Stepped Spillways Research Articles

Gabion Stepped Spillway: Interactions between Free-Surface, Cavity, and Seepage Flows

AbstractOn a gabion stepped chute, the steps contribute to the dissipation of turbulent kinetic energy, free-surface aeration may be intense, and there are complex interactions between the free-surface flow and seepage motion. Detailed measurements were conducted in a relatively large gabion stepped spillway model. Using a combination of high-speed movies and phase-detection probe measurements, the air–water flow properties in the step cavities and in the gabions were documented. Strong air–water exchanges between seepage and stepped cavity flows were observed. The data showed a complex bubbly seepage motion in the gabions associated with a high level of interactions between seepage and free-surface flows, leading to a modification of the step cavity recirculation and lesser flow resistance.

Hydraulic Studies of Stepped Spillways of Various Design

Different stepped spillway designs are presented, along with the results of model hydraulic studies of these spillways, equipped with abutments of various design.

Interaction between free-surface aeration and total pressure on a stepped chute

Stepped chutes have been used as flood release facilities for several centuries. Key features are the intense free-surface aeration of both prototype and laboratory systems and the macro-roughness caused by the stepped cavities. Herein the air bubble entrainment and turbulence were investigated in a stepped spillway model, to characterise the interplay between air bubble entrainment and turbulence, and the complicated interactions between mainstream flow and cavity recirculation motion. New experiments were conducted in a large steep stepped chute (θ=45°, h=0.10m, W=0.985m). Detailed two-phase flow measurements were conducted for a range of discharges corresponding to Reynolds numbers between 2×105 and 9×105. The total pressure, air–water flow and turbulence properties were documented systematically in the mainstream and cavity flows. Energy calculations showed an overall energy dissipation of about 50% regardless of the discharge. Overall the data indicated that the bottom roughness (i.e. stepped profile) was a determining factor on the energy dissipation performance of the stepped structure, as well as on the longitudinal changes in air–water flow properties. Comparative results showed that the cavity aspect ratio, hence the slope, has a marked effect on the residual energy.

Self-aeration in the rapidly- and gradually-varying flow regions of steep smooth and stepped spillways

In high-velocity chute flows, free-surface aeration is often observed. The phenomenon is called self-aeration or white waters. When the turbulent shear stresses next to the free-surface are large enough, air bubbles are entrained throughout the entire air–water column. A rapidly-varied flow region is observed immediately downstream of the inception point of free-surface aeration. An analytical solution of the air diffusion equation is proposed and the results compare well with new experimental data. Both experiments and theory indicate that the flow bulking spans over approximately 3–4 step cavities downstream of the inception point of free-surface aeration on a stepped chute. Further downstream the void fraction distributions follow closely earlier solutions of the air diffusion equation. The application of the diffusion equation solution to prototype and laboratory data shows air bubble diffusivities typically larger than the momentum transfer coefficient. The result highlights however a marked decrease in the ratio of air bubble diffusivity to eddy viscosity with increasing Reynolds number. The finding might indicate some limitation of laboratory investigations of air bubble diffusion process in self-aerated flows and of their extrapolation to full-scale prototype applications.

Designing Manhole in Water Transmission Lines Using Flow3D Numerical Model

Using cascades and drops existing in flow path has a history of 3000 years. Particularly, Roman engineers employed stepped spillways with the same idea in several countries; however, there are few information about the hydraulic performance of aqueducts. Most of these channels have flat long cross sections with low torsions (variable slope) such that they can encompass cascade and steep spillways or dopshaft. Given that there are few studies conducted on dropshafts, the present paper attempted to discuss about such structures in flow path and water transmission lines as well as introducing the existing principles and relations and present, the obtained results of designing though Flow3D. The obtained error percentage was about 20% which is acceptable for numerical studies.

