Showing papers in "Journal of Hydraulic Engineering in 1997"

Pier and abutment scour: integrated approach

Abstract: This paper summarizes many of the results from an extensive program of bridge scour research undertaken at The University of Auckland, New Zealand. An integrated approach to the estimation of local scour depth at bridge piers and abutments, collectively termed bridge foundations, is presented. The design method is based on empirical relations, termed K-factors, that account for the effects on scour depth of flow depth and foundation size, flow intensity, sediment characteristics, foundation type, shape and alignment, and approach channel geometry. The K-factors are evaluated by fitting envelope curves to existing data for piers and abutments and a new extensive data set for abutments. The new data are discussed and presented. Application of the design method is illustrated in examples.

Simplified Settling Velocity Formula for Sediment Particle

Abstract: A new and simplified formula for predicting the settling velocity of natural sediment particles is developed. The formula proposes an explicit relationship between the particle Reynolds number and a dimensionless particle parameter. It is applicable to a wide range of Reynolds numbers from the Stokes flow to the turbulent regime. The proposed formula has the highest degree of prediction accuracy when compared with other published formulas. It also agrees well with the widely used diagrams and tables proposed by the U.S. Inter-Agency Committee in 1957.

Nonrigid, Nonsubmerged, Vegetative Roughness on Floodplains

Abstract: Individual pine and cedar tree saplings and branches were used to model the resistance to flow in a water flume for nonsubmerged and nonrigid vegetation to determine the amount that streamlining decreases the drag coefficient and reduces the momentum absorbing area. Currently, vegetation on floodplains is commonly assumed to behave as rigid roughness that can lead to large errors in the relationships between velocity and drag force. This presents a basic fluid mechanics problem. An extreme variation of roughness with depth of flow can result due to a large increase in the momentum absorbing area in nonsubmerged vegetation as depth is increased. This deems all the available roughness equations (which generally are based on relative roughness approach) useless for this application. In this paper a dimensional analysis, supported by experimental results, is developed to obtain a relationship between roughness conditions (i.e., density and flexural rigidity) and flow conditions (i.e., velocity and depth) for floodplains and vegetative zones of natural waterways.

Parameter Estimation of Nonlinear Muskingum Models Using Genetic Algorithm

Abstract: The application of the Muskingum model to river and channel flood routing may have some limitations because of its inherent assumption of a linear relationship between channel storage and weighted flow. Although nonlinear forms of the Muskingum model have been proposed, an efficient method for parameter estimation in the calibration process is still lacking. In this paper, the objective approach of genetic algorithm is proposed for the purpose of estimating the parameters of two nonlinear Muskingum routing models. The performance of this algorithm is compared with other reported parameter estimation techniques. Results of the application of this approach to an example with high nonlinearity between storage and weighted-flow, show that the genetic algorithm approach is efficient in estimating parameters of the nonlinear routing models. A supplementary analysis of the sensitivity of the parameters during the performance of genetic algorithm shows that a unique set of parameters exists that would result in t...

Mixing in Inclined Dense Jets

Abstract: Experiments were performed using laser-induced fluorescence (LIF) and a microconductivity probe on turbulent dense jets inclined upwards at an angle of 60° into stationary environments. Such jets are frequently used to discharge industrial wastewaters. Time-averaged LIF images show concentration profiles that vary smoothly in space but instantaneous images show considerable patchiness. Dilution at the jet impact point was found to be higher than previously reported. Downstream from the impact point, the flow becomes predominantly horizontal with a complex additional mixing process that results in ultimate dilutions considerably higher than the impact dilution. The flow is highly turbulent in the vicinity of the falling jet with large concentration fluctuations. These fluctuations decay with distance due to turbulence collapse under the influence of density stratification. The end of the mixing zone is defined as the location where the intensity of the concentration fluctuations falls to 0.05 (i.e. 5%) of ...

