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.

Unexpected, unknown or unused side pipe branches, termed dead-end side branches in this study, commonly exist in water pipe systems because of incorrect device installations or illegal connections. These connections are detrimental to the water quality and the operation and management of pipe networks. This paper investigates a transient-based frequency domain method for detecting side branches in pipe systems. The frequency response function for a pipe system with a single dead-end side branch is first derived by the transfer matrix method and the side branch was found to cause shifts in the system resonant frequencies. The nature of the resonant frequency shifts can be used to inversely determine the location and size of the side branch. A two-step genetic algorithm based optimization is proposed in this study to efficiently solve the derived analytical expression for the resonant frequency shifts. The developed method is validated through numerical simulations, and the results demonstrate the feasibility of this method for detecting side branches. The accuracy for locating the side branch is higher than the accuracy in sizing the branches. The sensitivity of the method to the magnitude and bandwidth of the transient wave signal is also discussed in the paper.

Fast geomorphic transients may involve complex scenarios of sediment transport, occurring near the bottom as bed load (i.e., saltating, sliding, and rolling) or as suspended load in the upper portion of the flow. The two sediment transport modalities may even coexist or alternate each other during the same event, especially when the shear stress varies considerably. Modeling these processes is therefore a challenging task, for which the usual representation of the flow as a mixture may result in being unsatisfactory. In the present paper, a new two-phase depth-averaged model is presented that accounts for variable sediment concentration in both bed and suspended loads. Distinct phase velocities are considered for bed load, whereas the slip velocity between the two phases is neglected in the suspended load. It is shown that the resulting two-phase model is hyperbolic, and the analytical expression of the eigenvalues is provided. The entrainment/deposition of sediment between the bottom and the bed load layer is based on a modified van Rijn transport parameter, whereas for the suspended sediment a first-order exchange law is considered. A numerical finite-volume method is used for the simulation of three dam break experiments found in the literature, which are effectively reproduced in terms of both free surface elevation and bottom deformation, confirming the key role played by the solid concentration variability even for two-phase models.

A global energy-auditing approach is used to describe the complex transient flow associated with sudden pressurization of a confined pipe system containing entrapped air pockets. The key concept is that the greatest pressures following pressurization are directly related to the amount of energy absorbed by the air pockets. The energy approach leads to an insightful understanding of the transient pressurization event, one that clarifies the underlying physics and that better explains several previously published observations. Moreover, the energy formulation leads to analytical algebraic expressions that can be numerically confirmed.

Sediment traps with open check dams are widely used structures in flood hazard mitigation. This paper reviews the literature dedicated to their design. First, the general context in which sediment traps are built and their functions are presented. The second part proposes hydraulic design criteria for classical types with details on the opening shapes and dam crest spillway. The third part details sediment deposition dynamics: its initiation, its controls through the trap basin and open check dam shapes, the effect of hydrographs, and the control of trap self-cleaning. The methods to determine the deposit slope and height are discussed. Finally, a step-by-step design procedure is proposed and future research challenges are highlighted. Field feedback has shown that driftwood can substantially influence sediment trap behavior. A companion paper thoroughly covers the production and transfer of driftwood and the interactions with open check dams.

Sediment traps with open check dams are widely used structures in flood hazard mitigation. This paper and its companion review the literature on their design. The companion paper examines hydraulic and deposition processes associated with sediment transport. However, field feedback has shown that open check dam behaviors during floods are dramatically influenced by the presence or absence of driftwood. To better assess large woody debris hazards and influences, this paper first reports the methods available to estimate driftwood production in terms of volume and dimensions. Information is given on their recruitment and transfer in the catchment. The presence of driftwood and the relevance of trapping them strongly influence the choice of the suitable shape and type of the open check dam. The performance of the different open check dam shapes in terms of driftwood management is detailed. Design criteria to estimate clogging probabilities, trapping efficiencies, volume capacities to trap driftwood, and hydraulic head losses due to driftwood accumulations are detailed. A step-by-step design procedure is proposed and finally suggestions to complete today’s knowledge are outlined.

