Journal of Hydraulic Engineering

This paper presents the results of an experimental and numerical investigation of the mixing of stratified flow around bridge pier structures. In this study, which was carried out in connection with the Fehmarnbelt Fixed Link environmental impact assessment, the mixing processes of two-layer stratification was studied in which the lower level had a higher salinity than the upper layer. The physical experiments investigated two different pier designs. A general study was made regarding forces on the piers in which the effect of the current angle relative to the structure was also included. This was done in uniform flow with no stratification. Following this, a study was performed in which the mixing efficiency was investigated in the case of a stratified flow. The numerical investigations supplemented the findings of the physical experiments and gave information on scale effects, drag coefficients for low velocities, and natural background mixing. The present study provided a general understanding and knowledge about the mixing processes around bridge piers as well as a direct measure of the impact of the proposed designs on the natural stratification.

The entrances to the two lowest-survival migration routes for juvenile Chinook salmon in the tidal Sacramento River are located in the outside of a river bend where secondary circulation occurs. Three-dimensional simulations are conducted, in the Eulerian and Lagrangian frame, to understand tidal and secondary circulation effects on salmon migration route selection within this river reach. Fish were assumed to behave as neutrally-buoyant particles. Findings show that simulated particle entrainment rates into these routes tend to be larger than those expected from flow entrainment calculations alone, particularly during ebb tides, due to several factors. First, the fraction of the flow diverted to these routes tends to be higher in the shallowest layers, as a result of the secondary circulation that develops in the main river. Second, and supporting previous work done at the study site, the secondary circulation acting upstream also causes the surface-biased salmon distribution to be skewed toward the outside of the bend as they approach the entrance to the migration routes. As a result of these effects, the risk of particles being entrained is maximal near the surface, remaining higher than 50% during the course of a tidal cycle.

Water generally leaves a supply reservoir through a narrow rectangular concrete channel and enters a large trapezoidal earthen channel. The two channels are connected by a transition. Among the most common types of transitions, a wedge transition is relatively simple to construct. Even in well-designed transitions, the flow is often accompanied by a large zone of flow separation, causing considerable head loss. For the first time, this experimental study has explored modifications to a wedge transition to reduce the separation and head loss. The characteristics of three-dimensional flow in both the wedge and the modified wedge systems were investigated using non-intrusive measurements of the flow field. Transition flow parameters, including longitudinal flow profile, transition energy loss, maximum velocity at the transition outlet, and turbulence intensity in the downstream trapezoidal channel, were determined. Flow separation characteristics were recorded by tracing the separation zone and the points of reattachment based on velocity data, and were confirmed by visual observation involving dye tests. For both systems, separation occurred on one side and the flow was three dimensional. Compared with the wedge transition, the modified wedge transition effectively guides the flow and prevents abrupt changes in flow direction. The modified wedge transition results in a much earlier recovery of the flow in the downstream channel, lower levels of turbulence, and a less strong secondary flow. It also achieves a small gain in the energy at the flow recovery location and reduces the head loss coefficient as a result of the reduced separation zone.

This paper presents an experimental study of local scour at submerged weirs with different downstream slopes. Thirty-three tests of coarse sand ($d_{50}=0.85\mathrm{mm}$) and 24 tests of fine sand ($d_{50}=0.26\mathrm{mm}$) were carried out in a sand-recirculating flume. The tests for each sand type used four weirs with different downstream weir slopes ($\pi /6$, $\pi /4$, $\pi /3$, and $\pi /2$ to horizontal for the fine sand; $\pi /12$, $\pi /6$, $\pi /4$, and $\pi /2$ to horizontal for the coarse sand). The results show that the upstream scour depth is independent of the downstream weir slope $\beta$. For both sand types, the downstream weir slope has no effect on the downstream scour depth when it is greater than the upstream slope of the downstream scour hole $\theta$. For $\beta <\theta$, the downstream weir surface is partly exposed to flow, resulting in a downstream scour depth at the weir surface $d_w$, which can be increased by the increase in either downstream scour depth or $\theta$. In this situation, the upstream water level rises because of the reduced flow cross-section area, and the overflow jet is deflected to be approximately aligned with the downstream weir surface. For the coarse sand tests, when $\beta =\pi /6$, the effect of the rising upstream water level is predominant because $\beta$ is close to ${\theta }_a$ (time-averaged value of $\theta$). This results in larger downstream scour depths. When $\beta$ is considerably smaller than ${\theta }_a$, the effect of jet deflection starts to be predominant in the coarse sand tests, generating lower downstream scour depths when $U_0/U_c<2.6$. Those two effects balance each other when $2.6<U_0/U_c<3.1$ for $\beta =\pi /12$, resulting in the same downstream scour depths as those for a rectangular weir. For the fine sand tests, the jet deflection effect is predominant because the downstream weir slope has limited effect on the upstream water level. A conservative design method accounting for both effects is proposed for the local scour at downstream sloped submerged weirs.

