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.

The paper reviews in condensed form and from a historical perspective the various methods for treating and simulating turbulence and its effects in hydraulic flows. After highlighting the main characteristic features of turbulence and the role it plays in hydraulics, a necessarily brief overview is given of the main methods used in hydraulic flow calculations for dealing with turbulence and its effects. These are (1) empirical relations, (2) methods solving the Reynolds-averaged Navier-Stokes (RANS) equations with the aid of statistical turbulence models, (3) direct numerical simulations (DNS), and (4) large-eddy simulations (LES). Brief comments are made on the historical development of the different methods, and for RANS, DNS, and LES methods some application examples are presented. For details on the individual methods and further application examples, the reader is directed to the very extensive literature.

This 2015 Hunter Rouse Hydraulic Engineering Lecture reflects on the synergy between facts/knowledge and imagination/creativity. Our hydraulic engineering profession deals primarily with hard facts, and the complementarity between theory and practice is emphasized. We nevertheless have to face an increasing number of “soft” facts introduced by the stochastic variability of field observations linked to turbulence and climate change. A triangular relationship between observations, physics, and mathematics is highlighted to guide future research initiatives. In recent decades, tremendous developments in computer modeling of surface runoff from watersheds have been achieved. Physically based computer models should replace the remaining black boxes of our field. Experimental research should be pursued to continue to gain physical understanding. Answers to questions from the audience lead to a promising outlook for the future challenges in hydraulic engineering.

Effect of tank size on the radial flow from a large cylindrical tank into a vertical drain was studied experimentally. Different flow regimes are distinguished and the factors that caused these differences are discussed. Firstly, a weir flow regime occurred, in which the coefficient of discharge varied linearly with the ratio of the water depth to the outlet diameter. Then, a transition flow was observed for larger tank diameters. It was then followed by the full-pipe flow regime. In the weir flow regime range, some gulping was observed that resulted in the production of circulation. This could lead to an orifice flow regime or a filling and emptying process that produces different water depths for the same discharge. It is shown that the application of the principle of maximum discharge and vortex theories can predict the head-discharge curves for cases in which the control section was at the entrance or end of the vertical outlet pipe. A critical ratio of the air core to the outlet diameter was found corresponding to the maximum stage in the filling and emptying process. The water depth variation with discharge in the Harspranget Dam in Sweden in the literature was analyzed to show the importance of this phenomena in practical problems.

Boulders are widely used in fish habitat enhancement structures or fish passes. This study provides a detailed measurement of mean and turbulent flow around a boulder in a boulder array under a wake-interference flow condition. It was found that the vertical profiles of normalized streamwise mean velocity and streamwise turbulent intensity were approximately similar around the boulder, but both had different shapes in the upstream, downstream, and left and right sides of the boulder. The profiles of streamwise mean velocity deviated substantially from the classic logarithmic law but agreed with the Coles’ wake law fairly well. The profiles of turbulence intensity appeared to have minor deviations from the reported exponential decay profiles in the entire depth downstream of the boulder but only within a certain depth in the upstream and both sides of it. The correlation coefficient of velocity fluctuations was found to differ slightly from the reported exponential relation in the downstream and both sides of the boulder, whereas the difference was substantial in the upstream of it. Empirical relations were proposed for the profiles of streamwise mean velocity, turbulence intensity, and the correlation coefficient of velocity fluctuations.

This study aims at investigating the mixing layer located at the interface between the free stream and recirculation zone downstream from a sudden lateral open-channel expansion. Specific attention is paid to the interaction of the shallowness of the flow, characterized by the bed-friction number, with the lateral confinement due to the side wall. The velocity field for four flows with the same geometry but very different bed-friction numbers is measured in detail in order to characterize the mean velocity fields and Reynolds stresses across the mixing layers and evaluate the width of the mixing layers and their growth rates along with the typical oscillation frequencies. In the upstream region of the recirculation zone, the mixing-layer characteristics for the examined configurations are analogous to the ones of classical—laterally unbounded—mixing layers. In this region, the shallowness modifies the shape of the streamwise velocity profiles, extends the mean velocity gradient magnitudes and lowers the Reynolds stress terms, but hardly affects the mixing layer’s expansion rate. On the other hand, in the region near the flow reattachment, the mixing layer adopts very different behavior, with an abrupt drop of the its expansion. This change in behavior is linked to the dynamics of the two-dimensional (2D) vortices within the mixing layer, not a damping effect of the bed friction on these vortices, as the local bed-friction numbers remain much lower than the critical values reported in the literature. It is rather due to the interaction of the coherent structures with the side wall; the characteristics of this interaction itself are influenced by the flow shallowness. It is also shown that the damping effect due to bed friction is not responsible for the huge variations reported on the recirculation zones length. This imposes a distinction between local mixing-layer shallowness—derived using the mixing-layer width as length scale and governing the mixing layer’s characteristics—and a global flow shallowness—derived using the expansion width and governing the recirculation length.

