Showing papers in "Journal of Hydraulic Engineering in 2001"

Spatially Averaged Open-Channel Flow over Rough Bed

Abstract: In this paper it is suggested that the double-averaged (in temporal and in spatial domains) momentum equations should be used as a natural basis for the hydraulics of rough-bed open-channel flows, especially with small relative submergence. The relationships for the vertical distribution of the total stress for the simplest case of 2D, steady, uniform, spatially averaged flow over a rough bed with flat free surface are derived. These relationships explicitly include the form-induced stresses and form drag as components of the total stress. Using this approach, we define three types of rough-bed flows: (1) Flow with high relative submergence; (2) flow with small relative submergence; and (3) flow over a partially inundated rough bed. The relationships for the double-averaged velocity distribution and hydraulic resistance for all three flow types are derived and compared with measurements where possible. The double-averaged turbulent and form-induced intensities and stresses for the case of regular spherical-segment-type roughness show the dominant role of the double-averaged turbulence stresses and form drag in momentum transfer in the near-bed region.

Mean Flow and Turbulence Structure of Open-Channel Flow through Non-Emergent Vegetation

Abstract: The ability of turbulence models, based on two equation closure schemes (the k-e and the k-ω formulations) to compute the mean flow and turbulence structure in open channels with rigid, nonemergent vegetation is analyzed. The procedure, developed by Raupach and Shaw (1982), for atmospheric flows over plant canopies is used to transform the 3D problem into a more tractable 1D framework by averaging the conservation laws over space and time. With this methodology, form/drag related terms arise as a consequence of the averaging procedure, and do not need to be introduced artificially in the governing equations. This approach resolves the apparent ambiguity in previously reported values of the drag-related weighting coefficients in the equations for the turbulent kinetic energy and dissipation rates. The working hypothesis for the numerical models is that the flux gradient approximation applies to spatial/temporal averaged conservation laws, so that the eddy viscosity concept can be used. Numerical results ar...

Leak Detection in Pipes by Frequency Response Method

Abstract: The frequency response method is used to determine the location and rate of leakage in open loop piping systems. A steady-oscillatory flow, produced by the periodic opening and closing of a valve, is analyzed in the frequency domain by using the transfer matrix method, and a frequency response diagram at the valve is developed. For a system with leaks, this diagram has additional resonant pressure amplitude peaks (herein referred to as the secondary pressure amplitude peaks) that are lower than the resonant pressure amplitude peaks (herein called primary amplitude peaks) for the system with no leaks. Several piping systems are successfully analyzed for all practical values of the friction factor to detect and locate individual leaks of up to 0.5% of the mean discharge. The method, requiring the measurement of pressure and discharge fluctuations at only one location, has the potential to detect leaks in real-life pipe systems conveying different types of fluids, such as water, petroleum, and so on.

Development of Integrated Sediment Rating Curves Using ANNs

Abstract: Correct estimation of sediment volume being carried by a river is very important for many water resources projects. Conventional sediment rating curves, however, are not able to provide sufficiently accurate results. Artificial neural networks (ANNs) are a simplied mathematical representation of the functioning of the human brain. Three-layer feed-forward ANNs have been shown to be a powerful tool for input-output mapping and have been widely used in water resources problems. The ANN approach is used to establish an integrated stage-discharge-sediment concentration relation for two sites on the Mississippi River. Based on the comparison of the results for two gauging sites, it is shown that the ANN results are much closer to the observed values than the conventional technique.

