Supercritical Flow over a Dentated Sill

Experimental analysis concerning supercritical flow over a dentated sill in nonsubmerged conditions and the local scour downstream of this sill is presented. In this case, the energy dissipation is incomplete so that a scour control downstream of the hydraulic structure could be required. The use of this type of dissipation device could be needed in cases where the maximum flow depth upstream of the sill must be limited to satisfy physical or structural conditions. This could happen, for instance, in the restoration of stilling basin in gravity dams and at the outlet of diversion systems. Experimental and theoretical results, useful in the design of this type of stilling basin and scour control, are presented in the paper, with an application to the case of the Molato Dam in Italy.     

Risk Analysis for Dam Overtopping—Feitsui Reservoir as a Case Study

Risk and uncertainty analysis by mathematical and statistical methods is often used to assess systematic risks and uncertainties. This research presents the procedure and application of risk and reliability analysis to dam overtopping. Annual maximum series of peak discharges of Feitsui Reservoir in northern Taiwan are used to analyze five extreme flood events with different frequencies. The highest water levels of the five extreme flood events were computed by using reservoir routing and considering seven factors subject to uncertainty. Afterward, the overtopping risk of Feitsui Dam was assessed by five uncertainty analysis methods: Rosenblueth’s point estimation method (RPEM), Harr’s point estimation method (HPEM), Monte Carlo simulation, Latin hypercube sampling, and the mean-value first-order second-moment (MFOSM) method. The results show that values of overtopping risk computed by different methods are similar. One may apply some approximated methods (MFOSM, HPEM and RPEM) to avoid the computational burden by applying sampling methods. Furthermore, the accuracy of results by approximated methods compared with that by sampling methods may differ from case to case. The selection and application of the uncertainty methods depend upon the information availability of the model parameters and model complexity. One may need to examine the model parameters and model complexity before determining appropriate methods to be used in a study.     

Skimming Flow in the Nonaerated Region of Stepped Spillways over Embankment Dams

Traditionally, research on stepped spillway hydraulics has been focused on the air-water flow region but for the hydraulic design of small embankment dams experiencing relatively large overtopping flows, the nonaerated region can be very important. Empirical formulas are presented for predicting skimming flow properties upstream of the point of inception of air entrainment for 1V:2H sloping stepped spillways, and the location and flow depth at the point of inception. Particular emphasis is placed on the clear-water depth, velocity distribution, and the energy dissipation characteristics in the developing nonaerated flow region. The velocity distribution is well described by a power law. The normalized clear-water depth and the normalized specific energy varied with the relative distance along the spillway and the effect of the normalized critical depth was negligible. Finally, the rate of energy dissipation was small, which has direct implications for the design of the downstream energy dissipator.     

Seepage Velocity and Piping Resistance of Coir Fiber Mixed Soils

In the context of sustainable watershed management, natural fibers mixed with soil have applications in irrigation and drainage projects such as river levees, contour bunds, temporary canal diversion works, temporary check dams, soil structures, stream restoration, etc., for controlling seepage. In this study, a number of experiments were carried out for determining the seepage velocity and piping resistance of different types of soils mixed randomly with coir fibers. Three types of soils are used in this study. The experiments were carried out for various hydraulic heads, fiber contents, and fiber lengths. Discharge velocity and seepage velocity of flow of water through soil is calculated in each case and compared with plain soil. It is observed that fibers reduce the seepage velocity of plain soil considerably and thus increase the piping resistance of soil. Regression equations based on experiments are developed for quantifying the seepage velocity and piping resistance considering hydraulic gradient, fiber contents, and fiber lengths. Suitability of coir fibers for field applications with typical examples is also highlighted. The results show that coir fiber mixed soil can be used to increase the piping resistance and reduce seepage velocity in the above mentioned applications.     

Inception Point and Air Concentration in Flows on Stepped Chutes Lined with Wedge-Shaped Concrete Blocks

Stepped channels lined with wedge-shaped concrete blocks are a promising solution to provide overtopping protection for embankment dams if the discharge capacity of existing spillways is not adequate. The paper addresses the characteristics of the two-phase transition and skimming flows in stepped channels lined with this type of block. An experimental setup was developed with two flumes designed with a relative scale of 1:2.5. Air concentration was measured with an optical probe in several cross sections of both flumes. The scale effects are analyzed. An expression for the location of the inception point is proposed. The vertical air concentration profiles and their longitudinal variation are studied, considering data and models proposed by other researchers. The establishment of the uniform flow regime is analyzed.     

