Showing papers in "Journal of Hydraulic Engineering in 2000"

3d numerical modeling of flow and sediment transport in open channels

Abstract: A 3D numerical model for calculating flow and sediment transport in open channels is presented. The flow is calculated by solving the full Reynolds-averaged Navier-Stokes equations with the k − e turbulence model. Special free-surface and roughness treatments are introduced for open-channel flow; in particular the water level is determined from a 2D Poisson equation derived from 2D depth-averaged momentum equations. Suspended-load transport is simulated through the general convection-diffusion equation with an empirical settling-velocity term. This equation and the flow equations are solved numerically with a finite-volume method on an adaptive, nonstaggered grid. Bed-load transport is simulated with a nonequilibrium method and the bed deformation is obtained from an overall mass-balance equation. The suspended-load model is tested for channel flow situations with net entrainment from a loose bed and with net deposition, and the full 3D total-load model is validated by calculating the flow and sediment tr...

Estimating bottom stress in tidal boundary layer from acoustic doppler velocimeter data

Abstract: Bed stresses in the bottom boundary layer of the York River estuary, Virginia, were estimated from three-dimensional near-bottom velocities measured by Acoustic Doppler Velocimeters (ADVs) and also by a profiling array of electromagnetic current meters. By assuming the measurements were made in a constant stress layer, four methods of stress estimation were evaluated using ADVs: direct covariance (COV) measurement, turbulent kinetic energy, inertial dissipation utilizing the Kolmogorov spectrum, and log profile. The four methods yielded similar estimates of frictional velocity (U*) based on ADV output from both 14 cm and 44 cm above bed. All eight estimates of average U* were consistent with the overall mean of 1.10 cm/s to within the 95% confidence interval for individual burst estimates. The COV method worked slightly better nearer the bed, possibly because of the sensitivity of COV to the upper limit of the constant stress layer. The inertial dissipation method performed marginally well at 14 cm above bed, likely because of sediment induced stratification and insufficient separation of turbulent production and dissipation scales. The log profile method was the most variable and appeared most sensitive to stratification and to the thickness of the constant stress layer. The turbulent kinetic energy method was the most consistent at both heights and appears most promising for further development. Results encourage future use of the ADV in estuarine environments but also favor the simultaneous use of several methods to estimate bottom stress.

Flow Turbulence over Fixed and Weakly Mobile Gravel Beds

Abstract: Characteristics of turbulence structure in quasi-2D flows with static and weakly mobile gravel beds are presented. Three sets of measurements with acoustic Doppler velocimeters in an irrigation canal were used: two with subcritical bed shear stress (static beds) and one with the bed shear stress τo close to critical τoc (weakly mobile bed). The analyses included vertical distributions of local mean velocities, turbulence intensities, turbulent shear stresses, velocity auto- and cross-spectra, the quadrant method, and high-order velocity moments. A number of properties of turbulence intensities, high-order moments, streamwise bursting parameters, and velocity spectra appeared to be similar for all three flows, but some properties were different. The most important one was an observed reduction in the von Karman constant for the flow with weakly mobile bed. Comparison of these results with other studies and analogies with drag-reducing flows suggest that at τo/τoc ≈ 1 the drag on the bed for a given granula...

Probabilistic Exner Sediment Continuity Equation for Mixtures with No Active Layer

Abstract: The Exner equation of sediment continuity is the foundation of river morphodynamics. Generalization of this equation to mixtures of grain sizes has required the introduction of an active layer (i.e., a buffer layer between the sediment moving in the water column and the immobile substrate below). The active layer is defined to be a well-mixed layer, with no vertical structure, that encompasses those grains available to exchange directly with the moving sediment. The sediment in the substrate below exchanges with the active layer only as the bed aggrades or degrades. The active layer concept is a useful one that has served the research community well for 3 decades. However, the division of the erodible bed into a discrete active layer and substrate must represent only an approximation of a more general formulation that contains no active layer and in which parameters pertaining to the entrainment from and deposition to the bed vary continuously with depth below the sediment-water interface. Here the probability density function of bed elevation is used to derive a general Exner equation of sediment continuity with no discrete layers. The formulation is applicable to both sediment mixtures and tracers in uniform sediment. Although the treatment requires more information than that of the active layer approach, it offers the prospect of a better understanding of how streams create a stratigraphic record of their activities through deposition.