Exchange Processes between a River and Its Groyne Fields: Model Experiments

Abstract: The exchange of dissolved matter between a groyne field and a main stream influences the transport and distribution of a pollutant cloud in a river. In forecasting models, groyne fields are represented as dead zones with effective properties like exchange coefficients and exchanging volume. Despite its relevance for such practical applications, little research has been done on the exchange process between a groyne field and the main stream itself. Therefore, this study is aimed at examining this exchange process and validating the dead-zone prediction model, which treats the exchange process as a first order system. A schematized physical model of a river with groynes was built in a laboratory flume. The exchange process was visualized quantitatively with dye in adjacent groyne fields. In order to couple the exchange process to the velocity field, particle tracking velocimetry measurements were performed. Two different types of exchange were observed. First, exchange takes place via the mixing layer that is formed at the river-groyne-field interface. The large eddies formed in the mixing layer are the major cause of this exchange. Second, under certain conditions, even larger eddies are shed from the upstream groyne tip. Distortions in the flow field caused by such intermittent structures cause a much larger exchange than that by the mixing layer alone. The occurrence of large shed eddies depends on the presence of a sufficiently large, stationary, secondary gyre located at the upstream corner of the groyne field. The overall exchange of matter could be characterized as a first-order process, in accordance with the dead-zone-theory. The corresponding exchange coefficients agreed reasonably well with the results of earlier experiments and the effective coefficients as found in experiments in real river flows.

Experiments on flow at a 90° open-channel junction

Abstract: Although open-channel junctions are common in many hydraulic structures, no comprehensive data set has been compiled that describes the 3D flow field within the junction itself. This physical model study examined a 90°, sharp-edged, open-channel junction for channels of equal width. Depth measurements were made using a point gauge while velocity measurements were taken using an acoustic doppler velocimeter over a grid defined throughout the junction region. The average velocity and turbulence intensity were calculated from a time series of velocities that was recorded at each location. In addition, a 2D mapping of the water surface was performed on a 76.2 mm square grid throughout the channel junction. This paper presents the details of the experimental procedure and the general flow characteristics observed. The full data set generated during this experimental work is available for downloading on the Internet. Using a small portion of the data recorded, an evaluation of several previously proposed theori...

Mean Flow and Turbulence in Open-Channel Bend

Abstract: Flow over a developed bottom topography in a bend has been investigated experimentally. The measuring section is in the outer-bank half of the cross section at 60° into the bend. Spatial distributions of the mean velocities, turbulent stresses, and mean-flow and turbulent kinetic energy are presented. The cross-sectional motion contains two cells of circulation: besides the classical helical motion (center-region cell), a weaker counterrotating cell (outer-bank cell) is observed in the corner formed by the outer bank and the water surface. The downstream velocity in the outer half-section is higher than the one in straight uniform flow; the core of maximum velocities is found close to the separation between both circulation cells, well below the water surface. The turbulence structure in a bend is different from that in a straight flow, most notably in a reduction of the turbulent activity toward the outer bank. Both the outer-bank cell and reduced turbulent activity have a protective effect on the outer ...

Longitudinal dispersion coefficient in straight rivers

Abstract: An analytical method is developed to determine the longitudinal dispersion coefficient in Fischer's triple integral expression for natural rivers. The method is based on the hydraulic geometry relationship for stable rivers and on the assumption that the uniform-flow formula is valid for local depth-averaged variables. For straight alluvial rivers, a new transverse profile equation for channel shape and local flow depth is derived and then the lateral distribution of the deviation of the local velocity from the cross-sectionally averaged value is determined. The suggested expression for the transverse mixing coefficient equation and the direct integration of Fischer's triple integral are employed to determine a new theoretical equation for the longitudinal dispersion coefficient. By comparing with 73 sets of field data and the equations proposed by other investigators, it is shown that the derived equation containing the improved transverse mixing coefficient predicts the longitudinal dispersion coefficie...

Flow over ogee spillway : Physical and numerical model case study

Abstract: A study was completed to compare flow parameters over a standard ogee-crested spillway using a physical model, numerical model, and existing literature. The physical model was constructed of Plexiglas and placed in a test flume. Pressure taps were installed along the entire length of the spillway. Discharge and pressure data were recorded for 10 different flow conditions. A commercially available computational fluid dynamics (CFD) program, which solves the Reynolds-averaged Navier-Stokes equations, was used to model the physical model setup. Data interpolated from U.S. Bureau of Reclamation and U.S. Army Corps of Engineers design nomographs provided discharge and pressure data from the literature. Nondimensional discharge curves are used to compare the results from the different methods. Pressures are compared at low, mid, and high flow conditions. It is shown that there is reasonably good agreement between the physical and numerical models for both pressures and discharges. The availability and power of existing numerical methods provides engineers with another tool in the design and analysis of ogee spillways.

