Journal of Hydraulic Engineering

Computational fluid dynamics (CFD) was used to investigate how residence time analysis based on passive tracer transport in environmental and chemical engineering fluid systems can be significantly affected by flow or tracer release unsteadiness. The unsteadiness may be caused by intermittent influent flow, time-varying operations, and nonuniform tracer release profiles. Oftentimes these unsteady conditions are considered negligible and their impact on residence time analysis is not considered. Systems analyzed included a full-scale stabilization pond, a full-scale oxidation ditch, and full-scale and lab-scale baffled tanks. In the case of the stabilization pond, it was found that time-dependent inflow based on realistic water consumption pattern does not significantly affect mean residence time (MRT) but does have a nonnegligible effect on baffling factor ${\theta }_{10}$ relative to constant inflow. In the case of a surface aerated oxidation ditch, aerator speed was found to have a nonnegligible impact on MRT, and thus it is recommended that CFD tracer studies should consider changes in aerator speed during daily cycles of operation. The slug and step methods were considered for tracer analysis in baffled contactors. It was found that the tracer release time in the slug method should be kept at less than 5% of the theoretical residence time in order to obtain accurate measurements of residence time characteristics. Furthermore, residence time distribution obtained from tracer analysis based on the step method can be affected by the initial transient and subsequent fluctuations in the time series of injected tracer concentration that often characterizes physical experiments.

This paper presents visible experimental results of three-dimensional scour beneath pipelines. A miniature camera installed inside a transparent pipeline was used to observe the scour propagation process under a pipeline in steady currents. The data showed that the average scour hole propagation rate decreased nonlinearly with increasing pipeline embedment. The scour propagation rate increased with the Froude number at an increasing climb rate. The critical flow velocity for scour propagation was investigated. The critical velocity increased with increased pipeline embedment and the flow depth. It was lower than the critical velocity of sediment incipient motion. An empirical equation for the variation of the Froude number with the scour propagation rate was proposed for pipeline embedment ratios $0.045\le e/D\le 0.1$ and relative flow depths $2.73\le y_0/D\le 4.08$.

This paper analyzes the mixing of a tee diffuser discharging in shallow water with currents. Experiments are conducted for tee diffusers with different designs discharging in a crossflow. Measurements of dilution are reported and analyzed along with the results obtained by previous studies to investigate the effects of different outfall and ambient parameters on dilution. The effect of ambient current speed, diffuser length, port spacing, port angle relative to horizontal, ambient depth, nearshore distance, and effluent buoyancy on dilution is examined. Previous investigations have shown markedly different effects of ambient current on the dilution of tee diffuser, characterized by the ratio of ambient momentum to discharge momentum. This apparent confusion is resolved by showing that much of the difference can be attributed to the different range of parameters used in different studies. While most studies have focused on minimum dilution within the plume, dilution measurements at the edge of a regulatory mixing zone are considered in this paper because they are important for satisfying regulatory requirements. Measurements of centerline and flux-averaged dilution at the edge of a mixing zone are reported. Recommendations are made for the design of tee diffusers in shallow receiving waters with crossflow.

Energy dissipation through a rectangular jet discharging into a plunge pool is a common approach in dams engineering. This paper presents the results of a new experimental study on the pressure field generated by a rectangular nonaerated jet impinging on the bottom of a plunge pool, with reference to plunge pools confined by the presence of a drop structure at the upstream boundary. The mean dynamic pressure and fluctuating pressure components at the bottom of a plunge pool were measured and analyzed to define the features that are required in the evaluation of the stability of rock blocks in the plunge pool. The forces acting on blocks in the impingement region were computed, and from this, it was found that the effect of the mean dynamic pressure can be neglected compared with the effect of pressure fluctuations. Because the block stability depends only on the fluctuating pressure coefficient in the jet impingement region, this allows the use of experimental data reported in the literature to extend the range of applicability of this experimental study. This finding can be applied to any block shape. To highlight the application of the results, a design example is presented in this paper where the scale effect due to aeration is taken into account.

This paper presents a theoretically based procedure for predicting the local scour depth and shape downstream of rectangular falling jets induced by high hydraulic structures. This approach, based on a theoretical analysis with the support of laboratory evidence, represents a development in local scour investigations around dams, spillways, drop structures, and other similar hydraulic structures. The adopted methodology computes the equilibrium scour depth using a sediment equilibrium approach, starting from the analysis of the flow field along the scour. It is suitable for application to high drop structure design in alluvial rivers and has accuracy compatible with the engineering practices in this field. In this context, the effect of jet air entrainment on the maximum expected scour depth is highlighted, as well as the time for scour evolution.

A multilayer system with bottom, intermediate, and upper zones is proposed in order to provide full, continuous sediment velocity and concentration profiles. The model is developed with respect to different physical processes that rule sediment transport at various distances from an immobile bed to the water surface. In particular, the variation of shear stress is proposed, characterized by the increase to a maximum value at the bed and its fade-out in the bed. Consequently, the participation of separate layers in total transport is different for various shear rates. Specifically, when bed flow is fully mobilized, the participation of the bottom layer in sediment transport is dominant, whereas its contribution is negligible under conditions close to incipient motion. Then, only the topmost grains of that layer are transported as bed load. The model works in a very wide range of grain mobility conditions, from incipient motion to a fully mobilized bed. The theoretical description was validated with a number of data sets for different grain diameters and densities.