Uncertainty of Predictions of Embankment Dam Breach Parameters

Risk assessment studies considering the failure of embankment dams often require the prediction of basic geometric and temporal parameters of a breach, or the estimation of peak breach outflows. Many of the relations most commonly used to make these predictions were developed from statistical analyses of data collected from historic dam failures. The prediction uncertainties of these methods are widely recognized to be very large, but have never been specifically quantified. This paper presents an analysis of the uncertainty of many of these breach parameter and peak flow prediction methods. Application of the methods and the uncertainty analysis are illustrated through a case study of a risk assessment recently performed by the Bureau of Reclamation for a large embankment dam in North Dakota.     

Temporal Development of Scour Holes around Submerged Stream Deflectors

Deflector structures used in many fish habitat rehabilitation schemes are frequently overtopped, yet few studies have examined the scour patterns created around submerged models. Furthermore, laboratory studies typically test smooth-surfaced structures, whereas those installed in natural rivers are generally made of logs or boulders. This study uses rough-surfaced paired deflectors to investigate the temporal evolution of scour for three overtopping ratios in identical approach flow conditions in a flume. Results show that maintaining identical discharge and raising the deflector height, which reduces the overtopping ratio (i.e., flow depth divided by structure height), generates increased depth and volume of scour next to the structures. The location of maximum depth and the rate of scouring with time is similar for the two highest deflectors (overtopping ratios of 1.22 and 1.83), but different for the lowest deflector model (overtopping ratio of 3.67). To improve the success rate of river restoration projects using in-stream structures, the overtopping ratio should be considered in equations that predict the scour depth evolution with time.     

Analytical Solution for Circular Gates as Flow Metering Structures

Water measurement in irrigation canals is frequently hindered by low head availability and high capital investment costs associated with construction of compatible hydraulic structures. Often irrigation systems have circular sliding gates in place used as diversion and flow control structures. The Fresno Irrigation District investigated the feasibility of using such circular gates (Armco Model 101) as flow metering stations in the 1920s. This early work demonstrated that circular gates could be used simultaneously for both flow control and as flow measurement structures. The original work is compiled by USBR as 10,500 data points and is presented in tabular fashion for gate diameters varying from 20.3?to?121.9?cm (8–48?in.). An analytical equation of the form Q = CD(y,D)yD, [where CD(y,D) is a discharge function which depends on the gate displacement y and the nominal gate diameter D, g represents the gravitational acceleration, and H is the hydraulic headloss through the crescent-shaped orifice] accurately predicts most tabulated values. Equations are provided to compute the discharge function for nominal gate diameters varying from 20.3?to?121.9?cm (8–48?in.) for gate displacements between 5.1?cm (2?in.) and fully open conditions. The precision of the proposed algorithms are excellent (predicted values are within ±5% of the corresponding reported values 95% of the time) for gates greater than 30.5?cm (12?in.).     

Junction Flow between Drop Shaft and Diversion Tunnel in Lyss, Switzerland

A diversion tunnel is planned for flood protection in the City of Lyss, Switzerland. This tunnel includes a junction, from which water from a side creek is added to the tunnel flow. The latter is sensitive to choking, so that the junction had to be optimized in a hydraulic laboratory investigation. Its setup finally consists of a drop shaft and an injector generating annular flow in the tunnel. Free surface tunnel flow then occurred for all relevant discharges. This setup is also of interest in sewer systems as well as in spillway and diversion tunnels, where similar problems may occur. The concept was optimized for a specific prototype. General recommendations may be derived for similar situations.     

Laboratory Measurements of Flow through Culverts

Laboratory apparatus to simulate flow through culverts has been used to collect discharge and water level measurements. Two different shapes of culvert barrels, namely square and circular, were tested. The measurements presented in this note are intended to provide useful information regarding the variety of flow regimes (including overtopping) through culverts, and the transitions from one flow regime to another. It is known that modeling the culvert flow regimes and capturing the transitions among these regimes numerically is a challenging task. To that effect, the laboratory measurements presented herein can provide a testing and validation data set for numerical modeling of hydraulic structures such as culverts.     

