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.

Due to a complex three-dimensional flow pattern, the outer banks of river bends are predisposed to erosion. When endangering civil structures, preventing measures to mitigate this erosion are thus required. Vertical ribs at protection walls for scour reduction have been applied to several flood protection projects in mountain rivers; nevertheless, no systematic and intensive study has been presented so far to evaluate their effect. This paper investigates experimentally the effect of vertical ribs, placed as macroroughness elements on the outer vertical wall of a 90° laboratory channel bend. Systematic tests were performed using a wide and coarse grain-size distribution. Scour formation and velocity distribution were assessed in the channel in the presence of a macrorough outer bank, materialized in the laboratory by vertical elements placed on the outer vertical wall along the channel bend. Experiments showed that the macrorough outer bank changed considerably the bed morphology under equilibrium conditions. Maximum scour depth is considerably reduced by the vertical ribs placed at an optimal spacing on the bend outer wall. A considerable grain sorting process occurs across the cross section in the bend, which influences the scour process; differences are observed between situations with and without macrorough banks. The distribution of the time-averaged velocity field across the section shows the influence of the channel rough wall. An optimal macroroughness configuration in terms of scour reduction is discussed and proposed. It was observed that when spacing between vertical ribs is too reduced, these ribs act as uniformly distributed wall roughness, contributing to the width reduction due to the occupation of the cross section and increasing consequently the flow velocity with negative effects in scour reduction.

Extensive Experiments on inclined dense jets typical of brine discharges into shallow water are reported. The experiments were conducted with nozzles oriented at 30°, 45°, and 60° to the horizontal and the spatial variations of tracer concentrations were measured by three-dimensional laser-induced fluorescence (3DLIF). Three flow regimes were identified: deep water, surface contact, and shallow water. The regimes depend on the value of $dF/H$, where $d$ is the nozzle diameter, $F$ the jet densimetric Froude number, and $H$ the water depth; criteria were presented for the transitions between them. Flow images revealed complex three-dimensional interactions with the free surface, especially for steep nozzle angles in shallow water. Dilutions at critical points and their locations were measured. For deep water, all results followed those previously reported for fully submerged jets. As the depth decreases (or $dF/H$ increases) to the surface contact regime, dilutions begin to decrease. Tracer concentration profiles are truncated at the water surface and in shallow water resemble half-Gaussian profiles similar to those of wall jets. The jets can cling to the water surface, although the locations of the bottom impact point and near-field length are not significantly affected by the water surface. In deep water and surface contact regimes, the impact point and near-field dilutions are highest for 60° nozzles. As the depth decreases further, however, the dilutions for the three nozzle angles become approximately equal, until, for shallow water, the 30°-nozzle gives slightly higher dilution. The 30°-nozzle may be preferable for this case because there is less surface interaction and, therefore, less visual impact on the water surface. Previous recommendations that dense jets be submerged so that the rise height to the jet’s upper boundary be less than 75% of the water depth to avoid surface effects appear to be overly conservative and the present results suggest that the rise can be as much as 90% of the water depth for all angles with no deleterious effect on dilution.

The purpose of this study was to quantify how vegetation influences the flow and sediment processes relevant to the design and management of environmental compound channels. Therefore, a two-year field investigation was conducted in a cohesive two-stage channel, focusing on the flow resistance and net deposition in five subreaches with different floodplain vegetation conditions. In the grassy subreaches, the cross-sectional blockage factor was the key vegetation property governing the flow resistance, with a process-based model providing reliable estimates under widely variable hydraulic and vegetative conditions. The net deposition of cohesive sediment was best explained by vegetation height while high stand length and density created supply-limited conditions on the inner floodplain. These results showed that the two-stage approach offers potential for controlling the sediment processes through appropriate vegetation maintenance. The novelty of this research is that straightforward analyses accompanied by a physically-based parameterization of the floodplain plant stands were successfully used to characterize the flow–vegetation–sediment interaction in a practical engineering application. The results are expected to be helpful in designing and managing comparable channels.