Numerical modeling of mudflows

Abstract: To predict mudflow dynamics a numerical model based on shallow-water equations is developed. Previous work has shown that mudflow behavior can be well represented by a Herschel-Bulkley model. In a laminar regime, generally observed in practice, a wall friction force expression has been theoretically deduced for steady uniform flows. For unsteady flows the wall friction force is assumed to be equal to the resistance of a steady uniform flow with the same depth and mean velocity. The numerical model parameters, directly deduced from Herschel-Bulkley model, are determined independently by rheological measurements. Because of the lack of precise field data, experimental results obtained on a physical model are used in order to validate the numerical model. The main features of the experimental transient flows carried out in a laboratory flume are, for some identified conditions, in fairly good agreement with numerical model predictions, without any additional parameter fitting.

Velocity Profiles of Developing and Developed Open Channel Flow

Abstract: Using a Laser-Doppler anemometer, mean velocities are measured in developing and fully developed turbulent subcritical smooth open channel flows. Experiments are conducted in a rectangular laboratory channel for 12 different test conditions with Reynolds number ranging from 28,026 to 136,842. From the experiments it is found that the boundary layer along the centerline of the channel develops up to the free surface for a flow aspect ratio b/h≥ 3. Shear velocities are calculated using the measured velocity profiles in the viscous sublayer of the boundary flow. The experiments show that shear velocity varies in an oscillatory manner across the flow section around b/h= 3. In the turbulent inner regions of developing and fully developed boundary flows, the measured velocity profiles agree well with the logarithmic “law of the wall” distribution when the coefficients in the expression are 2.44 and 5.5, respectively. The “wake” effect becomes important in the velocity profiles of the fully developed boundary la...

Turbulence modulation and particle velocities over flat sand beds at low transport rates

Abstract: The presence of sand moving at low transport rates over a flat bed modulates the production of turbulence when compared to clearwater flow at similar mean flow conditions. Phase Doppler anemometry is used to discriminate the turbulence characteristics of the carrier fluid from the sediment grains (0.22 mm diameter) and shows that the presence of mobile sediment increases the near-wall velocity gradient and shear velocity when compared with the clearwater values. This increased shear velocity is associated with a greater bed roughness height and near-bed turbulence intensities and smaller mixing lengths. Quantification of slip velocities between the fluid and sediment phases reveals particle Reynolds numbers that range from 1 to 30. Turbulence enhancement is shown to occur at lower values of both the Stokes number and ratio of the particle size-to-turbulent length scale than in past work. Several mechanisms of turbulence modulation may be invoked to explain these changes, including increased bed roughness, eddy shedding from large grains, grain inertial effects, and particle-coherent structure interactions. These mechanisms may be significantly influenced by both particle-particle and particle-wall interactions. Since mobile sediment modulates the carrier fluid turbulence, there is a need for modification of existing theories of sediment suspension and for caution when interpreting velocity profiles that are obtained without discriminating the fluid and sediment phases.

Velocity Profiles for Particles and Liquid in Open-Channel Flow with Suspended Sediment

Abstract: Experiments were performed to measure the mean velocity and turbulence characteristics in open-channel flows with and without suspended sediment (alluvial sand of nearly uniform size). Velocity measurements were obtained by using a newly developed technique, the discriminator laser-doppler velocimeter (DLDV), which can distinguish both liquid and sediment particle velocities. The mean velocity of sediment particles was found to be lower than that of the water. While the velocity fluctuations in the water were not changed with the addition of sediment, those of the sediment were diminished. For the range of concentrations considered, the friction velocity and the free-surface slope increased with sediment concentration unlike other bulk flow parameters, which were practically constant. A uniform method of analysis was developed to facilitate data interpretation and comparison between similar studies.

Acoustic Velocity Profiler for Laboratory and Field Studies

Abstract: A monostatic acoustic Doppler velocity profiler (ADVP) for use in hydraulic research is presented including signal treatment algorithms. The 1-MHz ADVP used in the present study has an accuracy better than 2 mms˙s−1 with a range of up to 100 cm for velocities up to 290 cms˙s−1 and a resolution of 256 points in the profile. Results of ADVP measurements in uniform flow in two laboratory open channels with smooth and gravel beds compare favorably with established distribution laws for mean velocity, variance, and Reynold's stress, indicating that the turbulence scales are sufficiently resolved by the instrument. Measurements with the same instrument in a river and a lake illustrate that it is capable of producing excellent results in the field as well. It was demonstrated that, unaffected by water quality, the instrument is a good research tool for nonintrusive and accurate instantaneous profile measurements under turbulent flow conditions.

