Journal of Hydraulic Engineering

The effect of channel sinuosity on flow pattern in meandering streams is investigated. The centerlines of the idealized meandering streams under consideration follow sine-generated curves, and the banks are rigid; the flow is turbulent and subcritical. This study focuses on the vertically averaged flow over a flat (horizontal at any cross section) bed formed by a granular material. The “flat bed” is viewed as the initial surface of a moveable bed at the beginning of an experiment (at time $t=0$ ). A series of laboratory flow measurements involving the systematic variation of the deflection angle ${\theta }_0$ from 30 to $110°$ , is used. It is found that every different sinuosity (every different ${\theta }_0$ ) has its own convective flow pattern, i.e., its own distribution in plan of (the $L∕2$ long) convergence–divergence zones of flow. As ${\theta }_0$ increases, a gradual change in flow pattern is observed. Two expressions defining the observed ${\theta }_0$ variation of the convective flow pattern are introduced. It is shown, with the aid of the sediment transport continuity equation, that the geometry of the developed bed at the end of an experiment is strongly related to the convective behavior of the vertically averaged (initial) flow over the flat bed at $t=0$ . In particular, information on the ${\theta }_0$ variation of the convective pattern of the initial flow can be used to estimate the location of erosion–deposition zones and the location(s) of the most intense erosion–deposition corresponding to any ${\theta }_0$ .

Vegetation can impede flow in open channels. The control of vegetation by hydraulic means is a nonchemical approach that can be attractive. To control the vegetation, the calculation of the lodging velocity (velocity at which the plant stem breaks) is an important parameter. A theoretical method is developed here for a single stem fully exposed to the flow, assuming that the drag force is the sole failure-inducing force. Results are compared to field results and the trends analyzed. The comparison and field visual observations may indicate that vortex shedding (not accounted for in the model) could be the dominant failure-inducing force, and not the drag force, as assumed.

Mixing characteristics of conservative pollutants were examined two-dimensionally in a laboratory meandering channel, and a method to compute the dispersion coefficients was developed based on the measured concentration data. To investigate how the hydrodynamics influences pollutant mixing in meandering channels, both flow and tracer experiments were conducted in an S-curved laboratory channel. A two-dimensional routing procedure was presented to evaluate the longitudinal dispersion coefficient as well as the transverse dispersion coefficient under the unsteady concentration condition. The results of the tracer experiments showed that the tracer cloud behaved quite differently depending on whether or not the tracer cloud was transported following the filament of maximum velocity. Also, separation and reemerging of the tracer cloud were promoted by secondary currents. The observed transverse dispersion coefficients obtained by the routing procedure were close to those obtained by existing moment methods. The transverse dispersion coefficient tended to increase with an increasing aspect ratio, whereas it is not sensitive to the injection location. However, the longitudinal dispersion coefficient was sensitive to the injection location as well as the aspect ratio.

Model studies have been undertaken to study the spatial and temporal development of a brackish pool within an impoundment supplied by a freshwater inflow and bounded by an impermeable barrier that is overtopped periodically by a tidally generated saline water inflow. The results demonstrate the relative influence of the freshwater inflow and downstream tidal conditions in controlling the temporal development of this pool. In particular, the experimental data illustrate that the dimensions of the brackish pool reach equilibrium after a specific number of tidal cycles, with the normalized thickness of the pool being dependent primarily on the strength of the freshwater inflow. The density structure of the water within the impoundment is interpreted in terms of two contributory processes, namely, (1) turbulent entrainment of fresh receiving water into the saline intrusion; and (2) shear-induced interfacial erosion of the brackish water by the overriding fresh water during receding tidal conditions. A scaling analysis shows that the temporal growth of the brackish pool thickness can be parameterized successfully in terms of a volume ratio representing the dilution capacity of the freshwater stored in the impoundment during the saline intrusion phase.

A new technique for drawing isovel patterns in an open or closed channel is presented. It is assumed that the velocity at each arbitrary point in the conduit is affected by the hydraulic characteristics of the boundary. While any velocity profile can be applied to the model, a power-law formula is used here. In addition to the isovels patterns, the energy and momentum correction factors ( $\alpha$ and $\beta$ ), the ratio of mean to maximum velocity $(V∕u_{\max })$ , and the position of the maximum velocity are calculated. To examine the results obtained, the model was applied to a pipe with a circular cross section. A comparison between the profiles of the proposed model and the available power-law profile indicated that the two profiles were coincident with each other over the majority of the cross section. Furthermore, the predicted isovels were compared with velocity measurements in the main flow direction obtained along the centerline and lateral direction of a rectangular flume. The estimated discharge, based on measured points on the upper half of the flow depth away from the boundaries was within $\pm 7\%$ of the measured and much better in comparison to the prediction of one- and two-point methods. The prediction of the depth-averaged velocity values for the River Severn in the United Kingdom shows a good agreement with the measured data and the best analytical results obtained by the depth-averaged Navier–Stokes equations.

Despite the recent development of risk-based assessments in flood defense, in practice, flood levels in channels are generally estimated for a design return period by using the discharges of the same return period. The flood levels are also influenced by other factors such as bed roughness, flow area, wetted perimeter, and friction slope which are random in nature. The surveyed cross sections and calibrated values of roughness coefficients are generally used without any allowance for their variability to assess the water levels based on discharge of a selected return period. This simplifies a multivariate random process to a single-variate random process. When the uncertainties of selected influencing parameters were considered in sample studies using Monte Carlo simulation, it was found that the traditional procedures result in an underestimation of water level at high return periods and over estimation of water levels at low return periods. The overall effect is the diffusion of the density function from the center toward the tails of the distribution. Sample studies using the variation in channel roughness and friction slope indicated that the return period of the water level, based on a 100 year return period discharge, varied from 32 to 82 years, depending on the statistical properties of the influencing random parameters. The frequency analysis of water levels was undertaken by analyzing 25–800 million generated data sets.

