Journal of Hydraulic Engineering

This paper presents measurements of velocity profiles obtained from idealized dam-break experiments and results from numerical simulations of these experiments. Dam-break flows were generated in the laboratory by suddenly lifting a gate inside a flume for three different upstream heads with a dry-bed downstream condition. Ultrasonic Doppler velocity profilers were used for recording transient velocity profiles at eight different locations upstream and downstream of the removed gate. These experiments provided data on the spatio-temporal evolution of the flow field in an unsteady flow of relatively short duration. The two-dimensional experiments were simulated using a computational fluid dynamics solver. The following observations are made: (1) turbulence modeling does not affect the velocity profile in the upstream reservoir, but has significant influence on the prediction of downstream velocity; (2) the velocity magnitude at a specific location changes with time, but the shape of the velocity profiles remains similar; (3) an analytical solution for frictionless dam-break flow on a sloping bed and numerical simulation with the large eddy simulation (LES) modeling show satisfactory agreement with measured water surface profile; (4) nondimensionalization of reservoir-side velocity profiles resulting from different reservoir heads and at different locations from a specific head shows excellent collapse and reveals that the shear layer thickness of these profiles is approximately 5% of the initial reservoir head; (5) measurement and simulation with the LES model for turbulence show satisfactory agreement, suggesting that the LES modeling is a viable approach for an accurate prediction of dam-break flows.

A numerical model for simulation of embankment dams’ gradual breaching because of overtopping, named embankments breach software (EBS), is introduced in this paper. The model solves the equations for the depth-averaged flow (modified for shallow water flow on steep slopes), the $k\text{-}\epsilon$ turbulence model, the sediment transport, and the bed elevation change in a coupled manner. An upwind cell center finite volume method is applied for developing the discrete form of the differential equations. A linearized scheme with second-order accuracy in space and explicit time integration on unstructured triangular meshes is utilized in this model. To simulate the flow over dry sloping surfaces of the embankment dam configuration, the developed model considers a powerful wetting and drying technique. Furthermore, the abilities of the model enhanced simulation of sudden bank failure of the breach channel because of the base erosion and transport of the failed solid material.

Three-dimensional direct numerical simulations of gravity currents on different bottom slopes are presented in this paper. After the buoyancy closed in a lock is instantaneously released, the produced gravity currents go through an acceleration phase followed by a deceleration phase. In the acceleration phase, the tail current connects to and feeds buoyancy into the head for all cases considered here. The maximum buoyancy contained in the head, reached at the end of the acceleration phase, increases as the bottom slope increases. The maximum buoyancy in the head never reaches the total released buoyancy, and a significant portion of released heavy fluid is left in the tail current. In the deceleration phase, the tail current continues to join the head as the gravity currents propagate for lower slope angles ($\theta =0.2$, and 4°), but the head disconnects the joining tail current for higher slope angles ($\theta =6$, 8, and 10°). The gravity current head loses contained buoyancy less rapidly in the deceleration phase as the bottom slope increases. Structures of the gravity current indicate that the relative length of the head diminishes as the gravity currents propagate downslope for lower slope angles and remains approximately constant for higher slope angles. The maximum front velocity increases as the bottom slope increases. In the deceleration phase, the front location–time relationship follows the thermal theory power law for higher slope angles and for lower slope angles, and the inertial phase power-law asymptote is observed.

The emergent vegetation in open-channel flows is usually simulated using arrays of circular cylinders in laboratory experiments. Analysis of recent experimental data reveals that for a given Reynolds number, the drag coefficient of a cylinder in a dense array is larger than that of an isolated cylinder. A new approach is applied to parameterize the drag coefficient and Reynolds number for flows through arrays of emergent cylinders. The approach is developed based on the concept of pseudofluid, for which an analogy is made between the cylinder-induced drag in an open-channel flow and that induced by the cylinder settling in a stationary fluid. With the proposed parameterization, the experimental database is successfully reorganized in such a way that a generalized drag coefficient is related to a generalized Reynolds number by one single curve, which is valid for a wide range of solid fractions and Reynolds numbers. However, it should be mentioned that only rigid circular stems are considered in this study and their induced drag is assumed to be dominant in comparison with the channel bed resistance.