The Turn of the Screw: Optimal Design of an Archimedes Screw

Abstract: The geometry of an Archimedes screw is governed by certain external parameters (its outer radius, length, and slope) and certain internal parameters (its inner radius, number of blades, and the pitch of the blades). The external parameters are usually determined by the location of the screw and how much water is to be lifted. The internal parameters, however, are free to be chosen to optimize the performance of the screw. In this paper the inner radius and pitch that maximize the volume of water lifted in one turn of the screw are found. The optimal parameter values found are compared with the values used in a screw described by the Roman architect and engineer Vitruvius in the first century B.C., and with values used in the design of modern Archimedes screws.

Friction Factors for Coniferous Trees along Rivers

Abstract: A physically based model for estimating the resistance coefficient for coniferous trees in open-channel flow is modified to account for variations in the flexibility between species. The new model is based on the assumption of a linear increase in foliage area with height and a dimensional analysis. It is supported by experiments in air and water. The advantage of the new model over existing methods for estimating resistance factors is its ability to account for the interaction between the vegetation and the flow, taking into account velocity, depth of flow, and vegetative conditions (including type, size, stage of maturity, and density of vegetation). Based on the mathematical model, a table is provided to estimate Manning's n value for flow through vegetation.

Two-Dimensional Solution for Straight and Meandering Overbank Flows

Abstract: This paper presents a practical method to predict depth-averaged velocity and shear stress for straight and meandering overbank flows. An analytical solution to the depth-integrated turbulent form of the Navier-Stokes equation is presented that includes lateral shear and secondary flows in addition to bed friction. The novelty of the present approach is not only its inclusion of the secondary flows in the formulation but also its applicability to straight and meandering channels. The analytical solution is applied to a number of channels, at model and field scales, and is also compared with other available methods such as that of Shiono and Knight and the lateral distribution method. The present formulation gives much better predictions of velocity and shear stress, particularly in those cases where the secondary flows are dominant.

Numerical analysis of river channel processes with bank erosion

Abstract: This paper presents a numerical analysis of river channel processes with bank erosion. The model can be used for investigating both bed-deformation and bankline shifting in 2D plan form. The basic equations are used in a moving boundary fitted-coordinate system, and a new formulation of nonequilibrium sediment transport is introduced to reproduce the channel processes. The model was applied to examine the morphological behavior of experimental channels. Temporal changes in the plan form in a meandering channel can be classified into two patterns: meander developing and meander straightening. Comparison of the observed and calculated results indicates that the model is applicable to both channel changes under various hydraulic conditions. On the basis of the numerical findings, the paper clarifies the influence of hydraulic variables on the location of bank erosion and bed scouring. The model also was used to investigate the effect of alternate bars on bank erosion and to investigate the development of channel meandering from an initially straight channel.

Field Techniques for Suspended-Sediment Measurement

Abstract: The measurement of suspended sediment, particularly in field settings, is important in the documentation of sediment transport and deposition. Many measurement techniques have been used with varying degrees of success. The techniques, including their operating principles, advantages, and disadvantages are discussed. The techniques discussed include acoustic, bottle, pump, focused beam reflectance, laser diffraction, nuclear, optical backscatter, optical transmission, and spectral reflectance. Emphasis is placed on instrumentation techniques, as this is the area of suspended-sediment measurement that has the greatest potential for improving sediment data. Acoustic technology (if further developed) emerges as a promising technology because of its ability to measure the concentration profile without intruding into the flow. This technology-transfer information will be valuable to practitioners and researchers needing to choose a means of measuring suspended sediment. The choice of a measurement technique has...

