Abstract of Papers - JHR Volume 40, 2002
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ISSUE NO. 1

An experimental study of Roman dropshaft hydraulics
by H. Chanson
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In Roman aqueducts, series of vertical dropshafts were used to dissipate the kinetic energy of the flow : i.e., the dropshaft cascades. A re-analysis of Roman dropshaft hydraulics is conducted with physical model tests. Three basic flow patterns are observed. The results demonstrate that the vertical dropshafts could be very efficient energy dissipators and re-oxygenation structures, under appropriate flow conditions. The optimum operation of Roman dropshaft is discussed and an analytical model is developed to predict these conditions. Further the performances of aqueduct dropshafts are compared with modern dropshaft designs, and the operation of dropshaft cascades is discussed. Additional material is available upon request.

Flow pattern in the scour hole around a cylinder
by W.H.Graf and I. Istiarto
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Local scour around a bridge pier (cylinder) is a problem of much concern to hydraulic engineers. This is a complex phenomenon resulting from the interaction of the three-dimensional turbulent flow field around the cylinder and the mobile channel bed (see Fig. 1). In the vicinity of the cylinder, scouring is the consequence. In this paper, the three-dimensional flow field in an established (equilibrium) scour hole is experimentally investigated. An acoustic-Doppler velocity profiler (ADVP) was used to measure instantaneously the three components of the velocities in the vertical symmetry (stagnation) plane of the flow before and after the cylinder. The vorticity of the flow field was calculated. Results of the study show, that a vortex-system is established in the front and a trailing wake-vortex system of strong turbulence is formed in the rear of the cylinder.

Experiments on incipient channelization of submarine fans
by Jasim Imran, Gary Parker and Peter Harff
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Experiments on the formation of channelized submarine fans by the passage of successive turbidity currents are presented. Channels similar to subaerial rivers are found in many submarine fans. The inception of channels on submarine fans, which are essentially depositional environments, can be explained in terms of the lateral distribution of the rates of deposition and erosion of suspended sediment associated with a passing turbidity current. The experiments reported here supplement the findings of a numerical model developed earlier to study the inception of channel-levee systems on submarine fans. The experimental setup is a simple replication of a surface at the base of a continental slope receiving a turbidity current from a canyon. The turbidity current was generated by releasing a sediment-water mixture of low sediment concentration from an overhead tank into a quiescent water body held in a rectangular tank. The bottom slope of the tank was varied between 3 and 10%. Two sediment sizes, with geometric mean sizes of 71 µm and 110 µm were used. The bed profile was measured after the completion of each run. A 2-D depth-averaged numerical model of turbidity current was then run with the input conditions from the experiment. The resulting bed profiles were compared with the measured data. Generally good agreement between the experimental and numerical findings is observed.

Undular bores and secondary waves-Experiments and hybrid finite-volume modelling
by Sandra Soares Frazão and Yves Zech
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Secondary free-surface undulations (Favre waves), appearing for example after the opening of a sluice gate or at the head of a bore, cannot be reproduced by numerical models based on the hydrostatic pressure assumption. The Boussinesq equations take into account the extra pressure gradients but are difficult to integrate due to the high-order derivative terms. The paper describes the physics of wave initiation and proposes a demonstration of the Boussinesq equation based on relatively wider assumptions than usually adopted. A linear stability analysis is developed in finite-difference frame to
highlight some potential source of numerical instabilities. These conclusions are transposed in a new hybrid finite-volume / finite-difference scheme, which reveals a better accuracy in period and amplitude when evaluated against experiments.

A transient shear stress model for the analysis of laminar water-hammer problems
by Ricardo A. Prado and Axel E. Larreteguy
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A transient shear stress model for the solution of water-hammer problems for laminar flow in pipes is presented. The model is based on the polynomial expansion of the radial profiles of axial velocities, and the solution of the resulting set of equations by the method of characteristics. This approach, as compared to the usual quasi-steady model (which can be regarded as a particular case of the new method), allows for a better
representation of the shear stress at the wall during the pressure transients. The present model can be included with only minor modifications into any existing code for solvingwater-hammer problems that uses the characteristics method and the quasi-steady model. The test of the new model against experimental results of Holmboe and Rouleau, given in [8], and mathematical models and numerical simulations of other authors [5][8] show that it is less cpu and memory demanding, and is able of obtaining comparable results.

Leak detection in pipes by frequency response method using a step excitation
by Witness Mpesha, M. Hanif Chaudhry and Sarah L. Gassman
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This paper presents a new procedure utilizing transient state pressures to detect leakage in piping systems. Transient flow, produced by opening or closing a valve, is analyzed in the time domain by the method of characteristics and the results are transformed into the frequency domain by the fast Fourier transform. This method is used to develop a frequency response diagram at the valve end. The frequency response diagram of a system with leaks has additional resonant pressure amplitude peaks (herein called the secondary pressure amplitude peaks) that are lower than the resonant pressure amplitude peaks for the system if there were no leaks (herein called primary amplitude peaks). The location of a leak is determined from frequencies of the primary and secondary pressure amplitude peaks and the leak discharge is determined from the maximum and minimum discharge amplitudes. This method is applicable for practical values of the friction factor over the range 0.01 to 0.025 and can be used to detect leaks in real-life pipe systems conveying different types of fluids, such as water and petroleum. It can be used directly by comparing the frequency response diagram of a modelled system without leaks to the frequency response diagram developed by gradually opening or closing a valve at the downstream end of a pipe and taking measurements of pressure head and discharge at only one location.

Unsteady two-dimensional orifice flow: a large-size experimental investigation
by Hubert Chanson, Shin-ichi AOKI and Mamoru Maruyama
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Orifice flows were used as water clocks since the Antiquity up to the 16-th century. Today orifices and nozzles are used for measuring discharges. Most works were conducted with steady flow conditions and there is little information on the unsteady flow pattern. In this study, the writers describe an experimental investigation of an unsteady orifice flow discharging vertically. The study was conducted in a large-size facility with a rectangular orifice (0.75-m by 0.07-m) discharging up to 1.2 m3 in about 10 seconds. The study presents new information on the unsteady flow patterns, the discharge capacity and the velocity field in the reservoir. The results are compared with ''classical'' orifice flow results.

Numerical simulation of the draft tube and tailwater flow interaction
by Weixing Yuan and Rudolf Schilling
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Thiswork focuses on the numerical simulation of the interaction between the draft tube and tailwater flowin lowhead hydro power plants. An approach to the solution of incompressible free surface flow problems is developed. The flow field and the free surface location are calculated by coupling the free surface kinematic and dynamic conditions with the equations of motion for the bulk flow. A pressure correction method is applied to the flow calculation in amoving grid system. TheRNG k-å model is used for turbulence modelling. In order to reduce the computational time, thePVM(Parallel
Virtual Machine) subroutines are implemented.

Air demand behind high head gates during emergency closure
by Ismail Aydin
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Pressure drop and consecutive air demand behind high head gates during emergency closure is studied by physical and mathematical models. Measurements are done on hydraulic model of a leaf gate installed in the intake structure of a penstock. Local loss coefficients are determined as functions of Reynolds number and gate openings from measurements of discharge and piezometric levels at static positions of the gate. A mathematical model for the unsteady flow due to closing gate is formed by applying the integral continuity and energy equations on control volumes upstream and downstream
of the gate. Dimensionless numbers relevant to the problem are obtained by dimensional analysis of the governing equations. Timewise variations of air discharge in the ventilation shaft and pressure behind the gate are obtained from numerical solution of the model equations. The relative air demand is computed over substantial ranges of dimensionless parameters and some design considerations are discussed.