A method of achieving a dimensionless collapse of erosion-rate data for cohesive sediments is proposed and shown to work well for data collected in flume-erosion tests on mixtures of sand and mud (silt plus clay sized particles) for a wide range of mud fraction. The data collapse corresponds to a dimensional erosion law of the form $E\sim {(\tau -{\tau }_c)}^m$, where $E$ is erosion rate, $\tau$ is shear stress, ${\tau }_c$ is the threshold shear stress for erosion to occur, and $m\approx 7/4$. This result contrasts with the commonly assumed linear erosion law $E=k_d(\tau -{\tau }_c)$, where $k_d$ is a measure of how easily sediment is eroded. The data collapse prompts a re-examination of the way that results of the hole-erosion test (HET) and jet-erosion test (JET) are customarily analyzed, and also calls into question the meaningfulness not only of proposed empirical relationships between $k_d$ and ${\tau }_c$, but also of the erodibility parameter $k_d$ itself. Fuller comparison of flume-erosion data with hole-erosion and jet-erosion data will require revised analyses of the HET and JET that drop the assumption $m=1$ and, in the case of the JET, certain simplifying assumptions about the mechanics of jet scour.

Wastewater pump systems decompose relatively rapidly compared to other pump systems because of the demanding properties of the pump medium. Only a systematic maintenance of the systems can prevent a significant and continuous decrease of the energy consumption per unit volume pumped (the specific energy). This article presents a method for a continuous supervision of the performance of both the pump and the pipeline in order to maintain the initial specific energy consumption as close as possible to the original value from when the system was commissioned. The method is based on pressure measurements only. The flow is determined indirectly from pressure fluctuations during pump run-up.

A vertical two-dimensional numerical model is employed to study the scour beneath a vibrating pipe exposed to steady currents. The ability of the model is first validated against previous scour experiments concerning both fixed and vibrating pipes. Both the development of scour depth and the final bed profile are generally well reproduced by the model. Subsequently, a broad range of the vibration parameters, including the amplitude and frequency, are tested to reveal their influences on the equilibrium scour depth. A generic relationship between the vibration frequency, amplitude, and scour depth is established by constructing suitable nondimensional groups. The maximum scour depth is shown to increase with the vibration amplitude and frequency. At small frequencies, both the frequency and amplitude have a pronounced influence on the scour depth; with the increase of the vibration frequency, the influence of the frequency gradually diminishes and the scour depth is mainly decided by the vibration amplitude. Two empirical formulas are proposed to describe the quantitative relationship between the frequency, amplitude, and scour depth.

The primary results of this paper are two exact analytical solutions of the Colebrook-White equation (1939), one by an infinite recursion and the other by an integral. These solutions are the first exact analytical solutions of the Colebrook-White equation that do not use a special transcendental function such as the Lambert $W$ function. An explicit approximate formula for the Colebrook-White equation, called the $n$th formula, is also developed on the basis of one of the exact solutions by truncation. The key result in this development is the closed-form expression for an associated function of the Lambert $W$ function, called the $Y$ function, expressed by an infinite recursion. Once the $Y$ function is obtained in a closed-form using a recursive function, it can be applied to the $W$ function and to the Colebrook-White equation. It is shown numerically that the absolute error of the Darcy friction factor decreases geometrically toward zero as the recursion depth of the $n$th formula increases. Additionally, the recursion depth of the $n$th formula may be preferentially chosen to yield a solution that is consistently larger or smaller than the exact solution.

An experimental campaign of 25 long-duration (13–47 days) laboratory tests was carried out with three complex pier models under steady clear-water flow conditions. Each model, characterized by a different relation between the column and the pile-cap widths, $D_c/D_{pc}$, was tested for a variety of pile-cap positions relatively to the initial bed, $H_c$. The experimental data were used to describe the temporal evolution of the scour depth as a function of $H_c/h$ ($h$ = approach flow depth). The common criterion to stop experimental tests on complex piers was analyzed, and a new criterion was introduced. The equilibrium scour depth, $d_{se}$, was calculated by extrapolation of data series. The results are used to evaluate the effect of $D_c/D_{pc}$ and $H_c/h$ on $d_{se}$ when the pile cap is above the bed (Situation 1), partially buried in the bed (Situation 2), and completely buried in the bed (Situation 3). The analysis includes the definition of $H_c$ at which the maximum $d_{se}$ occurs through an equation that takes into account the $D_c/D_{pc}$ ratio, the relative pile-cap thickness, $T/h$, and the column and pile-cap shapes.