Three-dimensional computation of flow around groyne

Abstract: The three-dimensional (3D) numerical simulation of a steady, shallow turbulent flow around a groyne in a rectangular channel is presented. A method used to track a moving surface boundary and to follow its evolution, using a one-phase 3D Reynolds solver with rigid grids, by means of a transient fraction of fluid or “porosity” field is applied. Some results are compared with experimental data, such as the isolines of water depths and the mean velocity field. A comparison of the reattachment length prediction using the free-surface model and the rigid-lid assumption is given. Other results concerning the pressure field, turbulence, or shear stress distribution are presented and analyzed.

Subcritical Junction Flow

Abstract: The main flow features of subcritical junction flows are explored, based on an extended hydraulic model study. The flow conditions chosen resulted in flows that are governed by the Froude similarity law. So-called simple junctions with junction angles of 30°, 60°, and 90° were tested. The main emphasis was made in determining the characteristics of the lateral flow and the flow contraction in the tailwater branch. Further, expressions for the momentum correction coefficients, the lateral wall pressure force, and the ratio of flow depths in the lateral and upstream branches were provided. A rational approach for the momentum contribution of the lateral branch is presented and applied for the prediction of the backwater effect across a simple junction. The complex flow pattern is further documented with selected photographs such that a rather complete description of junction flow is now available.

Experiments on Downstream Fining of Gravel: I. Narrow-Channel Runs

Abstract: We present the results of three laboratory experiments in which longitudinally sorted deposits were formed by feeding poorly sorted sediment at the upstream end of a narrow, 45-m-long channel. The input sediment had a median size of 6 mm and included significant amounts of material up to 64 mm and down to 0.2 mm. Water discharge was constant at 49 L/s and sediment discharge varied from 0.048 to 0.19 kg/s. Downstream fining was produced in all three runs; the variation in sediment-feed rate had relatively little effect on the fining profiles. In all three runs, the formation of a longitudinally sorted deposit was mediated by the formation of a coarse surface layer. The surface layer remained at the top of the deposit during aggradation by continually reforming itself at the deposit surface. The coarse surface layer fined by approximately a factor of 2 in D90 and D50 consistently in all three experiments. The deposit (subsurface) fined less, with D90 fining more strongly than D50. The short channel length a...

Ripples on stream bed

Abstract: Some 30 years ago, John F. Kennedy laid the foundations for the description of the geometry of sand waves on stream beds by the potential flow model. Since then, the analysis has been extensively refined and extended, but no rigorous description of the small features, called ripples, exists. The present study attempts a “stock-taking” of the data and the various behavioral concepts with the aim of encouraging researchers to improve our knowledge of the physics of features called ripples. Various trends are identified from the data. It is deduced that ripples act as roughness elements of the boundary. The ripple troughs are “filled” with the energy-consuming lee vortices. The main flow is over the ripple crests and lee vortices and does not interact with the bed geometry. The “roughness layer” of ripples and vortices is within the constant shear layer and is affected by the flow depth only as far as it affects the velocity distribution and alters the shear stress on the bed.

Equilibrium Clear-Water Scour around an Abutment

Abstract: The author presents a semiempirical analysis of equilibrium local clear-water scour at a bridge abutment that is based on the flow-continuity equation, scour geometry, and a generalized form of the power-law formula for flow resistance in an alluvial channel. Methodology is based on the premise that the flow obstruction and subsequent increases in bed shear stress due to projection of the abutment into the flow are responsible for the scouring action around the bridge structure. The scour-depth equation is determined for an abutment located perpendicular to the flow direction and involves only the approach flow quantities, the median sediment size, and the projecting length of the abutment. From the present study and other sources, the author analyzed and compared 252 data on clear-water maximum scour depth with the proposed method. The equation is extended to include the effects of abutment shapes and nonuniform sediment mixtures. For mixtures, an effective sediment size that corresponds to the critical armor layer in the scour hole is employed. Generally, comparisons of calculated and measured maximum equilibrium scour depths reveal that the estimates are satisfactory and the methodology is acceptable.