On the incipient aerated flow in chutes and spillways
by Antonio Moñino Ferrando and Jaime Riera Rico
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Natural self-aeration of water flows in open channels protects surfaces in contact with the flow from cavitation damage if enough air content is reached (Falvey [6] [7], Peterka [10], Russel & Sheenan [11]), although it could lead to an increase in both flow depth and velocity. Also, self-aerated flow enhance the process of atmospheric gases exchange within the fluid, improving water quality downstream of hydraulic structures (Chanson [5]). So it is of great interest to evaluate accurately the critical point where air entrainment begins, that is, the location of the inception point. The note first shows a review on some methods to evaluate the inception point location. Second, a brief explanation on calculation development is made and an expression is obtained which enables to compute the point of incipient self-aeration as a function of unit discharge, bottom slope of the chute and uniform surface roughness in a more simple and accurate way. Finally, comparison with results provided by other methods is made and conclusions are obtained.

Use of resistance coefficients derived from single planes to estimate time of concentration of two-plane systems
by Tommy S.W.Wong
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By coupling the Darcy-Weisbach friction formula with the kinematic wave time of concentration formula, a kinematic-Darcy-Weisbach time of concentration formula for a series of planes is derived. The formula is applicable to a cascade of planes, to planes of different roughnesses, to planes of different flow regimes, to planes of different soil types and infiltration rates resulting in different net rainfall intensities, to planes subject to different rainfall intensities, and to planes with a combination of all these variables. By applying the observed times of concentration to the formula for single planes, the Darcy-Weisbach resistance coefficient for a concrete surface and for an artificial grass surface are derived. Based on the derived resistance coefficients, they indicate that the flow regime on concrete was transitional and the flow regime on artificial grass was laminar. Further, by applying the derived resistance coefficients to two-plane systems (i.e. planes with combinations of concrete surface and artificial grass surface in series), the comparisons of the estimated times of concentration from the formula with the observed values show very good agreement. In one particular two-plane system, even with the occurrence of kinematic shocks, the agreement is still good. This study shows that the resistance coefficients that are derived from uniform planes can be applied to two-plane systems.

 

ISSUE NO. 2


Wave effects on blockwork structures: model tests
by G.Muller, P.Hull, W.Allsop, T.Bruce, M.Cooker and L.Franco
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Up to the middle of this century many coastal structures were built from blockwork, using either natural stone blocks or concrete blocks. Those blockwork structures subjected to breaking wave impacts often show a particular damage type, whereby individual blocks are shifted out of their position and moved into the sea. Engineers have suspected for a long time that wave impact pressures can travel into the water filled cracks and joints of such structures, building up pressures inside of the structure and thus destroying the structure from within. In order to verify the damage mechanism,
and to investigate the characteristics of impact induced pressure pulses, model tests on the propagation of wave impact pressures into water filled cracks were conducted. It was found that impact generated pressure pulses can enter water filled cracks and that they exhibit wave-like characteristics such as finite propagation speed, reflection, superposition and attenuation. Changes in cross section were found not to affect pressure pulse magnitude or duration. The possibility of wave impact pressures to damage or destroy cracked or fissured structures from within the structure could thus be verified.

Bed load motion and grain sorting in a meandering stream
by Pierre Y.Julien and Deborah J.Anthony
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A three-dimensional moment analysis defines both particle mobility and the average orientation angle of moving bedload particles in meander bends. Accordingly, under identical bed shear stress and near-bed streamline orientation angle on a side slope, bedload particles of different sizes move in different directions. This sorting mechanism has been verified in the sharp meander bends of Fall River, Colorado. The extensive field data set includes near-bed sediment transport measurements by size fractions from 0.125 to 32 mm using a Helley-Smith sampler.The field measurements in two
meander bends corroborate the theoretical model: particles finer than d50 preferentially move up the point bar and particles coarser than d50 move toward the thalweg.The measured deviation angle between the mean trajectory of 0.125 mm and 32 mm particles reaches 20 degrees near the bend apex.

Modeling of three-dimensional velocity field in open channel flows
by W.Czernuszenko and A.Rylov,Dr.
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A comparatively simple model for calculation of the three-dimensional, stationary velocity field is presented.The model is able to calculate the streamwise velocity distribution as well as the secondary flow in a cross-section of regular channel. The Reynolds equations are closed by a new anisotropic turbulence model which consists of two sub-models: one for the shear stresses and the other for normal stresses. The numerical solution of the parabolic approximation of the model equations gives reasonably good secondary flow patterns as well as the longitudinal velocity distribution in the channel cross-section.

Modelling of supercritical flow conditions revisited; NewC Scheme
by Vedrana Kutija and Caspar J M.Hewett
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A hydrodynamic numerical model for one-dimensional free-surface flows,named 'NewC', is presented. NewC is a finite difference scheme which has a major advantage over schemes used currently in engineering applications in that, while the algorithmic structure is of the subcritical-flow-type, it is capable of modelling subcritical, supercritical and transcritical flow conditions without requiring any changes to the governing equations.The scheme is shown to be unconditionally stable for a range of Courant numbers even for Froude numbers greater than or equal to one. The computational effort expended compares favourably with the finite difference schemes used currently. The NewC scheme has the additional advantage that it is relatively straightforward to incorporate into algorithms for the solution of flows in free-surface networks.

Hydraulic geometry of straight alluvial channels and the principle of least action
by He Qing Huang, Gerald C.Nanson and Simon D.Fagan
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Natural rivers exhibit regular hydraulic geometry relationships for which no widely accepted explanation has been given. This paper applies the physical principle of least action to the determination of stable alluvial-channel form. For steady, uniform alluvial-channel flow, both theoretical inferences and a case study show that least action occurs when the criteria of minimum potential energy and MFE (Maximum Flow Efficiency, defined here as the maximum sediment transporting capacity per unit available stream power) are satisfied. The consistency between bankfull hydraulic geometry relationships of natural channels and those of maximally efficient or 'least action' channels identified in this study demonstrates that alluvial channels commonly adjust to a maximally efficient section. Support for the use of the extremal hypotheses of maximum sediment transporting capacity and minimum stream power is provided by illustrating that they are essentially expressions of, and hence subsumed by, the more general principle of MFE.

Simulation of flow around piers
by Kamil H.M.Ali and Othman Karim
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FLUENT CFD was used to predict the three-dimensional flow field around a circular cylinder. Solutions were obtained for rigid beds and for scour holes of different sizes resulting from different time-durations. The numerical results were used to obtain the variation of bed shear-stress around the cylinder. These results were used in the sediment continuity equation to obtain an expression for the variation of scour depth with time. The asymptotic scour depth was found to depend on three dimensionless numbers: the pile number, the sediment size number and the duration time number. The theoretical relationship was calibrated using various laboratory and field results.

Hydrodynamic pressures acting on rigid gravity dams during earthquakes
by Yong-Sik Cho and Philip L.-F.Liu
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A boundary integral equation model is developed based on the analytical integrals for three-dimensional potential problems. All necessary integrals are first converted into line integrals around a target element and then integrated analytically. The developed model is applied to a practical problem concerning computation of the hydrodynamic pressure acting on a dam face of a dam-reservoir system during earthquakes. The obtained numerical solutions are compared with available two-dimensional experimental data and analytical solutions. A very good agreement is observed. The model is then used to investigate three-dimensional effects of a complex dam-reservoir system.

