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 spatial distribution of emergent vegetation in rivers is frequently nonuniform. Here, spatial nonuniformity is simulated in the laboratory by varying longitudinally the stem areal number density ($m$). Time- and space-averaged flow variables, corresponding to different values and gradients of $m$ and different stem Reynolds numbers, are calculated from instantaneous velocity maps obtained with particle image velocimetry (PIV). Results show that Reynolds stresses are completely determined by the local spatially averaged number of stems per unit area; they are not sensitive to local spatial gradients of $m$. These variables seem to adjust locally to the spatial variations of the stem distribution. Contrarily, the spatial distribution of the time-averaged flow is influenced not only by the local value of $m$ but also by its spatial gradients. Form-induced stresses express this influence. They reveal a subsistence of flow complexity generated by the upstream stem distribution as a form of spatial memory. Thus, the effect of spatial gradients of $m$ is always in the direction of increasing form-induced stresses although at different rates depending on the signal of the former.

The determination of the creep (compliance) function of viscoelastic pipelines is essential for modeling their hydraulic behavior and accurately predicting pressure responses under transient events. This paper proposes a novel frequency-domain technique for the determination of the creep function of viscoelastic pipelines using hydraulic transients. A viscoelastic pipeline system, when compared with a frictionless elastic pipeline under the same system configuration, has nonuniformly shifted resonant frequencies. Analytical analysis shows that the shift in the resonant frequencies of a viscoelastic pipeline system is related to both the pipe wall viscoelastic compliance effects and the unsteady wall shear stress effects. A technique is developed to determine the elastic wave speed and the viscoelastic creep compliances based on the shifted system resonant frequencies. To improve the accuracy of the calibration for the viscoelastic parameters, an approach is proposed to correct the shifting in the resonant frequencies induced by the unsteady friction before the calibration. Numerical simulations conducted on a high-density polyethylene (HDPE) pipeline verify that the elastic wave speed and viscoelastic compliance can be determined with relatively high accuracy.

Longitudinal dispersion processes are often described by the advection dispersion equation (ADE), which is analogous to Fick’s law of diffusion, where the impulse response function of the spatial concentration distribution is assumed to be Gaussian. This paper assesses the validity of the assumption of a Gaussian impulse response function, using residence time distributions (RTDs) obtained from new laboratory data. Measured up- and downstream temporal concentration profiles have been deconvolved to numerically infer RTDs for a range of turbulent, critical, and laminar pipe flows. It is shown that the Gaussian impulse response function provides a good estimate of the system’s mixing characteristics for turbulent and critical flows, and an empirical equation to estimate the dispersion coefficient for the Reynolds number, $\mathsf{R}$, between 3,000 and 20,000 is presented. For laminar flow, here identified as $\mathsf{R}<\mathrm{3,000}$, the RTDs do not conform to the Gaussian assumption because of insufficient available time for the solute to become cross-sectionally well mixed. For this situation, which occurs commonly in water distribution networks, a theoretical RTD for laminar flow that assumes no radial mixing is shown to provide a good approximation of the system’s mixing characteristics at short times after injection.

Rigid water column (RWC) models simulate the unsteady-incompressible hydraulics of pressurized pipe networks. They conceptually lie between water hammer and quasi-steady models, yet despite their intrinsic strengths, existing RWC formulations suffer efficiency-, stability-, and interpretation-related challenges; thus, they are typically overlooked as a modeling alternative. To address the aforementioned limitations, this article presents the RWC global gradient algorithm (GGA), a novel formulation for pipe networks that has greater efficiency and overcomes the numerical challenges. The RWC GGA extends the generalized GGA (G-GGA) to consider inertial effects in addition to variable-area tanks and mixed (i.e., demand and pressure-dependent) outflows. Two pipe networks of simple and moderate complexity are used to compare the new approach against two other RWC algorithms, the G-GGA, and a water hammer model: the current work is shown to have improved stability and efficiency relative to previous work. The RWC GGA is also found to have a computational cost only slightly greater than that of the G-GGA for the same time-step size. Overall, this work highlights the practical utility of RWC models to simulate slow transient events and controlled operations.

Cohesive sediment detachment is typically modeled for channels, levees, spillways, earthen dams, and internal erosion by using a linear excess shear stress approach. However, mechanistic nonlinear detachment models, such as the Wilson model, have recently been proposed in the literature. Questions exist as to the appropriateness of nonlinear relationships between applied shear stress and the erosion rate. Therefore, the objective of this research was to test the appropriateness of linear and nonlinear detachment models for cohesive sediment detachment using three data sets: (1) rill erodibility studies across a limited range of applied shear stress (0.9–21.4 Pa), (2) hole erosion tests (HETs) across a wide range of applied shear stress (12.6–62.0 Pa), and (3) streambank erodibility as quantified by jet erosion tests (JETs) for the linear excess shear stress equation and the nonlinear Wilson model across a small range of shear stress (1–4 Pa). The Wilson model was also incorporated into the bank stability and toe erosion model (BSTEM) as an option for simulating fluvial erosion and used to simulate bank retreat in the streambank erodibility study. The Wilson model was shown to be an appropriate particle detachment rate model from previously published data on rill erodibility, HETs, and JETs. Using a nonlinear detachment model also alleviated questions about the most appropriate solution technique for deriving erodibility parameters from JETs. In situ and laboratory tests sometimes use a limited range of applied shear stress, and therefore users of these measurement techniques should be aware of the potential nonlinear behavior of cohesive sediment detachment especially at higher shear stress. The results suggest advantages for the nonlinear Wilson detachment model and also identify the need for additional research to evaluate the various detachment models for laboratory HETs and in situ JETs across a wider range of soil types and additional reach-scale streambank erosion studies.