Large-Eddy Simulation of Sediment Transport: Currents over Ripples

Abstract: A large-eddy simulation has been performed to study the detailed sediment dynamics that occur in channel flow over ripples. The code solves the Navier-Stokes equations and an advection-diffusion equation for the sediment, and has been verified to produce quantitatively accurate hydrodynamic results for the flow domains used in this study. The main features of interest in this flow are seen because of the high resolution of the grid, especially near the ripple boundary. This yields a detailed bottom shear stress distribution, properly identified coherent structures, and resolved sharp gradients in the sediment concentration. Globally, the sediment is carried up into the flow from locations where the shear stress is high—on the upslopes of ripple crests—and is advected downstream by the current and upward by the vertical velocity component. Regions of positive vertical velocity are associated with Gortler vortices, which represent a significant sediment transport mechanism in this flow.

Scour around Pile in Combined Waves and Current

Abstract: This paper presents the results of an experimental study on scour around a pile subject to combined waves and current. Irregular waves were used in the experiments, which were carried out both for ...

Riprap protection at bridge piers

Abstract: Although riprap is the most commonly employed countermeasure against scouring around bridge piers, few studies exist of riprap performance under live-bed conditions. In this study, failure mechanis...

Impact of turbidity currents on reservoir sedimentation

Abstract: All lakes created on natural rivers are subjected to reservoir sedimentation. The construction of a dam significantly modifies the flow conditions of natural streams inside and downstream of an artificial lake. The sediment concentration is often high during the flood season, and the entering flow shows a greater density than the ambient fluid. Suspended load can therefore be carried along the reservoir bottom to the dam in the form of turbidity currents. This paper presents research results that help to better understand this physical phenomenon, which contributes to reservoir sedimentation. It is based on in situ measurements, a laboratory scale model of turbidity currents and numerical flow simulations. The study of a thousand-year flood in the Luzzone Reservoir in the Swiss Alps using the developed computer model revealed the potential of such a tool. In particular, the impact on the sediment deposits was analyzed. A valuable evaluation of the incidence of such a turbidity flow is presented and its effects are compared to observations. Significant progress has been made in understanding the importance of turbidity currents in reservoir sedimentation.

Role of Bed Discordance at Asymmetrical River Confluences

Abstract: This paper studies laboratory open-channel confluences using a 3D, elliptic solution of the Reynolds-averaged Navier-Stokes equations, including a method for approximating the effects of water surface elevation patterns and a renormalization group modified form of the k-e turbulence model. The model was tested by comparison with laboratory measurements of an asymmetric tributary junction. This suggests that although the model is unable to reproduce the quantitative detail (notably upwelling velocity magnitudes) of the flow structures as measured in laboratory experiments, statistically significant aspects of the experimental observations are reproduced. The model is used to (1) describe and explain the characteristic flow structures that form in a confluence with one of the tributaries angled at 45°, both with and without an elevation difference (bed discordance) in the angled tributary; and (2) investigate the relative importance of junction angles (30°, 45°, and 60°), bed discordance, and ratio of mean ...

1D Numerical Model of Muddy Subaqueous and Subaerial Debris Flows

Abstract: A 1D numerical model of the downslope flow and deposition of muddy subaerial and subaqueous debris flows is presented. The model incorporates the Herschel-Bulkley and bilinear rheologies of viscoplastic fluid. The more familiar Bingham model is integrated into the Herschel-Bulkley rheological model. The conservation equations of mass and momentum of single-phase laminar debris flow are layer-integrated using the slender flow approximation. They are then expressed in a Lagrangian framework and solved numerically using an explicit finite difference scheme. Starting from a given initial shape, a debris flow is allowed to collapse and propagate over a specified topography. Comparison between the model predictions and laboratory experiments shows reasonable agreement. The model is used to study the effect of the ambient fluid density, initial shape of the failed mass, and rheological model on the simulated propagation of the front and runout characteristics of muddy debris flows. It is found that initial failure shape influences the front velocity but has little bearing on the final deposit shape. In the Bingham model, the excess of shear stress above the yield strength is proportional to the strain rate to the first power. This exponent is free to vary in the Herschel-Bulkley model. When it is set at a value lower than unity, the resulting final deposits are thicker and shorter than in the case of the Bingham rheology. The final deposit resulting from the bilinear model is longer and thinner than that from the Bingham model due to the fact that the debris flow is allowed to act as a Newtonian fluid at low shear rate in the bilinear model.

