Abstract of Papers - JHR Volume 39, 2001
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ISSUE NO. 1

Swiss Contribution To Water Hammer Theory
Willi H. Hager
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Swiss hydraulic engineers have significantly contributed to the understanding of water hammer, based partly on the advances of the Italians Ménabréa and Allievi. The contributions of Michaud, Strickler, Schnyder and Jaeger are particularly discussed in the light of modern developments, and biographies on the latter two individuals are added. It is concluded that the phenomenon of water hammer has been developed within a short period, mainly due to the mathematical methods furnished by mechanical engineers and the experience collected by civil engineers for the design and execution of dams during the golden age of dam engineering.

KEY WORDS Fluid Transients, History, Hydraulics, Water Flow, Water Hammer.

Turbulent velocity profiles in sediment-laden flows
Junke Guo, Pierre Y. Julien
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A theoretical analysis shows that velocity profiles in sediment-laden flows are similar to those in clear water. The modified log-wake law, which is developed for clear water by Guo, is also valid in sediment-laden flows. The analysis of the effects of sediment suspension on turbulent kinetic energy and turbulent diffusion shows that: (1) sediment suspension increases mean flow energy loss; (2) sediment suspension weakens turbulent diffusion in the vertical direction and then increases velocity gradient; and (3) sediment suspension affects velocity profile in two ways: average concentration and density gradient. The comparison with narrow-channel laboratory data confirms the theoretical analysis and shows that: (1) the modified log-wake law agrees well with experimental data for sediment-laden flows; (2) both average concentration and density gradient reduce the von Karman constant; and (3) for a given width-depth ratio, sediment concentration slightly increases the wake strength while density gradient has little effect on it. In addition, the modified log-wake law can reproduce experimental data where the maximum velocity occurs below the water surface.

Numerical simulation of sediment mixture deposition part 2: a sensitivity analysis
Ph. Belleudy
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This paper presents a series of numerical simulations, with the objective of testing the sensitivity to different parameters in the numerical modeling of previously published laboratory experiments. Different options and formulations are analyzed, especially when they concern and influence down-stream grading of sediment in rivers. A final discussion deals with the transposition of such investigations to environmental and river engineering studies : refined lab experiments are valuable, despite a direct applicability to real problems which is very limited.


Numerical modeling of one-dimensional solute transport in an unsaturated porous media
M. Hami, K. Gueraoui, A. Hammoumi, And G. Zeggwagh, M. El Hatri
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This survey treats the mathematical pattern that describes the solute transport in the unsaturated zone of a porous media conducting the contamination to the groundwater. This problem is the result of the combination of three essential processes: water flow in the porous media, the heat transfer and the transport of the polluting agent. The governing equations are non-linear and present a very strong coupling. A numerical approach based on the finite difference method, for the resolution of one-dimensional problem is proposed.

 

Numerical modelling of bore propagation and run-up on sloping beaches using a MacCormack TVD scheme
S. Vincent And J-P. Caltagirone, P. Bonneton,
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A McCormack TVD scheme is presented for the computation of Saint-Venant equations, in the context of coastal hydrodynamics. The dam-break problem on wet and dry bottoms is used to evaluate and discuss the performances of the scheme. A run-up simulation on a sloping beach is then presented and a comparison using an analytical solution is made. Finally, a bore propagation on a sloping beach is computed.

Coupled physical-numerical analysis of flows in natural waterways
Marian Muste, .Ehaba. Meselhe, Larry J. Weber, Allen A. Bradley,
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The recent digital-electronic revolution has helped experimental hydraulics benefit from a new generation of acoustic-, laser-, and imaging-based instrumentation. These newly developed techniques are not only of superior accuracy, but they have also expedited data collection. Powerful visualization software has been used increasingly to present and interpret experimental results. In addition, numerical models have become increasingly available in some cases providing turnkey solutions to complex flows. The outcome of this intensive development is powerful computer-based research tools that allow an unprecedented interaction between physical and numerical experiments. This integrated approach is considerably improving our under-standing of numerous aspects and practical consequences of flow mechanics and allows a comprehensive treatment of space-time processes in fluid flows which is difficult to obtain using alternative means. This holistic experimental-numerical approach is readily available for integration as expert-systems or decision-making programs in hydroinformatics systems. The present paper discusses the beneficial synergy between laboratory measurements and computational models of different levels of complexity. A study, conducted at the Iowa Institute of Hydraulic Research (IIHR) is presented herein as an example to demonstrate the interaction among the three investigation components, namely, laboratory measurements, the kinematic model, and the hydrodynamic model, as well as the benefits and limitations of each of them. The laboratory velocity measurements were made using three-component Acoustic-Doppler Velocimeters. A simple numerical model based exclusively on flow kinematics was used to empower results visualization and to provide insight in several flow features. The kinematic model feedback was used to optimize the data acquisition scheme for the ensuing measurements. The detailed hydrodynamic flow analysis for regions with complex three-dimensional flows was obtained by a numerical model that solves the Reynolds Averaged Navier-Stokes (RANS) equations in general curvilinear co-ordinates.

Parametric study of riprap failure around bridge piers
Foo-Hoat Lim, Yee-Meng Chiew,
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This paper presents a parametric study on riprap protection around a cylindrical bridge pier with uniform bed sediments. Riprap layers with different characteristic parameters: thickness, cover width, and placement level of the riprap layer; median grain size and density of the riprap stone; and undisturbed approach flow depth were tested under a sequence formation of ripples, dunes and transition flat bed. Observations show that a riprap layer
will eventually degrade to a maximum level which is the same as the maximum pier scour depth when the riprap layer is not present. This study proposed a criterion to determine the maximum embedment level and the embedment velocity, i.e., the flow velocity at which the riprap layer has embedded to its maximum level. The experimental results show that variations of the characteristic parameters have no influence on the embedment
failure at the upper end of the dune regime. The study also proposes a maximum embedment velocity, which defines a critical flow velocity at which all riprap layers, irrespective of the characteristic parameters will fail.