When pipelines are rapidly filled, dynamic and sometimes dramatic air–water interactions often occur. These air–water interactions, along with their associated pressure and temperature oscillations, are explored in this paper both experimentally and numerically. A computational fluid dynamics (CFD) approach based on a volume of fluid (VOF) formulation is used to simulate the flow field in three-dimensions (3D). Thermal conduction and convection in three different media (air, liquid water, and the pipe wall) are all considered in order to account for the key thermal processes potentially influencing an entrapped air pocket during rapid filling. The simulation accounts for the compressibility of water and the roughness of the pipe wall, considerations sometimes neglected in previous studies. Simulated pressures and air–water profiles are compared to measured data and to the dynamic images obtained through a high-speed camera. The relatively good agreement between the numerical and experimental results confirms that the proposed model can accurately simulate transient flow and also reasonably represents the associated physical processes. Significantly, the observed phenomena of white mist and of a hot pipe wall are explained through the physics represented in the 3D simulations. Indeed, the model shows that extremely intense air–water interactions appear sufficient to account for the observed efficient heat exchange and the dramatic rates of energy dissipation. Overall, the proposed model provides insight into the physical mechanisms of rapid filling and particularly those thermal effects associated with entrapped air.

Research on erosion behavior of sediments usually treats sand and mud separately. However, natural soil often occurs as a sand–mud mixture. An expression for cohesive forces in sand–mud mixtures, which considers the effect of the sand component on the mud component, is first proposed. Then the balance of forces under the initial motion condition is analyzed, and a unified formula for the critical shear stresses of sand, mud, and sand–mud mixtures is derived. The formula reproduces well the way critical shear stress varies with mud content and is more accurate than other formulas that are widely used for sand–mud mixtures and pure cohesive sediments. The proposed formula also promises to be more convenient, being based on the dry bulk density of the mixture and the stable dry bulk density of the mud component. The effect of consolidation on the erosion thresholds of both sand–mud mixtures and pure muds is considered in the formula, which makes the formula capable of predicting the erosion thresholds of sediments in the process of consolidation.

The paper deals with discrete vapor cavity models (DVCM) to reproduce transient cavitating pipe flows. Since features and limits of the classic DVCM are well known from literature, some developments are here considered and analyzed: the continuity equation for the cavity is written in terms of mass balance instead of volume balance, allowing calculations with appropriate computational fine grids; a lower threshold is introduced for pressure, in order to prevent it from dropping below vapor pressure; prediction-correction steps are introduced in the numerical solution procedure; and the two-dimensional (2D) description of the flow field is considered assuming axial-symmetric flow, in order to evaluate unsteady friction without the need of parameters calibration. Computational results, which are sensitive also to a weighting parameter used in the numerical process, are compared with each other, with results of a bubble flow model, and with experimental measurements reported in the literature. In order for such comparisons to be meaningful, the numerical simulation must not be limited to the first few cavitation events but must be extended in time. All one-dimensional (1D) models are weakly sensitive to grid size, except for one case with severe cavitation, whereas 2D model results are practically grid-independent. The comparison of numerical and experimental results shows that the effects of the physically based corrections (mass balance and lower threshold for pressure) are generally small but not negligible and consist of a slight increase of dissipations. In general, 2D models improve the performance of DVCMs; in particular, their results better reproduce the shape of the head function and reduce numerical spikes, but also provide an excess of energy dissipation which causes numerical results to be anticipated with respect to experimental traces. The comparison of results of DVCMs and a bubble flow model, in their 1D and 2D forms, with experiments shows that the latter performs better. As a result of the present study, all DVCMs should therefore be used with care and be limited either to cases of weak cavitation or to the first phases of the phenomenon.

Ice strain dominates the ice thrust and dynamics on reservoir dams and retaining structures. An exclusively designed laser range finder was deployed to measure the surface ice displacements along six directions at a reservoir in northeastern China. The incompletely confined boundary (ice-boundary bonding), ice cracks development, water level fluctuations, parallel crack dynamics, and ice creep allow the surface ice to move rather than keep still in response to thermal deformation/pressure, and thus cause the ice strain to deviate from thermal strain. Consequently, a residual strain was introduced and calculated from the recorded displacements. Observations showed that the residual strains were anisotropic and showed diurnal patterns following the air/ice temperature. A scale-dependence of crack development was observed to cause potential scale-effects to residual strains. The real ice strain consists of thermal strain and residual strain. The proportion of the latter increased as time went by. A modified constitutive law accommodating the residual strains was developed to evaluate the impacts of the residual strains and to estimate the surface ice stresses. Modeling results underlined the role of the residual strain in determining both the principal stress and the stress perpendicular to and parallel with the dam face. The residual strain is probably the reason why the observed ice stress is always much lower than the single thermal stress.