Reliable and efficient pipeline condition assessment in water transmission mains is required to locate deteriorated sections, such that water authorities can rehabilitate or replace vulnerable sections to prevent pipe failure. In this paper, a novel method is developed to estimate pipeline impedance and pipeline wall thickness through hydraulic transient testing. The recently developed reconstructive method of characteristics (RMOC) algorithm is generalized in the current research by relaxing the requirement of a dead-end boundary. Instead, the generalized RMOC as proposed requires two adjacent pressure transducers placed at any interior locations along a pipe to record head variations in a controlled transient event. The parameters along the pipeline can be analytically determined though the smart use of a method of characteristics (MOC) analysis backwards in time. The configuration required by the proposed method makes it applicable in the real world. The proposed approach is first verified by a numerical experiment, where three sections with different wall thicknesses (representing deteriorated sections) are successfully identified. The new technique is then verified by a laboratory experiment, where wall thickness and location of two sections with wall class changes are identified.

The drag reduction method using polymer additives is a common strategy to minimize friction losses when carrying fluids (water, oil, or slurries) in pipes over long distances. Previous studies showed that the interactions between the polymer and turbulent structures of the flow tend to modify the streamwise velocity profile close to the walls by adding a so-called elastic sublayer between the classical viscous and log layers. The gain in linear head losses can reach up to 80% depending on the roughness of the walls and the concentration of polymers. The application of this technique to sewers and the subsequent gain in discharge capacity motivated this work to quantitatively measure the drag reduction in classical open-channel flows. Three measurement campaigns were performed in a dedicated long flume for several water discharges and several polymer concentrations: backwater curves over smooth and rough channel walls (including velocity and turbulent shear-stress profiles) and flows around emerging obstacles. The addition of polymers, even in limited concentrations, allowed a high friction decrease with the typical Darcy-Weisbach coefficient reduced by factors of 2 and 1.5, respectively, in smooth and rough walls configurations without obstacles, but without strong modifications of the nondimensional velocity profiles. In contrast, when adding emerging obstacles, the flow was unaffected by the inclusion of polymers, in agreement with the prediction of the literature. The drag reduction method by addition of small concentrations of polymers thus appears to be a promising technique to increase flow conveyance in open-channel flows.

One of the most uncertain hydraulic designs in urban stormwater management concerns sediment control wet ponds as the event mean total suspended sediment concentration that is discharged into a receiving watercourse depends on many complex factors. The primary factors that influence the performance of wet ponds include design storm event volume, sediment load, suspended sediment particle size distribution, pond permanent pool volume, pond drawdown time, pond length-to-width ratio, and number of berms. In this study we developed a new empirical equation using monitoring data from the Greensborough pond (21 events) and the Ballymore pond (16 events), both of which serviced active construction sites in the Greater Toronto Area, and the numerical simulation results using a calibrated hydrodynamic and sediment transport model. The new equation for both study areas predicts outflow event mean total suspended sediment concentration based on pond hydraulic, geometric characteristics, storm event size, and the influent event mean sediment concentration as input parameters. Sensitivity analysis is performed to investigate the effect of each input parameter on the performance of the pond. Based on the results of these case studies, a series of recommended guidelines for pond hydraulic characteristics are proposed. Moreover, this paper proposes a design methodology on pond design sediment control during the construction period to better ensure protection of the aquatic life in the receiving streams.

A moment stability analysis method which computes a safety factor using shear stress and flow velocity was previously developed for assessing the stability of articulated concrete block (ACB) systems during overtopping flow. The present study used the shear and velocity stability assessment (SVSA) method to derive safety factor equations for evaluating channelized flow. The stability analysis method for channelized flow excludes several assumptions used in previous methods, including calculating the rotation angle for movement and using a ratio of the boundary shear stress to critical shear stress to account for all hydrodynamic forces. The SVSA channelized flow equations provide a reliable stability assessment approach for ACB systems. Performance of the SVSA method for channelized flow was evaluated using a database that included overtopping tests for three ACB systems with varying embankment slopes and lengths, and channelized tests for one ACB system. The previous design methodology predicted stability for 71% of the tests and the instability threshold for 50% of the installations. The SVSA approach predicted stability for 92% of the tests and the instability threshold for 83% of the installations. Therefore, the SVSA method provides an enhancement for predicting stability of ACB systems in channelized flow conditions.

Streams of water in channels that have a compound cross section interact with each other. The most characteristic feature of such channels is the transfer of momentum and mass from the main channel to the floodplains. A laboratory experiment was conducted in a two-stage channel to investigate the impact of stream interactions on discharges. Various roughness conditions of the channel bottom and sloping banks were considered. An original idea was to eliminate stream interactions using glass walls placed in the interface between the main channel and the floodplains. This resulted in significant differences in water levels between these parts of the channel. Measured mean-time velocities were used to analyze changes of discharges in the main channel and floodplains in cases when the streams interact with each other and when these interactions are eliminated. A comparison of the measured velocity distributions for various parts of the compound channel with measurements taken when the floodplains were disconnected from the main channel showed that the mean velocity increased in the main channel and in the floodplains. The results obtained could constitute significant information for the increasingly popular two-stage channel building projects for hydraulic engineering purposes.