Journal of Hydraulic Engineering

Computational modeling of open-channel flows at low Froude ($\mathsf{F}$) numbers is often carried out using the rigid-lid assumption to prescribe the water surface location. However, the presence of wall-mounted hydraulic structures could cause $\mathsf{F}$ to locally exceed unity, inducing a transition from subcritical to supercritical flow, locally accelerate the flow generating high-energy turbulent eddies at a constriction region, and lead to the formation of backwater at upstream regions. Under such circumstances, the rigid-lid assumption may not be adequate to obtain reliable flow field simulations. Previous investigation to examine the effect of a rigid-lid assumption on large-eddy simulation (LES) of a low-Reynolds open-channel flow showed that the second-order turbulent statistics are altered depending on the free-surface resolving method. In this work, we seek to study the rigid-lid effect on high-Reynolds LES applicable to natural river flows. We employed an in-house Virtual Flow Simulator (VFS-Geophysics) model in its LES mode to simulate the flow field in a similar test case that is scaled up to obtain a high Reynolds of $7.9\times {10}^4$. We performed LES evaluating the two free-surface resolving approaches: rigid-lid and level-set. The flow field and free-surface computations were validated using a series of measured data that we obtained by carrying out laboratory experiments. Our LES results indicate that at near-bed regions of the flume and around the bridge abutment, both the first- and second-order turbulent statistics are influenced by the method selected to prescribe the water surface.

This study focuses on the output variation and dynamic behavior of hydroturbines in a hydraulically coupled system under hydraulic disturbance. The theoretical formula predicting the output variation of operating turbines after hydraulic disturbance is presented, and the influential factors are discussed. To investigate the dynamic characteristics of the hydraulic turbine governing system (HTGS) and verify the accuracy of the formula, a mathematical model of the hydropower plant with multiple turbines sharing a common hydraulic system was established, and the transient process caused by partial load rejection was simulated on the basis of the operation of a real hydropower plant. The results show that the output variation of the operating turbine is in agreement with the theoretical formula, and the amplitude of overload is significant under the conventional regulating model, i.e., opening control, in the low-head hydropower plants with long headrace tunnel during hydraulic disturbance. These methods and results will provide a theoretical basis for the safe operation and control of such hydropower plants.

The focus of this paper is to study transient flow processes in a trunk oil pipeline initiated by a sudden stoppage of a pumping station equipped with an overpressure protection system. Our analysis is based on a dimensionally reduced model, in which a one-dimensional (1D) system of partial differential equations (PDEs) is used to simulate flow in the pipeline. The pumping units, valves, and overpressure protection systems are modeled by algebraic systems of equations and ordinary differential equations (ODEs) in zero-dimensional (0D) models. Our numerical results demonstrate the efficiency of overpressure protection systems. In addition, the numerical results are compared with field measurements.

Based on the Reynolds-averaged Navier–Stokes equations and the Shiono and Knight method, this paper describes the development of an analytical model to predict the lateral distribution of depth-averaged velocities in steady uniform flows in rectangular ice-covered channels, including the effect of river bed resistance, ice sheet resistance, eddy viscosity, and secondary flows. The analytical model has three working conditions: full ice cover, symmetrical shore ice, and asymmetrical shore ice. The modeled results agreed well with the available experimental data, thereby indicating that the proposed model can accurately predict the lateral distribution of depth-averaged velocity in rectangular ice-covered channels. The application of dimensionless eddy viscosity, resistance coefficient, and secondary flow coefficient was analyzed. Results illustrate that the calculation method for the dimensionless eddy viscosity and resistance coefficient in open channels is also applicable to ice-covered channels. The study shows that secondary flow, which has a close relationship with flow depth, plays an important role in ice-covered channels. In the application of the model, ignoring the secondary flow will lead to a large computational error.

Strategies to reduce leakage based on integrated systems of monitoring and management of the pressure levels and related anomalies are acknowledged as the most reliable and cost-effective techniques to improve the efficiency of water networks. Because of their relevant impact at the large scale and the crucial roles currently played by programs envisaging ideas of smart cities capable to control/solve problems of homeland security and prevention and early monitoring of disasters and natural hazards, assessment and optimization of water networks have gained interest in recent years, overcoming the scientific fences of civil and hydraulic engineering and now being at the center of a wider vivid debate on how to maximize theoretical and experimental methods to increase security, reliability, and the so-called resilience of urban centers. Within this general scenario, this paper approaches the problem of the monitoring of leakage phenomena by focusing on the possibility of tracing the effects of the presence of not-necessarily circular holes on pipes subjected to combined water pressure and bending regimes, conceiving a parametric model which includes size, shape, orientation, and positioning of the hole, as well as different boundary conditions in the analyses to establish how they influence the final leakage. Sensitivity finite-element–based analyses are conducted to derive curves which could in principle serve, in combination with other consolidated strategies, to better identify position and amount of leakage phenomena. The outcomes are compared with the literature and in-house full-scale experimental findings, demonstrating good agreement between theoretical outcomes and laboratory results and gaining insight into advantages and limitations of the proposed strategy. It is felt that the developed approach, integrated with consolidated monitoring systems, might be useful to scientists and civil engineers in the management of existing hydraulic networks to estimate leakage volumes and predict more-realistic scenarios and related solutions for designing and planning practical interventions. This paper principally focuses on the hole geometry and position aspects, without addressing the hydraulic topic of discharge coefficient, in order to better understand the behavior of a leaking pipe.