Computation of energy dissipation in transient flow

Abstract: A new model for the computation of unsteady friction losses in transient flow is developed and verified in this study. The energy dissipation in transient flow is estimated from the instantaneous velocity profiles. The ratio of the energy dissipation at any instant and the energy dissipation obtained by assuming quasi-steady conditions defines the energy dissipation factor. This is a nondimensional, time-varying parameter that modifies the friction term in the transient flow governing equations. The model was verified for laminar and turbulent flows and the comparison of measured and computed pressure heads shows excellent agreement. This model can be adapted to an existing transient program that uses the well-known method of characteristics for the solution of the continuity and momentum equations.

Tidal Flow and Transport Modeling Using ULTIMATE QUICKEST Scheme

Abstract: The paper describes an accurate numerical scheme for simulating two-dimensional solute transport in coastal and estuarine waters. The operator-splitting algorithm and a conventional discretization (i.e., no splitting) algorithm have been used to solve the solute transport (or advective-diffusion) equation. To avoid the occurrence of numerical oscillations, the ULTIMATE algorithm and the third-order accurate QUICKEST scheme have been used to represent the advective terms in the equation. A boundary-fitted curvilinear coordinate system has also been deployed to obtain a more accurate solution of the tidal currents. The model has been verified for a series of numerical tests, and has been applied to predict the water quality constituent and sediment transport fluxes in the Humber Estuary, U.K.

Geomorphology and Kinematic-Wave–Based Hydrograph Derivation

Abstract: Geomorphology-based instantaneous unit hydrographs have been proposed by several engineers as a tool to produce runoff hydrographs from rainfall for ungauged watersheds. A difficulty in applying the geomorphology-based instantaneous unit hydrographs is the determination of travel time that is actually a hydraulic problem. In this paper, kinematic-wave theory is used to analytically determine the travel times for overland and channel flows in a stream-ordering subbasin system. The resultant instantaneous unit hydrograph is a function of the intensity of rainfall excess; hence the linearity restriction of the unit hydrograph theory is relaxed. In applying the instantaneous unit hydrographs for hydrograph simulation, the model deals with temporally nonuniform rainfall through convolution integration of the instantaneous unit hydrographs applied to the rainfall excess of varying intensities with time. The proposed model is tested by comparing the simulated and observed hydrographs of an example watershed for several rainstorms with good results. Sensitivity of surface runoff unit hydrographs to the model parameters is also investigated.

Analytical Solutions for Advection and Advection-Diffusion Equations with Spatially Variable Coefficients

Abstract: sidered here are consistent with the problem of the transport of pollutant in an open channel where the flow in the channel is augmented by steady, unpolluted lateral inflow distributed along the whole length of the channel, such as a steady inflow of ground water. Therefore the analytical solutions are solu­ tions to a practical problem. The simple expressions considered here for the spatial var­ iation of the coefficients facilitate the process of obtaining an­ alytical solutions to these equations. The spatially variable co­ efficient equations reduce to constant coefficient equations through a simple transformation. Consequently, many of the ~nown analytical solutions to the constant coefficient equa­ tions can be used to obtain analytical solutions to the spatially variable coefficient equations. Analytical solutions are provided for the advection of a sud­ den release of pollutant into the channel and for the solution of advection-diffusion equation. The advection of an initial quasi-Gaussian concentration profile in the channel is also considered. The analytical solutions are simple to evaluate and are useful for validating numerical schemes for solving the advection and advection-diffusion equation with spatially var­ iable coefficients written in either conservative or nonconser­ vative form (Zoppou and Knight 1994). The conservative and nonconservative forms of the equa­ tions are valid equations describing different physical prob­ lems. The analytical solutions to the conservative and noncon­ servative forms of the governing equations will be used to illustrate the importance of selecting the equation relevant to the physical problem, when spatially variable coefficients are involved.

Turbulence Characteristics of New Zealand Gravel-Bed Rivers

Abstract: Gravel-bed river turbulence is investigated on the basis of field measurements in New Zealand rivers. The data include instant longitudinal velocities obtained by two methods: point measurements an...

