Journal of Hydraulic Engineering

A study was completed to compare flow parameters over a standard ogee-crested spillway using a physical model, numerical model, and existing literature. The physical model was constructed of Plexiglas and placed in a test flume. Pressure taps were installed along the entire length of the spillway. Discharge and pressure data were recorded for 10 different flow conditions. A commercially available computational fluid dynamics (CFD) program, which solves the Reynolds-averaged Navier-Stokes equations, was used to model the physical model setup. Data interpolated from U.S. Bureau of Reclamation and U.S. Army Corps of Engineers design nomographs provided discharge and pressure data from the literature. Nondimensional discharge curves are used to compare the results from the different methods. Pressures are compared at low, mid, and high flow conditions. It is shown that there is reasonably good agreement between the physical and numerical models for both pressures and discharges. The availability and power of existing numerical methods provides engineers with another tool in the design and analysis of ogee spillways.

Lack of field data on the thickness and roughness of river ice jams inhibits reliable calibration of ice jam models and can result in erroneous evaluation of important physical parameters. Detailed thickness data that have been obtained on several ice jams by means of a remote profiling technique reveal a highly irregular underside. Statistical analysis points to a normal probability distribution of the irregularities and allows direct assessment of the absolute roughness of each jam. The latter varies linearly with the average jam thickness, at least to a thickness value of 3 m, and is shown to be consistent with previously reported roughness values deduced from purely hydraulic considerations. Using the present findings in conjunction with past data and modelling experience, simple equations are developed to compute composite-flow resistance characteristics in terms of jam thickness and flow depth.

This paper presents a study on choked flows through short, lateral contractions in subcritical open channels. A new discharge equation is developed based on the conservation of energy and experimental data with a wide range of opening ratios (ratio of contracted width-to-total width). The theoretical derivation accounts for the critical flow conditions that take place in the contraction under choking conditions. The effects of opening ratio σ, encroachment structure shape, inlet angle α, and relative contraction length L* on discharge coefficients are considered. The coefficients of the newly developed discharge equation are determined using a total of 186 sets of choking experiments conducted by various researchers. The analysis shows that the discharge coefficient is mainly affected by σ and may vary between 0.29 and 0.54 for the range of σ values used in the study (0.12–0.88). It is shown that the discharge coefficient (correspondingly the discharge) can vary up to 16% for encroachment-structure shape changing from sharp corner to rounded corner conditions—up to 11% for α going from 30° to 90° and up to 6% for L* ranging from 0 to 1.33. This study extends the traditional choking analysis to severe contractions. The proposed equation fits previous experimental data well and can be used to predict either the water discharge through a contraction for a given upstream depth or the water depth upstream from a contraction corresponding to a given discharge under choking conditions.

In China many large-scale hydraulic projects have been designed with multistage orifice tunnels to dissipate a large amount of energy at the design discharge and water level. Based on two hydraulic-model tests and theoretical analysis about multistage orifice tunnels, this study further examined orifice geometry to meet energy-dissipation requirements and to minimize cavitation risk, as well as to systematically investigate factors influencing the energy dissipation and cavitation coefficients of the multistage orifices. The results indicate that measures, such as enlarging the tunnel diameter, decreasing the tunnel outlet area, and lowering the design water level of the reservoir, could be taken into consideration to increase the orifice cavitation coefficient σ for a given orifice geometry when the incipient cavitation coefficient σ_i of the orifice is large. For the same orifice contraction ratio β, the values of orifice dissipation coefficient k and σ_i are closely related to orifice geometry, especially to the radius r at the top edge of the orifice and to the ring fixed in front of the orifice plate.

A model that directly determines the required source concentration(s) to meet specified chlorine residuals at selected nodes is developed for unsteady flows. The model considers even, over-, and underdetermined cases in which the number of available equations is equal to, greater than, or less than the number of unknowns, respectively. The model computes the unsteady-state flows and chlorine concentrations in the system first. Then it utilizes the inverse method to determine the required source concentration(s) so that specified residuals at selected nodes match the calculated ones. To verify the model, chlorine concentrations computed by the forward method are input to the inverse model to determine the source concentration(s) in two hypothetical networks. Results show that the model computed accurately the source concentrations for the even and the overdetermined cases, but the results were less than satisfactory for the underdetermined case.

Open-channel flow simulations require values of friction parameters that are determined through formulating their inverse problems using a sample of historic events. However, there is a risk of the parameter values being biased toward the events used because of the sample size of historic events. Traditional sample-size determination approaches are known to suffer from a conflict between reliability and costs. In addition, there is no objective approach to determine a minimum size. These problems are solved through formulating the new “confidence calculation method,” which builds on the following: (1) the data points of a “parent sample” of friction parameters are resampled into “subsample populations,” where their “subsample means” contain latent information governed by the “central limit theorem”; (2) a rigorously derived formula makes it possible to calculate standard deviations of subsample populations, replacing direct resampling operations with mathematical ones; and (3) the method is parameterized for quantitatively trading off between sample sizes and reliability, minimizing the risk of the dependence of parameter values on their size. This paper illustrates the application of the method using synthetically generated samples of open-channel friction parameters.

The solution to the problem of induced recharge where pumping is planned near a stream, requires determining effective distance to the line of recharge. This is generally accomplished by analyzing the drawdowns observed during the pumping test conducted near a recharge boundary. Traditional methods of estimating the distance between observation well and image well either use the concept of fully developed straight lines or make use of the type curves for matching. Hantush's method requires the locating of the inflection point on the drawdown curve. The curve matching or the Hantush method is subjective and hence involves errors due to personal judgment. A long-duration pumping test is required to be conducted in order to obtain a fully developed second straight line. In many situations, such a long-duration pumping test is not feasible. In this paper, a robust optimization method is presented that allows the use of shorter-duration pumping test data, for estimating aquifer parameters, and distance to the effective line of recharge. Thus, it saves time and costs involved in conducting a long-duration pumping test. Application of the method on published data sets shows that the new method yields reliably accurate estimates of the parameters.

In the literature on turbulent flow various combinations of velocity, turbulence kinetic energy, and eddy viscosity models have been proposed for the inlet boundary to a flow field. There appears to be no rational criterion for specifying inlet boundary conditions. The present study proposes a criterion to select the inlet boundary conditions by treating the inlet boundary as a part of the flow field. Using this criterion, any prescribed variation of velocity, turbulence kinetic energy k, and rate of dissipation of kinetic energy ε, must satisfy the governing flow field equations at the inlet boundary. Analysis of previously used profiles of velocity, kinetic energy, and dissipation of kinetic energy in the literature indicates that most of these do not satisfy the flow field equations. To substantiate the importance of inlet boundary conditions, several reported numerical simulations using k-ε turbulence models are reconsidered to determine if there is any linkage between the residual errors at the inlet boundary and the errors in the flow field simulation. Based on such an analysis, it is both logical and practical to hypothesize the inlet boundary as a part of flow field.