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.

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.

Particle image velocimetry was used to measure the flow structures in evolving scour holes caused by a plunging jet downstream of a weir. The results clearly depict that four different types of roller were involved in scour development and the scour wes mainly caused by the major roller. The average kinetic energy (AKE) distributions reveal that the rollers were maintained by jet impingements on both the concave scoured bed and the water surface. The gap gradually enlarged between jet regions with high transport capability, which adversely affected scour development. The turbulence kinetic energy (TKE) dissipated along the main jet direction. As the scour hole developed, the regions of significant TKE moved successively away from the bed with a reduction in scour rate. The plunging jet zone had negative Reynolds shear stress (RSS), and absolute values there were larger than in other zones in the scour hole. Meanwhile, the regions of significant positive RSS changed in size and location with scour development.

An experimental study on flow-induced clear-water local scour at complex bridge piers was conducted using two typical pier models, nine pile-cap elevations, and seven pier skew angles from 0° to 90° with a 15° interval. Results show that a slight skew angle $\alpha$ imposed on an originally aligned pier can significantly increase the equilibrium scour depth $d_{se}$. When a pier was skewed to the flow, the column made the largest contribution to the scour depth because its wide-pier feature became dominant. Three categories of the pier skewness were defined, namely, an aligned pier (= 0°), a slightly skewed pier ($\alpha \approx 15°$), and a highly skewed pier ($\alpha \ge 30°$). The influences of the pile cap and the pile group on the scouring process were weakened with increasing $\alpha$. Additionally, for highly skewed piers with fully submerged pile cap, the equilibrium scour holes tended to equally expose the pier components, no matter the location of the undisturbed bed level. A new scour prediction method is presented, involving modifications for the effect of sediment coarseness. The new method leads to a higher safety margin with significantly reduced underestimation.

Low-head discharge over weirs and crest gates can produce a nappe instability in curvilinear weir nappe flow, referred to as nappe oscillation or vibration, that is characterized by horizontal waves (sinusoidal wave pattern in the longitudinal cross section) and significant low-frequency sound pressure waves. This topic has previously received minimal documentation in the literature. The current study documents weir-flow nappe oscillation research conducted using a prototype-scale quarter-round linear short-crested weir ($4.67\mathrm{m}\mathrm{wide}\times 3.63\mathrm{m}$ tall with a 152-mm radius) with confined and unconfined nappe conditions. The objective of this research was to develop a better understanding of the mechanisms behind nappe oscillation, evaluate oscillation frequencies (three different potential sources), and investigate potential mitigation techniques. Crest roughness modifications were evaluated as potential nappe vibration mitigation alternatives. The results suggest that the source of nappe oscillation is located at or near the weir crest downstream edge. Some modifications effectively suppressed nappe oscillation, whereas other factors functioned as oscillation amplifiers.

This paper investigates the effect of density differences on the forces acting on a moored vessel during lock operations, focusing on the effect of the position of the moored vessel in the lock chamber in the presence of density currents. The extensive scale-model research performed for the new sea lock in IJmuiden, The Netherlands has shown that the combination of density differences and an asymmetric layout of the moored vessel in the lock chamber may lead to high forces on the vessel that can largely exceed the allowable force limit. In particular, as a result of the density currents, forces in the transverse direction build up, pushing the vessel away from the chamber wall, during leveling and after opening the lock gate, leading to higher loads on the mooring lines. The forces caused by the density difference are the dominant forces and the performance of the leveling system cannot be assessed without taking this into account. Based on the results of the performed tests, criteria for achievable leveling times and allowable hydrodynamic forces during leveling are determined for the new sea lock of IJmuiden. Furthermore, the obtained results can be used for calibration or validation of numerical models that are valuable tools for the design of future locks.

This paper performs a three-dimensional, unsteady, detached-eddy simulation (DES) of a classical hydraulic jump with an inlet Froude number of 8.5. The volume of fluid (VOF) method with a high-resolution interface capturing (HRIC) scheme is used for free-surface tracking. The computational results are validated using available experimental results and by ensuring that details of the flow physics based on existing knowledge are properly captured. The three-dimensional nature of the flow in the developed zone of the hydraulic jump is well demonstrated, and a better understanding of the interaction between the wall-jet flow and the roller region above it is revealed. The paper also resolves the internal turbulent structure of the classical hydraulic jump, which is not completely realized in the experimental results. Quadrant decomposition of the Reynolds shear stresses reveals that inward and outward interactions dominate the flow field. This is further ascertained by the analysis of the third-order moments of the velocity field. It is also revealed that the expanding shear layer interacts with the free surface resulting in intense undulations and breaking up of the free surface.