Developing generic hydrodynamic models using artificial neural networks
by Yonas B.Dibike
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Possibilities for the development of a new modelling paradigm, namely allowing models to 'construct themselves' by learning from existing numerical-hydraulic models, was investigated by extending previous works to encompass schemes that can be applied over arbitrary bathymetries with variable distances and time steps. For the simplest possible cases of one and two dimensional flow problems considered in this study, the relatively elementary technology of artificial neural network was found to provide acceptable results. Moreover, it was demonstrated that the well-trained networks could be substituted in place of the finite difference schemes in the hydrodynamic model formulation and could perform like numerical operators. This new paradigm is intended in future to supplement, and even in some instances to replace the current one.

A self-adaptive boundary search genetic algorithm and its application to water distribution systems
by Zheng Y.Wu and Angus R.Simpson
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The success of the application of genetic algorithms (GA) or evolutionary optimization methods to the design and rehabilitation of water distribution systems has been shown to be an innovative approach for the water industry. The optimal design and rehabilitation of water distribution systems is a constrained non-linear optimization problem. Constraints (for example, the minimum pressure requirements) are generally handled within genetic algorithm optimization by introducing a penalty cost function. The optimal or near optimal solution is found when the pressures at some nodes are close to the minimum required pressure or at the boundary of critical constraints. This paper presents a new approach called the self-adaptive boundary search strategy for selection of penalty factor within genetic algorithm optimization. The approach co-evolves and self-adapts the penalty factor such that the genetic algorithm search is guided towards and preserved around constraint boundaries. Thus it reduces the amount of simulation computations within the GA search and enhances the efficacy at reaching the optimal or near optimal solution. To demonstrate its effectiveness, the self-adaptive boundary search strategy is applied to a case study of the optimization of a water distribution system in this paper. It has been shown that the boundary GA search strategy is effective at adapting the feasibility of GA populations for a wide range of penalty factors. As a consequence, the boundary GA has been able to successfully find the least cost solution in the case study more effectively than a GA without the boundary search strategy. Thus a reliable least cost solution is guaranteed for the GA optimization of a water distribution system.

Structure of the turbulent hydraulic jump in a trapezoidal channel
 by Noor Afzal and A.Bushra
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The axial flow structure of turbulent hydraulic jump has been analysed and the general equation valid for a channel of arbitrary cross section has been proposed. Based on the Reynolds equations of mean turbulent motion in two dimensional steady incompressible flow subjected to hydrostatic pressure distribution, the integral equations of depth averaged flow over a channel of arbitrary cross sectional area are obtained. An integral method has been developed where inertia, pressure gradient and depth averaged normal Reynolds stress play the dominant role. The closure model for variation of depth averaged normal Reynolds stress has been expressed as product of the constant eddy viscosity and the gradient of the depth averaged axial velocity with respect to axial distance. In the trapezoidal channel the closed form solution for the upper surface profile and axial length of the hydraulic jump have been obtained. The comparison of the theory with experimental data is remarkably good. The theory shows that for F1 larger than a fixed value, the surface profile approaches a limiting universal solution provided the variables are appropriately non-dimensionalized. Further, the present predictions on the roller length are also supported by experimental data in rectangular and triangular channels.

Design of minimum water-loss canal sections
by Prabhata K.Swamee, Govinda C.Mishra and Bhagu R.Charar
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The canal water losses constitute of seepage and evaporation losses. Where as seepage loss depends on the channel geometry, evaporation loss is proportional to the area of free surface. On account of complexities of analysis, the design of minimum water loss section has not been attempted as yet. In this investigation explicit equations for the design variables of minimum water loss sections for triangular, rectangular, and trapezoidal canals have been obtained using non-linear optimization technique. The proposed equations along with tabulated section shape parameters facilitate easy design of the minimum water loss section and computation of water loss from the section without going through the conventional and cumbersome trial and error method. A design example has been included to demonstrate the simplicity of the method.

 

ISSUE NO. 3

(Special Issue)

Wave propagation modeling in coastal engineering
by PHILIP L.-F. LIU and INIGO J. LOSADA
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In this paper we review various numerical models for calculating wave propagations from deep water to surf zone, including wave breaking.
The limitations and the approximations for each model are briefly discussed. The main focus of the discussions is on the unified depth-integrated model, which can describe fully nonlinear and weakly dispersive waves, and the Reynolds Averaged Navier-Stokes equations model, which can calculate breaking waves and associated turbulence. Several applications of various models are also presented.

The joint probability of waves and water levels in coastal engineering design
by PETER J. HAWKES, BEN P. GOULDBY, JONATHAN A. TAWN and MICHAEL W. OWEN
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On coasts with high tidal ranges, or subject to high surges, both still water levels and waves can be important in assessing flood risk; their relative importance depends on location and on the type of sea defence. The simultaneous occurrence of large waves and a high still water level is therefore important in estimating their combined effect on sea defences. Wave period can also be important in assessing run-up and overtopping, and so it is useful also to have information on the joint distribution of wave height and period.
Unless the variables are either completely independent or completely dependent, multivariate extremes are difficult to predict directly from observa-tional data, as there may be too few events of the relevant type amongst the observations.In the past, the fitting and extrapolation of the dependence functions between the variables has often involved complicated and/or subjective approaches. This paper presents a method for joint probability analysis, using a Monte Carlo simulation approach, based on distributions fitted to water level,wave height and wave steepness, and to the dependence between them.

Directional wave parameter interpretation and related statistical uncertainties
 by CARL TRYGVE STANSBERG
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The theoretical background and the interpretation of directional parameters derived from the first two complex Fourier coefficients of the directional distribution, or corresponding circular moments, is reviewed. It is found that for a four-parameter bi-modal spectrum type, cases with small secondary peaks can be clearly distinguished from cases with almost equal peaks. Some other relationships are also noticed. A simple and robust parametric estimation method is suggested and discussed. Statistical errors in parameters estimated from finite time series are investigated. Through a numerical case study, the resulting variability in directional parameters is illustrated. For the actual, limited set of conditions included in the case study, the main structures of the spectral characteristics are reasonably well identified from a given set of records, with a limited level of accuracy given by the variability.

Stochastic description of sea waves
by CONSTANTINE MEMOS with contributions from K. TZANIS and K. ZOGRAPHOU
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An alternative to spectral description of sea waves in water of any depth is proposed based on the stochastic nature of the sea surface. The suggested short-term representation of a sea state displays a probabilistic structure in terms of joint densities of wave heights and periods that is advantageous in the design of maritime structures. Models have been developed and supported by laboratory and field data studies that are capable of providing the said stochastic description in a manageable form and with acceptable accuracy. Lines of further research are described for investigating this relatively unexplored field in maritime hydraulics.