Hydraulic features of supercritical flow along prismatic side weirs
Giuseppe Oliveto, Vittorio Biggiero, Mauro Fiorentino,
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Side weirs are key structures often located in combined sewer systems, provided the upstream flow is subcritical. At present, knowledge of the outflow process when the flow along the weir is subcritical can be considered satisfactory. The same cannot be said when, as often happens in practice, the upstream flow is subcritical while the flow along the weir is supercritical. In this paper the local flow features along prismatic side weirs in a circular channel were investigated. In particular, the condition of supercritical flow along the weir was analysed. Based on experimental work and on data available in the literature, some characteristics of both the flow that remains in the main channel and the flow that leaves it were investigated. These include: the distribution of the discharge in the main channel, the lateral outflow angle and the lateral outflow velocity. The results combine to provide a better understanding of the outflow process and are also readily applicable for design. Finally, a theoretical global approach is presented to estimate the head loss due to the flow partition.

Numerical and experimental study of unsteady salt water purging in Hong Kong sea outfall model
Joseph H. W. Lee, Zhen-Ren Guo, Tony W. C. Yau,
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Based on experiments on the 1:20 Hong Kong model diffuser, a numerical model for simulating seawater intrusion and purging process in an invert-connected outfall is developed. An extension of the theory of Guo and Sharp (1996), this is essentially a one dimensional unsteady flow model with locally 2D techniques to account for stratification and density changes at riser-tunnel junctions. Numerical predictions of purging flow and times are
in good agreement with measurements; key features of the purging sequence are also well-supported by the synoptic observed riser flows. Both experiments and calculations demonstrate that, unlike a soffit-connected outfall, the seawater at the bottom of the tunnel is initially purged, and the mixing between effluent and seawater is much stronger. The purging sequence in invert-connected situation is usually from seaward end to land-ward
end for unsteady and quasi-steady purging; however for steady purging the sequence is from landward end to seaward end as in soffit-connected outfalls. The presence of an effluent layer at the top of the tunnel (due to e.g. pump shutdown) results in a persistent circulation in the system. The numerical model also shows that shortening the outfall tunnel length by an artificial bend has an effect on the purging sequence and time, but not the purging flow. Two-layer purging occurs in steady purging process and can help reduce purging discharge rate, especially with a large number of risers. Salt-assisted purging is effective only if the duration of the process is sufficiently long; however, the effluent volume required can be larger than that in the pure effluent purging process.

Concurrent two-phase downflow measurement with an induced voltage electro-magnetic flowmeter
U. Opara, M.Sc., I. Bajsic
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With a set of polynomial approximations a possibility is shown of the use of an induced voltage electromagnetic flowmeter in the area of measuring cocurrent two-phase downflow in tubes. The principle of the meter operation remains hereby unchanged; due to the fluid flow through the meter magnetic field, the movement of electric charges induces the electric field and electrodes sense the difference of electric potentials on two separate points as an induced voltage. The flow is hereby considered as a two-phase one also in terms of electric conductivity since it consists of the electric conductive liquid phase and electrically non-conductive gas phase. Dependence of the output signal of the electromagnetic flowmeter on the volumetric quality and the total gas-liquid volumetric flow is experimentally confirmed through the master-slave method at a measuring air-water system-testing site.

Recognition and measurement of dispersed oil droplets in a water column
S.K. Tan, A.F. Yao
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A non-intrusive method, based on laser-sheet illuminated imagery and image processing techniques, was used to explore the phenomena of natural dispersion of oil in a water column. The method was used in a laboratory experiment to determine the distribution of oil droplets in the water column. The findings of this study suggest that this approach is a fast and accurate way to sample oil droplets in a water column without disturbing the flow.

 

ISSUE NO. 2

Beginning of sediment transport of incoherent grains in shallowshear flows
MARCO PILOTTI, GIOVANNI MENDUNI
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In this paper the main results of an experimental investigation on the beginning of sediment transport of incoherent
granular materials in shallow viscous flows are presented. The experiments, performed in a laboratory flume in both laminar and smooth turbulent .ow conditions, complement the data already available in literature, and confirm that
the hyperbolic trend shown by the Shields curve when the grain Reynolds number X decreases to zero is not supported
by experimental evidence. In addition, they confirm the Yalin and Karahan (1979) hypothesis on the existence of a distinct curve for the inception of sediment transport in viscous dominated flows. On the basis of the observed phenomenology, we propose to interpret the process as a function of the probability distribution of the repose angle of the grains of the erodible bed. Accordingly, a simple mechanical relation is derived that explains the pattern exhibited by the experimental data.

 

The continuous measurement of bedload discharge in a large alpine gravel bed river
H.M. HABERSACK, H.P.NACHTNEBEL , J.B. LARONNE
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The objective of this paper is to analyze the ef.ciency of a bedload-measuring trap for continuous sediment measurements. The trap is implemented into the river-bed of the Drau, located in Southern Austria. At the trap site the mean annual discharge is 64 m3s-1, the slope is about 0.002 and the mean subsurface grain diameter is about 28 mm. Two criteria, hydraulic ef.ciency and sampling efficiency, are used to evaluate the performance of the trap.
The hydraulic efficiency as well as the sampling ef.ciency are shown to be high. The advantage of the trap is that continuous and automatic bedload transport measurements can be obtained even in wide, alpine gravel bed rivers. We demonstrate that continuous bedload data are useful with respect to identifying initiation of motion. The comparison with formulas shows significant deviations between measurements and calculations. In such a supply limited river-bed, where bedload often moves over an armour layer, it is apparent that local hydraulic conditions explain only a fraction, about one third of the variation in the weak bedload flux.