Two-dimensional turbulent free-surface flow with Froude numbers close to 1 is considered. The roughness of the bottom of the channel is allowed to vary with the space coordinate, leading to small variations in the bottom friction coefficient. In a predecessor of the present paper, an extended Korteweg-deVries (KdV) equation for the surface elevation was derived by means of an asymptotic analysis which is free of turbulence modeling. The steady-state version of the extended KdV equation has solutions that describe stationary solitary waves. In the present paper, previous results of the analysis are extended with the aim of providing a basis for the experimental investigations. A comparison of measured surface elevations with theoretical predictions is shown, and possible sources of small discrepancies are discussed. In an appendix, the structure of solutions that describe solitary waves with a tail is investigated.

It has been hypothesized that negative pressures caused by transients within water distribution systems may result in ingress of contaminated groundwater through leaks and hence pose a risk to public health. This paper presents results of contaminant ingress experiments from a novel laboratory facility at The University of Sheffield. An engineered leak surrounded by porous media was subjected to pressure transients resulting from the rapid closure of an upstream valve. It has been shown that a pollutant originating externally was drawn in and transported to the end of the pipe loop. This paper thus presents the first fully representative results proving the occurrence and hence, risk to potable water quality of contaminant ingress.

From a linearized form of the equations of unsteady (transient) motion in an open channel of uniform cross section using the well-known technique of the characteristic lines, it is shown that under a set of not-too-restrictive conditions, the sudden occurrence of abrupt flow discontinuities, such as a sudden leak, burst, or whatever flow perturbation, can be detected and localized by monitoring, and the pressure variations in a purposefully designed array of transducers located along the channel bottom can be suitably processed in real time. This study, which belongs to the general theme of transient analysis, extends the techniques of burst detection, localization, and quantification to the case of open-surface flows, which are tested for the first time by the authors in the case of single pressurized pipes. The proposed methodology can easily be applied to instantaneously detect and assess unexpected events that can occur in the more sensible and vulnerable channel stretches.

An experimental study evaluated the local scour caused by flow past bridge piers in a channel covered by ice. Results show that ice cover affects the scour characteristics, especially at lower flow depths. In the range of test conditions used in the study, with the increase in relative bed coarseness, dimensionless scour depth decreases correspondingly. Including scour radius as an indicator brings forward a new perspective for scour estimation. Furthermore, empirical equations for both scour depth and scour radius are developed under both open channel and ice-covered flow conditions. Present equations are also supported by previous findings related to the influence of relative bed coarseness, flow shallowness, and pier Froude number.

Determination of longitudinal dispersion coefficient (LDC) using artificial intelligence (AI) techniques can improve environmental management strategies for river systems. However, the uncertainty involved in AI models has rarely been reported. The main objective of this paper was to investigate the reliability of three AI-based techniques, including the artificial neural network (ANN), adaptive neuro-fuzzy inference system (ANFIS), and support vector machine (SVM), for predicting the LDC in natural rivers. To that end, LDC predictions were first carried out using ANN, ANFIS, and SVM techniques. Then, a forward selection (FS) and gamma test (GT) were conducted to sort input variables according to their importance and effects on LDC prediction. Finally, uncertainties in the model predictions were analyzed to answer the question, “How reliable are ANN, ANFIS, and SVM techniques?” It was found that model inputs could not be satisfactorily sorted by a linear method (i.e., FS) due to the complex and nonlinear nature of LDC. Thus, the nonlinear GT technique was chosen as a suitable input selection method for prediction of LDC. The results or model input variables selected from the GT technique showed good consistency with previous researches. Furthermore, the reliability of ANN, ANFIS, and SVM models was calculated and tabulated by an uncertainty estimation for LDC prediction. A high uncertainty was found in the models although they predicted LDC appropriately. It was also found that the uncertainty in the SVM model was less than those in the ANN and ANFIS models for estimating the LDC in natural rivers. The ANFIS model performs better than the ANN model.