Simple Design Criterion for Residual Energy on Embankment Dam Stepped Spillways

AbstractThe stepped spillway design is associated with significant flow resistance and associated energy dissipation on the steps, yielding smaller, more economical downstream dissipation structures. A number of design guidelines were developed for steep stepped spillways typical of concrete gravity dams. The focus of this study is on embankment stepped spillways. A large set of air-water flow data is compared with reanalyzed data sets to provide a simple unifying design approach for the residual energy at the stepped chute’s downstream end and to highlight the uncertainties involved. The results provided some simple design criteria in terms of the dimensionless residual energy of stepped chutes with flat steps. It is believed that a stepped design with a 1V:2.5H slope (θ=21.8°) might be optimum in terms of energy dissipation performances. The Darcy-Weisbach friction factors were close for all stepped data ranging between 0.1≤fe≤0.4.

Effects of Stepped Spillway Geometry on Flow Pattern and Energy Dissipation

In order to pass surplus water and inundation from upstream to downstream of dams, structure called spillway is used. Spillway and chutes are among important hydraulic structures which play a significant role in stability of dams. In some cases and when the slope is too steep to build a chute, in order to transfer water from upstream to downstream, a stepped spillway which is remarkably effective in energy dissipation is used. In the present study, in order to ascertain the effect of different parameters such as number of steps (Ns), step height (h), step length (L), and discharge in width unit (q) on energy dissipation in the simple stepped spillway, the model of Flow-3D was used, and the relationship between energy dissipation and flow critical depth in the stepped spillway was investigated. Further, the method of finite volume was used to solve the extant equations, and the model of K − ɛ was also used to investigate the flow turbulence. The results revealed that as the flow discharge increases, energy dissipation decreases and as the number of steps increases and their height decreases, energy dissipation decreases. Besides, the obtained findings were compared with the other researchers, empirical and mathematical studies and finally an acceptable coincidence was obtained.

Predicting the Influence of a Water-Ice Cloud Behind Boguchanskaya HPP Spillways Nos. 1 and 2 on Hydroproject Facilities in Winter Under Emergency Operating Conditions

Amathematical model is developed for the generation and transport of condensed moisture in a stepped spillway. Estimates were made for solid and liquid precipitation at Boguchanskaya HPP facilities for winter operation of spillways Nos. 1 and 2.

Numerical investigation of the configuration of the pools on the flow pattern passing over pooled stepped spillway in skimming flow regime

Spillways are hydraulic structures which transfer water to the downstream. Stepped spillways are seriously attended to, since they are highly efficient regarding energy dissipation. In this study, the flow pattern over stepped spillways was simulated with five different pool configurations. The VOF method was used to simulate the flow free surface, and K−ɛ(RNG) was employed to model turbulence. The results obtained from the numerical model were fairly consistent with the experimental results. The results demonstrated that the flow velocity and the residual head showed significant differences in the spillway width with regard to different pool configurations. The pool configuration in which the pools were installed as staggered configuration of flat and pooled steps showed the least residual head at the downstream and as a result the greatest energy dissipation.

Closure to “Cavitation Potential of Flow on Stepped Spillways” by K. Warren Frizell, Floriana M. Renna, and Jorge Matos

Closure to “Cavitation Potential of Flow on Stepped Spillways” by K. Warren Frizell, Floriana M. Renna, and Jorge Matos

Physical modeling of a stepped spillway without sidewalls

The interest of a consulting company in designing stepped spillways in roller compacted concrete (RCC) dams led us to propose the possibility of building this type of spillway without sidewalls. Previous research on stepped spillways has focused on characterizing the complex hydraulic behavior of flow on these structures, as well as design criteria. Such studies have usually been conducted on stepped spillways with a constant width along the spillway, that is, with sidewalls. In the present work, we report the results of the physical modeling of a generic stepped spillway without sidewalls (slope 1v:0.8h). In general terms, the lack of sidewalls produces a lateral expansion of water and therefore a non-uniform longitudinal and transversal discharge distribution. Consequently, the flow type, characteristic water depth, velocity, air concentration, and pressure fields change along and across the spillway. The resulting data demonstrate that the distribution of the different variables studied depend on the s...