Discharge Coefficient for Sharp-Crested Side Weir in Subcritical Flow

To estimate the outflow over a rectangular sharp-crested side weir, the discharge coefficient in the weir equation needs to be known. Although this type of structure has been designed and used extensively by hydraulic engineers, a universally acceptable discharge coefficient does not exist. In this study over 250 laboratory tests were conducted, and the results were analyzed to find the influence of the flow hydraulics and the geometric, channel, and weir shapes on the coefficient. The results show that for subcritical flow the De-Marchi assumption of constant energy is acceptable, and the weir discharge can therefore be used. Furthermore, it was discovered that the De-Marchi coefficient of discharge is a function of the upstream Froude number and the ratios of weir height to upstream depth and weir length to channel width, whereas the channel slope in subcritical flow can be ignored. Hence, an accurate equation for the coefficient of discharge is introduced.     

四流中间包流场影响因素分析与优化

采用正交试验法建立1:2.5水力学模型,选取L12(22×31)混合正交表设计12组水模试验方案,对水模试验结果进行极差分析,分析挡墙类型、挡墙导孔孔径和挡墙导孔倾角对中间包流场特性的影响大小,并找出合理的控流装置配置.结果表明,挡墙导流孔倾角对中间包流场特性影响最大,其次是导流孔孔径和挡墙类型.采用Y型挡墙,挡墙导孔...     

Risk Indexing Tool to Assist in Prioritizing Improvements to Embankment Dam Inventories

A risk indexing tool is proposed to assist in the prioritization of maintenance, repair, and evaluation tasks on embankment dams that are generally less than 33 m (100 ft) high and for which there is little or no instrumentation, limited or no information concerning as-built conditions, and little or no information on the performance history. Under such circumstances, there may not be enough information available to perform anything other than an “indexing” type of analysis to assist in prioritization. The risk indexing tool is based upon identifying potential deficiencies in the physical condition of the dam and rating the overall importance of these deficiencies to the safety of the structure. It is meant as an index of risk (indication) and not as a direct measure of risk. Checklists are presented for onsite inspections to determine current physical condition. Condition is defined in terms of a condition function that is based upon a condition indexing scale. Four potential failure modes are considered: (1) overtopping; (2) external erosion; (3) piping; and (4) mass movement (slope instability). Absolute probabilities are specifically not factored into the analysis. Conditional probabilities are estimated for each failure mode using a Bayesian updating procedure based on dam attributes. A simple failure criticality analysis is performed wherein specific changes in physical condition of the dam are considered to contribute to the probability of failure for each mode. From this failure criticality analysis and the conditional probabilities of failure, the relative importance of the various changes in physical condition is determined. The physical condition and the relative importance are then combined for each observable deficiency to form a risk index. These risk indices are used to prioritize expenditures for improvements on the premise that actions to address the most significant physical deficiencies are preferred.     

Effect of Flood Recession on Scouring at Bed Sills

The effect of the flood recession time on the local scour depth at bed sills in gravel deposits is examined. Experiments were carried out to study the development of scour holes under time-varying hydraulic conditions with no upstream sediment feed. Triangular-shaped hydrographs, having recession times up to three times the duration of the rising limb, were used. Traditionally, the peak water discharge in any flood event is used as a design value in estimating the final depth of scour formed by a flood. This approach is overly conservative when the flow hydrograph is steep, i.e., during the occurrence of flash floods. The actual reduction of the scour depth from this estimated value is dependent on both the characteristics of the flood event and the characteristics of the stream. The results show that the maximum potential scour depth can be achieved only for hydrographs with long recession times, while the rate of this process can be estimated as a function of the ratio between a characteristic flood time and the steady-state temporal scale of scour development. A method is proposed for the prediction of the scouring process under unsteady flows in terms of two dimensionless temporal parameters. Results obtained for clear-water boundary conditions can be extended to sediment-supply tests if specific supply input conditions hold.     

