free-surface flow called Free Surface. This research has been continuously analyzed. To perform the validation and capability of software, at first a broad-crested weir mode were modeled and the results of model were compared with the laboratory model. Then the laboratory model of stepped spillway was selected and analyzed for various discharge. Consider to results of the energy dissipation in stepped spillway and broad-crested weir and comparing with experimental results, it can be concluded that numerical method as well as laboratory method has the ability to provide acceptable results in field analysis of flow in different types of stepped spillways and their results can be used to predict flow behavior for different conditions. The results of the presented numerical model showed the appropriate efficiency of stepped overflows in energy dissipation. For the model with the ogee crest, the dissipation rate for the selected flow discharge obtained from 80 to 90% and for the model with the broad cast weir, the energy dissipation obtained from 55 to 80%.

The purpose of the design of energy dissipaters is to dissipate part of the kinetic energy of the inflowing flow in order to return safely the flow to the downstream channel or river and prevent scour below spillways, chutes and sluices. However, surveys show that despite the numerous researches, there is a lack of research on the comprehensive study of hydraulic performance of stepped spillway with curve axis under downstream transition channel slope variation. Therefore, in this study, several physical models of curve axis stepped spillway with converging training walls were made and the impact of downstream transition channel slope variation on the hydraulic characteristics of this type spillway was assessed.

This study was conducted with the aim of investigating the hydraulic performance of stepped spillway with curve axis under downstream transition channel slope variation. The experiments were carried out in a rectangular flume with length of 15 m, height of 1 m and width of 2 m. The experiments were performed at different discharge rates from 0.015 to 2.1 times the design discharge and the physical model was tested under four different transition channel slopes of m=0, m=1:33, m=1:30, and m=1:27.

The results of the experiments indicate that in the converging steeped spillway with downstream transition channel slope variation, by decreasing slope of transition channel, the discharge flood in the maximum head allowed will go up. Also, it is find that in the range of H/Hd = 0.7, the discharge coefficient in the stepped physical model for all transition slopes was less than the smooth USBR model and in the range of 0.7 < H/Hd < 1. 3 there was a good consistency with the USBR model. However, with increasing the total water head, due to the spillway submergence the discharge coefficient for all transition slopes showed a descending trend and the spillway efficiency decreased in compared with the standard USBR ogee spillway. Moreover, the results showed that the models with slope of 0 and 1:33 are two models which can pass the probable maximum flood discharge in the maximum allowable height successfully But, the model dimension of physical model with downstream transition Channel slope of m=1:33 is smaller than that of m=0.Therefore, model with slope of m=1:33 can be selected as the most efficient model.

General qualitative and quantitative results of the present study are summarized as the fallowing: 1- In the converging stepped spillway by increasing total upstream head, the discharge coefficient will go up for each of the transition Channel slope (m) and until the downstream flow is at either supercritical or critical stages, the discharge coefficient is independent of variation of transition Channel slope. By contrast, at the submergence stage for the spillway, the difference in the discharge coefficient can be due to tailwater submergence occurring in the spillway. 2- Energy dissipation over converging stepped spillway decreases with increasing the discharge ratio, but model with smaller amount of transition slope (m) lead to decline more energy dissipation in higher discharge. 3- The model with slope of m=1:33 can be selected as the best model due to it’s ability to pass the probable maximum flood in the Maximum allowable head.