Buoyant Velocity of Spherical and Nonspherical Bubbles/Droplets

Abstract: An integrated formulation is presented to calculate the buoyant velocity of bubbles/droplets of various sizes. The bubble/droplet shape can be a sphere, ellipsoid, or a spherical-cap. This formulation can be applied to solids, liquids, or gases. The comparison of the calculated results with experimental data shows a good match and that the formulation presented is better than the Stokes law and Reynolds law combination when dealing with bubbles/droplets in a wider range of sizes. This work was developed in connection with oil and gas spill models that have buoyant oil, gas, or gas hydrates, although they can also be applied to other hydraulic engineering problems.

Application of three-dimensional hydrodynamic model for lake Okeechobee

Abstract: A 3D hydrodynamic and heat transport model was developed for Lake Okeechobee. Continuity, momentum, and temperature transport equations were solved. Dynamically coupled transport equations for turbulent kinetic energy and turbulent scale also were solved. The numerical scheme used spatial finite differencing and a three-time-level, external-internal mode splitting procedure. A 28-day calibration was conducted, using measured bathymetry, rainfall, relative humidity, total solar radiation, wind velocity, inflow, and outflow data. During the calibration period, little rainfall occurred, and lake water levels receded. Water surface elevation, horizontal velocities, and temperature were computed. Agreement between observed and simulated values was based on graphical comparisons, minimizing mean absolute and root-mean-square errors, and spectral analysis. Comparisons showed that the model reproduced general observed trends and short-term fluctuations. The model's heat transport and turbulence closure schemes be...

Direct Determination of Rheological Characteristics of Debris Flow

Abstract: (4) 0 dy where t = shear stress; t0 = Bingham yield stress; m = dynamic viscosity; and du/dy = shear rate. Then, adding coarse particles successively, the concentration of coarse particles was 0.1, 0.2, 0.25, 0.3, and 0.35, respectively, and the average diameter was 0.55, 1.01, 1.18, and 1.63 mm, respectively. The sedimentations were observed at the range of shear rate 20‐56 s 21 . The larger the effective concentration of slurry, the larger the shear rate of sedimentation. The larger the diameter of coarse particles, the smaller the shear rate of sedimentation. The model of O’Brien and Julien as described by Julien and Lan (1991) was fitted to the rheological curves of nonsedimentation: 2 du du t = t 1 h 1 z (5) y SD dy dy

Evaluation of Unsteady Flow Resistances by Quasi-2D or 1D Models

Abstract: This note considers the evaluation of resistances in unsteady flow by means of a 1D unsteady friction model. The model is applied for the case of a water hammer, both in a single gravity pipeline and in a single pumped pipeline. Comparisons with experimental results show that the model does not predict the exact shape of the oscillation, but it gives, with high precision, the maximums and minimums of the pressure head oscillation if correct values of the parameter of the model are used. Because experimental evaluations can be made only for a few cases and can give rise to very different values depending on experimental conditions, the sensitivity of the model to physical and numerical parameters that influence the phenomenon is examined by comparison with the results of a quasi-2D model previously proposed.

Three-Dimensional Mathematical Model of Suspended-Sediment Transport

Abstract: This paper presents the basic equations for a mathematical model of sediment-laden flow in a nonorthogonal curvilinear coordinate system. The equations were derived using a tensor analysis of two-phase flow and incorporate a natural variable-density turbulence model with nonequilibrium sediment transport. Correspondingly, a free-surface and the bottom sediment concentration are employed to provide the boundary conditions at the river surface and the riverbed. The finite analytic method is used to solve the equations of mass and momentum conservation and also the transport equation for suspended sediment. To demonstrate the method, the sediment deposition for the Three Gorges Project is considered. The mathematical model specifies the boundary conditions for the inlet and outlet using data from physical model experiments. The results for the mathematical model were tested against laboratory measurements from the physical model experiment. Good agreement and accuracy were obtained.