Sleeve forces on inclined cylinders due to long and short crested waves
by V. SUNDAR,V. VENGATESAN and K.U. GRAW
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The wave-induced pressures around an inclined cylinder due to long and short crested waves are measured in a wave basin at an elevation of 0.8 m below still water level in a water depth of 3 m. The sleeve force normal to the cylinder axis was obtained by integrating the circumferential pressures and resolved in each of the three directions.The variations of the spectral density of the sleeve forces are presented. The effects of wave directionality on the pressures and sleeve forces are presented in the form of root mean square pressure and force ratios defined as normalized root mean square pressures/forces in short crested waves to that in long crested waves. The variations of the pressure and force ratios are discussed for the different mean wave directions and for different angles of inclinations of the cylinder with respect to the vertical plane. The pressure ratio is observed to be minimum when it is inclined at 45 degrees along the wave direction. The maximum normal force ratio is about 25 to 37% less when cylinder is exposed to directional waves compared to being subjected to long crested waves.

Field and numerical comparisons of the RIBS floating breakwater
 by M. BRIGGS, W. YE, Z. DEMIRBILEK and J. ZHANG
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A new concept in floating breakwaters was successfully field-tested offshore Cape Canaveral, FL in May 1999. The Rapidly Installed Breakwater System (RIBS) has a hybrid design of rigid and membrane components, similar to a ' Venetian blind' to permit rapid and expedient deployment. The objective of the RIBS is to reduce Sea State 3 (SS3) wave conditions to Sea State 2 for safe vessel operations. This field study was probably one of the most heavily instrumented floating breakwaters ever deployed. Measured wave transmission coefficients and dynamic wave pressures are compared to two different numerical models. This is the first verification of the numerical model WAMIT with field data.

( Regular Issue )

Flow analysis of the Arctic Ocean with a complicated density field
by AKIRA WADA
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In recent years, research has been underway to clarify the fate of radioactive wastes dumped into the Arctic Ocean (especially, the Kara Sea and the Barents Sea). These sea areas are very narrow, shallow and located close to land. To analyze the diffusion of radionuclides and carry out exposure dose assessment by determining the circulation of seawater in these sea areas, it is necessary to identify the flow characteristics of the seas. As the first step of research, the mechanism of flows in the Barents Sea and the Kara Sea in the Arctic Ocean was investigated. As the second step, the entire Arctic Ocean was studied. Using the observation data (water temperature and salinity), the flow was analyzed using a hybrid box model, taking into account river flows and density structures in the seas. The results thus obtained agreed with the observed features in many aspects. Especially, stream flows in the Norwegian Sea, Barents Sea and Kara Sea showed fairly realistic features.The flow field in the surface layer in the central Arctic Ocean agreed with that in previously known data. In the intermediate and deep layers, there was a stream flow that agreed with the known cyclonic circulation. East of Greenland, a stream flow equivalent to the East Greenland Current was recognized.

Accuracy of solitary wave generation by a piston wave maker.
by GUIZIEN KATELL and BARTHÉLEMY ERIC
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A new experimental procedure to generate solitary waves in a flume using a piston type wave maker is derived from Rayleigh's (1876,[18 ]) solitary wave solution. Resulting solitary waves for dimensionless amplitudes epsilon ranging from 0.05 to 0.5 are as pure as the ones generated using Goring's (1978, [7]) procedure which is based on Boussinesq (1871a,[1]) solitary wave,with trailing waves of amplitude lower than 3 %of the main pulse amplitude. In contrast with Goring's procedure, the new procedure results in very little loss of amplitude in the initial stage of the propagation of the solitary waves. We show that solitary waves generated using this new procedure are more rapidly established.This is attributed to the better description of the outskirts decay coefficient in a solitary wave given by Rayleigh's solution rather than by a Boussinesq expression.Two other generation procedures based on first-order (KdV) and second order shallow water theories are also tested. Solitary waves generated by the latter are of much lower quality than those generated with Rayleigh or Boussinesq-based procedures.

An experimental study on prevention of saline wedge intrusion by an air curtain in rivers
by MASANORI NAKAI and MASAMITSU ARITA
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Prevention of saline wedge intrusion by an air curtain in rivers was experimentally studied from a physical viewpoint. The behavior of steady saline wedges around an air curtain was classified into the three types and their appearance conditions were clearly expressed by the two parameters A/B and A/R :A ,B , and R represent the buoyancy due to an air curtain, the intrusion force of a saline wedge, and the inertial force of a fresh water flow (a river flow), respectively. In addition,the effectiveness of this method was evaluated using the newly introduced parameter alpha defined by (A/B )/(A/R).

Exchange flow through an opening
 by DAVID Z. ZHU, HESHAM FOULI and YAW A. OKYERE
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This paper presents a theoretical and experimental study of the exchange of .uids of different densities through an opening. Three types of openings are examined: a bottom opening (the opening is at the bottom of a gate), a middle opening (in the middle) and a window opening (the opening is in the middle but does not extend across the width). Simultaneous measurements of velocity field and interface position were obtained using flow visualization and image processing techniques. Experimental results confirm the predictions of the internal hydraulic theory that there are two internal hydraulic controls in the flow through bottom openings, but one control in the middle and window opening experiments. The neglect of non-hydrostatic forces and interfacial mixing in the theory, however, results in a significant underestimate of the exchange rate by more than 50 %in the middle and window opening experiments. The fluctuations in the interface position were caused by Kelvin-Helmholtz instabilities as well as basin-scale internal seiche, and the transition of internally supercritical flow to subcritical flow was caused by the mixing generated by these instabilities.

Large eddy simulation of dispersion in free surface shear flow
by C.W. LI and J.H. WANG
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Wastewater and waste heat are frequently discharged into ambient water and affect the water quality there. An accurate evaluation of the turbulent mixing and dispersion processes is one of the key factors for assessing the environmental impact of these discharges. To achieve this objective a three-dimensional numerical model incorporating the method of Large Eddy Simulation has been developed. In this method the large scale turbulence is computed explicitly and the subgrid scale turbulence is modelled. The empiricism incurred for the specification of the dispersion and turbulent mixing coefficients is thus reduced to minimal. The governing equations are split into three parts in the finite difference solution: advection, dispersion and propagation. The advection part is solved by a characteristics-based scheme. The dispersion part is solved by the central difference method and the propagation part is solved implicitly by using the Gauss-Seidel iteration method. The model has been applied to simulate a continuous line source in free surface shear flow. The computed results demonstrate the existence of the non-Fickian diffusion and dispersion region close to the source.Further downstream the transverse diffusion process obeys Fick's Law and the transverse diffusion coefficient is in agreement with the empirical value measured in laboratory. The initial non-Fickian diffusion and dispersion region is further analysed based the coherence of the velocity data. The dispersion coefficient is found to follow the Okubo's 4/3 power law, but with a much larger coefficient owing to the shear effect.

Combined-free flowover weirs and belowgates
by ABDEL-AZIM M. NEGM, A.M. AL-BRAHIM and A.A. ALHAMID
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The results of an experimental investigation on the characteristics of the combined flow over contracted sharp-crested rectangular weirs and below contracted sharp-crested rectangular gates are presented. The experiments are carried out in a laboratory flume using various geometrical dimensions under different flow conditions. The basic principles are employed to correlate the discharge to the relevant geometrical and hydraulic parameters in nondimensional form. The experimental data are then used to develop a general nondimensional equation for predicting the discharge through thecombined system knowing its geometry and the head of water over the weir. It was found that only one equation describes both horizontal and sloping channels with either mild or steep slopes. Also, the modular limit for combined flow is discussed and an equation for its calculation is presented as well. The effects of viscosity and surface tension are addressed in terms of Reynolds number and Weber number.