Transport of sediment in large sand-bed rivers
ALBERT MOLINAS, BAOSHENG WU
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KEYWORDS Large river, Sediment transport, Alluvial river, Stream power, Universal stream power, Amazon river,
Mississippi river, Deep river, Bed material load, Sediment transport capacity

A sediment transport equation based on universal stream power is presented for the prediction of bed-material concentrations in large sand-bed rivers. The universal stream power, which is derived from the energy concept, has the advantage of eliminating the energy slope as a parameter. The energy slope, which is in the order of 10-5 for large rivers, is a major source of uncertainty in measurements. The analysis shows that relationships derived from flume experiments with shallow flows cannot be universally applied to large rivers with deep flows. Also the use of dimensionless homogeneous
parameters in an equation is not suf.cient to ensure its applicability to flow conditions where .ow depths are several orders of magnitude larger. The comparisons between computed and measured sediment concentrations indicate that the commonly used Engelund and Hansen, Ackers and White, and Yang equations which were developed using mainly flume experiments are not applicable for large rivers with flow depths and Reynolds numbers up to 100 times larger than those found in flumes. The Toffaleti’s method which was developed mainly from field data gives reasonable predictions
of sediment transport rates for large rivers. Using the proposed equation, the computed sediment transport rates are in much closer agreement with the actual measured values in large and medium rivers.

Height and wavelength of alternate bars in rivers: modelling vs. laboratory experiments
M.A.F. KNAAPEN, S.J.M.H. HULSCHER, H.J. DE VRIEND, A. VAN HARTEN
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Alternate bars are large wave patterns in sandy beds of rivers and channels. The crests and troughs alternate between the banks of the channel. These bars, which move downstream several meters per day, reduce the navigability of the river. Recent modelling of alternate bars has focused on stability analysis techniques. We think, that the resulting models can predict large rhythmic patterns in sandy beds, especially if the models can be combined with data-assimilation techniques. The results presented in this paper confirm this thought.
We compared the wavelength and height of alternate bars as predicted by the model of Schielen et al. [14], with the values measured in several flume experiments. Given realistic hydraulic conditions > 2*10³, (R the width-to-depth ratio and Re the Reynolds number), the predictions are in good agreement with the measurements. In addition, the model predicts the bars measured in experiments with graded sediment. If < 2*10³, the agreement between model results and measurements is lost. The wave height is clearly underestimated, and the standard deviation of the differences between predictions and measurements increases. This questions the usefulness of small flume experiments for morphodynamic problems.

Flowstructure of the plane turbulent impinging jet in cross flow
Meilan Qi, Zhicong Chen, Prof., Renshou Fu, Prof.
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KEY WORDS: Normal impinging jet, cross flow, turbulence, velocity similarity

This paper presents both experiment and numerical simulation on Two-dimensional vertical impinging jet in cross flow. Intake pond of the cooling water system is a requisite component of thermal or nuclear power plant. But in intake pond sediment deposition is heavy. Impinging jet was equipped in a nuclear power plant in China and has a significant impact on sediment suspension. However, before this investigation, little attention has been paid to the mechanism of this application case. In response to the demands of the engineering design, started this study. Experiments are performed for different Reynolds number ratio of the jet-to-cross-flow R varying from 0.13 to 0.22. The measurement results of turbulent parameter and flowstructure are obtained. Moreover the turbulent parameters play an important role on sediment transportation. As well the experiment results agree well with thecalculations provided from . – e turbulence model.

Mixing with multiple circular turbulent jets
AHMED K. MOAWAD, N. RAJARATNAM, S. J. STANLEY
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This paper presents the results of a laboratory study investigating chemical mixing in an open channel with multiple circular turbulent jets. The experiments were carried out for ƒ¿, which is the ratio of the velocity of the jets to that of the cross.ow, varying from 8 to 16, with a combination of coflowing and cross flowing jets and only cross flowing jets. The number of ports for the jets was varied for 20 to 36 and the spacing between the ports was varied from 12d to 24d where d is the diameter of the ports. Concentration measurements covered the relative distance x/d=2000, where x is the longitudinal distance from the diffuser. The dimensionless mixing distance ƒ¿xm/d at which the standard deviation of the concentration field reached
a value of 5% was equal to 25000 where xm is the mixing distance. The results indicate that a diffuser with turbulent cross flowing jets could be an alternative to mechanical methods for achieving mixing of chemicals in open channels in water treatment plants.

Integral model of shallowmixing layers
ROBERT BOOIJ, JAN TUKKER
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Mixing layers that develop downstream of the con.uence between two .ows of different velocity determine the lateral exchange of pollutants and sediment between both flows. The shallowness of most flows in the natural environment restricts the development of those mixing layers in two ways.
First, the bottom friction has a stabilizing influence on the generation of large-scale turbulent structures in the mixing layer and in this way reduces the growth of the mixing layer. Second, the bottom friction leads to a decrease of the velocity difference between both flows, also leading to a reduced growth. This reduced growth results in a reduced lateral exchange between both .ows which has consequences for the pollution and sedimentation of harbours and flood plains and for the longitudinal dispersion in rivers. To predict the development of mixing layers in shallow flows a simple integral
model was developed in which both reduction mechanisms are incorporated. The model gives a fair reproduction of the downstream evolution of the width and the transverse displacement of investigated shallow mixing layers on laboratory scale. Simulation of river con.uences yields very narrow mixing layers which corresponds to the often observed phenomenon that merging rivers hardly mix, but appear to flow alongside each other over a long reach.


Friction velocity associated to a non-uniform flow and an intermediate scale roughness
HOSSEIN AFZALIMEHR and FRANCOIS ANCTIL
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This work consists of an experimental study of the resistance associated with non-uniform flows over cobble-bed channels, leading to an intermediate scale roughness. Friction velocity is estimated for twelve cases of accelerating and decelerating flows. Four estimation methods are compared: Reynolds stress, logarithmic law, parabolic law and a global approach. They are all in close agreement. The observed mean longitudinal velocity profiles are consistent with observations reported in the literature for non-uniform flows over gravel-bed channels with small relative roughness. Accelerating flows produce smaller friction velocities than decelerating flows, which is contrary to observations reported in the literature.