Evaluation of unsteady flow resistances by quasi-2d or 1d models. technical note

Abstract: This note considers the evaluation of resistances in unsteady flow by means of a one-dimensional (1D) unsteady friction model. The model is applied for the case of a water hammer, both in a single gravity pipeline and in a single pumped pipeline. Comparisons with experimental results show that the model does not predict the exact shape of the oscillation, but it gives, with high precision, the maximums and minimums of the pressure head oscillation if correct values of the parameter of the model are used. Because experimental evaluations can be made only for a few cases and can give rise to very different values depending on experimental conditions, the sensitivity of the model to physical and numerical parameters that influence the phenomenon is examined by comparison with the results of a quasi-2D model previously proposed.

Finite-difference tvd scheme for computation of dam-break problems

Abstract: A second-order hybrid type of total variation diminishing (TVD) finite-difference scheme is investigated for solving dam-break problems. The scheme is based upon the first-order upwind scheme and the second-order Lax-Wendroff scheme, together with the one-parameter limiter or two-parameter limiter. A comparative study of the scheme with different limiters applied to the Saint Venant equations for 1D dam-break waves in wet bed and dry bed cases shows some differences in numerical performance. An optimum-selected limiter is obtained. The present scheme is extended to the 2D shallow water equations by using an operator-splitting technique, which is validated by comparing the present results with the published results, and good agreement is achieved in the case of a partial dam-break simulation. Predictions of complex dam-break bores, including the reflection and interactions for 1D problems and the diffraction with a rectangular cylinder barrier for a 2D problem, are further implemented. The effects of bed s...

Failure behavior of riprap layer at bridge piers under live-bed conditions

Abstract: Experiments conducted under live-bed conditions show that a riprap layer at a cylindrical bridge pier will fail in either one of the following two modes: Total disintegration or embedment. The former refers to the break-up of the entire riprap layer where the stones are washed away by the flow field generated at the pier. The latter relates to the embedment of the riprap layer where it is buried in the sediment bed. The study proposes a criterion to demarcate the limiting condition between the two types of failure. It also identifies that embedment failure is a more common failure mode of riprap layer under live-bed conditions. The causes of embedment failure are twofold: (1) bed feature destabilization; and (2) differential mobility. Bed level fluctuations caused by the propagating bed features resulted in bed feature destabilization, whereas differential mobility is due to the different response of the riprap stones and bed sediments to the flow field. Experimental results also show that the riprap layer can degrade to an equilibrium level for a given flow condition. Finally, the study proposes a semiempirical equation to compute the maximum depth of riprap degradation, which occurs at the upper end of dune regime.

Boundary Shear in Circular Pipes Running Partially Full

Abstract: The distribution of boundary shear stress in circular conduits flowing partially full, with and without a smooth flat bed simulating deposited sediments, has been examined experimentally ranging from 0.375 < F < 1.96 (F = Froude number) and 65,000 < R < 342,000 (R = hydraulic radius), using the Preston tube technique. The invert level of the flat bed and the water depth have been varied to simulate a wide range of possible flow conditions that may occur in culverts, sewers, and hydropower tunnels. The distribution of boundary shear stress around the wetted perimeter is shown to be highly sensitive to changes in cross-sectional shape. The results have been analyzed in terms of the variation of local/global shear stress versus perimetric distance, and the percentage of the total shear force acting on the wall or bed of the conduit. The %SFw results (%SFw = the percentage of the average wall shear force of the overall shear force) have been shown to agree well with Knight's empirical formula for prismatic channels. The influence of secondary flows on the distribution of boundary shear stress and the implications of this for sediment transport have also been examined.

Reliability as tool for hydraulic network planning

Abstract: This paper presents a methodology to evaluate the reliability of water distribution systems that can be used in the design phase and for identifying repair works to be carried out on existing systems. The methodology is based on the statistical analysis of dimensionless performance indices (\Ihydraulic performance indices\N) derived from a large number of simulations of various water system demand scenarios and/or operating conditions. The \Ihydraulic reliability index\N is assumed as the probability that, under a given operating condition, the hydraulic performance index will be above a certain threshold. Finally, the system’s overall reliability (mechanical + hydraulic) is estimated using the \Ioverall reliability index\N, which is defined by the weighted mean of the hydraulic performance indices obtained for the various operating conditions. A case study using this methodology shows the concrete possibilities of applying this approach to a wide spectrum of cases, and the small influence on overall system reliability normally exerted by such events as the failure of links, pipes, and valves.