The time-averaged turbulent flow characteristics in a wall-wake flow downstream of a wall-mounted vertical cylinder (free-surface piercing) are analyzed from the viewpoint of self-similarity in their vertical profiles. In a wall-wake flow, the deficits (differences between upstream and downstream values at a given elevation) in streamwise velocity, Reynolds shear stress, turbulence intensities, and third-order moments exhibit a certain degree of self-similarity in their vertical profiles when they are scaled by their respective maximum deficit values. For scaling the vertical distance to achieve the self-similarity, the velocity deficit profiles are scaled by the half-thickness of maximum velocity deficit, whereas the profiles of all other aforementioned turbulence quantities are scaled by the half-thickness of Reynolds shear stress deficit. The values of the maximum deficit of all the quantities diminish with the downstream distance, signifying the recovery of their uninterrupted upstream profiles. The deficits in mixing length and turbulent length (the Taylor microscale) scale also exhibit a certain degree of similarity in their profiles, when they are appropriately scaled.

This paper presents a methodology for estimating the bias limit of moving-boat acoustic Doppler current profiler (ADCP) streamflow measurements based on the classical error analysis techniques. The standard deviation (i.e., one-sigma) of the probability distribution of bias errors is defined as the bias limit. The presented method was applied to two types of data sets, as examples of applications. The first example uses 110 data sets collected in the comparison tests of different ADCP models on a wide variety of rivers and streams, located mostly in the United States. The second example uses 44 data sets collected in an ADCP regatta conducted with different ADCP models on a stream in New Zealand. The estimated bias limits from these data sets should have accounted for most, if not all, unknown and unknowable bias errors encountered in these measurements. The results from this study may be used to determine a nominal bias limit for moving-boat ADCP discharge measurements.

Plastic pipes [e.g., polyvinyl chloride (PVC) or high density polyethylene (HDPE)] are commonly used to replace aging metallic pipe sections (e.g., cast iron) in water-distribution networks. However, our understanding of how a combined metallic-plastic-metallic system responds to hydraulic transients is limited. This research studies such a system and focuses on answering whether the use of a plastic section to replace a metallic section can contribute to pressure surge suppression. The response of a metallic-plastic-metallic system to step and pulse pressure waves generated in the plastic section is investigated through theoretical and numerical analyses. It is found that the low impedance of plastic pipes (relative to metal pipes) plays a key role in determining the shape and amplitude of the pressure response. The metallic-plastic-metallic configuration can be used for surge suppression, especially for pulse waves that are commonly experienced in real networks attributable to customer activities. A generalized analysis finds that larger diameters and longer lengths of the plastic section contribute to better surge suppression, and the normalized results provide the selection criteria for system design. The findings are verified and demonstrated by numerical simulations using the method of characteristics, and some are also supported by the results of field trials. Some practical issues related to field applications are discussed.

Flow structure and dispersion characteristics change significantly in ice-covered channels and rivers, and the longitudinal dispersion coefficient is an important parameter for water pollution control and environmental protection. Considering the secondary flow in ice-covered channels and based on the Shiono and Knight method (SKM), the lateral distribution of the depth-averaged velocity in ice-covered channels is solved in this study by adopting a power series. The result agrees well with the original analytical solution and experimental data. The longitudinal dispersion coefficient caused by nonuniform distribution of transverse velocity is derived using Fischer’s triple integral formula, and the longitudinal dispersion coefficient formula in a rectangular experimental ice-covered channel is obtained by regression analysis, which coincides with the experimental results. The computational formula of longitudinal dispersion coefficient in natural ice-covered rivers is obtained by logarithm linear regression of the longitudinal dispersion coefficient in an experimental ice-covered channel based on the mapping relationship between the data obtained from experiments and natural rivers. The measured data and the corresponding dispersion formula in natural ice-covered rivers show a mean error rate of 28.4%, which verifies the correctness and the rationality of the proposed formula. The proposed formula can be used to estimate pollutant transport in broad and shallow ice-covered channels and natural ice-covered rivers. The comparison with ice-free rivers shows that the longitudinal dispersion coefficient of ice-covered rivers is considerably smaller than that of ice-free rivers.

This study investigates the unsteady pressure fluctuations in a hydraulic turbine observed during a dangerous case of steep ramping interrupted by an unexpected transition into total load rejection. Although hydraulic turbines are expected to experience such events only a few times over their lifetime, the resulting pressure amplitudes are so significant that they take a toll on a machine’s operating life. The focus of the present study is to experimentally measure and numerically characterize time-dependent pressure amplitudes in the vaneless space, runner, and draft tube of a model Francis turbine. To this end, 12 pressure sensors were integrated into a turbine, including four miniature sensors mounted in the runner. Steep ramping was performed by changing the rotational speed using a frequency controller. After a few seconds, as the load increased, total load rejection was initiated. This resulted in the generation of strong vibrations throughout the entire structure and strong pressure fluctuations in the turbine. The data analysis shows that pressure amplitudes are in the order of 10–20% of hydraulic energy in the vaneless space and runner, with high-amplitude fluctuations occurring at expected characteristic frequencies, including those associated with rotor-stator interactions, water hammer travel times, and standing waves in the turbine. Various stochastic frequencies were also observed, especially at the runner outlet.