Transitional Flow between Orifice and Nonorifice Regimes at a Rectangular Sluice Gate

The hydraulic transition between nonorifice and orifice flow regimes at a rectangular sluice gate was analyzed to determine the value of a coefficient (Co) used to define the threshold between the two regimes. The transition coefficient was defined as the ratio of vertical gate opening to upstream water depth. Several dozen data sets were collected in a hydraulic laboratory, each including the measurement of upstream and downstream water depth for five different vertical gate openings, and 17 different steady-state discharges from 0.02?to?0.166?m3/s. Various approaches were tested to define the limits of the nonorifice-to-orifice regime transition, but the one presented herein uses the specific-energy equation for open-channel flow. After the transition limits were defined, an estimation of the nonorifice-to-orifice transition coefficient, Co, was made. The experimental results indicate that orifice flow always exists when Co is less than 0.83, and nonorifice flow always exists when Co is greater than 1.00. A procedure was developed to determine the flow regime and the discharge at a rectangular gate in the range 0.83相似文献   

Hydraulics of Tangential Vortex Intake for Urban Drainage

A tangential vortex intake is a compact structure that can convey storm water efficiently as a swirling flow down a vortex dropshaft. It has been studied in physical models and successfully employed in urban drainage and hydroelectric plant applications, but a comprehensive account of the key flow characteristics has not been reported and a theoretical design guideline of a tangential intake is not available. In this study the hydraulics of tangential slot vortex intakes is investigated via extensive experiments. It is found that the flow in the tapering and downward sloping vortex inlet channel is strongly dependent on the geometry of the inlet and dropshaft. Under some conditions, hydraulic instability and overflow can occur, rendering the design ineffective. It is shown that the hydraulic stability depends on the discharge at which flow control shifts from upstream to downstream (Qc), as well as the free drainage discharge (Qf). A theoretical design criterion for stable flow is developed in terms of Qf and Qc as a function of the vortex inlet geometry. For a “stable” design, the flow in the tapering inlet evolves from supercritical flow to subcritical flow smoothly as the discharge increases. Fifteen different tangential vortex intake models are tested. The experimental observations are in excellent agreement with the theoretical prediction. The present study provides a general guideline for designing a tangential vortex intake that can convey the flow smoothly without unstable fluctuating flow associated with a hydraulic jump.     

Structure of Flow Upstream of Vertical Angled Screens in Open Channels

This paper presents the results of a laboratory study of the structure of flow in a diversion structure with a vertical angled wedge-wire fish screen. This screen had a 10×25?mm mesh and was tested at three angles of 10.4, 17.5, and 26.8°, to the direction of the approaching flow, for two mean velocities of 0.5 and 0.8?m/s, with a depth of flow of about 0.75?m. In this water and fish diversion (channel or) structure, it was found that the depth of flow at any section is approximately constant with a drop at the screen on the side of the canal and decreased towards the bypass located at the downstream end. The distribution of the velocity component u in the direction of the approaching flow as well as the perpendicular component w and the resultant velocity V was uniform in the vertical direction. The depth averaged mean velocity for different verticals at any section in the diversion structure increased with the longitudinal distance x and was correlated with the relative width, bs/b (in the diversion structure) for all five experiments. Correlations have been found for the depth averaged transport velocity and the impinging velocity on the screen in terms of the approach velocity U. A general relation has also been developed for the attack angle of the flow on the screen. The downstream part of the screen carried more flow into the canal compared to the upstream part as a result of the uniform mesh size used in this study. The results of this hydraulic study should be useful, particularly for freshwater adult fish, in designing screens in irrigation canals and for micro-hydro sites that use diversion canals.     

Earthquake Hydrodynamic Pressure on Dams

Hydrodynamic pressures on the vertical upstream face of straight dams during horizontal earthquakes were studied by Westergaad in 1933, and an analytical solution was obtained. Assuming that water is incompressible, an approximation can be made to reduce Westergaad’s mathematical formulation to the Laplace equation. The computer program SEEP2D, from the U.S. Army Corps of Engineers (COE), is available for the study of seepage flow in porous media; this flow can be expressed mathematically in a form of the Laplace equation. Therefore, we can use this computer program to study the hydrodynamic pressure on dams during a horizontal earthquake in the upstream/downstream direction. In practice, the proposed procedure is not limited to SEEP2D but can also be applied to any computer model capable of solving Laplace equations in bounded domains. Two examples are presented to show the application of the COE’s computer program, and the accuracy of the proposed method is discussed.