Cleaning of fine sediment out of gravel streambeds has become an important method to restore affected stream habitats. Introducing the increased flows needed to entrain fine sediments without eroding the coarser fractions of the bed and potentially destroying its usefulness as a habitat requires careful selection of flow strength. Toward this end, a series of flume experiments was conducted on the entrainment of sand from immobile gravel beds to determine the depth to which the sand could be eroded without the gravel being mobilized. A series of steady flows was imposed on three gravel beds filled with sand with median sizes of 0.2, 0.3, and 0.9 mm. The shear stress of the series of flows ranged from just above the critical shear stress for initiation of motion of the sand up to 0.9 times the critical stress of the 36.1-mm gravel. The experiments continued until the rate of fine sediment exiting the channel was very small. It was found that the cleanout depth of the sand was reasonably predicted by estimating the shear stress in the gravel as the product of bed shear stress and the value of the cumulative probability distribution function of the elevations of the gravel surface evaluated at the surface of the sand layer. The appropriate initiation of sediment motion criterion was found to be a function of the shear velocity of the flow and the fall velocity of the fine grains.

A two-dimensional steady potential flow theory is applied to calculate the flow in an open channel with complex solid boundaries. The boundary integral equations for the problem under investigation are first derived in an auxiliary plane by taking the Cauchy integral principal values. To overcome the difficulties of a nonlinear curvilinear solid boundary character and free water surface not being known a priori, the boundary integral equations are transformed to the physical plane by substituting the integral variables. As such, the proposed approach has the following advantages: (1) the angle of the curvilinear solid boundary as well as the location of free water surface (initially assumed) is a known function of coordinates in physical plane; and (2) the meshes can be flexibly assigned on the solid and free water surface boundaries along which the integration is performed. This avoids the difficulty of the traditional potential flow theory, which seeks a function to conformally map the geometry in physical plane onto an auxiliary plane. Furthermore, rough bed friction-induced energy loss is estimated using the Darcy-Weisbach equation and is solved together with the boundary integral equations using the proposed iterative method. The method has no stringent requirement for initial free-water surface position, while traditional potential flow methods usually have strict requirement for the initial free-surface profiles to ensure that the numerical computation is stable and convergent. Several typical open-channel flows have been calculated with high accuracy and limited computational time, indicating that the proposed method has general suitability for open-channel flows with complex geometry.

The USGS Office of Surface Water (OSW) previously validated the use of Teledyne RD Instruments (TRDI) Rio Grande (in 2007), StreamPro (in 2006), and Broadband (in 1996) acoustic Doppler current profilers (ADCPs) for streamflow (discharge) measurements made by the USGS. Two new ADCPs, the SonTek M9 and the TRDI RiverRay, were first used in the USGS Water Mission Area programs in 2009. Since 2009, the OSW and USGS Water Science Centers (WSCs) have been conducting field measurements as part of their stream-gauging program using these ADCPs. The purpose of this paper is to document the results of USGS OSW analyses for validation of M9 and RiverRay ADCP streamflow measurements. The OSW required each participating WSC to make comparison measurements over the range of operating conditions in which the instruments were used until sufficient measurements were available. The performance of these ADCPs was evaluated for validation and to identify any present and potential problems. Statistical analyses of streamflow measurements indicate that measurements made with the SonTek M9 ADCP using firmware 2.00–3.00 or the TRDI RiverRay ADCP using firmware 44.12–44.15 are unbiased and, therefore, can continue to be used to make streamflow measurements in the USGS stream-gauging program. However, for the M9 ADCP, some important issues must be considered in making future measurements. Possible future work may include additional validation of streamflow measurements made with these instruments from other locations in the United States and measurement validation using updated firmware and software.

