Journal of Hydraulic Engineering

Turbidity current is formed as subaerial open-channel sediment-laden flow plunges into a reservoir. The whole process of reservoir turbidity current, i.e., formation, propagation, and recession, is generally controlled by the water and sediment inputs from upstream and also the reservoir operation scheme specifying the downstream boundary condition. Enhanced understanding of reservoir turbidity current is critical to effective sediment management in alluvial rivers. However, until now there has been a lack of physically based and practically feasible models for resolving the whole process of reservoir turbidity current. This is because the computing cost of three-dimensional modeling is excessively high. Also, single layer-averaged models cannot resolve the formation process characterized by the transition from open-channel sediment-laden flow to subaqueous turbidity current, or the upper clear-water flow as dictated by the operation scheme of the reservoir, which has significant impacts on turbidity current. Here a new two-dimensional double layer-averaged model is proposed to facilitate for the first time whole-process modeling of reservoir turbidity current. The two hyperbolic systems of the governing equations for the two layers are solved separately and synchronously. The model is well balanced because the interlayer interactions are negligible compared with inertia and gravitation, featuring a reasonable balance between the flux gradients and the bed or interface slope source terms and thus applicable to irregular topographies. The model is benchmarked against a spectrum of experimental cases, including turbidity currents attributable to lock-exchange and sustained inflow. It is revealed that an appropriate clear-water outflow is favorable for turbidity current propagation and conducive to improving sediment flushing efficiency. This is significant for optimizing reservoir operation schemes. As applied to turbidity current in the Xiaolangdi Reservoir in the Yellow River, China, the model successfully resolves the whole process from formation to recession. The present work facilitates a viable and promising framework for whole-process modeling of turbidity currents, in support of reservoir sediment management.

The ability of a bubble screen to redistribute the flow field and bed morphology in shallow rivers and open channels has been investigated in laboratory experiments. Rising air bubbles generated by a pressurized porous tube situated on the bed induced secondary flow perpendicular to the porous tube. The secondary flow redistributed the longitudinal velocity, which caused also morphological redistribution under mobile-bed conditions. The strength and size of the bubble-induced secondary flow were independent of the base flow velocity and increased with water depth. The size of the secondary flow cell ranged from $3\times$ (immobile bed) to $7\times$ (mobile bed) the water depth. Similar sizes of bubble-induced secondary flow cells have been reported in literature for water depths ranging from 0.1 to 5 m, indicating that the laboratory experiments are relevant for natural rivers and open channels. A mutually strengthening interplay occurred between the bubble screen, the bubble-induced secondary flow, and the morphology. The bubble-induced secondary flow considerably increased the rising velocity of the air bubbles, which on its turn strengthened the secondary flow. The morphological redistribution increased the flow depth in the region covered by the secondary flow cell, which on its turn increased the size and strength of the secondary flow cell, and its effect on the morphological redistribution. This coupled hydrodynamic-morphologic behavior explains the larger size and strength of the secondary flow over a mobile bed than over a flat immobile bed. The results demonstrate the potential of the bubble screen as a technique to modify the morphology in a variety of applications in shallow rivers and open channels.

Weirs or bed sills are low-head hydraulic structures used for bed stabilization, raising upstream water level, and reducing flow velocity. During high-flow events, the weir is fully submerged in the river and scouring occurs both upstream and downstream of the weir. For a fully submerged weir, the scour mechanism around the weir is dependent on approach flow intensity (clear-water scour conditions or live-bed scour conditions) and flow regimes (surface-flow regime or impinging-jet regime) over the weir. The fast evolution of underwater mobile topographies and propagating bedforms increase the complexities of the scour process and the difficulties for scour measurement at the submerged weir under live-bed scour conditions. This paper develops a measurement and data-processing technique for the study of scour at submerged weirs under extreme measurement environments and investigates the scour process both upstream and downstream of submerged weirs under live-bed scour conditions. The experiments are carried out with uniform sediment in a tilting sediment recirculating flume. Different flow rates and weir heights are used. For all the tests, the flow upstream of the weir is subcritical. Bed elevation changes are measured in the approach flow reach and in the scour zones both upstream and downstream of the weir using a Seatek multiple transducers array (MTA) (SeaTek Instrumentation, Florida). The highly contaminated raw bed-elevation data are filtered. Scour depths and bedform characteristics are extracted in data postprocessing. During live-bed conditions, a scour-and-fill process occurs immediately upstream from the weir in response to periodic approaching bedforms. The influence of the flow regimes on the scour mechanism downstream of the weir is discussed. Based on dimensionless analysis and experimental data, equations for prediction of the scour depth at the weir are proposed.