Scour at culvert outlets as influenced by the turbulent flow structure
by SARAH L. LIRIANO, RODNEY A. DAY and W. RODNEY WHITE
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Scouring downstream of culvert outlets is a common problem that can lead to damage to the culvert structure and neighbouring land. This paper presents measurements of the turbulent flow structure within scour holes downstream of pipe culvert outlets at different stages of development. Velocities have been recorded in scour holes at four stages of formation to enable changes in flow structure to be observed as the scour hole develops. The analysis of mean velocities, turbulence intensities, Reynolds stresses and near-bed bursting structure has led to a foundational understanding of the flowstructure in scour holes. Initial scour hole development is a result of high velocities exceeding the critical velocity for sediment transport whilst further development results in a reduction in the magnitude of the near-bed velocities and an asymptotic increase in scour depth associated with the turbulent structure of the flow. Towards the downstream end of the scour holes the jet comes into contact with the bed and flow structures similar to those observed downstream of backward facing steps are noted.


ISSUE NO. 4

Computational fluid dynamics modelling of flow and energy fluxes for a natural fluvial dead zone
by B.G.HANKIN, M.J.HOLLAND, K.J.BEVEN AND P.CARLING
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This study links together information that characterizes the energy and momentum fluxes across the shear zone between the main flow and a dead zone in the UK on the River Severn [1-3 ].The depth-averaged flow in and around the dead zone is modelled using TELEMAC-2D,with semi-distributed eddy viscosity and bed roughness, and compares well with some distributed field measurements within the reach. The resulting velocity field is then used to provide momentum fluxes for a finite difference model incorporating finite volumes (FDFV model) numerical scheme that has been developed to solve depth averaged advection-diffusion of thermal energy in a body fitted co-ordinate system. Assuming that buoyancy forces can be neglected, the gross hydraulics of the system explains much of the temperature distribution that was observed using infra-red aerial imagery.

Three-dimensional transient shallow water flow simulation using a boundary integral equation model
by D.L.YOUNG, Y.F.WANG AND T.I.ELDHO
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This paper describes the applications of a model based on the boundary integral equation method (BIEM) for the analysis of three-dimensional (3-D) transient flow in large-scale shallow water flow fields. Under the assumption of hydrostatic pressure distribution and non-stratification, Ekman type linear barotropic models are adopted in the development of the BIEM model.The BIEM model has been compared with analytical solutions and other numerical schemes and found to be feasible and accurate. The circulation structures in hypothetical shallow-water flow fields are simulated under long-duration, intermittent and periodic wind blowing with various kinds of bottom configurations. The computed results for large-scale hypothetical shallow water flow fields are a good realization of actual behaviors at least in terms of qualitative aspects.

3-D Multiple-level simulation of free surface flows
by MARCELO REGGIO, ANDRÉ HESS AND ADRIAN ILINCA
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A 3-D multiple-layer model to describe and compute free surface flows has been applied to rivers and seashore parts. Vertical convective fluxes and shear stress at the interface are kept in the x -y momentum equation. The hydrostatic pressure of the top layer, is taken into account. The flow is calculated via a finite volume numerical scheme. The solver is based on Roe's Flux Difference-Splitting (FDS) method. The spatial discretization uses triangular elements as the basic control-volume cells. Data obtained from references, analytical solutions and real configurations have been used to evaluate the performance of the proposed method. The numerical scheme has been shown to be capable of predicting the behavior of Ekman's classical surface layer or of a simple tide wave fixed on an open boundary of a rectangular domain, for example. The evolution of a tidal constituent on the St. Lawrence estuary is also presented.

Simulation of fan formation using a debris mass model
by SHAO SONGDONG, EDMOND LO YAT-MAN AND WANG GUANGQIAN
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Based on the particle-in-cell computing method, a debris mass model has been established to simulate debris flow fan formation over large downstream areas. Under the assumption that the debris medium is an assembly of many small, identical debris particle masses, the overall flowbehavior is obtained by averaging the flow parameters of neighboring debris masses at fixed grid points. The equation of motion for each debris mass is based on the depth-averaged Navier-Stokes equation in two horizontal dimensions. The friction slope of debris flow is modeled by combining the effects of both the liquid phase (slurry composed of water and fine particles) modeled as a Bingham fluid and solid phase (coarse particles) in the debris mixture. The rheological parameters are evaluated according to the density and particle size distribution of the debris material.Convergence of the method is demonstrated by repeatedly doubling the number of debris masses employed in the computation until insignificant change is observed. The debris mass model is demonstrated through a prototype application to a documented 1991 debris flow deposited in the lower reach of the Shawan Ravine in Yunnan Province, China. The final alluvial fan was formed by eight consecutive debris flow events, each lasting about 2000 seconds with a discharge rate of 250 cubic meters per second. The simulation results are in good agreement with field observations. The general features of debris fan development and configuration are well predicted.

Suspension flows in open channels; experimental study
by W.H.GRAF AND M.CELLINO
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The vertical distribution of the concentration of suspended particles is usually determined by solving the diffusion-convection equation under appropriate boundary conditions. This study focuses on the experimental determination of the depth-averaged ß-value, given by the ratio of the sediment, epsilon sub-s, and the momentum, epsilon sub-m, diffusion coefficients.These coefficients were evaluated under laboratory conditions from measurements of instantaneous velocity and concentration profiles, obtained simultaneously by using a non-intrusive sonar instrument, the Acoustic Particle Flux Profiler (APFP). In suspension flows over a movable bed without bed forms the measured depth-averaged ß-values at capacity condition are smaller than unity, depth-averaged ß<1.However, it could also be shown that for flows over a movable bed with bed forms the depth averaged ß-values are larger than unity, depth averaged ß>1; this is notably the case in natural waterways.

Free surface instability of non-Newtonian laminar flows
by ZHAO-YIN WANG
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The mechanism of free surface instability of non-Newtonian laminar flows is studied theoretically and experimentally in the paper. Development of surface waves and roll waves of non-Newtonian flow has been reported in many cases, such as the river clogging in the hyperconcentrated flows, intermittent viscous debris flows and fluctuation in mud flows. Theoretical analysis from the equation of motion incorporating the non-Newtonian nature of the fluid demonstrated that the free surface instability is essentially caused by the yield stress. Two dimensionless numbers, Sy and Svis, representing the effects of yield stress and viscosity are calculated and compared for various flows. It is concluded that the free surface is unstable and roll waves may develop even at constant incoming flow rate if Sy is much larger than Svis and is stable if Sy is smaller than Svis. Experiments were conducted to study the phenomena of river clogging, development of a perturbation wave in non-Newtonian laminar flow, and development of roll waves. The results agree well with the theoretical formula showing exponential law of growth of wave height. The growth rate of wave height depends essentially on the parameter Sy. If Sy is large enough a series of roll waves develop from stable flow and the larger is the parameter, the higher the waves.

Numerical simulations of inviscid three-dimensional flows at single-and dual-pump intakes
by MATAHEL ANSAR, TATSUAKI NAKATO AND GEORGE CONSTANTINESCU
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Three-dimensional inviscid solutions for pump-approach flow distributions within both a single-pump and a dual-pump sump model were developed. The single-pump sump model consisted of a rectangular pump bay with a vertical circular pipe located at the downstream end of the bay. The two-pump sump model consisted of a wider rectangular pump bay with two vertical circular pipes located at the downstream end of the bay. The equations of motions were solved in generalized curvilinear coordinates on a non-staggered grid. For the single-pump model, the simulations were carried out for two cases, cross-flow and no-cross-flow. The results are in good agreement with laboratory flow measurements obtained from a 1:10-scale model using an Acoustic Doppler Velocimeter. For the two-pump sump model, simulations were also carried out for two cases. In Case 1, an equal pumping discharge was delivered through the two pipes, and in Case 2 the total discharge was split in a 7-to-3 ratio between the two pipes. The results for the two cases were compared with a focus on the formation of free-surface and subsurface vortices surrounding the pumps.