A momentum-impulse model that accounts for the critical impulse of a particle bridge that is balanced with a fluid pulse resulting from turbulent pumping is presented and applied for predicting the clogging depth of fine sand in gravel streambeds overlain by turbulent open-channel flow. The model was tested against the literature-derived database of clogging depth with conditions defined by hydraulic flume experiments characterized by fine sand infiltrating into gravel substrates, with hydraulically-rough open-channel flow ranging from subcritical to critical conditions. Results show the efficacy of the momentum-impulse model and support the hypothesis that particle bridging and intragravel flow due to fluid pumping control the clogging depth. Model results show improvement over previous empirical modeling of the clogging depth phenomena due to the reduction of unknown parameters from four coefficients to one coefficient and an increase in model predictability as quantified using k-fold validation and model comparison. A set of curves developed from simulations of the validated momentum-impulse model is provided, which will be useful for stream-restoration practitioners interested in estimating embeddedness. In addition, the clogging profile is shown using clogging depth predicted with the momentum-impulse model, which is an additional application of this work.

A lattice Boltzmann method with two relaxation times for shallow-water equations (${\mathrm{LABSWE}}^{\text{TRT}}$) without turbulence is proposed. The described model is validated through simulations of three typical cases with laminar flows: 1D steady flow over a bump, 2D unsteady dam-break flow, and flow around circular cylinder. Good agreement between prediction and analytical or experimental solutions are obtained. In addition, the performance of the ${\mathrm{LABSWE}}^{\text{TRT}}$ and the lattice Boltzmann method for shallow-water equations using a single relaxation time (LABSWE) is compared in detail. Studies have shown that the former is more stable than the latter for 2D cases. Different combinations of the two relaxation times are also studied and the optimized one is recommended for good numerical stability. This study demonstrates that the additional relaxation time in the ${\mathrm{LABSWE}}^{\text{TRT}}$ can improve the stability of simulations for laminar shallow flows using a procedure almost as simple as that in the LABSWE.

Several studies have demonstrated that in abruptly expanding channels phenomena of flow asymmetry can arise under certain tailwater conditions. These undesired situations are characterized by high-velocity concentrations and backward flows in the tailwater channel. This paper presents some preliminary results of an experimental investigation carried out to study the effects of a system of crossbeams for improving the flow patterns in the tailwater channel. Four different beam spacings, two system orientations and slopes, two locations, and two beam heights were tested in order to define the most adequate geometry of the dissipator. It was found that, for almost all tested configurations, the presence of the crossbeams produced a significant improvement in the flow field in the channel, in terms of velocity distribution and stability under variable downstream conditions.

The use of low-cost, low-profile, and highly portable sidescan sonar is on the ascendancy for imaging shallow riverine benthic sediments. A new automated, spatially explicit, and physically-based method for calculating lengthscales of bed texture elements in sidescan echograms (a 2D plot of acoustic intensity as a function of slant range and distance) is suggested. It uses spectral analysis based on the wavelet transform of short sequences of echograms. The recursive application of the transform over small overlapping windows of the echogram provides a robust measure of lengthscales of alternating patterns of strong and weak echoes. This textural lengthscale is not a direct measure of grain size. Rather, it is a statistical representation that integrates over many attributes of bed texture, of which grain size is the most important. The technique is a physically-based means to identify regions of texture within a sidescan echogram, and could provide a basis for objective, automated riverbed sediment classification. Results are evaluated using data from two contrasting riverbed environments: those of the Colorado River in Grand Canyon, Arizona, and the West Branch of the Penobscot River, Maine.

New experimental data on pressure-pipe flow of slurries, together with some literature data, are used to evaluate the performance of four resistance models based on different interpretations of the process. The experiments were performed on a mixture of water and uniform sediment showing a shear-thinning rheological behavior. The literature datasets refer to a Bingham plastic mixture and to a shear-thickening mixture. Two of the tested models assume a fully stratified flow and express the total resistances as the sum of the bed and of the vehicle contribution. The third and the fourth models are based on a rheological approach that considers an equivalent homogenous fluid.