Calibration assessment and data collection for water distribution networks

Abstract: Calibration of a water distribution network is intended to develop a model that mimics field conditions under a range of demand distributions. In this paper, a three-step calibration procedure is developed that considers the uncertainties in measurement and estimation and provides a measure of the quality of the calibration. The approach can also be used to identify preferable conditions for data collection. The procedure's steps are parameter estimation, calibration assessment, and data collection design. Parameter estimation considers input uncertainty and the resulting uncertainty in model parameters. Calibration assessment analyzes the propagation of the parameter errors on model predictions. The trace of the covariance matrix of the predictive heads is used to measure the model uncertainty. Based on this uncertainty and using a sensitivity-based heuristic analysis, data collection experiments can be designed for systemwide tests and critical pipe for individual pipe tests. An example system is analyz...

Neural networks as routine for error updating of numerical models

Abstract: This paper describes a somewhat alternative approach to combining observations and numerical model results in order to produce a more accurate forecast routine. The approach utilizes artificial neural networks to analyze and forecast the errors created by numerical models. The resulting hybrid model provides very good forecast skills that can be extended over a forecasting horizon of considerable length. The method has been developed for the purpose of operational forecasting of current speeds in the Danish Oresund Strait. The forecast system was used as a planning tool during the construction of the 16 km-long fixed link across the Oresund Strait, linking the countries of Denmark and Sweden.

Use of Vanes for Control of Scour at Vertical Wall Abutments

Abstract: Rock vanes are single-arm structures angled to the flow with a pitch into the streambed such that the tip of the vane is submerged even during low flow. Vanes have primarily been used in recent years for treatment of bank erosion in stream stability projects. These structures roll the water away from the eroding banks, thus limiting erosion of the channel banks. They have proven to be very effective treatments over a range of flow conditions. In this project, the effectiveness of vanes for preventing scour at single-span bridges with vertical wall abutments was evaluated based on laboratory experiments. The vanes were tested in small-scale experiments in a recirculating flume and subjected to a range of flow conditions, including bank full and a number of overbank flows, which were forced to return to the channel at the abutment. The results showed that the vanes were highly effective in moving the scour away from the abutment into the center of the channel under all flow conditions tested. Based on the experimental results, optimum design settings for the vane angle and height, most effective number of vanes, and distance upstream for placement of the first vane were determined.

3d numerical model for pearl river estuary

Abstract: A 3D numerical model with an orthogonal curvilinear coordinate in the horizontal direction and sigma coordinate in the vertical direction has been developed. This model is based on POM (Princeton Ocean Model). In this model a second moment turbulence closure submodel is embedded and the stratification caused by salinity and temperature is considered. Furthermore, to adapt to estuary locations where the flow pattern is complex, the horizontal time differencing is implicit with the use of a time-splitting method instead of the explicit method in POM. This model is applied to the Pearl River estuary, which is the largest river system in South China, with Hong Kong at the eastern side of its entrance. The computation is verified and calibrated with field measurement data. The computed results mimic the field data well.

Rational Criterion for Designing Opening of Slit-Check Dam

Abstract: This paper presents a theoretical approach to the problem of designing the opening in a slit-check dam. The approach is based on the conservation of the mass of water and sediments and on the energy balance under steady conditions. It leads to a relationship among opening width, sediment characteristics, mountain river geometry, and water and sediment discharge. The final relationship can be simplified to make it suitable for practical applications. Also, the problem of unsteadiness of both water and sediment is considered, as well as the possibility of treating the unsteady flow as a sequence of steady states. The results of the theory were checked in a laboratory investigation using a scale model. Different opening widths were tested under conditions of steady and unsteady supplies of water and sediment. The mean grain size of the sediment, as well as the rates of sediment and water discharge, were changed in the experiments. The results of the experiments confirm the theory quite well.

