Journal of Hydraulic Engineering

Although open-channel junctions are common in many hydraulic structures, no comprehensive data set has been compiled that describes the 3D flow field within the junction itself. This physical model study examined a 90°, sharp-edged, open-channel junction for channels of equal width. Depth measurements were made using a point gauge while velocity measurements were taken using an acoustic doppler velocimeter over a grid defined throughout the junction region. The average velocity and turbulence intensity were calculated from a time series of velocities that was recorded at each location. In addition, a 2D mapping of the water surface was performed on a 76.2 mm square grid throughout the channel junction. This paper presents the details of the experimental procedure and the general flow characteristics observed. The full data set generated during this experimental work is available for downloading on the Internet. Using a small portion of the data recorded, an evaluation of several previously proposed theories of combining flow in open-channel junctions is presented. This has revealed that the simplified mathematical model gives reasonable prediction of the experimental results. The complete data set describing combining flows at a 90° open-channel junction is presented as a resource for the validation of 3D computational fluid dynamics codes that utilize a free-surface model.

This paper studies laboratory open-channel confluences using a 3D, elliptic solution of the Reynolds-averaged Navier-Stokes equations, including a method for approximating the effects of water surface elevation patterns and a renormalization group modified form of the k-ε turbulence model. The model was tested by comparison with laboratory measurements of an asymmetric tributary junction. This suggests that although the model is unable to reproduce the quantitative detail (notably upwelling velocity magnitudes) of the flow structures as measured in laboratory experiments, statistically significant aspects of the experimental observations are reproduced. The model is used to (1) describe and explain the characteristic flow structures that form in a confluence with one of the tributaries angled at 45°, both with and without an elevation difference (bed discordance) in the angled tributary; and (2) investigate the relative importance of junction angles (30°, 45°, and 60°), bed discordance, and ratio of mean velocities in the tributary channels upon flow structures. This shows that bed discordance significantly enhances secondary circulation because of the effects of flow separation in the lee of the bed step, which significantly increases lateral pressure gradients at the bed and reduces water surface superelevation in the center of the tributary and water surface depression at the downstream junction corner. Extension to consideration of a number of junction angles, levels of bed discordance, and velocity ratios suggests that a small (10%) reduction in tributary depth can significantly increase the intensity of secondary circulation, albeit in a relatively localized manner. Simulations involving a numerical tracer illustrate the importance of bed discordance for mixing between the two flows and question the use of simple 2D parameterizations of mixing processes that do not consider bed discordance when the latter is present.

This paper investigates the discharge of a vertical round jet into an unsteady cross-flow that consists of a mean flow and a sinusoidally oscillating component. An experimental technique is devised to simulate the unsteady cross-flow situation in the laboratory. A vertical jet-pipe-nozzle assembly is physically oscillated backward and forward in the steady flow stream of a laboratory flume, and the flow patterns are viewed by an observer moving with the jet. Dispersion patterns of the dye-marked jet fluid are studied with a phase-locked analysis of digitized flow images. Oscillations in cross-flow velocity are found to organize the jet fluid into regular large-scale fluid patches, which, after time averaging, lead to a widened jet width and enhanced dilution. Experiments are also carried out on a vertical jet discharging into a current with surface waves, a situation that approximates to a genuine oscillating cross-flow with a nonzero mean velocity. The two sets of experimental observations are found to match with each other. The validity of the experimental simulation technique is further supported by a computational fluid dynamics study of the flow problem, in which it is possible to produce an idealized oscillating cross-flow situation. The numerical results agree well with the experimental observations.

A new methodology for the solution of shallow water equations is applied for the computation of the unsteady-state flow in an urban drainage network. The inertial terms are neglected in the momentum equations and the solution is decoupled into one kinematic and one diffusive component. After a short presentation of the DORA (Double ORder Approximation) methodology in the case of a single open channel, the new methodology is applied to the case of a sewer network. The transition from partial to full section and vice versa is treated without the help of the Preissmann approximation. The algorithm also allows the computation of the diffusive component in the case of vertical topographic discontinuities, without the introduction of any internal boundary conditions and without any change in the structure of the linear system matrix. The algorithm is tested on one field example, and its performance is compared with the performance of other popular commercial codes.