This study was conducted to investigate the flow restructure and resistance in vegetal flow under wind shear action in shallow lakes. A special racetrack-style flume with two sets of driving devices installed on the top was designed to simulate the wind-driven current. Two typical types of live vegetation, Vallisneria natans and Acorus calamus of Taihu Lake in China, were planted in the system. The results revealed that the velocity distribution of vegetal flow under wind action differed from that in open channels. The velocity distribution can be described as logarithmic-curve shaped and contained a compensation current in the lower layer. The area within the effective height of vegetation in which the velocity was obviously affected comprised an effective friction region. When compared with V. natans, A. calamus displayed better resisting capacity. Based on measured data, a new empirical relationship between drag coefficient and wind speed was presented, which indicated the resistance ability of vegetation under different intensities of wind action. Variations in the equivalent roughness coefficient were also discussed.

The experimental study investigates how the application of point suction at different bed locations affects local scour around a cylindrical bridge pier under clear-water conditions. The results show that suction has a profound effect on pier-scour development, and the extent of the influence is related to the location of the suction source. When the suction source is located at or upstream of the pier, the equilibrium scour depth is reduced by up to 50%, with $Q_s/Q_o=2\%$, where $Q_s$ and $Q_o$ = suction flow rate and undisturbed approach flow rate, respectively. The data also show that for the suction source that is located downstream of the pier, the equilibrium scour depth is reduced by up to 30% with the same suction flow rate. The extent of pier-scour depth reduction at different locations from the suction source is closely associated with changes to the local effective shear and critical shear velocity. Results of this study show that bed suction may be used as an effective pier-scour countermeasure.

Cavitation on spillways has proven to be an undesirable condition. The formation of vapor-filled bubbles and cavities and their eminent collapse has led to significant damage to major spillway components and appurtenant structures worldwide. Although stepped spillways are thought to be less prone to cavitation damage than smooth spillways, designers continue conservative practices regarding specifying stepped spillways at many sites. Using laboratory experiments in a specialized reduced ambient pressure chamber, cavitation was shown to form on stepped geometries that are representative of typical stepped spillways currently in service. Experiments in a nonaerated closed conduit revealed the strength and extents of the highly intense shear layer that forms above the step tips, and the friction characteristics were determined and compared with results from previous researchers. Advanced techniques for detecting cavitation characteristics along with high-speed videography have given additional insight into the flow features that drive the formation of cavitation. Finally, a correlation between the critical cavitation index and the common friction factor is shown, extending the data that also includes shear layers resulting from uniformly distributed roughnesses, jets, and wakes.

This paper develops an analytical solution to assist in designing and sizing stormwater infiltration structures. As original elements, our solution allows to estimate the hydraulic head in the infiltration structure as a function of (1) storm temporal dynamics, making use of an appropriate intensity duration frequency (IDF) curve, (2) the model adopted to describe the response of the catchment to a given input rainfall, and (3) the evolution of the wetted front advancing in a homogeneous soil where the infiltration device is located. Our solution allows highlighting the effects of the various simplifications associated with existing formulations employed in the common engineering practice. Our results indicate that typically adopted methodologies based on the assumptions of a uniform infiltration rate and/or negligible flow rate along the lateral surface of the structure may lead to overestimating the key design parameters.

Hydraulic transients (water hammer waves) can be used to excite a pressurized pipeline, yielding the frequency response diagram (FRD) of the system. The FRD of a pipeline system is useful for condition assessment and fault detection, because it is closely related to the physical properties of the pipeline. Most previous FRD-based leak detection techniques use the sinusoidal leak-induced pattern recorded on the FRD, either shown on the resonant responses or the antiresonant responses. In contrast, the technique reported in the current paper only uses the responses at the first three resonant frequencies to determine the location and size of a leak. The bandwidth of the excitation only needs to be five times that of the fundamental frequency of the tested pipeline, which is much less than the requirement in conventional FRD-based techniques. Sensitivity analysis and numerical simulations are performed to assess the robustness and applicable range of the proposed leak location technique. The proposed leak location technique is verified by both numerical simulations and by using an experimental FRD obtained from a laboratory pipeline.