Three-dimensional numerical simulation for an open channel flow with a constriction
N. J. SHANKAR, E. S. CHAN, Q. Y. ZHANG
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Flow characteristics in an open channel with a constriction are investigated numerically by using a three-dimensional multi-level hydrodynamic model.
In this model, the two-step Euler predictor-corrector algorithm is introduced to predict and correct free surface water level. The momentum equations in horizontal directions are discretized in an explicit scheme for the component velocities u and v, and the continuity equation is discretized for the velocity w in an implicit scheme in uniform and non-uniform grid systems. The model has been tested against analytical solutions for two standard cases and used to study sudden contraction and expansion open channel flow. The numerical results are in good agreement with analytical and experimental data. Simulated flow patterns for the sudden contraction and expansion flow show reasonable flow characteristics downstream of an opening.

Hydraulic condition for undular-jump formations
IWAO OHTSU, YOUICHI YASUDA, and HIROSHI GOTOH,
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This paper presents the upper limit of the in.ow Froude number for undular-jump formations in smooth rectangular channels. It has been found that the formation of undular jumps depends not only on the inflow Froude number but on the boundary-layer development at the toe of the jump under conditions in which the effects of the aspect ratio and the Reynolds number on the .ow condition are negligible. The velocity of the first wave crest immediately before the breaking is at a maximum near the water surface and becomes a critical velocity. For the undular jumps with the developing inflow, the upper limit of the Froude number F1limit has been shown experimentally as F1limit = 1.3-2.3. For the fully developed inflow, F1limit 1.7 =. . has also been obtained, and it shows the same value as described in many textbooks. The upper limit of the inflow Froude number for undular-jump formations has been derived by taking account of the boundary-layer development and considering the flow along the water surface immediately before the breaking. The predicted values agree with experimental results.

Velocity profile of sediment suspensions and comparison of log-law and wake-law
XINGKUI WANG, ZHAO-YINWANG,MINGZHONG YU, DANXUN LI
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The log-law and wake-law of velocity pro.le of open channel .ow of sediment suspensions are discussed and compared in the paper. Data from 9 literatures are employed for comparison of the two laws and regression analyses are conducted on the main factors affecting velocity profile. Empirical formulas are obtained for estimation of the factors from the .ow conditions. The elevation of the maximum velocity and the deviation of velocity from the logarithmic formula at the water surface are functions of the aspect ratio of the channel. The log-law is developed into Eq. (20) applicable to the whole
flow including the region near the water surface for various boundary conditions. The wake law describes the velocity distribution below the maximum velocity point. The relative error of wake-law (11%) is larger than that of log-law (6%). Moreover, the wake coefficient must be determined by using the measured velocity profile because there is no reliable formula to estimate its value from the flow conditions.

 

ISSUE NO. 3

Interplay between turbulence and periphyton in rough open-channel flow
GODILLOT R. and CAUSSADE B., AMEZIANE T. and CAPBLANCQ J.
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During the low flow period, important development of periphyton occur in rivers. The understanding of the mutual ‘periphyton-flow’ influences have induced, through recent studies, conflicting conclusions which underlined the particular complexity of these interactions. An experimental flume has been build to show that the near-bed regime (which depends on the water discharge, the nature and form of the substrate, the presence or absence of blowing, ...) and the development of the periphyton are strongly dependent on each other. Indeed, different experiments show that the periphytic characteristics differ depending on the hydrodynamic conditions and that the flow properties are modified by the presence of the periphytic matrix. The periphyton induces a smoothing of the roughness of the bottom which is significant depending on the thickness of the periphytic matrix. This thickness has been taken into account in terms of a gap of the y-axis in the interpretation of measurements of the hydrodynamic parameters, contrary to recent papers.

Solution for spillway flowby finite difference method
TUFI M. ASSY
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The stream function is used to analyze the irrotational flowover spillway crests, with and without gate. Items of interest include the free surface location and determination of the discharge and pressure fields. The approach, which is based on the finite difference method with a new representation of Neumann’s problem on boundary points, is mathematically simple and requires only simple programming. Results thus obtained are in agreement with those obtained by way of experiments.

Developments in unsteady pipe flow friction modelling
ANTON BERGANT, ANGUS ROSS SIMPSON and JOHN VÍTKOVSKÝ
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This paper reviews a number of unsteady friction models for transient pipe flow. Two distinct unsteady friction models, the Zielke and the Brunone models, are investigated in detail. The Zielke model, originally developed for transient laminar flow, has been selected to verify its effectiveness for "low Reynolds number" transient turbulent flow. The Brunone model combines local inertia and wall friction unsteadiness. This model is verified using the Vardy’s analytically deduced shear decay coefficient C* to predict the Brunone’s friction coefficient k rather than use the traditional trial and error method for estimating k. The two unsteady friction models have been incorporated into the method of characteristics water hammer algorithm. Numerical results from the quasi-steady friction model and the Zielke and the Brunone unsteady friction models are compared with results of laboratory measurements for water hammer cases with laminar and low Reynolds number turbulent flows. Conclusions about the range of validity for the three friction models are drawn. In addition, the convergence and stability of these models are addressed.

Regime theory and the stability of straight channels with bank full and over bank flow
E. M. VALENTINE, I. A. BENSON, C. NALLURI and J. C. BATHURST
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Experiments have been carried out at a large laboratory scale to test a rational regime theory and to study the development of regime channel morphology for straight loose-boundary channels (bed and banks) in fixed flood plains for a range of flowconditions. Results are presented for straight channels which have been developed with bankfull flows and then subjected to overbank flows, and include friction and sediment transport data. These indicate reasonable theoretical agreement with the friction data but the sediment transport rate is underestimated. Stability of the main channel was assessed in response to bedforms, bank erosion and hydraulic friction and sediment transport. For overbank flows, rates of bank erosion have been measured. These measurements constitute unique overbank widening data for the main channel.