The ability of turbulence models, based on two equation closure schemes (the k-ε and the k-ω formulations) to compute the mean flow and turbulence structure in open channels with rigid, nonemergent vegetation is analyzed. The procedure, developed by Raupach and Shaw (1982), for atmospheric flows over plant canopies is used to transform the 3D problem into a more tractable 1D framework by averaging the conservation laws over space and time. With this methodology, form/drag related terms arise as a consequence of the averaging procedure, and do not need to be introduced artificially in the governing equations. This approach resolves the apparent ambiguity in previously reported values of the drag-related weighting coefficients in the equations for the turbulent kinetic energy and dissipation rates. The working hypothesis for the numerical models is that the flux gradient approximation applies to spatial/temporal averaged conservation laws, so that the eddy viscosity concept can be used. Numerical results are compared against experimental observations, including mean velocities, turbulence intensities, Reynolds stresses, and different terms in the turbulent kinetic energy budget. The models are used to further estimate vegetation-induced flow resistance. In agreement with field observations, Manning's coefficient is almost uniform for some critical plant density and then increases linearly.

This paper presents the results of an experimental study on scour around a pile subject to combined waves and current. Irregular waves were used in the experiments, which were carried out both for codirectional waves and for waves propagating perpendicular to the current. The measured scour depth is plotted as a function of U_cw = U_c/(U_c + U_m) for various values of the Keulegan-Carpenter number KC in which U_c is the undisturbed current velocity and U_m is the maximum value of the undisturbed orbital velocity at the sea bottom. In the experiments, KC ranges from 5 to about 30 and U_cw from 0 to 1. The results show that the scour depth increases with increasing current component of the flow. The scour depth attains its steady-current value for U_cw > 0.7.

Although riprap is the most commonly employed countermeasure against scouring around bridge piers, few studies exist of riprap performance under live-bed conditions. In this study, failure mechanisms, stability, and placement level effects for riprap at bridge piers are considered experimentally. Under clear-water conditions, riprap is subject to shear, winnowing, and edge failure. Under live-bed conditions, a fourth failure mechanism, destabilization by bed-form progression, becomes important. Destabilization by bed-form progression is dependent on the destabilizing influence of bed-form troughs as they pass the pier. Experiments were used to assess the ability of riprap stones to protect bridge piers under a wide range of flow conditions. The effects of placing the riprap layer at depth within the sediment bed, rather than level with the bed surface, were investigated also. The study showed that, as the flow velocity increases, the ability of riprap stones to protect a pier decreases asymptotically until the scour depth in the riprap layer reaches that of an equivalent unprotected pier. In addition, it was found that the deeper the placement level the less exposed the riprap was to destabilizing bed forms and the better the protection against local scour. Lowering the placement level also meant that the riprap performed better than for surface-placed layers as the flow velocity increased. The mode of riprap failure is also changed as the placement level below the bed surface is lowered. A pier riprap size-prediction equation is proposed, including a parameter to account for placement level.

A new modeling framework for internal erosion in heterogeneous stratified soils is developed by combining existing methods used to study sediment transport in canals, filtration phenomena, and stochastic processes. The framework is used to study the erosion process in sediments caused by a flow of water through a covering filter layer that deviates from geometrical filter rules. By the use of spectral analysis and Laplace transforms, mean value solutions to the transport rate and the variance about the mean are derived for a transport constraint at the upstream boundary and a constant initial transport. Increasing the heterogeneity of a defective filter layer in a canal retards the propagation of the expected bed degradation. An implication is that the heterogeneity of the riprap in this respect prolongs the expected degradation process and the expected lifespan of the structure. The increased variance and decreased correlation length of the riprap reduces the confidence of the mean value solution. The stochastic analysis is therefore suitable as part of risk analyses of extreme events such as failure of embankment dams, for which empirical observations are rare.