Traditional Manning-Strickler and Keulegan formulas underestimate open channel flow resistance in the presence of large-scale roughness. How to theoretically evaluate resistance coefficients for large-scale roughness remains challenging in spite of significant efforts made in recent decades. The research reported in this paper provides an alternative understanding of energy losses associated with large-scale roughness. This yields a new resistance formula, of which two empirical coefficients were calibrated with laboratory and field data available in the literature. The results show that the new formula applies for both shallow and deep flows, and also agrees well with the best of previously proposed formulas.

Undesired bank erosion and migration of channel bends has led to the development of transverse in-stream structures as a mitigation technique. Currently, state-of-the-art procedures for structure design focus upon ranges of geometric parameters and do not provide information regarding hydraulic effects due to parameter alteration. A mathematical framework for the description of induced transverse in-stream structure hydraulics as a function of geometric parameters was proposed. The framework was empirically calibrated using normalized velocity data from a physical model study of various transverse in-stream structure types and geometries. Developed methodologies represent a fundamental improvement over traditional design methods, allowing for the prediction of induced hydraulic effects and the optimization of structure geometries to meet project design criteria.

A non-intrusive fault detection technology for real time condition assessment of pipelines is highly desirable for pipeline operators and the use of artificially induced fluid transients for this purpose has been the topic of many studies in the past. Fluid transients are stress waves in the fluid that can propagate through pipelines at high speeds and can collect information on the pipe condition during its travel. Whilst many transient-based fault detection methods have been proposed in the literature, little work was done on the characteristics of the transient signal that is best suited for fault detection. Such a study is critical for the development of this technology as the generation of controlled pressure waves is a challenging key step of the procedure. This paper presents analytical, numerical and experimental results to illustrate the effect of signal bandwidth (frequency content) of the induced transient signals has on the spatial resolution, accuracy and range of transient based fault detection. The results demonstrate that higher bandwidth signals provide more accurate fault detection at the expense of detection range.

This paper presents the results of an investigation on the turbulence characteristics of a rock-ramp-type, nature-like fish pass with a staggered arrangement of boulders for slopes of 1.5 and 3% for different discharges. This study showed some major differences between the flow with cluster of boulders (wake-interference flow) and the flow with isolated boulders (isolated-roughness flow) in the decay rate of turbulence and turbulence magnitude. General correlations were derived for the turbulence intensity and turbulent kinetic energy (TKE) as a function of normalized streamwise distance in a rock-ramp fish pass. Turbulence statistics (skewness and kurtosis), energy dissipation rate, and dissipative eddy size in this fish pass were also discussed in relation to fish passage.

Flow behavior upstream of a circular orifice with the invert at the bed level (initially) is investigated under fixed bed and equilibrium scour (mobile bed) conditions. The experiments are performed under three different constant heads and sediment sizes. Longitudinal and vertical components of velocity are measured in the vertical and horizontal planes passing through the center of the orifice. The velocity decay upstream of the orifice (along the centerline of the orifice) is slower for the fixed bed case than for the unbounded orifice. Under equilibrium scour condition, the velocity decay transitions from the velocity profile for the unbounded orifice to the fixed bed condition. The longitudinal velocity profiles in the horizontal plane at different locations upstream of the orifice are found to be similar for both fixed and mobile bed cases. The longitudinal velocity profiles in the vertical plane are also found to be similar for each case. The maximum velocity is found to occur below the centerline of the orifice for both cases. The location and decay of maximum velocity with distance upstream of the orifice are quantified. A three-dimensional flow model is used to simulate the flow behavior upstream of the orifice under fixed bed and equilibrium scour conditions. The model accurately predicts the velocity field upstream of the orifice under both fixed bed and mobile bed conditions.