Selective bedload transport during the degradation of a well sorted grade sediment bed
GARETH PENDER, TREVOR B. HOEY,CHRIS FULLER, IAN K. McEWAN
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The paper presents an analysis of the composition of bedload transport and changes to bed structure and topography during three graded sediment degradation experiments. The analysis suggests that variations in channel hydraulics and active layer composition alone may not explain the observed reductions in sediment transport. Further, the experiments appear to cover a crucial range of mean bed shear stresses for armouring studies, ranging between a condition of passive winnowing, to one of more active armour development in which the coarse grains play a role in determining bed structure. This indicates that the active layer concept, commonly applied in computer models of graded sediment transport, may be limited in its application.

Stabilising the sediment bed in laboratory flumes
I.A.BENSON,E.M.VALENTINE, C.NALLURI and J.C.BATHURST
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Recent laboratory investigations of alluvial channels with loose bed and banks of coarse sand required a method for immobilising or ‘freezing’ the loose boundary to permit time consuming hydraulic measurements without changes in the boundary geometry. A technique involving spraying separate solutions of sodium silicate and sodium bicarbonate on to a drained channel has been rediscovered, and is reported here. After freezing the changes in geometry and hydraulic roughness are negligible and some results are presented to illustrate this. The equipment and materials costs are low.
Furthermore, for freezing a large area the method described has advantages, from a health and safety point of view, over using adhesives or dry cement powder as alternatives.

Resistance studies of overbank flowin rivers with sediment using the flood channel facility
DONALD W. KNIGHT, FENELLA A. BROWN
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Some large scale experiments concerning equilibrium sand channels with overbank flow are described. The experiments were undertaken in the UK Flood Channel Facility (FCF) configured into a straight compound channel with fixed banks, a mobile main channel composed of uniform sand with a d50 of around 0.8mm, and two symmetric floodplains. The main channel alluvial resistance changed with flow depth and discharge in a complex way arising from the effects of both bedforms and the floodplain/main channel interaction processes. Overall, zonal and local resistance coefficients for this type of channel are reviewed and the data discussed in relation to practical modelling of alluvial channels with overbank flow.

3D numerical modelling of open-channel flow with submerged vegetation
T. FISCHER-ANTZE, T. STOESSER, P. BATES, N.R.B. OLSEN
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Velocity distributions in channels partially covered with vegetation have been computed using a three dimensional model. The Navier-Stokes equations were solved, using the SIMPLE method and the k-å turbulence model. The vegetation was modeled as vertical cylinders. A formula for the drag force on the vegetation was included as a sink term in the Navier-Stokes equations. The advantage with this method compared with using large roughness is that effects of the vegetation over the whole water depth can be taken into account, instead of only affecting the velocity near the bed. The numerical
model was tested against three laboratory experiments from straight flumes with uniform flow, where vegetation partially covered the cross-section. The velocity and vegetation density varied in both vertical and horizontal directions in the different cases. The experiments also included varying crosssectional shapes. All tests gave fairly good correspondence between computed and measured velocity profiles.

Influence of boundary roughness on velocity and discharge in compound river channels
W.R.C MYERS, JF LYNESS & J CASSELLS
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Results are presented of an experimental compound channel research programme carried out at the UK Flood Channel Facility including fixed and mobile main channel boundaries together with two flood plain roughnesses. For comparison data from a natural compound river channel are also presented. Velocity and discharge relationships are explored illustrating the complex behaviour of compound river channels and calling attention in particular to the errors incurred in applying conventional methodologies to discharge assessment in overbank flows. Relationships are presented for velocity and discharge ratios which could form the basis of mathematical modelling of overbank flow estimation methods. The research also represents a step towards prototype conformity by the introduction of mobile boundaries.

High-resolutionandnon-oscillatory solution of the St.Venant equations in non-rectangular and non-prismatic channels
BRETT F. SANDERS
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A scheme to model open channel flow over wet and dry beds in non-rectangular and non-prismatic channels is presented. The scheme is second-order accurate, stable for Courant numbers up to unity, and monotonicity preserving. The scheme solves the St.Venant equations using a Godunov-type finite volume method. Mass and momentum fluxes are computed using a Roe-type Riemann solver, theMUSCL(Monotone Upwind Scheme for Conservation Laws) approach is applied for second-order spatial accuracy, and a treatment is introduced to model the hydrostatic pressure force exerted by the channel
walls in the stream wise direction. The treatment permits momentum fluxes and the channel wall force to be balanced to numerical precision, preventing the artificial acceleration of the flow. Comparisons between model results, exact solutions, and experimental data show that the scheme is robust. Accurate and monotone results are obtained in the presence of discontinuities, supercritical flow, subcritical flow, transcritical flow, and dry-bed flow problems without the need for special front-tracking approaches or deforming grids. In addition, the scheme will conserve mass to numerical precision in all applications.



ISSUE NO. 4





An implicit scheme for steady two-dimensional free-surface flow calculation
A.J. Klonidis, J.V. Soulis
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An implicit numerical scheme has been developed and subsequently applied to calculate steady, two-dimensional depth averaged, free-surface flow problems. The implicit form of the scheme gives fast convergence. The scheme is second order accurate and unconditionally stable. The free-surface flow equations are transformed into a non-orthogonal, boundary-fitted coordinate system so as to simulate with accuracy irregular geometries. The model is used to analyze a wide variety of hydraulic engineering problems including subcritical flow in a converging-diverging flume, supercritical flow at a channel expansion with various Froude numbers, and mixed sub- and supercritical flow in a converging channel. The computed results are compared with measurements as well as with other numerical solutions and satisfactory agreement is achieved.