Turbulent Open-Channel Flow in Circular Corrugated Culverts

Abstract: Circular corrugated pipes are used as culverts at road crossings, and in many regions in Canada, the United States, and other countries, they have to be designed to allow fish passage during periods of fish migration. In many of these culverts, if the velocities are too large to allow fish passage, some type of baffles need to be installed to reduce the velocities. This paper presents the results of a laboratory study of the velocity field in turbulent open-channel flow in a circular corrugated pipe of diameter 0.622 m for three slopes of 0.55%, 1.14%, and 2.55% and a range of discharges from 30 l/s to 200 l/s. The Manning n was found to be equal to 0.023. Velocities were relatively small in some portion of the flow near the boundary of the pipe, and these low velocity regions may be useful for fish passage upstream. In the region of fully developed flow, in the central vertical plane, the longitudinal velocity was described by the Prandtl equation for rough turbulent flow, with a dip in the velocity profiles near the water surface. The velocity profiles in the noncentral planes were also described by the Prandtl equation for rough turbulent flow, but with a significant dip in the upper part of the flow. An empirical method was devised to describe the geometrical and kinematical properties of this velocity dip. The general findings of this study were also found to be valid for flow in a large corrugated pipe of diameter of 4.27 m with two slopes of 0.14% and 1.42%.

Investigations of Continuous Bed Load Saltating Process

Abstract: Particle saltating motions during bed load transport are dominated by the forces acting upon particles and the random process of the particles impacting on and rebounding from the channel bed. A real-time flow visualization technique is developed in this study to measure particle saltating trajectories, corresponding velocities, and impacting and rebounding angles. Based on the experimental data, regression equations for the dimensionless saltating length, height, and velocity were obtained. A numerical model that is able to simulate the continuous saltating process of a single particle was developed. The model was calibrated and verified with the experimental data, and the results were satisfactory. The model was also used to generate a series of synthetic data. Based on these data and the flume data collected by previous investigators, a bed load equation was derived.

Flow interaction of meandering river with floodplains

Abstract: A series of laboratory test results are presented concerning the boundary shear stress, shear force, and discharge characteristics of compound meandering river sections composed of a rectangular main channel and one or two floodplains disposed off to its sides. Five dimensionless parameters are used to form equations representing the total shear force percentage carried by floodplains. A set of smooth and rough sections is studied with an aspect ratio varying from 2 to 5. Apparent shear forces on the assumed vertical, diagonal, and horizontal interface plains are found to be different from zero at low depths of flow and change sign with an increase in depth over the floodplain. A variable-inclined interface is proposed for which apparent shear force is calculated as zero. Equations are presented giving proportion of discharge carried by the main channel and floodplain. The equations agree well with experimental and river discharge data. Using the variable-inclined interface, the error between the measured and calculated discharges for the meandering compound sections is found to be the minimum when compared with that using other interfaces.

River Width Adjustment. I: Processes and Mechanisms

Abstract: In 1993 a Task Committee (TC) of the ASCE was established to study the hydraulics, bank mechanics, and modeling of width adjustment in alluvial channels. The work of the TC in reviewing width adjus...

Settling of Fine Sediment in a Channel with Emergent Vegetation

Abstract: The potential for enhanced removal of sediment from flowing water in a channel by means of settling on vegetation was studied theoretically and experimentally. Settling on vegetation was modeled as a distributed sink of sediment with vegetation spread throughout depth and length of the channel. Sediment-capture experiments were performed in a laboratory channel using model vegetation and well-sorted silt. The theory predicts that settling on vegetation has the potential to increase sediment-capture efficiency considerably, and this was demonstrated in the experiments. The enhancement of sediment removal was greater for artificial vines than for inclined strips. For the runs with flow velocities >10 mm/s the sedimentation efficiency was less than predicted. The reduced efficiency could be related to bed destabilization (which was observed in some runs), re-entrainment from the vegetation, or sediment sliding off the vegetation.