Turbulent flow and energy dissipation in plunge pool of high arch dam*
by XU WEILIN, LIAO HUASHENG, YANG YONGQUAN and WUCHIGONG
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The 3-D flow fields in the plunge pools of the Xiaowan high arch dam and Laxiwa high arch dam in China are simulated by the turbulence mathematical model and measured by the five-hole Pitot sphere combined with the pressure sensors and automatic data sampling and processing system. The typical 3-D flow pattern and energy dissipation characteristics in plunge pool are obtained. The simulated results of energy dissipation show that the water body in plunge pool can be divided into three regions:shear, impact and mixing dissipation regions of energy.

A study of a buoyant axisymmetric jet in a small co-flow
by M.J.DAVIDSON, S.GASKIN, and I.R.WOOD
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This paper deals with measurements of and an integral model for a buoyant axisymmetric jet in a very small co-flow. The integral model is based on the insights gained from the measurements presented here and other recent experiments, which suggest that the turbulent flow is simply advected and the entrainment is that normal for a jet or plume in a still ambient fluid. General equations for the turbulent flow trajectories are also developed. The predictions of the theory are verified for the no cross-flow case and the cases where the jet or plume is ejected vertically or horizontally into a very small cross-flow.
The results of experiments in which a buoyant jet is released in the same direction as the horizontal ambient flow, show that outside the turbulent region the entrainment velocities can be represented with uniform flow and the appropriate sink. Direct measurement of the strength of the sink allows the transition from weakly-to strongly-advected behaviour to be determined. The departure of the trajectory measurements from the theory also provides information about the transition. The transition location is then compared with recent measurements in which the jets and plumes are ejected vertically into a very small cross-flow.

Hydraulic jumps at drop and abrupt enlargement in rectangular channel
by GIOVANNI B.FERRERI and CARMELO NASELLO
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The different types of hydraulic jumps that occur in a rectangular channel at an abrupt increase in section are experimentally studied. The abrupt section increase is due to both a drop and an increase in the channel width. Experiments were carried out with three different values of the ratio L/l between the channel widths respectively downstream and upstream of the abrupt section increase. For each L/l value five values of Froude number F sub-1 of the supercritical flow upstream of the section increase were considered, and for each of them five values of the depth Y sub-1 of the same flow. The experiments showed that, as the depth Y sub-2 of the downstream subcritical flow increases, several types of hydraulic jumps occur. The sequence of hydraulic jump types and several characteristics of hydraulic jumps of the same type change with the flow parameters L/l, F sub-1 and s/y sub-1 ,with s the drop height.Physical explanations of these changes are proposed, based on both direct observation of phenomena and comparison with results of other authors relative to the cases of either drop only or enlargement only.

Prediction of critical submergence for an intake pipe
 by NEVZAT YILDIRIM and FIKRET KOCABAS
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In this study, the effects of the blockage of the intake pipe and impervious flow boundaries on critical submergence of an intake are presented. The potential solution based on the Rankine stagnation point is found to be an approximate method for the prediction of the critical submergence for an intake pipe. It is found that a critical spherical sink surface with a radius equal to the radial distance of the stagnation point (which is 1/square root of 2 times the critical submergence of the intake) can also be used to predict the critical submergence. The agreement between theoretical results and available experimental data indicates that this critical spherical sink surface gives good results especially for the intake when the distance of the impervious vertical dead-end wall to the center point of the intake is smaller than or equal to the diameter of the intake.

Darcy and post-Darcy flows within different sands
by IMAM WAHYUDI, AGNÈS MONTILLET and ABDERAHMANE O.A.KHALIFA
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The present paper reports a study conducted to examine several sands with a large spread of particle size in order to validate the modelling of both Darcy's and Forchheimer's law parameters, which could be applied to any kind of sand. An experimental set-up has been specially conceived to generate hydraulic gradients within fine sands higher than 600. We also present an attempt at using a capillary-type flow model to determine structural parameters of sand and predict pressure gradients. The specific surface areas calculated appear to be consistent with surface areas estimated from screening experiments. Similarly, tortuosity values calculated from pressure drop experiments stand within the range of values found in the literature, measured using an alternative method, the conductometric method. Using one dimensionless equation of the capillary-type flow model, a single value of the pore Reynolds number makes it possible to determine, whatever the porous medium, the limits of Darcy's flow regime. This Reynolds number value is 4.3.

Turbulent sand jets in water
by K.A.MAZUREK,K.CHRISTISON AND N.RAJARATNAM
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This Technical Note presents photographic observations of turbulent sand jets in water. Four sands ,with mean diameters ranging from 0.17 to 1.47mm, were used in combination with three nozzles of 8,12.7 and 25.4 mm diameter. It was found that the linear growth rate of these sand jets increases with the parameter F sub-0 which is proportional to the ratio of the momentum flux of the sand jet at the nozzle to buoyant force on the sand particles. For the largest value of F sub-0 equal to about 15, the growth rate of the sand jet was 0.19 which is about 20% larger than that of turbulent water jets with Reynolds number larger than about 10,000.


ISSUE NO. 5

Artificial neural networks for streamflow prediction
by OSCAR R. DOLLING and EDUARDO A. VARAS
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This paper presents monthly streamflow prediction using artificial neural networks (ANN) on mountain watersheds. The procedure addresses the selection of input variables, the definition of model architecture and the strategy of the learning process. Results show that spring and summer monthly streamflows can be adequately represented, improving the results of calculations obtained using other methods. Better streamflow prediction methods should have significant benefits for the optimal use of water resources for irrigation and hydroelectric energy generation.

Logarithmic matching and its applications in computational hydraulics and sediment transport
by JUNKE GUO
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This study presents an asymptotic matching method, the logarithmic matching. It states that for a complicated nonlinear problem or an experimental curve, if one can find two asymptotes, in extreme cases, which can be expressed as logarithmic or power laws, then the logarithmic matching can merge the two asymptotes into a single composite solution. The applications of the logarithmic matching have been successfully tried in several cases in openchannel flows, coastal hydrodynamics and sediment transport such as: 1) the inverse problem of Manning equation in rectangular open-channels, 2) the connection of different laws in computational hydraulics, 3) the solution of linear wave dispersion equation, 4) criterion of wave breaking, 5) wavecurrent turbulence model, 6) sediment settling velocity, 7) velocity profiles of sediment-laden flows, and 8) sediment transport capacity. All these applications agree very well with numerical solutions or experimental data. Besides, it is pointed out that there are several other cases where the logarithmic matching has potential applications.