3D layered-integrated modelling of mass exchange in semi-enclosed water bodies
C.W.Li and J.Gu
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The flow patterns in semi-enclosed water bodies are generally complicated and have significant effect on the water quality and fluid exchange there. The exchange of of fluid between the semi-enclosed water body and outside is due to two physical mechanisms. The first is the flushing due to tidal effect. The second is the shear induced by the velocity difference between the flow within the water body and outside. To predict the flow and solute transport in these water bodies a three-dimensional layer-integrated numerical model has been developed. To account for the turbulence which consists of the free shear component and the bottom friction component the k-e model is employed. The numerical model has been applied to the cases of tidal flow as well as steady river flow outside a rectangular harbour with or without breakwater. Compared with the available experimental results, the gross mass exchange both due to tidal effect and shear can be estimated satisfactorily by the numerical model.


Dynamic orifice model on water hammer analysis of high or medium heads of small hydropower schemes
H. Ramos and A.B. Almeida
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The most severe hydropower transients are induced in long hydraulic circuits due to extreme operating conditions. A computational model was developed in order to on one hand ensure waterhammer system control and, on the other hand, provide a more reliable and easier analysis for different specific speed turbines and alternative solutions of the system as a whole, through interaction between different hydraulic components. In reaction turbines, runaway conditions and guide vane closure cause significant discharge variations and pressure fluctuations that can affect the design of conveyance systems. A new approach for groups modelling as dynamic orifices concept was developed enabling the characterisation of the integrated system. The simulation results were compared with laboratory tests. This model can be used in the initial stages of civil works design as an efficient way to better characterise the hydrodynamic behaviour of the system when equipped with reaction turbines.


Numerical simulation and prevention of water freezing in outdoor penstocks
I. Sârbu, F. Kalmar
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Pipes laid in open air and conveying water can freeze in winter times. A blockage due to freezing can be effectively prevented by providing a minimum discharge. This precaution ensures the free flow of water through the pipe, but does not stop the forming of an ice layer on the inner pipe wall. In this paper a mathematical model is developed to determine minimal protection discharge and simulation of variation in time along the pipe of ice layer formed inside outdoor pipes during non-stationary atmospheric regime. The model allows the study of the pipe capacity to transport the normal discharge in operation and minimal protection discharge without affecting the hydraulic characteristics of the flow. Also, it gives the possibility to adopt economical solutions for the problem of protecting these pipes from frost. The performance of the developed model is illustrated using a numerical example.


Scour around spur dikes and bridge abutments
U.C. Kothyari and K.G. Ranga Raju
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Realistic estimation of scour depth around spur dikes and bridge abutments in alluvial rivers is important for safe and economic design of their founda-tions. Procedures have been developed by previous investigators for determination of design scour depth in steady flows at abutments and spur dikes by making use of the design discharge. However, the time required by the design discharge to scour to its full potential is generally much larger than the time for which it runs. Therefore, computations on temporal variation of scour depth are also important for design purposes. Scour processes at bridge piers, abutments and spur dikes have been found to be similar except that the boundary layer effect induced by the channel wall upstream of the abutment or spur dike causes less scour around these as compared to the case of piers. In the present study, therefore, the concept of an analogous pier is developed. The analogous pier would have the same equilibrium scour depth as the given abutment or spur dike under similar hydraulic conditions. The parameters relating to drag due to flow around abutment/ spur dike and bridge pier have been found to be useful in establish-ing a relationship for the diameter of the analogous pier. The temporal variation of scour depth and the equilibrium scour depth at the spur dike and the abutment are then computed using pier scour equations with size of the analogous pier being taken as the pier diameter. Results obtained are verified
using laboratory data of several investigators for both clear-water and live-bed scour conditions.


Adaptive quadtree model of shallow-flow hydrodynamics
A. G. L. BORTHWICK, S. CRUZ LEÓN
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Natural shallow-flow domains have irregular boundaries which can strongly influence the interior flow field. Here, the nonlinear shallow water equations are solved on adaptive quadtree grids that can approximate any two- dimensional boundary topology and are easy to enrich or coarsen. A special indexing system matches the quadtree structure to conventional finite volume notation. Grid adaptation is controlled by a cell circulation parameter. Simulations of standard test flows are in close agreement with analytical and other numerical data. The sample application of wind-induced circulation in Lake Balaton, Hungary, demonstrates the ability of the model to deal with a complicated shallow-flow geometry.


Hydraulics of simple habitat structures
H. Shamloo, N. Rajaratnam, C. Katopodis
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Habitat structures are built in rivers to provide feeding and resting areas for fish. At the present time, only rough guidelines are available for the design of these structures. This paper presents the results of a laboratory study on the flow and erosion around simple habitat structures. Hemispheres with diameter D of 74 and 130 mm were placed on smooth, rough as well as erodible beds and Froude number of the approaching flow was in the range
of 0.074 to 0.6. The relative depth d/h where d is the depth of and h is the height of the body was found to be the important parameter and was varied from about 0.6 to 4.3. Four different regimes of flow were found, which were classified based on the relative depth. Downstream of the body, there was a recirculation region (closed wake) with a length of about 2D which was followed by an open turbulent wake. The structure of flow in this open wake was analyzed in two layers using the concept of the wall wake. In the plane of symmetry, the inner layer was analyzed using the law of the wall whereas the outer layer was analyzed using the wake equation of Schlichting. The variation of the velocity in the transverse direction was also analyzed using the concept of similar profiles. Further an empirical correlation was found for the velocity scale. The amplification of the bed shear stress near the body, especially for the rough bed was significant. Some observations were also made on the nature of erosion around the hemisphere placed on erodible beds of two sand sizes of 1.11 and 2.1 mm. It was found that the pattern of erosion was different for the different flow regimes. The maximum equilibrium clear water scour depth occurred in front of hemispherical bodies and was approximately equal to 0.67 D.