Entrainment and turbulence in saline underflow in Lake Ogawara

Abstract: Under certain tidal conditions, a saline underflow originating in the Pacific Ocean moves into Lake Ogawara, Japan. The underflow consists of a uniform saline bottom layer that is slightly warmer than the ambient and an interfacial shear layer in which the velocity and density are decreased. Within the experimental area the underflow is confined to a channel approximately 1 km wide and is essentially two-dimensional. The underflow had a bulk Richardson number, defined in terms of the mean properties, between 1 and 2. The rate of entrainment into the bottom layer was calculated using two distinct methods. The first method used the change in the maximum salinity of the underflow measured at two stations along the path of the underflow to infer the amount of ambient water entrained. The second method made direct measurements of vertical mass fluxes with a profiler. The agreement between the two methods was excellent. The measured entrainment coefficients were consistent with the derived entrainment law. The turbulent structure of the flow was mapped for a 3-h quasi-steady period of the flow. Turbulence is predominantly generated on the bottom boundary and is transported vertically to the density interface where it leads to mixing.

Discrete Particle Modeling of Entrainment from Flat Uniformly Sized Sediment Beds

Abstract: The stability of randomly deposited sediment beds is examined using a discrete particle model in which individual grains are represented by spheres. The results indicate that the threshold shear stress for flat beds consisting of cohesionless uniformly sized grains cannot be adequately described by a single-valued parameter; rather, it is best represented by a distribution of values. Physically, this result stems from the localized heterogeneity in the arrangement of surface grains. For uniformly sized beds, geometric similarity exists such that the critical entrainment shear stress distributions scale directly with grain size. A Shields parameter of 0.06 is commonly used to define “threshold conditions,” and it was found that this corresponds to a point on the distributions where approximately 1.4% by weight of the surface is mobile. Furthermore the analysis includes a comparison of the contributions of sheltering to variation in critical entrainment shear stress. It was found that remote sheltering, ind...

Simulation of Scour Process in Plunging Pool of Loose Bed-Material

Abstract: The scouring process in a plunge pool of loose bed with uniform bed-materials due to a two-dimensional plane impinging jet was simulated computationally. The finite-element-based unsteady three-dimensional model, CCHE3D, with \Ik-e\N turbulence closure was employed to solve the flow field. It has long been recognized that the unsteady behavior of the turbulent jet fluctuation plays an important role in scouring and transporting sediment in the plunge pool. In order to model this phenomenon realistically, one has to consider the effects of both shear stress and the lift force on sediment particles due to pressure fluctuation. The latter has been taken into account by using empirical relationships of flume data. Both of these effects have been incorporated in the nonequilibrium sediment transport model consisting of sediment pickup rate and step length adopted for the jet scour problem. The model constant relating to the fluctuating lift force was calibrated using an empirical equation to predict the quasi-equilibrium scour depth. The results simulated by the model proposed here agree reasonably well with the experimental data.

Onset of skimming flow on stepped spillways

Abstract: Assuming that skimming flow occurs when the jet leaving a step has a slope equal to that of the stepped spillway when it impinges on the pool behind it on the next step, an equation has been developed to predict the incipient value of \Iy\dc/\ih, where \iy\dc = critical depth and \ih = step height. This equation was found to agree well with most of the experimental observations for \ih/\il greater than about 1.0, where \il = step length. For \ih/\il = 1.7, the incipient value of \iy\dc/\ih = 0.24, whereas for values of \ih/\il smaller than about 0.8, \iy\dc/\ih is almost constant at 0.8.