The paper presents the results of calculations of the water-hammer course accompanied by the column separation caused by a rapid closure of the inline valve. The most important problem which the paper is dealing with is taking into account the cavitation characteristics of the inline valve. The calculation method contains a special way of including valve characteristics that have been determined on the laboratory setup in the Szewalski Institute of Fluid-Flow Machinery (IFFM) in Gdansk, Poland. In addition, calculations have been conducted using the writers’ own discrete vapor cavity model (DVCM), the new single-zone DVCM. The paper presents a comparison between these calculation results and the experimental results obtained at the laboratory setup for investigation of the water-hammer phenomenon in the pump discharge pipeline. The comparison of the numerical and empirical results is a basis for the verification process and assessment of the computational method that has been developed. The results and conclusions presented in this paper are of great importance from the theoretical and practical point of view, and thus give the basis for better understanding of the pipeline transients accompanied by the column separation phenomenon.

Reservoir sedimentation is considered as a sustainability problem. A concept getting down to the root of the cause was developed and tested in laboratory experiments. The idea is to maintain sediment in suspension, i.e., avoiding its settling near the dam by generating a jet induced artificial flow field and related turbulence (rotational flow), enabling the release of suspended sediment through the power intake. Therefore, a perpendicular jet configuration consisting of four jets arranged in a horizontal plane was investigated in a rectangular basin equipped with an outlet structure. The influence of its geometric parameters and the jet discharge on the sediment release was analyzed in detail. The flow pattern and its effect on the sediment release efficiency were evaluated, by measurements of turbidity and flow velocity. Depending on experiment duration and discharge, an ideal parameter set was identified resulting in a release efficiency between 1.5 and 2 compared to the reference case without jets.

Several proofs for the existence of flocculation in the Three Gorges Reservoir (TGR) are found through field measurements and laboratory experiments. The suspended particles and bed sediments are fine, and the particle diameters stay the same along the water depth, indicating that flocculation may occur. Both the vertical distributions of sediment concentrations (at low flow velocities) and measured settling velocities suggest that the effective settling velocities are larger than those of the primary particles, providing strong evidence for flocculation. The flow velocity zones of deposition and transportation are found, and the demarcation velocity of the two zones is approximately 0.5–$0.6\mathrm{m}/\mathrm{s}$. It is thought that flocculation occurs when the flow velocity is smaller than the demarcation velocity; otherwise, the flocs will be disrupted by turbulent shear. Laboratory experiments are conducted by using the sampled bed sediments, and floc structures are also observed, confirming the existence of flocculation in the TGR.

In this paper, modeling the spatial evolution of roll waves with diffusive Saint Venant equations is examined. A momentum diffusion term is added to the equations to describe turbulent normal stress occurring in extremely nonuniform roll waves. Roll-wave flows with the same hydraulic conditions as in Brock’s experiment are simulated so as to obtain time-averaged wave crest and trough depths. The magnitude of the diffusion effect is found to have significant influence on the spatial growth rate of the wave amplitude. Simulated wave crest and trough depths along the channel agree with experimental data only when the proper turbulent viscosity value is selected. This study indicates that the nondiffusive Saint Venant equations are unable to describe roll-wave flows because the wave amplitude is largely overestimated. The Saint Venant–type model should incorporate comprehensive turbulent stresses and appropriate closure approaches for predicting the spatial evolution of roll waves.