Progress and Recent Developments in Storm Surge Modeling

Abstract: The field of storm surge modeling has developed and matured considerably over the past 30 years. Several operational surge models have been implemented in Europe over the past decade. These operational advances have been accompanied by research into new areas, chiefly coupled surge-wave models and data assimilation, both of which are covered in this review. Surge-wave models attempt to provide more realistic model physics for the crucial area of air-sea interaction. Assimilation techniques use available data to generate improved numerical solutions, despite model inadequacies. The current activity in surge modeling and related areas is highlighted by the large proportion of recent papers cited in this work. Despite this activity, there is a general recognition that inadequate meteorological inputs remain the weak link in surge modeling. In addition, the advances in midlatitude modeling are not mirrored in the tropics. This is largely due to difficulties in predicting the paths and properties of tropical storms, but it also serves to emphasize the importance that has been attached to storm surge modeling in Europe.

Turbulent Bursting-Based Sediment Entrainment Function

Abstract: One of the basic impediments to a clear understanding of a variety of fundamental problems in the context of sediment transport has been the lack of a well-grounded formulation of bed sediment entrainment. This is dealt with herein, physically based on the mechanism that bed sediment particles are actually entrained by the bursting process inherent in wall turbulent flows. A simple theoretical model for sediment entrainment from flat, loose bed is established using the averaged bursting period scaled on inner variables and the spatial scales of turbulent bursts. Sediment entrainment is shown to depend strongly on bed-shear velocity. The theoretical entrainment flux is compared to available laboratory data sets covering both hydraulically smooth and transitional bed situations. Generally good agreement is obtained, representing the best performance of the present model in relation to existing entrainment functions. It appears to characterize a rather encouraging aspect for a new approach to sediment transport, given the enhanced understanding of turbulent bursting.

Relation between Mean and Maximum Velocities in a Natural River

Abstract: The cross-sectional mean velocity is an important variable in open-channel hydraulics, whereas the maximum velocity in a channel cross section is mentioned very little. However, the maximum velocity in a channel cross section might be as important as the cross-sectional mean velocity. This is because the measurable maximum velocity can be considered as a “signal,” from which one can know the range of the velocity in a channel cross section. This study explored the relation between the cross-sectional mean and maximum velocities in a natural river by using velocity data collected from the Mississippi River. The relation of the maximum velocity to the cross-sectional mean velocity on different straight reaches in the Mississippi River is perfectly linear. The relation of the maximum velocity to the cross-sectional mean velocity on different river bends is still linear and changes with the rc/B value slightly. The conclusions are preliminary because of the limited amount of data.

Depth-averaged hydrodynamic model in curvilinear collocated grid

Abstract: A two-dimensional (2D) depth-averaged model of turbulent flows has been developed in a boundary-fitted curvilinear coordinate system. Based on the finite-volume method (FVM), the convection terms are discretized by the Roe’s scheme of second-order accuracy, as well as the Power-law scheme. The governing equations are solved in a collocated grid system by the efficient fractional two-step implicit algorithm that is regarded superior to the SIMPLEC algorithm. The “time marching” technique has been used to obtain a steady-state solution as a limiting case. The new model has been applied to several problems including pure convection of a sharp step profile, side discharge into an open channel, flow in a meandering channel, and flow in a Parshall flume with supercritical outflow. Comparison with the available data shows that the model is efficient and robust.

Effect of Concentration on Settling Velocity of Sediment Particles

Abstract: The drag coefficient for dispersed particles settling in a fluid is related to the particle Reynolds number with the same function as that used for a single particle falling in a clear fluid, after taking into account the effects of concentration both on the effective density of particles and on the viscosity of fluid-sediment mixture. A new method is developed for evaluating the settling velocity of natural sediment particles dispersed in a fluid; it agrees well with the available experimental data. The differences among various empirical expressions are illustrated using the present relationship.