Despite extensive research on bridge scour and channel erosion, predicting the occurrence and rate of cohesive soil erosion remains problematic. The lack of standard procedures for sample preparation and testing has resulted in wide variations in testing conditions, devices, and soil properties across erosion studies, ultimately preventing the synthesis of cohesive erosion studies and progress in understanding the fundamental processes of cohesive soil erosion. Therefore, the objective of this study was to evaluate the effects of sample holding time on the fluvial erosion of remolded cohesive soils to inform the development of standard testing procedures. Three different soils (fat clay, lean clay, and silty sand) were tested in a flume following multiple sample holding times. Results show that erosion rate can decrease 85–95% within 72 h of soil wetting, depending on clay mineralogy. These results highlight the importance of maintaining a consistent soil preparation protocol in cohesive soil erosion experiments and reporting soil sample holding durations when conducting cohesive erosion research using remolded samples.

Previous literature has shown unsatisfactory validation of the existing equations predicting pier-scour depth with laboratory and field data. In an attempt to find the reasons for the unsatisfactory comparisons, we examined the performance of the Chinese equations with several other equations through in-depth analyses of the errors and explored the possible error-causing factors both in field measurement data and the proper use of parameters. Results show that the Chinese equations underpredicted scour depth for all the laboratory data and for field data under clear-water conditions. The Chinese equations performed better for field data under live-bed conditions compared to the other (i.e., laboratory, or clear-water field) conditions. The other equations overpredicted scour depth for both laboratory and field data. Uncertainty of the field data associated with measurement accuracy results in large errors, especially when scour depth is less than 1 m. The flood duration in the field is usually far shorter than what is needed to reach the equilibrium scour depth that the equations are attempting to predict for safe design, which contributes to the larger predicted scour depth than measurements. The effect of some parameters (e.g., sediment size, pier width, and incipient velocity for local pier scour) should be modified in the Chinese equations under fine-sediment and/or wide-pier conditions.

Experiments were performed in the symmetric compound channel with diverging and converging floodplains for different relative flow depths to investigate the resistance characteristics of overbank flow in nonprismatic sections. The effectiveness of Manning’s $n$, Darcy-Weisbach’s $f$ and Chezy’s $C$ are analyzed. Manning’s roughness coefficient relies upon the nondimensional parameters like width ratio, aspect ratio, relative flow depth, angles (diverging and converging angles), relative distance, bed slope, Reynolds number, and Froude number. A multivariable regression model has been developed by taking care of the aforementioned geometric and hydraulic parameters to estimate the Manning’s roughness coefficient for nonprismatic compound channels. The nonlinear regression models are developed using pertinent experimental data obtained from laboratory experiments and the data from other researchers on the compound channel with nonprismatic floodplains. The present model gives satisfactory results as compared to other approaches for different experimental channels and field data by providing less error.

There is increasing interest in the application of Archimedes hydrodynamic screws (AHS) to generate hydroelectric power. Although numerous theoretical and experimental investigations have been published on this application of AHS over the last decade, there have been no experimental evaluations in the literature of blade shapes other than the helicoid. This paper provides the results of bench-scale testing of three-dimensional (3D)-printed models to evaluate the performance of various shapes and parameters for AHS. Three different screws are investigated to experimentally compare the new blade shapes: the helicoid, strake, and reverse strake.

This paper explains the root cause of instabilities that tend to arise in pressure-reducing valves (PRVs) under low-flow conditions. It was found that the loss of stability in PRVs is a direct result of an increase in the static valve-network gain as the valve position gets smaller, thus making pressure changes more sensitive to valve position adjustments. If the valve controller is tuned at medium valve openings characteristic of normal operating conditions, the increased gain at low valve openings can cause the control system to be too aggressive in its valve position adjustments, leading to oscillations. This paper provides a mathematical derivation of the gain equation for a simplified pipe-PRV-pipe model. The obtained gain equation curve was then used to derive the formula for a gain compensator whose purpose is to keep the static gain constant across an entire range of permitted valve openings. A simplified network transient model was then used to recreate a real-life PRV instability event and show the remedial effects of the gain compensator.

Extended partial blockages in urban water supply systems (UWSS) are formed from complicated physical, chemical, and biological processes; thus, these blockages are commonly in random and nonuniform geometries. The transient-based blockage detection method (TBBDM) has been evidenced in many applications to be a promising way to diagnose these blockages. Despite the successful validation and application of the TBBDM in the literature, pipe blockages used in these studies were idealized and simplified to regular and uniform shape, which are however not common in practical UWSS, and thus invalidity and inaccuracy of this TBBDM has been widely observed in practical applications. This paper presents fundamental research on understanding the influence of more realistic and nonuniform blockages on transient wave behavior and the accuracy of current TBBDM. The blockage with a linearly varying diameter (termed as nonuniform blockage) is firstly investigated by the frequency domain analytical analysis for its impact on transient wave behavior, which is thereafter incorporated in the overall transfer matrix of transient frequency response for reservoir-pipeline-valve systems. The results indicate the nonuniform blockage may induce very different modification patterns on the frequency shift and amplitude change of transient waves from the uniform blockage situation.