A stepped spillway consists of steps that start from near the crown and continue to downstream heel. Generally, the amount of energy dissipation in stepped spillway is more than the one of flat spillways (with no steps) with the same dimensions. The high amount of energy dissipation caused by steps allows reducing the depth of drilling of downstream stilling basin, length of stilling basin, and the height of sidewalls. This way, a considerable economic saving is achieved in the implementation of dams. Since spillways help reduce flow rate and increase intake airflow rate, they can be referred to as a combination of a spillway and an energy dissipater. Some of the studies on stepped spillways are as follows:Launder and Spalding (1972); Olsen and Kjellesvig (1998); Chen et al. (2002); Tabbara et al. (2005); Dermawan et al. (2010); Sori and Mojtahedi. 2015; Haji azizi et al. (2016) So far, the studies on stepped spillways have been mostly on experimental and physical models. However, experimental and physical models are costly and sometimes have limitations such as required space and a large number of tests. This necessities consideration of further mathematical and numerical models. Several numerical models can be introduced to analyze flow on a stepped spillway. CFX is one of the numerical models. CFX has been known as one of the most robust software for fluid flow analysis and heat transfer. The finite volume method (FVM) is a numerical method to solve the governing differential equations, which is capable of simulating complexities of a turbulent flow on a spillway appropriately. On the other hand, CFX numerical model uses the coupled model, which increases the speed to achieve results considerably as compared to other numerical models. This research aims at simulating a stepped spillway using CFX numerical model and assessing the amount of energy dissipation under geometric parameters (such as spillway height, spillway gradient, number of steps, and height of steps) and hydraulic parameters (such as flow rate). The difference between this research and other studies can be attributed to the coherence and correlation in studying various components (geometric and hydraulic parameters) and the ability of CFX numerical number in replacing some of the costly and time-consuming tests.

For safe pass of flood flow from upstream to downstream of dams, the spillways are used. Among the various spillways, in stepped spillways, the flow is dissipated when passing through the structure. This will reduce the cost of downstream energy-dissipaters structures such as stilling basin. In addition to energy dissipation, the volume and cost of stepped spillways can affect its design. The best design of the stepped spillway is that the remaining energy and the volume and cost of the spillway will be minimized. Therefore, the stepped spillway design is a multi-objective optimization problem. In the present study, a two-objective simulation-optimization model based on the NSGA-II algorithm was used to minimize the possible remaining energy and spillway volume. The results showed with increasing the relative height of spillway stairs, the amount of remaining energy is increased initially and after reaching a peak point, it would be reduced to a constant value. From the perspective of volume and cost of spillway, by increasing the relative height of the spillway stairs, its volume is increased linearly and by increasing the spillway angle, its volume decreased. The results of multi-objective optimization model showed that in the current plan of Siah-Bisheh Dam spillway, the remaining energy and the volume of spillway criteria were considered well. So that 83.7 percent of the flow energy is dissipated.

A ‘spillway’ is a structure used to provide the controlled release of flood water from upstream into downstream area of a dam. As an important component of every dam, a spillway should be constructed strongly, reliably and efficiently to be used at any moment. Labyrinth and stepped spillways are presented as appropriate modifications to those spillways hardly capable of managing the maximum potential discharge. Owing to their nonlinear crests for a given width, labyrinth and stepped spillways have a larger discharge rate than linear- crest spillways at an identical height. Compared to other energy dissipaters, the combination of stepped and labyrinth spillways is known as a very strong energy dissipater. In the following part, the combination of these two structures and their dimensional change for increasing the water- energy dissipation are addressed. To conduct this study, an experimental flume with a 90- degree bend in the Islamic Azad University of Ahwaz was used. In total, 90 experiments were conducted on three different labyrinth- shape stepped spillway models with two different lengths, three different widths, and five different discharges. Analysis of the results showed a greater energy loss reduction in triangular rather than rectangular or trapezoidal labyrinth- shape stepped spillways. In addition, energy loss was greater in labyrinth spillways with two cycles than those with one cycle. Energy loss was increased by raising the Froude number from 0.05 to 0.1; in contrast, energy loss was decreased with increasing the Froude number from 0.1 to 1.0, which was due to the submergence of steps, a decrease in the roughness of steps and an increase in the intensity of aeration.