Leakage is a major factor impacting the evolution of flood in a highly permeable river. By integrating a mathematical model with data provided by a monitoring database, a flood routing system was developed for the Dagu River in China. The aim of this study was to calculate the leakage discharge and to discuss the effect of leakage for flood modeling within a high-leakage riverbed. This system was implemented for application of a flood-diversion experiment in 2003, which accessed data automatically and used estimated coefficients to calculate leakage discharge. The results showed that simulated discharge values fit well with the measured values, with an average relative error less than 10%, and the leakage average relative error was approximately 2%. The infiltration experiments showed that the leakage coefficient could be treated as a constant when the dynamics in the initial phase of a short flood event was stimulated. The initial water level and leakage and roughness coefficients were the main factors required to accurately determine the discharge of river seepage.

There are quite a few studies investigating low Reynolds number turbulence models for predicting the near-wall flow over smooth walls. However, analogous work over rough walls is sparse despite the fact that many flows of practical interest, particularly in geophysical flows and hydraulic engineering, are over rough walls. This paper presents a priori and a posteriori evaluations of low Reynolds number Reynolds-averaged Navier-Stokes (RANS)–based modeling approaches for flows over rough walls with the help of high-resolution large-eddy simulation (LES) data. The LES data allow the calculation of the vertical distribution of the turbulent kinetic energy, the dissipation rate, the specific dissipation rate, and the turbulent eddy viscosity, so that the assumptions inherent of RANS-based turbulence models can be tested a priori. A posteriori testing of the models through RANS simulations showed that the three low Reynolds turbulence models evaluated in this paper provide a fairly accurate prediction of the velocity profile, turbulent eddy viscosity, and wall shear stress.

This paper is focused on the sedimentation processes of fine-grained particles in a 14-km-long section upstream of the Three Gorges Dam, referred to as the dam area, during the period from March 2003 to October 2006. Field measurements indicated that sediment deposits in the dam area were mostly found at the bottom of the main channel, where a new river bed was formed with a flat, nearly horizontal surface. In contrast, very little deposits could be found on the bank slopes of the channel. The observations disagree with the previous predictions based on numerical models, which suggested more evenly distribution of deposits over the cross section. This paper explains the discrepancy by identifying the motion of muddy deposits by gravity as a key process in the dam area. A three-dimensional (3D) full Reynolds-Averaged Navier-Stokes (RANS) model is applied to calculate the flow and sediment transport processes. A critical slope method (CSM) is used to estimate the downslope movement of muddy deposits in the bottom by gravity. The formation of bottom deposits is successfully simulated using this approach. The simulated sedimentation patterns agree well with field observations.

This study examines the dangerous countercurrent that can form at the downstream face of a low-head dam for certain flow conditions, utilizing computational fluid dynamics software and laboratory observations. The objective of this project was to identify easily measured parameters that can be used to accurately distinguish between the possible high-risk and low-risk states of flow. The research was carried out on two common low-head dam shapes (ogee-crested and flat-topped) and three dam heights (0.61, 1.52, and 3.05 m) with simulations performed for various upstream and downstream water depths. It was found that the combination of parameters $(h_u-h_d)/P$ (named the risk factor) was capable of defining transition points between states of flow, and therefore the low-risk and high-risk conditions. With regard to the ogee-crested dam simulations, it was found that the most dangerous flow conditions occurred at risk factors between 0.09 and 0.80. For the flat-topped dams tested, the high-risk conditions occurred at risk factors ranging from 0.34–0.71. Countercurrent length was also examined, with rollers from the ogee dams ranging from $2.05–5.95P$ in length, and from $2.60–5.45P$ for flat-topped dams.