Extended thermodynamics derivation of energy dissipation in unsteady pipe flow

Abstract: Extended irreversible thermodynamics (EIT) provides a framework for deriving extensions to phenomenological equations (e.g., Newton's law of viscosity, Fick's law of mass transport, and Darcy's law for porous media flow) for problems involving high frequencies (i.e., rapid transients). In this paper, a phenomenological equation is derived for energy loss in 1D unsteady pipe flow using an EIT formalism. The resulting wall shear stress is equal to the sum of (1) the steady-state shear stress; (2) a term that is proportional to the local (i.e., temporal) acceleration; and (3) a term that is proportional to the product of the velocity and the convective (i.e., spatial) acceleration. The form of this EIT-based wall shear stress formula shows that EIT provides a physical basis for instantaneous acceleration based unsteady friction formulas. It also illustrates the limitations and underlying assumptions of these models. For example, instantaneous acceleration based unsteady friction formulas are limited to fast transients (i.e., transients in which the water hammer timescale is significantly smaller than the diffusion timescale). A characteristics solution for unsteady pipe flow is proposed in which the phenomenological equation is used to model energy dissipation. Comparison of numerical test results with measured data from upstream and downstream valve closure laboratory experiments shows excellent agreement.

Friction Factor for Gravel-Bed Channel with High Boulder Concentration

Abstract: This paper reports the results of an experimental investigation on flow resistance law for a gravel-bed channel having bed arrangements characterized by different values of the number of boulders. The dependence is studied of a parameter, proposed for describing the gravel-bed surfaces, on the boulder concentration and on the ratio between the median diameter of the coarser particles and the median diameter of the bed layer. Flume measurements carried out for the hydraulic condition of \Itransition\N and \Ilarge-scale\N roughness (1.5 ≤ \ih/\id\d5\d0 ≤ 6.9) show that the intercept of the semilogarithmic flow resistance law depends on the ratio and on boulder concentration. For concentration values less than 50%, a relationship is developed between the intercept and proposed parameter; for higher concentration values, the intercept is assumed constant and equal to -1.5. Further differences between the experimentally determined values of the friction-factor parameter and values calculated using the semi-logarithmic expression for flow resistance are explained in terms of the ratio between the Shields parameter and its critical value.

Aggregation rate of fine sediment

Abstract: A dynamical description of estuarial fine sediment aggregation has been developed. The spectrum of fine particle or agglomerate size is represented by a discrete number of classes based on particle mass. Aggregation and disaggregation, respectively, move mass up and down through the classes. The frequency of two- and three-body particle collisions due to Brownian motion, shearing and differential settling is expressed by simple statistical relationships, using a new form of the collision efficiency parameter. This parameter is defined by nondimensional terms accounting for the physical and chemical forces that determine whether close encounters result in collisions. Calculations of the rate of change of sediment mass for each class as the aggregation process approaches equilibrium, when compared with previously obtained data in a Couette chamber, highlight the need to characterize the sediment as to particle density and strength. Near equilibrium, three-body collisions seem to play a major role by disaggregating particles that have been aggregated by the more numerous but less forceful two-body collisions.

Hydraulics of Skimming Flow on Modeled Stepped Spillways

Abstract: Two sets of models of stepped spillways of slope 1 upon 0.6 with the same crest-shape and with a range of step sizes (0.25–2.0 m on the 1:10 scale and 0.5–2.0 m on the 1:20 scale models) were built and studied. In the range of prototype unit flows considered (0.8–3.8 m2/s on the 1:10 scale models and 1.8–21.7 m2/s on the 1:20 models), the residual specific energy is independent of these step sizes, at positions where measurements were made on the spillways (from 30 m below the crest) once fully aerated skimming flow is established. The residual specific energy at the toe of a 50-m-high (or higher) stepped spillway, within the range of step heights tested, is <60% of the residual specific energy at the same level on a similar smooth spillway experiencing flows up to 20 m2/s; at these flows and at this height, the stepped spillway was found to be in equilibrium. The reduction of specific energy is lower than that expected from previous studies; these findings are likely to impact on the design of stepped sp...