Stepped spillways are technically and economically regarded as one of the most efficient options for energy dissipation of spillway flows. A remarkable amount of the flow energy is dissipated due to the hydraulic resistance of rough elements or those steps. Therefore, the energy dissipation obviates the need to build an energy dissipation system at the end of the downstream and reduces its size to a large extent. Considering the mentioned point and the required technology to use RCC in building such spillways, it is indispensable to thoroughly examine the hydraulic processes and the flows passing through them. The flows passing through the stepped spillways are divided into three types of nappe, transition, and skimming. Most studies concerned with stepped spillways have focused on nappe and skimming flows, while transition flows have not been fully addressed. Moreover, most of the stepped spillways’ designs have been based on nappe and skimming flows types. Therefore, the present study employed laboratory methods to examine the flow formation in transition mode in order to determine the upper and lower limit of the transition flow regime, and finally, provides more space for studying its characteristics. The flow regimes’ type depends on the step discharge and geometrical shape. Due to variations in the depth and velocity of flow, air concentration and energy dissipation in the three different regimes, it is essential to evaluate the flow in the regimes. Thus, the flow regimes’ onset has been the subject of many laboratory studies. The previous experiments and studies have shown that the nappe flow onset is a function of the height and length of step and also, the critical depth. This research was conducted in Water Research Center of Power Administration in order to determine the range of transition flow regime and hydraulic parameters of various flow regimes, including dynamic pressure at the steps’ bottom and the depth of flow passing over the steps as a flow profile. To do so, spillways’ models of Siahbisheh dams were used, which were constructed with the scale of 1:15 and 1:20. The flow will be fully developed, after passing through the spillway crest and the aeration inception point. The upper and lower limits of the transition flow regime were determined by letting different discharges pass over the spillways. To detect the nappe flows’ entrance into the transition flows, i.e. the lower limit of transition flow regime, the inception of water spray can be used. The water spray level will be higher in sloped spillways. Moreover, the flow jet has more fluctuations in the transition flow regimes of pool; in other words, it will be in immersion mode. In addition, some flow rotation works can be noticed. The criterion for determining the threshold for the onset of skimming flow, i.e. the upper limit of transition flow, is the flow slipping from the edge of one step onto the next. It is worth mentioning that existence or lack of air holes does not play any role in proving this. This is in contrast to Chanson’s theory; however, it is in line with Chamani and Rajaratnam’s view. The onset of flow rotation in the pool below the step was also taken into account; these rotations are not still complete enough to enter the skimming flow section. After determining the flow regimes, the dynamic pressure and profile of passing flow were measured in transition regimes. In order to measure the pressure exerted on the steps’ bottom, 3 piezometers at (y/l), which are equal to 0.14, 0.45, and 0.71, were installed on four spillways. Transducer system adapters, computer systems and an information processing software were used in order to measure the dynamic pressure moment-by-moment. The pressure sensors used in the experiment were 0.150 kg/〖Cm〗^2 (150 mbar). The pressure fluctuations in each piezometer were recorded during 30 seconds with the frequency of 100 hertz. To measure the flow profile, without accounting for water spray, a point gauge was used. The results of the study revealed that the increase in the spillway slope reduces the area for occurrence of transition flow regime. Moreover, as the ratio of h/l increase the ratio of y_c/h decreases, which shows faster formation of a skimming flow. In the nappe flow regime, it is noticed that pressure increases from the step bottom to the edge, where it moves with a slight slope to the middle of the step and then, continues with a steep slope. In the nappe flow regime, increasing the spillway slope, increases the pressure from the step bottom toward the edge. Moreover, the maximum and minimum pressures occur near the step edge. With respect to the transition flow regime, the pressure remains almost constant from the step bottom up to y/l=0.4 and then continues with a steep slope toward the step edge. In addition, in the transition flow regime, the maximum and minimum pressures occur on the step edge. In the skimming flow regime, the mean pressure increases from the step bottom toward theedge. This increasing trend has a direct influence on the spillway slope. The maximum pressure occurs in the middle of the step and then decreases with a slight slope. Similarly, the minimum pressure occurs in the middle of the step and then, increases toward the step edge. Finally, in the skimming flow regime, increasing the spillway slope is followed by a decrease in the maximum pressure.