A large variety of hydraulic jumps on horizontal and sloping inverts at the end of an ogee standard weir is investigated. An ogee standard weir was used to create supercritical flow and slopes of 0.0, $-0.025$ , $-0.05$ , $-0.075$ , and $-0.10$ were built downstream of the weir. Based on the momentum equation in the horizontal direction, a method to predict the sequent depth ratio is presented. The theory agrees well with the results of the writers and previous investigators. A correlation was developed to predict the minimum Froude number needed to establish jumps on negative slopes. Observations showed that in those cases where the gravity force component in the jump was opposite to the flow direction, the water surface of the surface roller became undular and unstable. The hydraulic jump on an entirely adverse slope was almost impossible to control. The analysis of experimental data showed that the negative slope of the basin reduces the sequent depth ratio, while a positive slope increases the sequent depth ratio.

Scour at a downstream vertical gate of a dam is investigated using a fuzzy logic inference system. The application is presented for experimental data sets. A comparison is made between a regression formulation (RM) and a fuzzy logic approach. Some restrictive assumptions for RM are discussed. Here the Takagi–Sugeno (TS) approach is used with a constant function in the consequent part of the fuzzy rules. It is demonstrated that the TS model gives a lower error than the RM. Furthermore, scatter diagrams indicate that the TS approach has superiority over the RM.

Sewage pumping stations represent an element of the sewer system, which is directly responsible for affecting serviceability; i.e., failing pumps may result in combined sewer overflows or flooding. However, failures of sewage pumps are not yet incorporated in sewer assessments due to lack of data. This paper presents the analysis of pump failure data provided by two sewer management authorities in The Netherlands. Pump failures have been studied accounting for the nature of the failures, the operation and maintenance procedures of the management authority, the aging of the pumps, and the changes in the environment of pumps. The analysis shows that sewage pumps fail relatively often due to the composition of sewage and the discontinuous operation of the pumps. The interarrival time and the duration of failures are highly variable and independent of the specific function of the pump. Resulting pump failure characteristics are applied in a Monte Carlo simulation to calculate the impact of failures on combined sewer overflow volumes. The results indicate that the serviceability of sewer systems is significantly affected by failing pumps. Therefore, including pump availability in sewer system assessments should be considered.

An experimental and numerical study has been carried out to study the wave boundary layers under asymmetric waves. The experiments were conducted in an oscillating tunnel using a simple mechanical system to generate an asymmetric oscillatory motion similar to cnoidal waves. The velocities were measured by laser Doppler velocimetry and the bottom shear stress was calculated from the cross-stream velocity profile. A low Reynolds number $k–\epsilon$ model was used to predict the hydrodynamic properties of the cnoidal wave boundary layers. After validating the model with the experimental data, a series of numerical experiments were carried out to study the transitional behavior of these boundary layers by virtue of friction factor and phase difference between mean free-stream velocity and bottom shear stress. Finally a stability diagram was drawn to demarcate the laminar, transition, and fully turbulent regimes using the numerical results. The present study would be useful for the hydraulic and coastal engineers interested in calculating bottom shear stress in order to compute the sediment transport in coastal environments.

The Shields diagram remains the most widely used criterion for incipient motion of sediment. However its implicit nature makes applications rather inconvenient. By deploying Guo’s logarithmic matching method twice, this technical note develops an explicit formulation of the Shields diagram, enabling the critical Shields parameter to be determined directly from fluid and sediment characteristics without resorting to any trial and error procedure or iteration. An extended application of the logarithmic matching method is demonstrated.

Experiments of local scour around pile groups are carried out under steady clear-water scour conditions. A variety of conditions including different pile group arrangements, spacing, flow rates, and sediment grain sizes are considered. In total, 112 experiments are carried out. It is observed that the scour-hole depth for some cases of pile groups increases as much as two times more than its magnitude for the case of single piles. The data from this study and some laboratory experiment data from previous works are used to derive a correction factor to predict the maximum local scour depth for the pile groups. Two well-known equations, i.e., Federal Highway Administration, Hydraulic Engineering Circular No. 18, HEC-18 (reported by Richardson and Davis in 2001) and the New Zealand pier scour equation (reported by Melville and Coleman in 2000) are considered. The prediction of scour hole based on the present correction agrees well with the observations.

This technical note explores the applicability of the general nodal point relation to describe the distribution of sediments at channel bifurcation. Experiments were conducted in a physical model of channel bifurcation to estimate the coefficient and exponent of the nodal point relation for different nose angles and upstream discharges. It was found that the nose angle is the major variable for the distribution of sediments to the downstream branches. The value of the exponent $k$ found from experimental results was compared with that of a theoretical analysis. For a stable equilibrium, the value of $k$ is greater than $5∕3$ from theoretical consideration when the Engelund–Hansen sediment transport formula is used. This was confirmed from the experimental results. This suggests that the distribution of sediments at channel bifurcation can be expressed by the general nodal point relation.

Conventional stability analysis methods for determining rock armor size to protect steep $(\tan \theta >0.1)$ embankments from erosion due to downslope flow do not predict the correct dependency of armor size on the slope. Consequently, such methods become very conservative (discharge factor of safety $\sim 8$ ) at high slopes. We review published large-scale measurements recorded in the literature and show that adequate representation of flow aeration is important as proposed by Hartung and Scheuerlein in 1970. However, the assembled measurements show that the Hartung and Scheuerlein design method is not conservative. The aerated flow equations of Hartung and Scheuerlein, when combined with a correctly formulated stability equation, yield good collapse of the available large-scale data and a conservative design method.