The total dissolved gas (TDG) supersaturation that results from dam spillage may cause adverse effects, including increases in the risk of gas-bubble disease and mortality in fish. The accurate prediction of TDG levels is necessary in the exploration of measures for ameliorating the effects of TDG supersaturation. Based on an analysis of the mechanisms of hydropower projects with a plunging jet that produces high TDG levels, the process of TDG generation is divided into three stages. In Stage 1, TDG levels return to normal during jet spillage in air; in Stage 2, gas is dissolved in the stilling basin under high pressure; and in Stage 3, the TDG is abruptly released at the outlet of the stilling basin. According to previous research on Stage 1, the TDG level of water entering stilling basins can reach 100%. Experiments were carried out to estimate the TDG levels in Stage 2 under different pressures and retention times, and these experiments indicated that a TDG level above equilibrium saturation ($\mathrm{\Delta }{G}_0$) displays a linear relationship with the average pressure ($\mathrm{\Delta }P$) and a negative exponential relationship with retention time ($t_R$). Experiments were also conducted using physical models of the Songta and Yangfanggou dam projects in China to develop a method for estimating the retention time in stilling basins. The resulting formula for estimating the retention time is a function of the water depth in the stilling basin ($h_k$), length of the stilling basin ($l$), distance between the toe of the dam and impact point of the jet ($l_0$), and the dimensionless number at the stilling basin outlet $\lambda$. For Stage 3, in which the abrupt release of TDG occurs, field measurements were used to determine the values of the parameters used in the abrupt release expression contained in the model. By combining the results for the three stages, a predictive model of TDG levels was obtained. TDG observations collected at six different hydropower projects in China were used for validation. Substantial agreement between predictions and measurements was found. This work may provide a scientific basis for the production of precise predictive models of TDG levels, and it has considerable application value in assessing the effects of TDG and minimizing the risks posed by elevated TDG levels to aquatic life.

Debris flows are notorious geohazards existing in both subaerial and subaqueous environments. They may cause catastrophic destructions to adjacent life and properties along their overriding path. As such, predictions of their movement are critical to future geohazard mitigations, and there is a need to develop an effective numerical model to achieve this purpose. In this paper, a two-dimensional depth-averaged numerical model is presented to simulate the movement of subaqueous and subaerial muddy debris flows. The Herschel-Bulkley rheological model is used to describe the rheology of debris flow. The conservation equations of mass and momentum in conservative forms are numerically solved using an explicit finite difference scheme. The model is applied to a series of one-dimensional laboratory experiments in subaerial environments. The model is also applied to a field setting within the Na Kika Basin, Gulf of Mexico. Modeling results of deposit thickness of debris flow agree with those laboratory and field observations. Furthermore, the model is applied to two synthetic two-dimensional field conditions, one with a uniform slope and the other with a sinuous canyon. Sensitivity analyses are performed to explore the relative importance of yield stress, dynamic viscosity, bottom slope, initial failure height, and initial failure shape for runout distances of debris flow. For the application with a sinuous canyon, two different dimensions of canyon are used to demonstrate possible deposition patterns of debris flow.

To reduce bridge scour-induced collapse, experiments were performed to determine the optimal ratio of air to water velocity values for the least amount of scour at a cylindrical bridge pier. Clear-water experiments were done in noncohesive soils with air injected by means of a horizontal, semicircular air diffuser on the upstream half of the pier at the level of the original sediment prior to scour initiation. The diffuser location, number, and size of holes were determined previously to be optimal. Of the air-to-water velocity ratios tested, it was found that the least scour occurred at a ratio of 57.1. Scour was reduced 35% over the case with no air injection.

This paper addresses the sediment pick-up process in the high-velocity range of $2–6\mathrm{m}/\mathrm{s}$. An existing sediment pick-up function was recalibrated and modified using data of new experiments in a closed pipeline circuit with sand diameters in the range of 50–560 μm. The new pick-up function was used to simulate the generation and passage of a turbidity current along the submarine Congo canyon offshore from the coast of Zaire in Africa.

Uniform flow is important for physical model studies because generalizing flow data is possible only if the collected data is from locations where the flow is uniform, i.e., the test section. Presented in this paper are results from a physical model study of the channel length required for uniform flow to develop in a rough open channel with fully turbulent flow (i.e., the uniform flow development length, $L_{unif}$). The objective of this study was to develop a physically meaningful and parsimonious relationship for predicting $L_{unif}$. It is concluded that $L_{unif}$ is proportional to flow depth and channel width, and inversely proportional to bed material size. Also, neither Froude’s nor Reynold’s number is significant for predicting $L_{unif}$ in a rough open channel with fully turbulent flow.