To pass the excess water and floodwaters from upstream to downstream of the dams, a structure called "spillway" is used. This structure is vital and integral as they should be ready for operation at any moment. Stepped spillways are introduced as a viable option for improvement of spillways facing problem when flowing the possible maximum flow rate. Stepped spillways consist of stairs which start near the crown and continue to lower heels. Increase of roughness leads to a uniform and continuous distribution energy of the flow over the spillway. This is of great benefit for designers as there is no longer need to the toe of the spillway to create facilities reducing flow energy such as stilling basins. In this study, to increase the roughness on the stepped spillway, barriers are used to increase the energy dissipation. For experimental tests, a flume with a 90 degree bend was used in Islamic Azad University of Ahvaz (IAU-A) and several different types of barriers on the stepped spillway in three shapes with three different lengths and widths as well as using barriers individually and in combination, with 5 different flow rates, a total of 140 tests were performed. After analyzing the results, it was found that in the stepped spillway combined with respectively a triangular, rectangular, and trapezoidal barrier, a decrease in dissipation and energy loss is observed. Triangular barriers, on average, increase the energy consumption by 15.9%, rectangular barriers, on the average, increase the energy consumption by 13.7% and tipping barriers by an average 11.2% increase in energy depreciation compared to the control model. An increase in the length and width of the barriers results in an increase in dissipation and energy loss. The two-step barriers have the highest energy dissipation and loss. By combining barriers on the two-stair stepped spillway, there is an average of 14.4 percent increase in energy dissipation. Based on the observations, rise in the Froude number from 0.32 to 1.71 leaded to a decrease in dissipation and energy losses which is due to the immersion of the stairs below the water level and the reduction of the roughness of the stairs and with the increase of the intensity of the inflow phenomenon. The simulation results with the Flow-3D math model are close to the physical model, and on average only 6.3% of errors are acceptable. After analyzing the results, it was found that in the combination of stepped spillway with triangular, rectangular and trapezoidal obstacles, we see a decrease in energy depreciation and energy loss. Also, comparing the simulation results and the physical model shows that the Flow-3D mathematical model find less bias with the physical model and closer to reality.

Stepped spillways and stilling basins are the most important energy dissipation structures. Even though most part of energy would be dissipated by these two structure, the remaining part of energy of flow may be capable to grave the bed scour and to create the scour hole downstream of them. Structures could be destroyed if this part of energy is high. In this study, the effects of stilling basin slopes on bed scour at downstream of stepped spillway were investigated experimentally. Experiments were conducted in the laboratory of hydraulic structures at University of Kerman with six different discharges and five various stilling basin slopes. The following parameters were measured: maximum scour depth (ds), flow velocity (in three point), water depth in upstream and downstream stepped spillway and stilling basin, the distance from where  the maximum scour depth happened to sill (Ls), and the geometry of scour hole. Results showed 47 percent increase and 52.2 percent decrease in average of maximum relative scour depth when stilling basin slopes were 0.02 and -0.02, respectively. Also, it was shown that the distance between the point where maximum scour depth had happened to the end of stilling basin increased when Froude number increased and decreased when stilling basin slope decreased.