Towards the hydraulics of the hydroinformatics era
M.B. ABBOTT, V.M. BABOVIC and J.A. CUNGE
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Hydroinformatics is the study of the flows of knowledge and data related to the flow of water and all that it transports, together with interactions with both natural and man-made, or artificial, environments (Abbott, 1991). Hydraulics, understood as the study of flows of water, more recently extended to include the transport of matter in all its forms with these flows, is accordingly central to hydroinformatics. Without hydraulics, no hydroinformatics! From this situation it may at first appear as though hydroinformatics provides only a new periphery to hydraulics: a new way of transmitting hydraulics knowledge and data to society. In practice, however, the way in which hydraulics is viewed and practised is itself now changing as a result of its incorporation into the new paradigm that hydroinformatics provides. The first purpose of the present paper is to introduce some of the changes that are currently proceeding in hydraulics under the influence of developments occurring in hydroinformatics. The second purpose is to indicate the consequences of these changes for the application of hydraulics within society, and thus for the future direction of hydraulics and hydroinformatics themselves.


Effect of downstream control on stability and mixing of a vertical plane buoyant jet in confined depth
C.P. KUANG and JOSEPH H.W. LEE
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The effect of downstream control on the mixing of a plane turbulent heated jet discharging vertically into confined depth is studied using the buoyancy extended k-e model. The steady two-dimensional turbulent flow, temperature and turbulence fields are computed using the finite volume method on a high resolution grid. In the absence of a specific downstream control, the numerical predictions demonstrate three generic flow patterns for different jet discharges and environmental parameters: i) a flow with circulation cells of alternate rotation for non-buoyant discharge; ii) a stable buoyant discharge with the mixed fluid leaving the vertical jet region in a surface warm water layer; and iii) an unstable buoyant discharge with flow recircula-tion and re-entrainment of heated water. A stratified counterflow region always appears in the far-field for both stable and unstable buoyant discharges.
The near field interaction and hence discharge stability is governed by only two dimensionless parameters - the discharge densimetric Froude number F o and the depth to jet width ratio H/B. The computed velocity and temperature fields agree well with the laboratory flow-visualization and temperature measurements of Jirka & Harleman (1979). Numerical prediction of stability categories is in excellent agreement with experiments. For a given discharge and depth, it is found that the jet stability can be predicted regardless of downstream control, provided that the channel length exceeds about 6H. The effect of a strong downstream control close to the discharge primarily results in a flooded internal jump and the lowering of the interface level in the stratified counterflow region. Consistent with the detailed measurements of Andreopoulos, Praturi and Rodi (1986), the predictions show a clear reduction of the bulk dilution, although the effect of downstream control on the jet discharge stability is insignificant.


An example of computational approach used for aerodynamic design of a rain disdrometer
EMAD HABIB and WITOLD F. KRAJEWSKI
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The present work reports on the application of a computational fluid dynamics-based method as a tool to improve the aerodynamic design of rainfall measurement devices. The focus is on a newinstrument, a two-dimensional video disdrometer that provides information about raindrop size distribution. The distorted wind field around and inside the instrument's body is simulated using a three-dimensional numerical model. A modified geometry of the instrument, suggested for operational purposes, is tested numerically. Trajectories of raindrops are simulated to investigate the wind effect on the catchment efficiency of the instrument. A stochastic Lagrangian particle-tracking model that accounts for the turbulence effect is examined. General guidelines related to aerodynamic aspects of the design of in-situ rainfall measuring devices are discussed.



ISSUE NO. 5



The axisymmetric and the plane jet in a coflow
S.J. Gaskin and I.R.Wood
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The approximate variation of the mean properties in an axisymmetric jet and a two dimensional jet with an ambient coflow in an infinite duct have been determined using the excess momentum equation and an entrainment function. The turbulent velocity flux, consisting of a portion due to the excess jet velocity and a portion due to the turbulent coflow velocity, is carried by a velocity approximately equal to the top hat velocity. The entrainment into this flow is driven by the excess jet velocity and the entrainment function varies to allow for the change in entrainment from the strong jet to the weak jet. The entrainment constant is determined from the data of Nickels and Perry [14] for the axisymmetric case and the data of Bradbury and Riley [6] for the two dimensional case. All coflow experiments are in ducts and the effect of the duct is explored for the axisymmetric case. This paper is a prelude to the study of a buoyant jet in a coflow and a buoyant jet in a crossflow.

Flow structure and mixing dynamics of the advected line thermal
S.J. Gaskin and I.R. Wood
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Ocean outfalls are modelled as buoyant jets. The final behaviour in a deep ocean is the advected line thermal, which is dominated by the jet buoyancy and the ambient velocity. The dilution of the effluent along its path is of interest, in some cases the traditional time averaged information is needed and in others the instantaneous or peak concentrations are needed. Laser induced fluorescence (LIF) is used to study the instantaneous flow structure and the mixing dynamics of the advected line thermal. Large scale structures in the flow are identified as subthermals and their development influences the flow behaviour. Statistical measures of the magnitude of the concentration fluctuations and the intermittence of the flow are obtained from LIF
images.

The applications of the enhanced CCHE2D model to study the alluvial channel migration processes
Jennifer G. Duan, Sam S.Y.Wang and Yafei Jia
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This paper is to report a newly developed numerical-empirical model, the Enhanced CCHE2D (EnCCHE2D), and its application to simulating the alluvial channel migration phenomena. EnCCHE2D model is capable of predicting quasi-three-dimensional (3D) flowfield and shear stress distribution on the bed, because a set of empirical functions of 3D flow characteristics formulated by results of a 3D model, CCHE3D, was integrated with CCHE2D, a depth-averaged hydrodynamic model, the predecessor of EnCCHE2D. The processes of sediment transport and meander migration were predicted based on these quasi-3D flow solutions. The advance or retreat of bank is calculated by considering not only the hydraulic erosion of bank surface and toe, but also the mass balance of sediment flux in the near-bank zone. As a result, the simulation of bank erosion, bar/pool formation and shifting, bank advance and retreat, channel widening and migration and meander evolution phenomena agree well with the available measurements of physical experiments.