Two-Phase Flow Analysis of Concentration Profiles

Abstract: Two-phase flow analysis is used to analyze sediment concentration profiles in uniform open-channel flows over flat, sediment-starved beds that have high concentrations of single-sized sediment. Two-phase flow analysis can explicitly incorporate the effects of particle-particle interactions and particle inertia. Conventional convection-diffusion modeling cannot directly represent these phenomena and are thus limited. Both the two-phase flow formulation and the convection-diffusion modeling are compared against experimental data collected in sediment-starved sediment-laden flows. The two-phase flow model is shown to simulate the effect of both particle-particle interactions and particle inertia in these experimental flows. Simple criteria are given to determine when particle-particle interactions and particle inertia are important in sediment-laden open-channel flows over a flat bed. The current two-phase approach requires empirical formulas of the turbulence quantities and further experimental and analytical work is necessary to develop improved models for the velocity distribution and turbulence quantities.

Scour of Cohesive Soil by Submerged Circular Turbulent Impinging Jets

Abstract: This paper introduces a method for estimating the scour in cohesive soils produced by a submerged vertical circular turbulent impinging jet. Determining scour in cohesive soils is a complex problem, partly because the clay particles within the soil are held together by electrochemical forces that are not easily quantifiable. As well, erosion occurs in many forms, such as the removal of individual particles or as large chunks of soil. Results of a laboratory study of scour by a circular impinging jet of a cohesive soil, consisting of 40% clay, 53% silt, and 7% fine sand, are presented. Analysis based on the mechanics of the impinging jets shows that the dimensions of the scour hole at an equilibrium state of scour are a function of the momentum flux from the jet, the impingement height (for “large” impingement heights), the viscosity and density of the eroding fluid, and the critical shear stress of the soil. Mass erosion was the predominant type of erosion observed.

Flume tests for scour in clay at circular piers

Abstract: Local scour at circular piers founded on clay was studied experimentally in the laboratory to compare the depth of scour in sand and in clay and to investigate the effects of the Reynolds number, Froude number, and approach flow depth on scour depth. The depths of scour in front, at the side, and in the back of the piers were measured as a function of time under steady, gradually varied flow conditions. The measured scour-depth-versus-time curves were fitted with a hyperbola to estimate the equilibrium scour depths. The extrapolated equilibrium scour depths were compared with values predicted by the Federal Highway Administration equation. The results showed that although the rates of scour were much slower in clay than in sand, equilibrium scour in clay was about the same as in sand. It was found that the shape of the scour hole correlates with the pier Reynolds number. At low Reynolds numbers, the depth of scour was about the same all around the piers. At higher Reynolds numbers, the scour holes developed mainly behind the piers with much less scour in front of the piers. It was also found that the extrapolated equilibrium scour depth correlates well with the pier Reynolds number and that the Froude number and relative water depth did not have a significant effect on the scour depth for these experimental conditions.

Monitoring River Channel and Flume Surfaces with Digital Photogrammetry

Abstract: This paper describes and illustrates a technique for high resolution monitoring of the surface morphology of water-worked sediments. The monitoring uses close-range digital photogrammetry. While photogrammetry is a long-established technique, more recent developments in digital photogrammetry allow application in fluvial research to be highly cost effective in both flume and natural river channel studies. Results are presented that involve two scales of laboratory flume: a smaller-scale application associated with sediment sorting processes in a straight channel; and a larger-scale application involving sediment transport and bed material feedbacks in a meandering channel subject to overbank flows. A preliminary assessment of data quality is undertaken with encouraging results. The precision of elevation estimates corresponds to the scale of the imagery acquired and hence may be controlled by design of the image acquisition process.

Experimental study of 3D pump-intake flows with and without cross flow

Abstract: Detailed measurements of three-dimensional turbulent flows within a rectangular single-pump bay area of a right-angle water intake model with and without cross flow were obtained using an acoustic Doppler velocimeter (ADV) in order to elucidate swirling flow characteristics within the pump sump. Without cross flow, the pump-approach flow distributions were characterized by nearly uniform streamwise velocities in the pump bay and weak free-surface vortices near the pump column. With cross flow, the three-dimensional mean velocity measurements revealed the existence of a large recirculation zone upstream of the pump column such that strong streamwise velocities were present at higher depths and near the left sidewall, while the reverse current concentrated at lower depths along the right sidewall. Flow patterns in the latter case were also characterized by strong free-surface vortices in the vicinity of the pump column and a strong floor-attached subsurface vortex underneath the pump bell. Uncertainty analy...