Comparative analysis for estimation of the height of training wall of convergent stepped spillway

For a dam system with inadequate capacity of spillway, provision of (RCC) roller compacted concrete stepped spillways with converging training walls prove to be an appropriate alternative. Sufficient number of guidelines and literature references are available on the design of straight side walled stepped spillways. The need was felt to conduct the experiments on stepped spillways with convergent training walls, as limited literature is available. This paper presents the comparative analysis of an experimentation conducted on flow over convergent stepped spillways having 1:1 chute slope with 45o convergence angle for different step height ratios leading to different step height variations. The findings are presented in the form of expressions for the maximum depth of flow through the training walls required to accommodate the flow rate. The observed maximum depth of flow along the training walls also compared with estimated maximum height of the training wall as per the criteria available within literature and percentage variation is worked out taking into account the step height variation. Outcomes of the experimental findings are expected to be useful for assessment of height of training walls by the hydraulic design engineers involved in rehabilitation of existing dam system and also in design of the modified spillway system that is necessarily associated with convergent training walls and stepped chute.

Numerical Simulation of Air Entrainment for Flat-Sloped Stepped Spillway

Stepped spillway is a good hydraulic structure for energy dissipation because of the large value of the surface roughness. The performance of the stepped spillway is enhanced with the presence of air that can prevent or reduce the cavitation damage. Chanson developed a method to determine the position of the start of air entrainment called inception point. Within this work the inception point is determined by using fluent computational fluid dynamics (CFD) where the volume of fluid (VOF) model is used as a tool to simulate air-water interaction on the free surface thereby the turbulence closure is derived in the k –ε turbulence standard model, at the same time one-sixth power law distribution of the velocity profile is verified. Also the pressure contours and velocity vectors at the bed surface are determined. The found numerical results agree well with experimental results.

Assessment of the safe evacuation of people walking through flooding staircases based on numerical simulation

A numerical model was developed to evaluate the possibility of people walking in a flooding flow on a staircase with rest platforms. Commercial software was used and validated by experimental data for flows on staircases and stepped spillways. The effects of the rest platform, the staircase slope, and the staircase pattern on the flooding flow characteristics are discussed. A comparison of staircases with or without rest platforms shows that the flow velocity increases significantly downstream of the rest platform on a straight-run type, which would have negative effects on the safe evacuation of people walking through a flooding staircase. The slope of the staircase, ranging from 26.6° to 30°, has less effect on safe evacuation. A comparison of flows on straight-run (with or without rest platforms), 90°-turn and 180°-turn staircases (with rest platforms) shows that the rest platforms on the latter two staircases could induce a redistribution of the flow field on the rest platform and downstream. The distribution of evacuation indicators along the longitudinal planes of those staircases indicates that a 90°-turn staircase or a straight staircase without rest platform would be the first choice for trapped people evacuating from underground spaces.

Study of the Effect of the Rate Flow and the Slope of the Channel on the Energy Dissipation in the Stepped Channels: Proposing an Empirical Models

Many researchers have concentrated on experimental research of hydraulic characteristics of the flow over the stepped spillway. It is in this context that we operate in the present work to distinguish between the two flows regimes and to study the effect of rate flow and the slope of channel on the energy dissipation, the point of inception. And finally we present the empirical models.To show the importance of studying these flows, we made an experimental approach in the laboratory of civil engineering at the University of Laghouat (Algeria), in three (03) models: model A (4cm x 7.5cm 4cmx) and model B (8cm x 7.5cm 8cmx), and the third model C size (12cm x 12 cm x 7.5 cm) developed “Plexiglas.” In what follows, we set the rate of flow and vary the slope of the channel. The simulated flows range from 0.072 l/s to 2.569 l/s.As result, for a slope (45°), the energy dissipation is relative maximum and average around 95% in all three models. For low slope (15° or 20°), the energy dissipation is relative maximum and average of 93%, 86% and 87% for models A, B and C respectively. For an average slope (30°), the energy dissipation is relative maximum and average about 95%, 92% and 89% for models A, B and C respectively.

