Journal of Hydraulic Engineering

Bubbles entrained in the plunge pools of hydraulic structures are subjected to the hydrostatic pressure of the water column. The hydrostatic pressure increases the saturation concentration of gases within the bubbles and correspondingly alters the rates of gas transfer. An “effective saturation concentration” is introduced to account for the elevated pressure within the bubbles. The effective saturation concentration must be incorporated into gas-transfer calculations to accurately predict downstream concentrations of dissolved gases. A method of measuring the effective saturation concentration from simultaneous measurements of dissolved oxygen and methane is presented. The method is verified through similar measurements of dissolved nitrogen and methane. Applications demonstrate the use of the effective saturation concentration in the prediction of dissolved-gas levels downstream of hydraulic structures and the use of dissolved-gas measurements in the determination of the effective saturation concentration.

The fundamental behavior of unsteady flows in an open channel with a rectangular dead zone is investigated experimentally and numerically to clarify the mass-exchange mechanism near an embayment along a river bank. Typical flow features such as generation of a coherent vortex due to shear instability in the mixing layer between two flow regions (main-channel flow and dead zone) and water-surface oscillation (seiche) in the dead zone are shown by the findings of laboratory tests. These flow features can be reproduced numerically by plane two-dimensional (2D) open-channel flow equations in the variable grid system. Model performance was examined in detail by changing the grid size and constants. The calculated values for temporal and spatial variations in velocity and depth are compared with observed values. Examination of the calculated results shows that temporal velocity variations at the interface can be decomposed into two components due to seiche and shear instability; during one seiche, cycle-velocity variations caused by instability are amplified to the large-scale vortex selectively by the interaction between seiche and instability components. This selective amplification process can be detected in a series of photographs of the surface flow pattern as well.

A new model for the computation of unsteady friction losses in transient flow is developed and verified in this study. The energy dissipation in transient flow is estimated from the instantaneous velocity profiles. The ratio of the energy dissipation at any instant and the energy dissipation obtained by assuming quasi-steady conditions defines the energy dissipation factor. This is a nondimensional, time-varying parameter that modifies the friction term in the transient flow governing equations. The model was verified for laminar and turbulent flows and the comparison of measured and computed pressure heads shows excellent agreement. This model can be adapted to an existing transient program that uses the well-known method of characteristics for the solution of the continuity and momentum equations.

This paper presents the results of preliminary observations of turbulence at the inlet to a pipe culvert using a bidirectional electromagnetic current meter (ECM). Instantaneous longitudinal and vertical components of velocity have been sampled in the approach section to a pipe culvert. The results correspond to a pipe culvert with a sharp edged inlet, r/D= 0.015, with relative headwater depths of H/D= 1.62, 1.26, and 0.96, laid on a 0.3% slope. The distributions of mean velocities, turbulence intensity, and characteristic eddy scales are shown to vary with proximity to the culvert headwall. It is demonstrated that there is a significant increase in the magnitudes of the mean velocity components and that the distribution of the turbulence intensity levels and eddy scales undergo significant changes as the flow approaches the inlet.

This paper summarizes many of the results from an extensive program of bridge scour research undertaken at The University of Auckland, New Zealand. An integrated approach to the estimation of local scour depth at bridge piers and abutments, collectively termed bridge foundations, is presented. The design method is based on empirical relations, termed K-factors, that account for the effects on scour depth of flow depth and foundation size, flow intensity, sediment characteristics, foundation type, shape and alignment, and approach channel geometry. The K-factors are evaluated by fitting envelope curves to existing data for piers and abutments and a new extensive data set for abutments. The new data are discussed and presented. Application of the design method is illustrated in examples.

The application of the Muskingum model to river and channel flood routing may have some limitations because of its inherent assumption of a linear relationship between channel storage and weighted flow. Although nonlinear forms of the Muskingum model have been proposed, an efficient method for parameter estimation in the calibration process is still lacking. In this paper, the objective approach of genetic algorithm is proposed for the purpose of estimating the parameters of two nonlinear Muskingum routing models. The performance of this algorithm is compared with other reported parameter estimation techniques. Results of the application of this approach to an example with high nonlinearity between storage and weighted-flow, show that the genetic algorithm approach is efficient in estimating parameters of the nonlinear routing models. A supplementary analysis of the sensitivity of the parameters during the performance of genetic algorithm shows that a unique set of parameters exists that would result in the best performance for a given problem. In addition, genetic algorithm does not demand any initial estimate of values of any of the parameters, and thus avoids the subjectivity and computation time associated with the traditional estimation methods.

Analytical solutions are provided for the one-dimensional transport of a pollutant in an open channel with steady unpolluted lateral inflow uniformly distributed over its whole length. This practical problem can be described approximately by spatially variable coefficient advection and advection-diffusion equations with the velocity proportional to distance, and the diffusion coefficient proportional to the square of the velocity. Using a simple transformation, the governing equations can be transformed into constant coefficient problems that have known analytical solutions for general initial and boundary conditions. Analytical solutions to the spatially variable coefficient advection and advection-diffusion equations, written in conservative and nonconservative forms, are presented. The analytical solutions are simple to evaluate and can be used to validate models for solving the advection and advection-diffusion equations with spatially variable coefficients. The analytical solutions show that nonconservative forms of the equations can yield exact solutions that are not consistent with the physical problem.

A new and simplified formula for predicting the settling velocity of natural sediment particles is developed. The formula proposes an explicit relationship between the particle Reynolds number and a dimensionless particle parameter. It is applicable to a wide range of Reynolds numbers from the Stokes flow to the turbulent regime. The proposed formula has the highest degree of prediction accuracy when compared with other published formulas. It also agrees well with the widely used diagrams and tables proposed by the U.S. Inter-Agency Committee in 1957.

Outflow hydrographs for three dams with long lakes in narrow valleys were computed by both hydrologic routing (level-pool routing) and hydraulic routing. They were compared with three inflow hydrographs of different peaks with three unregulated ogee spillways of different capacities. In all these cases, the difference between outflow hydrographs was greatest at the peak value, growing larger as the spillway capacity became smaller, relative to the magnitude of the inflow hydrograph. The peak outflow by hydraulic routing was smaller than that by hydrologic routing for all the routing combinations, the difference varying between 2 and 11%.

A study to find the hydraulic behavior of galvanized iron and copper pipes, of ages between 15 and 50 yr and of up to 50.8 mm (2 in.) in diameter, is presented. It was found that in the galvanized iron pipes, the diameter is reduced and both the roughness and the losses coefficient increase with age. The increase in roughness is substantial and follows a parabolic law instead of the linear one usually found in the literature. For the copper pipes there is no appreciable change in these variables with age. The apparition of local narrowness, or stenosis, due to age, may cause considerable increases in the losses coefficient and eventually even close the pipe completely.