This paper proposes a physically based probabilistic model for predicting the longitudinal distance of sediment deposition in an open channel flow. Numerical simulations were performed using a stochastic particle-tracking model to obtain the probability density functions (PDFs) of the longitudinal deposition spots. A skewness–kurtosis analysis and probability–probability plot confirmed that the PDF of the deposition locations had a lognormal type distribution. The distribution parameters, which include the mean and variance, were examined to analyze the effect of two particle-related factors, namely, the particle size and initial release point, on the PDF of the deposition locations. Smaller particles and a higher release point were found to produce, on average, longer travel distances to the deposition location and a more-dispersive distribution of the locations. The physical relationships between the distribution parameters of the PDF and the particle-related factors were considered for better prediction of the distribution parameters. The findings of the present study confirm the robustness and accuracy of a probabilistic model parameterized by physically based regression for predicting the spatial patterns of sediment deposition locations.

Because pollutant dispersion in rivers is strongly influenced by the longitudinal dispersion coefficient ($K_x$), its accurate estimation is critical in the field of environmentally sound hydraulic engineering. In this study, a granular computing (GC) model was explored for the first time to overcome problems in accurately estimating $K_x$. Because GC is a black-box model that is not user friendly, an appropriate nonlinear regression (NLR) method was also applied to precisely predict $K_x$. The inclusion of the generally ignored parameter of river curvature in $K_x$ estimation significantly improved NLR model performance. In so doing, both GC and NLR model estimations of $K_x$ achieved high linear coefficients of determination ($R^2$) and small error indices [root mean square error (RMSE) and mean absolute error (MAE)] with respect to measured $K_x$ values. The same analysis showed that the GC model (with $R^2$, RMSE, and MAE values equal to 0.997, 8.11, and 2.18, respectively), outperformed the NLR model, particularly for extreme high values of $K_x$. Similarly to previous studies, it was also found that the most effective parameters on $K_x$ were the channel aspect ratio, friction term, and river curvature, respectively, in descending order of importance. Moreover, a comparison between some well-known $K_x$ models and the developed GC and NLR alternative presented here showed the latter to have outperformed the former, indicating that the GC and NLR models are a good choice for $K_x$ prediction.

Erosion of rock masses by water typically involves unraveling of blocks of rock along existing defects in a manner that is not represented by analytical solutions for sediment transport or rockfill (rip-rap). A pragmatic approach to this complex problem is found by using a rock mass index to represent the erodibility of the rock-mass. The likelihood of erosion is then assessed by comparison against case studies in similar rock masses. A new independent data set of rock-mass erosion was gained from inspection of 26 unlined spillways in Australia, South Africa, and the United States. It was found that erosion can be usefully correlated against various published rock mass indices and hydraulic indices. A modified rock mass index, developed specifically to represent erodibility, is presented, and is applied to prepare a new comparative erosion assessment method for rock masses.

The error of the spatiotemporally averaged (STA) velocities of moving-vessel (MV) acoustic Doppler current profiler (ADCP) measurements was estimated by comparison with time-averaged fixed-vessel (FV) ADCP measurements in a large laboratory flume. Controlled laboratory conditions ensured that only the movement of the ADCP differed between these two measurements. Three different MV speeds (surrogates of horizontal bin sizes), were investigated. Results indicate no statistically significant differences in the zero-order quantities, discharge, and bulk velocity among STA MV measurements, and between STA MV and FV measurements. However, the traverse speed does affect the accuracy of the measurement of the local velocity, a first-order quantity, including the transverse and vertical components needed to evaluate the turbulence-driven secondary flow in the channel. Although the present study is limited to a unique ADCP, it demonstrates the value of controlled laboratory experiments for quantifying STA MV errors.

An original numerical procedure based on the elastic water-hammer theory, with special solutions of continuity and momentum equations modeling the unsteady liquid flow in pipelines, has been developed for discharge calculation within the pressure-time method framework. The results of the discharge measurements carried out in two different hydroelectric power plant units using the pressure-time method with the newly developed procedure are compared with those obtained using the standard procedures. An analysis of the advantages of the proposed method is presented. Among others, broader universality of the proposed method and significantly lower sensitivity of calculation results to assumed integration limits are pointed out.