Conservation-form equations of unsteady open-channel flow
By C. LAI, R.A. BALTZER and R.W. SCHAFFRANEK
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The unsteady open-channel flow equations are typically expressed in a variety of forms due to the imposition of differing assumptions, use of varied dependent variables, and inclusion of different source/sink terms. Questions often arise as to whether a particular equation set is expressed in a form consistent with the conservation-law definition. The concept of conservation form is developed to clarify the meaning mathematically. Six sets of unsteady-flow equations typically used in engineering practice are presented and their conservation properties are identified and discussed. Results of the theoretical development and analysis of the equations are substantiated in a set of numerical experiments conducted using alternate equation forms. Findings of these analytical and numerical efforts demonstrate that the choice of dependent variable is the fundamental factor determining the nature of the conservation properties of any particular equation form.

Two dimensional vertically averaged and moment equations for rapidly varied flows
by HAITHAM K.GHAMRY and PETER M. STEFFLER
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The classical depth averaged De St. Venant equations, which are used for most of the computational models in open channels, are based on the fundamental assumptions of uniform velocity and hydrostatic pressure distributions. They are thus limited in their applicability to cases where vertical details are not of importance. Alternative two-dimensional vertically averaged and moment equations are developed, by a moment weighted residual method from the fundamental 3D Reynolds equations, to account for problems where more vertical details are significant and essential. The proposed model is applied to rapidly varied flow problems involved in open channel flow. These problems include flow in channel transitions with rapid contraction and/or expansion and flowover a hemispherical hump. Linear distribution shapes are proposed for the horizontal velocity components, while quadratic distribution shapes are considered for vertical velocity and pressure. The implicit Petrov-Galerkin finite element scheme is used in these simulations. A good agreement is attained. In addition, the obtained results show that more details are gained and the flow is better represented by the proposed model compared to the classical De St. Venant model.

Dynamic tracking of flow boundaries in rivers with respect to discharge
By MOURAD HENICHE, YVES SECRETAN, PAUL BOUDREAU and MICHEL LECLERC
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A new Eulerian approach is proposed to track the dynamic position of flow boundaries in rivers with respect to flow discharge or tides. Associated to a two dimensional (2D) transient horizontal hydrodynamic model, it allows to define the configuration of watercourses in a broad hydrological register varying from dry conditions to severe flooding. The finite element method is used to develop the numerical prediction tool. It is employed to estimate not only the classical flow variables such as water surface level and velocity field, but also the position of the shorelines. In this paper, the strategy followed for building this «drying-wetting» model consists in letting the water surface move freely, everywhere in the domain including the dry zones, allowing it to plunge under the ground. Two practical applications on rivers of Québec (Canada) are presented. The first one deals with steady state situations on St. Marguerite River. The second one deals with the reconstitution of flood propagation on Chicoutimi River according to the extreme flooding events of July 96 in the Saguenay region.

River bed deformation calculated from boundary shear stress
by ANDRÉ PAQUIER and SAEED R. KHODASHENAS
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Deformation of various kinds of cross-sectionswas computed with the hypothesis that scour or deposition were directly related to shear stress computed by the Merged Perpendicular Method. Final stabilised cross section agrees with theoretical stable shape. To estimate the deformation of a river bed, the results of a 1D model which computes the volume of sediment eroded or deposited between two cross sections are used as a basis. Then, these volumes are transversely distributed in every section in relation with shear stress. The method is then applied to a reach of River Rhône.

Roll waves simulation using shallow water equations and Weighted Average Flux method
 by BARBARA ZANUTTIGH and ALBERTO LAMBERTI
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The weighted average flux method is used to represent trough 1-D shallow water equations the development of natural roll waves in rectangular channels. The method is reformulated in such a way that all source terms are incorporated into the local Riemann problem by means of a useful transformation of the equations, maintaining second-order accuracy of the solution. Acode based on this method has been developed and results of numerical applications to aVenturi channel and to the attenuation ofwaves are presented to check for well known situations how accurately the source terms are evaluated. The code is applied to reproduce Brock’s experiments on roll waves performed in a rectangular laboratory channel. The numerical solution accurately represents roll waves evolution due to uniform flow instability as well as wave intensity increase due to instability and wave coalescence. A sensitivity analysis of the numerical solution to changes of computational parameters is performed, in order to explain the discrepancies among computed and experimental values.

A numerical evaluation of air bubbles as a potential explanation to the higher than expected pore pressures in the core o fWAC Bennett Dam
by MATS BILLSTEIN and URBAN SVENSSON
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The objective of this study is to numerically evaluate if the Air Hypothesis is a potential explanation of the unusual pressure behaviour of the core at WAC Bennett Dam. Up to now, the pore pressures have exceeded the expected normal steady state distribution for about 25 years. The Air Hypothesis describes the influence of air bubbles on the pressure distribution in the core. An increased water pressure will compress the air bubbles and increases the amount of air that can go into solution at the upstream side of the core. At the downstream side, the situation is reversed, i.e. the air volume will increase and cause a hydraulic blockage. A one-dimensional numerical model, that is based on relevant conservation laws, physical laws (Darcy’s, Boyle’s, and Henry’s laws) and the relationships between the relative hydraulic conductivity and water saturation level, is able to predict the pressure evolution in both a qualitative and quantitative way. The model is applied to both a hypothetical core and the core ofWAC Bennett Dam. Results from a plug flow analysis on the dissolution of the air in the hypothetical core are in fair agreement with the numerical results. Comparisons with pressure measurements from WAC Bennett Dam show that the Air Hypothesis is a potential explanation to the unusual pressure distribution in the core.

Numerical simulation for the coupled problem of temperature and seepage fields in cold region dams
by LAI YUANMING, LIU SONGYU, WU ZIWANG, WU YAPING and J.M. KONRAD
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In this paper, a mathematical mechanical model and the governing differential equations of the coupled problem of temperature and seepage fields, with phase change, are first derived from the theory of heat transfer and the theory of seepage. Then the finite element formulae of this problem are obtained from Galerkin’s method. Finally, considering the seepage influence, an illustrative example of the temperature field in cold regions dam is provided. The example shows that the effect of seepage field on the temperature field of cold region dams is large. So the effect of this factor on the dam temperature field should be taken into account in cold regions dam engineering design.

Transient flow in pipe networks
by HOSSEIN M. V. SAMANI and ALIREZA KHAYATZADEH
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In order to apply the fixed-grid method of characteristics to transient flow in multi-pipe systems, some pipe reaches may not satisfy the Courant condition. Various numerical techniques can be applied for these reaches. In the present study, the finite difference or interpolation techniques are coupled with the method of characteristics. The location of the disproportionate pipe, which does not satisfy the Courant condition, is changed along the pipe in different schemes. The accuracy of the developed schemes has been checked by comparing their results to available exact analytical solutions. The comparison indicated that the scheme in which the finite difference method is employed for an intermediate location of the disproportionate pipe reach is the best. A general computer program based on this method has been developed. This program is capable of analyzing pipe networks including pumps, valves, surge tanks, air chambers, etc.


ISSUE NO. 6

Multi-phase plumes in uniform and stratified crossflow
 By S. A. SOCOLOFSKY and E. E. ADAMS
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Laboratory experiments of multi-phase plumes in uniform and stratified crossflows are presented. In uniform crossflow, multi-phase plumes behave as mixed single-phase plumes up to a critical height, hS, where the entrained fluid separates from the dominant dispersed phase. From the experimental results, an empirical relationship for hS was calibrated giving /(B/hS)1/3 = 6.3(us/(B/hS)1/3)-2.4, where u is the crossflow velocity, B is the total u∞ kinematic buoyancy flux of the mixed plume, and us is the slip velocity. Above hS the separated continuous-phase plume behaves like a momentum jet and the bubble column follows the trajectory of the vector sum of us and . In stratified crossflow, the trap height in quiescent water, hT, was u∞ compared to hS. For hT « hS, the plumes are stratification-dominated and separation occurs at hT = (2.8 – 0.27us/(BN)1/4)(B/N3)1/4, where N is the Brunt- Vaisälä buoyancy frequency. For hT » hS, the plumes are crossflow-dominated, and separation occurs at hS. A simple single-phase model was modified to predict the fate of the separated plume above hS.