The spillway is one of the most important elements of dam building. So studying and designing of the spillway hydraulic should be considered in dams and other similar projects. The ogee spillway, because of its superb hydraulic characteristics, has been widely studied. Its ability to pass flow efficiently and safely, when has been properly designed, with relatively good flow measuring capabilities, has enabled engineers to use it in a wide variety of situations. In the last two decades, there has been an increasing interest in the stepped spillways in various laboratory experiments around the world. This is partly because of technical advances in the construction of Roller Compacted Concrete (RCC) dams and considerable amount of energy dissipation along the chute, leading to reduction in the size of the stilling basin. Flow analysis using laboratory experiments usually involves considerable time and cost requirements .Nowadays, availability of computational fluid dynamic (CFD) programs and powerful computers has resulted in increasing usage of numerical methods of flow analysis. Free surface flows are encountered in hydraulic engineering problems including water jets, weirs and around gates. For the numerical modelling of a steady flow, the area of calculation and therefore the position of the free surface have to be known. The main object of this study is numerical analysis of free surface profiles on ogee and stepped spillways by finite volume and finite element methods and comparison of the two methods. The physical laws governing a fluid flow problem are represented by a system of partial differential equations regrouping the continuity equation, the Navier-Stokes equations and any additional conservation equations. The numerical analysis resolves these equations by accurate and complex numerical schemes. A program or code, where the numerical algorithm is implemented, is then solved on a computer. In recent study experimental results offered by Tabara and CHatila (2005) have been used to investigate stepped spillway physics models of Tabara and Chatara (2004) for studying ogee spillway. FLUENT and ADINA software have been used to simulate flow field of types of stepped and ogee spillways. It should be noted that k-£ model has been applied in order to modeling turbulence. Gambit preprocessor software is a tool for networking flow field. In recent study we have used PRESTO scheme for discretization of pressure, quick plan for discretization of terms of displacement of momentum equations, turbulence formulas and PISO algorithm for coupling velocity and pressure. In finite element method, Galerkin relation has the second order accuracy. SUPG method is used in momentum equations and turbulence for discretization of term advection which is effective in problem convergence. Meshing type has been selected as triangle form in ADINA code, but quadrilateral in FLUENT code regarding the spillway geometry. In some area structured mesh and in some other ones non-structured mesh have been chosen. Due to existence of rotatory flows over the steps, more fine meshes have been used in both codes near to body of stepped spillway as well as ogee spillway, because of the velocity gradient nearby wall, where flow enters spillway channel from reservoir. In recent study, water surface profile has been simulated in four types of stepped spillways and 3 types of ogee spillways on the basis of finite element method using FLUENT code and finite volume method using ADINA code. Relatively close consistency has been observed between water surface profiles in both codes by comparing experimental results. Note that free surface profile simulated over the ogee spillway was closer to water surface profile evaluated in laboratory than the profile by finite element method. There were observed better consistency in results from free surface profile over stepped spillway with finite volume method. Maximum differences between results of water surface profile in numerical model of FLUENT and ADINA were nearly 1.2 cm and 1.6 cm, comparing with laboratory results respectively. This value in numerical simulation is acceptable regarding to different meshing in model networking. On this basis, numerical methods of FLUENT and ADINA are proper in order to simulate similar structures for saving costs due to construction of physical model. Flow surface can be determined considering the lower error of FLUENT model in measuring free surface flow; thereby, designing lateral walls of spillway is realized with more precise.

In a stepped spillway, the steps can considerably reduce the flow energy. The purpose of this study was to investigate and comparing the amounts of energy dissipation in stepped spillways with different longitudinal profiles using physical models. For this purpose the experiments were done in a channel with 12 meters of length, 0.5 meter of width and 0.8 meter of height. For doing this research, the stepped spillways that had plexi glass gender with 4 ratios of hL = 0.5, 0.67, 1 and 2 was designed and constructed. The results showed that the amount of energy dissipation in hL=1 have maximum value, in the other words, when step length and step height be equal causes energy dissipation will be more. The results showed for hL≤1, increasing height of steps has been increase the energy dissipation. In a constant ych, the increase in the number of steps Increases relative energy dissipation and increasing dimensionless parameter ych, reduces energy dissipation almost 5 %. Finally, third degree equations for relative energy dissipation extracted, that the calculated statistical parameters indicate high accuracy of equivalents. The results showed relative energy dissipation in our study was in agreement with Chanson equations and Fratino-Piccinni equations.

One of the most effective strategies for energy dissipation of hydraulic structures is hydraulic jump .The position of hydraulic jump play an important role in design of stilling basin. In this study, hydraulic jump in ogee spillways were simulated by using fluent software. The governing equations were solved through finite volume method and the standard model was applied for estimating the turbulence flow. The equations were discretized in structured mesh accommodate the well-defined boundaries and the volume of fluid (VOF) method was introduced to solve the complex free-surface problem. The study examined the effect of increasing the discharge and the slope on the hydraulic jump position and lentgh. the results suggest that increasing the slope and discharge caused the spatial delay in hydraulic jump. It was found that in ogee spillway with a constant slope, the hydraulic jump length increases up to 120% when the discharge increases. Additionally, for a certain discharge and a constant length of spillway, increasing the slope of spillway decreases the hydraulic jump length up to 43%.