Journal of Hydraulic Engineering

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.

Laboratory experiments were conducted to investigate the reduction of flood depth in a channel due to an engineered levee breach. Different combinations of inflow hydrographs, breach openings, and floodplain storage areas were considered. Inflow hydrographs with various peak discharges, hydrograph shapes, and flood durations were generated by a computer-controlled valve. A breach was created instantaneously on the channel sidewall as the flood wave approached the breach location. Regression analyses were conducted to express the reduction of flood depth as a function of the breach width, channel width, distance of the selected location from the breach centerline, hydrograph characteristics, and the area of the floodplain basin. The results show that the engineered levee breach provided an effective reduction of flood depth both upstream and downstream of the breach—more so upstream of the breach—by modifying the flood wave. The breach width and floodplain area are found to play a significant role in depth reduction.

Water hammer flows are almost simulated by using the Eulerian mesh methods including the most popular method of characteristics (MOC). In this paper, smoothed particle hydrodynamics (SPH) of the Lagrangian meshless method are introduced to simulate transient pipe flow considering the unsteady friction model (UFM). One special boundary treatment with virtual and mirror particles is proposed to improve the inherent boundary deficiency. Pressure results predicted by the SPH model are compared with those obtained from MOC scheme and experiments in a reservoir-pipe-valve system. The proposed model can accurately reproduce the experimental pressure histories. As the Courant number decreases (less than one), the SPH method is more accurate and more robust without numerical attenuation, whereas the Godunov scheme and MOC scheme produce obvious numerical damping. When the Courant number is less than one, for a similar level of accuracy, the SPH method could be more efficient than the Godunov scheme and MOC scheme. It is found that implicit and explicit solution schemes of the unsteady friction model, number of particles, artificial viscosity, smoothing length, and smoothing function significantly influence numerical stability and accuracy. Artificial viscosity can eliminate spurious numerical oscillations but may cause numerical dissipations. Cubic spline function and smoothing length close to the particle distance are suitable for the proposed model.

Understanding the air demand of a hydraulic jump in a closed conduit is important in hydropower operations and urban sewer designs. In this study, physical experiments are described to study flow regimes and the air demand of a hydraulic jump in a closed conduit with various submerged outlet depths. Flow regimes with a submerged outlet were defined following previous studies based on outlet depth. Free-surface supercritical flow with a hydraulic jump can induce a relative air demand (air flow rate to water flow rate) of approximately 3%–14%. If the hydraulic jump is followed by pressurized pipe flow, the air demand decreases with increasing outlet depth until the return roller is fully developed in the pipe. For the partially submerged hydraulic jump, the relative air demand is significantly reduced to less than 1%. Field measurements of the air demand at the Hugh Keenleyside Dam, British Columbia, were consistent with the experimental measurements for the partially submerged hydraulic jump. The dynamics of the air pocket upstream of the hydraulic jump was studied. If the air supply was constrained by nozzles of various sizes placed on the top of the air vent, the air pressure in the closed conduit decreased and the hydraulic jump was pushed upstream. The energy loss coefficient in the air vent was also studied.

Focusing on the effect of the meandering planform, two functions are investigated by a numerical experiment: the first represents the ratio between the intensities of the resisting forces acting on a meandering flow and its corresponding straight flow; and the second represents the ratio between the intensities of the resisted meandering flow itself and its corresponding straight flow. Both functions depend on the sinuosity (or relative curvature) and the width-to-depth ratio. The two functions and their numerical results are thoroughly analyzed, showing the following: (1) meandering causes an increase in the resistance to flow, with a greater resistance occurring in the stream having an “intermediate” sinuosity (or “large” relative curvature) and a “small” width-to-depth ratio; (2) there is an imbalance between the driving and resisting forces in a meandering stream, and the degree of this imbalance has a direct interrelation with the intensity of the “kinetic-energy-weighted” convergence of flow; and (3) the ratio of kinetic to potential energy of a meandering flow cannot be reflected by the square of the (Chézy) resistance factor alone, and the two things manifest different variation patterns versus the sinuosity and the width-to-depth ratio. These findings are likely to advance the understanding of the resistance to flow in meandering channels.

Salt water intrusion through the New Sea Lock of IJmuiden, Netherlands requires mitigation to ensure availability of enough fresh water further inland. For this purpose, a salt screen has been proposed for selective withdrawal of salt water from the Noordzeekanaal in the vicinity of the lock complex. Formulas to assess the withdrawal rate of selective withdrawal are based on idealized layouts and conditions. In the case of IJmuiden, the flow surrounding a salt screen has a strong nonuniform character, such that these formulas are not applicable to predict the correct withdrawal rate and the effectiveness of selective withdrawal accurately. In this case physical scale modeling or computational fluid dynamics (CFD) modeling can be applied. This article discusses the limitations of the formulas for a three-dimensional (3D) flow application near the locks of IJmuiden and presents the use of CFD and physical scale model research to assess the flow patterns around the salt screen and the effectiveness of selective withdrawal. The CFD model was validated against the physical scale model and represented the complex flow fields around the salt screen to within acceptable deviations for both steady and transient states. This gives confidence in applying these more advanced modeling tools for the design and positioning of salt screens in confined complex 3D flow areas.

The dynamics of bed configurations and the effect of the bedforms on flow in sand-bed rivers are among the most challenging aspects of fluvial hydraulics to quantify. The big unknown is the resistance coefficient, without an accurate knowledge of which there is not much point in striving for overly precise numerical simulations of water-surface elevations and velocities. Two basic approaches have been used to account for the influence of bedforms on flow resistance. The first method divides the total resistance into two components: one related to impedance generated by the channel boundary without bedforms (grain resistance) and the other related to the opposing force produced by the bedforms (form resistance). The second tactic predicts the total resistance based on the overall flow and sediment parameters. It is the second approach that is followed in this study in which a neural network model is devised based on the evaluation of 941 measurements of reach-averaged flow resistance in small to large sand-bed rivers ranging from shallow to extremely deep. The model considers the influence of the several controlling variables, including water temperature and its effect on fluid viscosity, to make high-quality predictions of a resistance coefficient.