Embankment overtopping protection systems

For the last decades, the design floods of numerous embankment reservoirs were re-evaluated, and the revised spillway outflows are typically larger than those used in the original designs. As a result, a number of overtopping protection systems were developed for embankments and earthfill dams, with applications encompassing river dykes, coastal barriers for storm surge and tsunami protections. Several design techniques were developed for embankments and earthfill dams. These include concrete overtopping protection systems, timber cribs, sheet-piles, riprap and gabions, reinforced earth, minimum energy loss weirs, embankment overflow stepped spillways and the precast concrete block protection systems. Various designs are reviewed herein and discussed based upon prototype experiences. This review highlights that a safe operation of embankment overflow protection systems relies upon a sound design and a good quality of construction, suitable flow conditions, together with regular maintenance.

Application of Displacement Height and Surface Roughness Length to Determination Boundary Layer Development Length over Stepped Spillway

One of the most uncertain parameters in stepped spillway design is the length (from the crest) of boundary layer development. The normal velocity profiles responding to the steps as bed roughness are investigated in the developing non-aerated flow region. A detailed analysis of the logarithmic vertical velocity profiles on stepped spillways is conducted through experimental data to verify the computational code and numerical experiments to expand the data available. To determine development length, the hydraulic roughness and displacement thickness, along with the shear velocity, are needed. This includes determining displacement height d and surface roughness length z0 and the relationship of d and z0 to the step geometry. The results show that the hydraulic roughness height ks is the primary factor on which d and z0 depend. In different step height, step width, discharge and intake Froude number, the relations d/ks = 0.22–0.27, z0/ks = 0.06–0.1 and d/z0 = 2.2–4 result in a good estimate. Using the computational code and numerical experiments, air inception will occur over stepped spillway flow as long as the Bauer-defined boundary layer thickness is between 0.72 and 0.79.

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 “Cavitation Potential of Flow on Stepped Spillways” by K. Warren Frizell, Floriana M. Renna, and Jorge Matos

The authors presented a very interesting contribution to the topic of stepped spillway hydrodynamics. Based on a water-tunnel study, they presented some detailed flow features, for example, in terms of the strain rate, which are valuable to designers. The authors stated that “stepped spillways have no documented damage.” It is partially correct. Indeed no cavitation damage and sign of cavitation pitting were reported on stepped spillways after more than 3,000 years of operation, but there were documented damages and failures of stepped spillway systems (Marsh 1957; Jansen 1983; Pravdivets 1993; Chanson 1995b, 2000, 2001a). In this discussion, a number of prototype stepped spillway tests are reviewed and the operational experiences are discussed with a focus on the stepped chute performances...

Phase-detection probe measurements in high-velocity free-surface flows including a discussion of key sampling parameters

Air–water high-velocity flows are characterised by strong interactions of air bubbles and water droplets. The void fraction ranges from a few percent in bubbly flows to up to 100% at the free-surface and a reliable measurement instrumentation is the phase-detection intrusive probe. Herein new experiments were conducted on a stepped spillway (θ=26.6°) in transition and skimming flow sub-regimes yielding new insights into the turbulent air–water flow properties including the turbulence intensities and integral turbulent time and length scales. The integral turbulent scales showed self-similarity independently of the flow regime. A sensitivity analysis was conducted on the phase-detection probe signals to investigate the optimum sampling duration and frequency as well as the data analysis parameters threshold, sub-sampling duration, histogram bin sizes and cut-off effects. The results provide recommendations in terms of optimum sampling and processing parameters for high-velocity air–water flows.