An important component of the management cost of aqueducts is the energy costs. Part of these costs can be recovered by transforming some of the many existing energy dissipations into electric energy by means of economical turbines. This paper describes an experimental study that has been carried out in order to (1) test the performance of an economical cross-flow turbine that maintains high efficiency within a large range of water discharges, and (2) validate a new approximate formula relating main inlet velocity to inlet pressure. It is demonstrated that the proposed formula, on the basis of some simplifying assumptions, exactly links inlet velocity to inlet pressure with any possible geometry of the cross-flow turbine. A specific facility was designed and constructed inside the hydraulic laboratory of the Dipartimento di Ingegneria Civile, Ambientale, Aerospaziale dei Materiali (DICAM) Department of the University of Palermo to carry out the experiments. Both numerical and experimental tests were carried out for the two cases of turbine impeller with and without the internal shaft. Experimental and numerical results showed a good agreement and give an excellent validation of the proposed formula, which can be readily used for a fast and reliable design of a cross-flow turbine.

This study presents an experimental and numerical study for an inclined (60° to horizontal) dense jet discharged into a coflowing current. The mixing and transport of the density current arising from the jet impingement on a horizontal bottom boundary is investigated. A light attenuation technique is employed to measure the layer-averaged concentration field over a region that extends $20FrD$ downstream and $9FrD$ laterally (where $F$ = jet densimetric Froude number, and $D$ = jet diameter). A comprehensive characterization of the resulting buoyant spread in both steady and unsteady cases is obtained. The concentration field is also computed using a three-dimensional (3D) shallow water equation model via the distributed entrainment sink approach that incorporates a near-field Lagrangian integral jet model (JETLAG) into the 3D model for dynamic simulation of the near-far field transition. The results show the occurrence of bifurcation in the gravitational spreading layer when the impinging dense jet is bent over, characterized by a crossflow Froude number $\mathbf{F}=U_{r}Fr$ of around 0.8 (where $U_r$ = ratio of ambient to jet velocity). The lateral concentration profiles are bimodal in shape; the concentration maximum is off-centered and 1.6–2 times the centerline value. The buoyant spreading is governed by buoyancy and inertia, and the spreading layer grows as $x^{2/3}$ with downstream distance. For $\mathbf{F}\approx 0.4$, the upstream intrusion of the buoyant layer past the source is arrested; lateral gradients of concentrations are small and the dilution becomes constant downstream. In the intermediate range $\mathbf{F}\approx 0.4–0.8$, the profiles evolve gradually from top-hat to bimodal. The numerical prediction of the salient flow features as well as the intermediate field dilutions and spreading layer thickness are in good agreement with data.

This paper deals with the analytic-theoretical derivation of the relationships between the entropic quantity $\mathrm{\Phi }(M)$, representing the ratio between the mean and maximum flow velocities, and the relative submergence and aspect ratios, using classical open-channel flow equations. $\mathrm{\Phi }(M)$ is found to be highly dependent on the relative submergence when large or intermediate roughness scales occur, whereas it might be assumed to be almost constant for a small roughness scale. Furthermore, considering the hydraulic geometry relationships, an attempt is made to relate the relative submergence to the aspect ratio of flow through a log relationship whose coefficients depend on the local bed slope, with an important implication for hydrological practices. Then a practical relation between $\mathrm{\Phi }(M)$ and the aspect ratio is proposed and validated in an operative chain for discharge assessment that shows high robustness and stability. The proposed model has been applied to a set of experimental velocity data collected at gauged river sites with different geometric and hydraulic characteristics as well as low, medium, and high flows.

The representative grain size of a sediment bed can be defined as the mean-volume diameter, which varies with the geometric standard deviation of sediment size. The obtained results explain why the representative sediment size can be approximated to be $D_{84}$ for typical gravel beds. The analysis is finally extended to quantify the dependence of the equivalent sand roughness on the geometric standard deviation of grain size distribution.