Journal of Hydraulic Engineering

Partially treated wastewater is often discharged into coastal waters through an outfall diffuser fitted with clustered ports on risers. On each riser the effluent is discharged through two to eight ports arranged circumferentially, in the form of a rosette-shaped buoyant jet group. The near field mixing of such a jet group in a tidal flow is determined by the merging and interaction of coflowing, oblique-flowing, cross-flowing, and counterflowing jets. Despite numerous studies, a general predictive method for such complex jet groups has not been reported; ocean outfall design is often based on comprehensive physical model experiments. The mixing of merging nonbuoyant and buoyant jets issuing from a rosette outfall riser into an ambient current is studied experimentally by using the laser-induced fluorescence technique. Detailed cross-sectional measurements of the scalar concentration field downstream of the bent-over jets are made. The trajectories of multiple and individual jets discharging at various angles are measured. For typical outfall designs, the dynamic interaction of adjacent jets is found to be negligible. The average dilution of the jet group can be predicted by accounting for jet merging and plume overlapping. Theoretical predictions using the Lagrangian VISJET model are in excellent agreement with the experimental data and also results of previous studies in a stratified crossflow. The model correctly predicts the changes in near field dilution as a function of the number of nozzles on a riser, or the number of risers on a diffuser, and helps to resolve observed anomalies in previous studies.

Many rivers in Taiwan have steep slopes, are subject to typhoon-induced flood flows, and contain soft bedrock that is exposed at many locations and easily erodible. The occurrence of extensive bedrock erosion has been a major threat to river infrastructure at many locations. Soft bedrock erosion, therefore, is an important process to consider for river projects in Taiwan. In this study, bedrock erosion models are reviewed. A specific model is proposed by combining two existing models incorporating both the hydraulic and abrasive scour mechanisms. The proposed bedrock erosion model is incorporated into a two-dimensional mobile-bed model, and the integrated model is tested by simulating bedrock erosion downstream of the Chi-Chi weir on the Choshui River in Taiwan. A calibration study is performed to determine appropriate values of the model parameters based on two and a half years of measured data. The model is then assessed based on a verification study that compares model predictions of bedrock erosion of the same reach to two additional years of measured data. The bedrock erosion model is found to be suitable for the river reach studied. Further improvement, however, is still necessary, which points to potential future research.

In this paper, the authors present experimental results of overbank flow in compound channels with nonprismatic floodplains and different convergence angles. The depth-averaged velocity, the local velocity distributions, and the boundary shear stress distributions were measured along the converging flume portion for different relative depths. The momentum balance is used to analyze the force acting on the flow in the main channel and for the whole cross section. Using the experimental data, various terms in the momentum equation are also calculated. The apparent shear forces on the vertical interface between the main channel and floodplains are evaluated for compound channels with nonprismatic floodplains, and the results are then compared with the prismatic cases. The energy balance in nonprismatic compound channels is also investigated by using the water surface elevation.

A new approach for studying forced hydraulic jumps below abrupt symmetric and asymmetric expansions is introduced. Intensive experiments for two expansion ratios, one asymmetry ratio, and different solid sill heights and locations were conducted specifically for the spatial jump case as a critical design case. A new parameter correlating sill height and sill location is presented and found to play an important role in the basin’s design, scour, and flow patterns. Using this parameter gives distinguished relationships for predicting sequent depth, energy dissipation, and basin length for the preliminary or operating design in expected or unexpected flow conditions. Empirical equations and experimental regression curves are introduced to predict the sequent depth and the basin length at any sill height and sill location. Scour and flow patterns are intensively studied experimentally and qualitatively described for the symmetric and asymmetric basins with or without sills for the spatial jump. A numerical example is presented to illustrate the applicability of the study in preliminary and operating designs.

A diversion tunnel is planned for flood protection in the City of Lyss, Switzerland. This tunnel includes a junction, from which water from a side creek is added to the tunnel flow. The latter is sensitive to choking, so that the junction had to be optimized in a hydraulic laboratory investigation. Its setup finally consists of a drop shaft and an injector generating annular flow in the tunnel. Free surface tunnel flow then occurred for all relevant discharges. This setup is also of interest in sewer systems as well as in spillway and diversion tunnels, where similar problems may occur. The concept was optimized for a specific prototype. General recommendations may be derived for similar situations.

This paper applies dimensional analysis to propose an alternative model for estimating the effective density of flocs ($\Delta {\rho }_f$). The model takes into account the effective density of the primary particles, in addition to the sizes of the floc and primary particles, and does not consider the concept of self-similarity. The model contains three dimensionless products and two empirical parameters (${\alpha }_f$ and ${\beta }_f$), which were calibrated by using data available in the literature. Values of ${\alpha }_f=0.7$ and ${\beta }_f=0.8$ were obtained. The average value of the primary particle size ($D_p$) for the data used in the analysis, inferred from the new model, was found to vary from 0.05 μm to 100 μm with a mean value of 2.5 μm. Good comparisons were obtained in comparing the estimated floc-settling velocity on the basis of the proposed model for effective floc density with the measured value.

United States Geological Survey streamflow data are commonly used for hydraulic model calibration and boundary conditions. The transitory nature of sand-bed rivers’ bathymetry is problematic for the traditional automated stream gauging methods used by the USGS. This note seeks to assess the limitations of streamflow measurements for use in hydraulic models. An overview of USGS rating-curve development and use is presented with a focus on the specific challenges of sand-bed rivers. Measurements from three consecutive USGS gauges for a storm event on the Rio Grande in Albuquerque, New Mexico, illustrate the outlined problems with rating curves. These gauges are utilized to study the impact of uncertainty in rating-curve discharges on hydraulic model results. A one dimensional hydraulic model of the study reach indicates up to 25% reduction in the calculated flow depth if questionable rating-curve discharges are used as model input. Recommendations for using USGS streamflow data in hydraulic models are outlined.

Looped water distribution systems are conventionally analyzed using iterative methods such as Cross, Linear, Newton-Raphson, and Gradient algorithm methods. Depending on the unknown (hydraulic head or discharge), the methods are characterized as $h$ or $Q$ methods. This paper focuses on the $h$-Newton-Raphson method, which uses the Darcy-Weisbach head loss equation. The paper presents a procedure for improving the $h$-Newton-Raphson iterative procedure by directly calculating the discharge of each branch by using the Swamee and Jain equation. The proposed procedure leads to a simplified algorithm and more accurate determination of the Jacobian matrix, which accelerates the convergence of the algorithm.