An improved light attenuation method [light attenuation method for two-dimensional data acquisition (LAM2D)] to quantify two-dimensional (2D) sediment depths with higher accuracy is developed in the present study. The working principle is to relate attenuated light intensities with depths of the sediment layer. Traditionally, calibration has been performed with the scraper method with either multiple constant sediment depth profiles or a slope with incremental depth profile. This method works well for coarse sediment but is shown to be inaccurate for fine sediment with shallow sediment depth in the near-zero limit regime. In the present study, a new method is established for the calibration of shallow sediment depth by using the correlation of particle plume deposition profiles. The practical application of LAM2D to determine sediment deposition profiles from a laboratory simulation of barged sediment disposal was demonstrated with both approaches. The errors in the overall determination of sediment mass were found to be significantly reduced with the current approach.

This paper presents the application of a computationally convenient and physically consistent numerical approach to specify boundary conditions for suspended sediment (SS) with low-Reynolds-number (LRN) turbulence models. This new approach is attractive when the turbulence model equations need to be integrated all the way to the wall without the use of wall functions. A previous paper that discussed the case of using wall functions presented a way to link the bed boundary and a reference height at which many empirical suspended sediment entrainment laws specify a reference concentration (entrainment capacity). This paper is a follow-up to demonstrate a similar approach when LRN models are used. The treatment method with LRN models has been proposed before based on the near-wall turbulent-flow structure. The governing equation for SS was integrated between reference height and wall to derive the new wall boundary conditions. Such an approach eliminates the need for ad hoc treatments of the gap in many sediment transport models. Two applications of the new approach are demonstrated. The results of two examples compare well with experiments and demonstrate the applicability of the new method to more realistic applications.

A transient-based leakage detection method using a matched-field processing (MFP) scheme is formulated and applied. It is found to be efficient, robust, accurate, and provides a unique solution. Its inherent efficiency stems from the fact that the MFP method decouples the search for the leak location and the leak size. Its robustness is demonstrated by showing that the MFP method is able to estimate the location and size of leak in the presence of (1) noise even for signal-to-noise ratio (SNR) as low as $\mathrm{SNR}=-3$ dB, and (2) uncertainty in the fluid–pipe system wave speed. Another positive attribute of the MFP method is that it is able to use all available frequencies, rather than just resonant frequencies, and does not need to identify resonant frequencies. As a result, the MFP method provides precise localization estimates even in noisy environments. It is also shown that a unique identification is achieved by the MFP method provided that the pressure signal is measured at two locations along the pipe. For the case of multiple leaks, MFP identifies each of the leaks provided that the distance between the leaks is of the same order or larger than the shortest probing half-wavelength.

The construction of eight consecutive weirs along the Nakdong River changed the erosion and sedimentation patterns after the Four Major River Restoration Project. This study examines the sedimentation problems upstream of Sangju Weir with a new procedure combining: (1) the flow-duration; (2) the sediment rating curve; (3) the series expansion of the modified Einstein point procedure; and (4) the trap efficiency. For weirs or low-head dams, trap efficiency is not only a function of sediment size $d_s$, but also varies with discharge $Q$ and water stage of the reservoir $H$. At Sangju Weir, trap efficiency varies from 24 to 99%, the incoming sediment load is about $\mathrm{425,000}\mathrm{t}/\mathrm{year}$ and the annual reservoir sedimentation rate is estimated at $\mathrm{332,000}\mathrm{t}/\mathrm{year}$, which corresponds to a 0.76% annual reduction in reservoir storage capacity. Two hydraulic threshold values for discharge and weir operation stage are obtained from a 20-year daily sediment transport simulation. The two threshold values balancing the hydropower production revenue and the sediment excavation cost are (1) a flow discharge threshold of $600{\mathrm{m}}^3/\mathrm{s}$ and (2) a stage threshold of EL 43.6 m. The stage should be kept low during floods to minimize sedimentation.

A vertical two-dimensional (2D) nonhydrostatic numerical model for dam-break flows is proposed. This model combines a depth-averaged shallow-water equations (SWEs) model and a nonhydrostatic core using mode splitting. The nonhydrostatic core is solved under the free surface provided by the SWE model, in which tracking the complex and discontinuous free surface of dam-break flows in vertical spaces is avoided. In a test of an ideal dam-break flow, the new model was revealed able to capture the discontinuous free surface of dam-break flows and produce reasonable simulation results for velocity and nonhydrostatic pressure. The proposed model was further tested using an experiment of dam-break flow over dry bed with a trapezoidal step. The new model with the first-order Riemann solver produces a discontinuous reflected wavefront in the simulated free-surface profiles. The simulated free-surface profiles are improved by the new model with the second-order Riemann solver, and further improvements by using the volume-of-fluid (VOF) three-dimensional (3D) model are slight.