This paper presents a new model, using existing consolidation theory, suitable for long-term morphodynamic simulations; we refer to the dynamic equilibrium consolidation (DECON) model. This model is applicable for muddy systems at small suspended particulate matter (SPM) concentrations, where the sedimentation rates are smaller than the consolidation rates and small fractions of sand can be accounted for. Thus, the model assumes quasi-equilibrium of the consolidating bed. It is derived from the full consolidation (Gibson) equation and is implemented in a mixed Lagrangian-Eulerian bed model guaranteeing stable and non-negative solutions, while numeric diffusion remains small. The erosion and deposition of sand and mud is accounted for, whereas internal mixing (e.g., bioturbation) is modeled through diffusion. The parameter settings for the new consolidation model (the hydraulic conductivity, consolidation coefficient, and strength) can be obtained from consolidation experiments in the laboratory. The model reproduces one-dimensional consolidation experiments and the qualitative behavior of erosion and deposition in a tidal flume. The DECON model was also applied to more natural conditions, simulating fine sediment dynamics on a schematized mud flat and in a schematized tidal basin under tide and wave forcing. The computational results of the mudflat simulations compared well with the simulations with the full Gibson equation. For the tidal basin simulations, DECON predicted the expected landward tidal transport of fine sediment during tide-dominated conditions, while the tidal basin withstood erosion during the more energetic wave-dominated periods. Computational times for the morphodynamic simulations of the tidal basin example without waves increased by a factor of 5 when consolidation was included. For the simulations with waves, this increase in computational times was only a factor of 2, as simulations with waves are always expensive. Applying a complete consolidation model would be prohibitive. The DECON model therefore serves as a useful tool to simulate fine-sediment dynamics in complex wave- and tide-dominated conditions, as well as the effects of seasonal variations.

Sedimentation in the Three Gorges Reservoir (TGR) has been studied with a primary focus on the amount due to the decrease in sediment inflow caused by water and soil conservation projects and sediment retention of recently constructed upstream cascade reservoirs. In this study, the rate and distribution of sedimentation in the TGR were investigated. A total of 1.6 billion tons of sediment was trapped in the TGR from 2003 to 2015. The sediment retention rate increased from 60% during the initial operation stage between 2003 and 2005 to 82% during the normal operation stage between 2006 and 2015. Sediment finer than 0.008 and 0.016 mm contributed 44 and 61% of the total sedimentation, respectively, and the median size of the sedimentation was 0.01 mm. The spatial distribution of the sedimentation was discontinuous, and 80% of the sedimentation occurred in one-third of the backwater region, with primary deposition zones being at the wide reservoir reaches. The sedimentation was assessed by the sediment transport capacity, which was embedded in the 1-D sediment transport and bed deformation equations. Due to the impoundment of the TGR, the sediment transport capacities decreased significantly at the wide reservoir reaches where flocculation occurred, resulting in deposition. However, the sediment transport capacities were still greater than the sediment concentrations at the gorges, which will be the fixed stretches with no deposition. The surplus sediment transport capacity at the gorge stretches would not allow cumulative sedimentation in the future, leading to a spatially discontinuous equilibrium sedimentation along the TGR instead of the predicted spatially continuous distribution. A uniform bed slope is not expected under the new sedimentation equilibrium state, and the actual sedimentation will be considerably less than the initially predicted value, generating a sustainable storage volume with about 84.7% of the initial volume, which will facilitate the engineering functions of the TGR significantly.

This study investigates experimentally the effect of nozzle type, including diffusers with single- and multiple-orifices with different diameters, on the hydrodynamics and mass transfer characteristics of air-water bubbly jets discharged at the bottom of a water tank. The results revealed that for the same flow rates of air and water, the total area of the orifices independent of the number of ports controls the initial momentum of the bubbly jets and the jet-to-plume length scale of the flow. An integral model was fitted to the experimental data in order to obtain entrainment coefficients for each test and at different water depths, which confirmed that merged multiple bubbly jets can be analyzed using an equivalent single jet integral model. This resulted in an entrainment relationship described as a function of the kinematic buoyancy flux, bubble slip velocity, and distance from the source, which is similar to that available in the literature for bubble plumes, but with different constants. Finally, the model proposed here is compared to previous models and applied to practical cases including mixing and aeration of tanks and water bodies, indicating that gas transfer efficiency can be significantly enhanced by using multiple-orifice arrangements.