Application of gradually-varied flow algorithms to simulate buried streams
 By RIZWANUL BARI and DAVID HANSEN
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This paper reports the underlying algorithm, implementation, and development details of the Flow Analysis of Buried Streams (FABS) hydraulic model. This model simulates water surface profiles for 1-D non-Darcy flow through buried streams. Buried streams are formed in open-pit coal mines due to the disposal of large volumes of coarse rockfill. In such buried streams the formerly open channel passes through very coarse porous media but the behavior of this flow does not follow Darcy’s law. Rather, it behaves in a manner similar to that of ordinary open channel flow. The longitudinal variation in the depth of water along the stream is not, however, governed by the roughness of the stream-bed, but by the characteristics of the coarse porous media that now fills the channel. Such flow is governed by non-Darcy flow operating under the Dupuit assumptions. The performance of the model in simulating steady non-Darcy flow profiles was evaluated under laboratory experimental conditions and found to be satisfactory. The model uses eitherWilkins’ or Stephenson’s equation to compute head losses. It was found that these equations performed equally well in simulating experimental water surface profiles. The performance of the model was also evaluated under three different friction-slope averaging methods, namely, the arithmetic, geometric, and harmonic average. Based on the results obtained in this study, it is suggested that any of these frictions-slope averaging techniques result in satisfactory flowprofiles, provided that reach lengths are not excessive. Although there are certain limitations and definite possibilities for further improvement, it is believed that the FABS model represents a significant step forward in providing more explicit assistance in the non- Darcy water surface profile simulation process.

Contribution on transient flow modelling in storm sewers
 By MUSANDJI FUAMBA
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Currently, the Preissmann slot model still enjoys popularity in modelling the transition between free-surface and fully pressurised flows in tail-race tunnels. However a fully dynamic and transient modelling technique is needed to predict the surge front location and velocity, the pressure rise in the full flow zone and the water depth change in the free-surface zone. In this paper, the transient flow is modelled and only one surge front is considered. Three 1-D models, which differ from each other by the computational method used to calculate either the free-surface or the full flow conditions, have been developed and applied successfully to both laboratory and field data. Predictions have been compared to measurements and good agreement found. Comparison between the three fully dynamic models was done and selective criteria were forwarded.

Sources of bias errors in flume experiments on suspended-sediment transport
By MARIAN MUSTE
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 Extensive research efforts in the last fewdecades have only partially elucidated the complexities of suspended-sediment transport. Lacking an adequate formulation and quantification of the interaction between suspended particles and the carrier liquid, it is common practice to combine sediment mechanics theory and empiricism to obtain predictive formulations. Flume data for suspended sediment transport, however, is incomplete and often inconsistent with respect to insights into sediment effects on water flow. Improvement of the data quality/reliability for future similar studies requires identification and evaluation of the sources of bias errors that might affect the experimental results. The present paper identifies and partially evaluates significant sources of bias errors in flume investigations of suspended-sediment transport. Bias errors are discussed in the order in which they typically arise in the conduct of flume experiments. The paper considers first conceptual errors associated with the governing equations used for the design and interpretation of the experimental results. Considered next are bias errors generated during data acquisition and data reduction. As an outcome of bias error discussion, the paper recommends guidelines to avoid and reduce such errors, hence, to increase the reliability of the experimental results. Special emphasis is placed on illustrating the potential of the non-intrusive measurement instruments for removing bias errors.

Erosion of a polystyrene bed by obliquely impinging circular turbulent air jets
By N. RAJARATNAM and K. A. MAZUREK
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 This paper presents the results of an experimental study of the erosion of a cohesionless bed by obliquely impinging circular turbulent jets. The jet of air, with velocity of 27.3 to 86.8 m/s and diameter at the nozzle of either 6.35 or 12.6 mm, was set to impinge on a bed of polystyrene particles at varying angles of impingement of 7.5 to 60°. Several characteristic dimensions of the scour hole were measured and analyzed and found to depend on the angle of impingement and the erosion parameter Fo/(H/d), where Fo is the densimetric Froude number, H is the impingement distance, and d is the diameter of the jet at the nozzle. Correlations were developed for the main characteristics of erosion at asymptotic state. Observations of th growth of the scour hole are also presented.

Influence of cohesion on scour around bridge piers
 By S. A. ANSARI, U. C. KOTHYARI and K. G. RANGA RAJU
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 Experimental results on temporal variation of scour around circular bridge piers founded in cohesionless and cohesive sediments under steady clear water flows are reported. The difference between scour patterns in cohesionless and cohesive sediments is brought out. Considering the horse shoe vortex to be the prime agent causing scour, a procedure is developed for computing the temporal variation of scour depth in cohesive sediments. Empirical relationships have also been obtained for maximum scour depth around bridge piers in cohesive sediments.

Local scouring in low and high gradient streams at bed sills
 By MARIO A. LENZI, ANDREA MARION, FRANCESCO COMITI and ROBERTO GAUDIO
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 The main characteristics of local scouring downstream of bed sills, forming a staircase-like system in high-gradient streams with non-uniform alluvium, have been investigated through 13 clear-water laboratory runs. Three initial longitudinal slopes and different flow rates were considered, keeping the same distance between the baffles. The grain size distribution of the sediment is that of a real alpine torrent scaled to the model dimensions. The measured scour depth, length and shape are compared to previous results concerning low gradient and uniform sediment gradings. A dimensional analysis approach appears to remain valid; nevertheless some simplifications cannot be made, since the jet regime plays an important role both for the depth and the length of the scour, and consequently affects the scour shape. Two equations are proposed for the estimation of the maximum scour depth and length. The equations are from previous data sets on low-gradient tests and a new one of experimental results on high-gradient cases.

Effect of circulation on critical submergence of an intake pipe
 By FŸKRET KOCABAS and NEVZAT YILDIRIM
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 In this study, the effect of circulation on the critical submergence of an intake pipe is presented. Experiments were conducted on a vertically flowing downward intake pipe in a circulation imposed still-water reservoir. The circulation imposed on the flowcauses an outward centrifugal force component in radial direction which increases the critical submergence considerably. For a given flow and geometry, the vane setting angle has a certain value which causes the same critical submergence as in the case of no-vanes (unsteady vortex in no-circulation imposed flow). As the vane setting angle gets larger than its certain value, the critical submergence becomes larger than that in the no-circulation imposed flow (no-vanes). The smaller the vane setting angle, the smaller is the critical submergence. The critical submergence attains its minimum value when the vane setting angle is zero (vanes are set radially). Flowvisualization for an intake pipe in a still-water reservoir has indicated that a spherical volume of fluid bounded by a stream surface of a sphere (SSS) develops. This spherical volume, hence SSS, shrinks radially and rotates about its vertical axis. The air-entraining vortex occurs as the SSS collapses. The SSS is a useful concept for the physical explanation of occurrence of the air-entraining vortex.