A portable particle image velocimetry (PIV) measurement system is developed for sensing flow surface velocities. Using the laser-based technique, four laser points are projected on the flow surface and utilized as reference points to simplify the image processing of orthorectification in PIV measurement. Furthermore, by combining a smart phone with laser modules, a portable PIV device has been developed that is capable of performing PIV measurement comprehensively. Two laboratory experiments were carried out to validate the proposed portable PIV measurement system. The experimental results show that this portable PIV system could be easily set up and measure the flow surface velocities immediately.

A maximum entropy (MaxEnt) method is developed to predict mean external and internal flow rates and mean pressure gradients (potential differences) in hydraulic pipe networks, without or with sufficient constraints to enable a closed-form solution. This substantially extends existing methods for the analysis of flow networks (e.g., Hardy Cross), applicable only to deterministic networks. The analysis is based on a continuous relative entropy defined on the space of flow rates, requiring one dimension for each edge and node within the network. This entropy is maximized subject to observable constraints on the mean values of certain flow rates and/or potential differences, and also physical constraints arising from Kirchhoff’s first and second laws (node and loop constraints) and the frictional properties of each pipe. A semianalytical and numerical algorithm is developed to solve the equation system, which involves implicit solution of multidimensional integrals and root finding with quasi-Newton and Newton methods. The analysis is demonstrated by application to (1) a simple three-node network, and (2) a 1123-node, 1140-pipe water distribution network in the suburb of Torrens, Australian Capital Territory, Australia.

It is shown that the quasi-steady-state approach to water-distribution-system simulation, which is adopted in most of the currently available hydraulic simulators, may result in nonunique solutions (in terms of nodal pressures) in the practically occurring case of a network section fed through a flow control valve whose setting is equal to the sum of the required demands in the section (this being true both for demand-driven and pressure-driven analyses). This paper presents an algorithm that allows one to determine whether nonunique solutions occur and, if so, to determine the nodal pressure range in which they lie. The algorithm is illustrated by the example of an operating city water distribution network.

In water-supply pipe networks, inherent uncertainties in the analysis parameters, including nodal demands, pipe friction factors, reservoir heads, etc., lead to imprecise hydraulic responses. This study introduces a methodology based on fuzzy set theory to analyze network hydraulics under uncertainty. The fuzzy approach results in a complex optimization problem that is neither single nor common multiobjective. To solve the problem efficiently to find extreme values of nodal pressures and pipe velocities, a novel many-objective particle swarm optimization (MO-PSO) model is developed and coupled to a network hydraulic simulation model from the literature. The coupled model is applied against a benchmark example and a real pipe network from the literature and the results are compared with those from the previous methods. The examples manifest that the proposed fuzzy MO-PSO is computationally efficient and reliable. Analyzing the real pipe network shows that, for instance, $\pm 15\%$ uncertainty in the pipes’ roughness and nodal demands could averagely result in $-11.2$ to $+6.4\%$ uncertainty in the nodal pressures and $-41.7$ to $+50.1\%$ in the pipe velocities.

Laboratory experiments were carried out to investigate local scouring around twin cylindrical piers in open-channel flows. The experimental results of clear-water scouring around the twin piers show that the scour hole formed around the upstream pier at different pier spacing was almost the same as that around an isolated pier. However, the local scour depth around the downstream pier was less than that around the upstream pier. The local scour depth around the downstream pier with the velocity can be segregated into four regions: a no-scour region, synchronous-scouring region, transition region, and a radical-deviation region. Finally, a relationship between the deviation in the radical-deviation region and the spacing of the two piers was obtained.

In this paper, confluence vertices in distribution networks are discussed from the point of view of network topology, to solve the problems that require a number of confluence vertices before hydraulic calculation. First, open and closed network models are introduced to represent the different types of distribution networks and constructed based on the general assumptions. Six propositions are proven according to graph theory to reveal the relationship between the number of confluence vertices and the network topology, not only based on the general assumptions but also the strict assumptions with additional limitation on the degree of vertices. Besides, the equations also reveal the relationship between the number of confluence vertices and the number of independent loops in the network models with the strict assumptions. Through discussion of hydraulic variations, the number of confluence vertices in the network model that follows the strict assumptions is dependent only on the network topology and can be calculated by the number of pipes, vertices, suppliers, and consumers; whereas in the network model with the general assumptions determination of the number of confluence vertices requires the support of hydraulic analysis.