Irrotational Flow and Real Fluid Effects Under Planar Sluice Gates

Abstract: A numerical method of solution of the inviscid flow under planar gates is developed and evaluated. The results of the calculations show good agreement with previous inviscid flow solutions and with experimental results for the water surface profile, velocity, and pressure distributions throughout the flow domain. The discrepancies between the experimental and numerical inviscid flow values of the coefficient of contraction are analyzed. After detailed examination of alternative explanations for the discrepancies, it is suggested that the most plausible explanation is the energy loss due to the vortex system attached to the upstream region of the gate.

Turbulent Structure in Unsteady Depth-Varying Open-Channel Flows

Abstract: Turbulence measurements over a smooth wall in unsteady depth-varying open-channel flows were conducted by the simultaneous use of a two-component laser Doppler anemometer (LDA) and a water-wave gauge. In the present study, velocity measurements in the viscous sublayer of highly unsteady flow have been first conducted to evaluate the friction velocity independently of the log-law. The von Karman constant κ was calculated using this friction velocity. It was proven then that the value of κ was really a constant of 0.41 even in unsteady open-channel flows. Next, statistical structures of mean velocity and turbulence characteristics were investigated across the whole flow from the near-wall region up to the free surface. In particular, turbulence measurements near the depth-varying water-surface zone of flood flow were conducted successfully for the first time with the LDA; such measurements have been impossible with conventional velocity instruments such as hot-film anemometers. Mean velocity profiles and turbulence characteristics were revealed in both the rising and the falling stages of the flood period.

A perturbation solution for Bingham-plastic mudflows

Abstract: An analytical solution is proposed for laminar mudflows and debris flows that can be modeled by a Bingham-plastic law. Two-dimensional, unsteady, nonuniform, Bingham flows released from a point source or a source of finite size (dam-break problem or mudslide problem) on a steep slope are considered. The method of matched asymptotic expansions was implemented to get a first-order solution. For the dam-break problem, the proposed model is found to be valid when the shock wave has advanced three reservoir lengths downstream. Also, it is found that the Bingham flow only propagates a finite distance downstream, with the shock depth asymptotically approaching the yield depth and the shock velocity asymptotically falling to zero. The hydrograph produced by a Bingham flow is seen to have a slower and lower flood peak and a longer and higher flow tail than that produced by Newtonian flow having the same dynamic viscosity. Comparison of the model predictions with laboratory observations shows reasonable agreement.

Fundamental properties of flows in open channels with dead zone

Abstract: The fundamental behavior of unsteady flows in an open channel with a rectangular dead zone is investigated experimentally and numerically to clarify the mass-exchange mechanism near an embayment along a river bank. Typical flow features such as generation of a coherent vortex due to shear instability in the mixing layer between two flow regions (main-channel flow and dead zone) and water-surface oscillation (seiche) in the dead zone are shown by the findings of laboratory tests. These flow features can be reproduced numerically by plane two-dimensional (2D) open-channel flow equations in the variable grid system. Model performance was examined in detail by changing the grid size and constants. The calculated values for temporal and spatial variations in velocity and depth are compared with observed values. Examination of the calculated results shows that temporal velocity variations at the interface can be decomposed into two components due to seiche and shear instability; during one seiche, cycle-velocity variations caused by instability are amplified to the large­ scale vortex selectively by the interaction between seiche and instability components. This selective amplification process can be detected in a series of photographs of the surface flow pattern as well.

Experimental Study of Reservoir Turbidity Current

Abstract: A series of experiments were conducted in a flume to study the hydraulic characteristics of the turbidity current in a reservoir. Kaolin was used as the suspended material. The plunge points were found to be unstable initially. As the experiment went on, it moved downstream from the incipient plunge location and finally reached a stable location. The densimetric Froude number of the incipient and the stable plunge points are approximately equal to 1.0 and 0.6, respectively. It is found that the length of the plunge region is 15 times the water depth at the stable plunge point. The volumetric flow rates in the plunge region increase 17% during the plunging process. About 70% of the water discharge and 80% of the sediment discharge are confined to the denser layer, a layer close to the channel bed with approximately constant concentration. The thickness of the turbidity current increases while the layer-averaged velocity and concentration decrease in the longitudinal direction; the layer-averaged velocity has the smallest variation rates. Equations for the dimensionless velocity and concentration profiles are obtained.