An arbitrary lagrangian-eulerian finite difference method for computations of free surface flows
Ming-Hsi Hsu, Chun-Hung Chen and Wei-Hsien Teng
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A numerical model based on a finite-difference scheme is presented for simulation of two-dimensional transient flow in open channels. The Arbitrary Lagrangian-Eulerian (ALE) kinematic description, and the techniques of moving grids and boundary-fitted coordinate system are adopted for the computation of free surface flows. An iterative flow field solver using an algorithm of velocity divergence with an adjusted pressure is employed to ensure mass conservation. The proposed model is demonstrated on three well-known problems, which include wave propagation caused by dam-break, viscous damping of a solitary wave, and reflection of a wave running against a vertical wall. The simulated results show good agreement with available experimental data or theoretical solutions. Keywords: finite-difference scheme, free surface flow, solitary wave, dam break, moving grids, boundary-fitted coordinate, ALE description.

Numerical calculation of submerged hydraulic jumps
F. Ma, Y. Hou and P. Prinos
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The turbulence characteristics of submerged hydraulic jumps have been investigated numerically by means of the standard k-e turbulence model. The concept of a fractional volume of fluid (VOF) is employed to track the moving free surface. Numerical predictions include surface profiles, hydrodynamic pressures, mean velocities, turbulence intensities and shear stresses, maximum horizontal velocities and friction coefficients along the channel bed. Computational results are presented for Froude numbers ranging from 3.2 to 8.2 and submergence factors ranging from 0.24 to 0.85. The results are compared with available experimental data. They provide insights into both the macroscopic structure and the turbulent structure of submerged
hydraulic jumps.

Spreading and deposition of particulate matter in uniform flows
Andrew J. Hogg, Herbert E. Huppert
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We consider the fate of a cloud of heavy particulate matter instantaneously released from either a line or a point source in a uniform ambient flow. The particles are advected by the flow as well as sedimenting from it. The behaviour of the current is determined by a non-dimensional parameter that represents the ratio of the advected flux of the ambient flow to the downward particle flux to the boundary. We determine the horizontal extent of the particulate matter and the density of the deposit as functions of time in terms of this parameter. In particular we evaluate the maximum upstream penetration distance of the particles.We also present the results of a series of experiments for a line release in a channel (the two-dimensional situation) and find excellent agreement between our theoretical predictions and the experimental data for: the upstream and downstream lengths of the current as functions of time; the final areal density of deposit on the floor; and the maximum upstream penetration of the current.

Automated extraction of grain-size data from gravel surfaces using digital image processing
Justin B. Butler, Stuart N. Lane and Jim H. Chandler
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This paper describes and tests a method for the automated extraction of grain-size data from digital imagery. It combines two basic image processing methods for this purpose: grey-scale thresholding to create a binary image and watershed segmentation to grow edges on the binary image to allow the identification of individual grains. The method is subject to rigorous testing in terms of edge detection and automatic measurement of grain-size information from the edge images, and is also compared with the results obtained from simple direct clast sampling. The edge detection methods are tested with respect to manually-identified edges. This suggests that simple thresholding of raw imagery produces grain-size estimates that are: (i) in excellent agreement with manual estimates, above a critical particle size defined by the scale of the photography; (ii) downgraded with the inclusion of additional edge information from analysis of high resolution digital elevation models (DEMs); and (iii) not affected by the use of raw imagery as opposed to imagery that has been rectified to deal with geometric, tilt and relief distortion effects. The automated ellipse-based measurement method is shown to produce a good estimate of two-dimensional a- and b- axes as they appear as long and short axes on the edge images. Thus, the research shows that it can be used to map and quantify very rapidly spatial variations in grain-size characteristics, although it cannot deal with the longrecognised problem of the relationship between two-dimensional planform grain-size estimates and actual a- and b- axes obtained by direct grain sampling.

Mapping of bridge pier scour with projection moiré
Gerald Müller, Rüdiger Mach and Klemens Kauppert
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During the construction works for an extension of an existing bridge pier severe scouring occurred after the placement of a sheet pile cofferdam. A detailed investigation of the situationwas conducted at the Bundesanstalt fürWasserbau (Federal Hydraulic Research Institute) in Karlsruhe, Germany. For the measurement of the scour development without protective layer, an optical measurement method - out-of-plane projection Moiré -was employed in order to investigate its suitability. This technique uses the Moiré effect to map surface contours which are indicated by superposition lines or fringes. The Moiré fringe patterns generated by the measurements were then digitised for further analysis. The contour model revealed that, although the scouring depth did not increase significantly with time after the initial scour had formed, scour size did in fact do so. In addition, the contour model allows to map the scour extension and to determine the maximum deposition height. A 3D model of bridge pier and scour showed, in combination with photographs, the validity of the measurement technique and pointed towards the generation of a virtual environment for the representation of experimental results.

Detecting leaks in pressurised pipes by means of transients
Bruno Brunone and Marco Ferrante
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Reliable and quick techniques are needed to locate and estimate leaks in pressurised pipe systems in order to reduce water loss as much as possible. The aim of the present paper is to show that the use of unsteady-state tests, which give rise to small overpressure, can be considered as an appropriate method. During these tests, it is necessary to record the pressure-time history - the 'pressure signal' - in just one section of the pipe, the 'measurement section'. On the basis of a thorough laboratory investigation, the influence on the pressure signal of size and shape of small leaks, along with discharge conditions and initial flow regime, is analysed. Some remarks about the possible applicability of the obtained results to pipes of different material can be found. A reliable criterion is provided for a first-approximation estimation of leak size, based on the results of an inverse method procedure concerning pressure signals.

Stability of two consecutive floors with intermediate filters
Dr. Adel A.S. Salem and Dr.Yousry Ghazaw
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A subsidiary weir is proposed to be constructed downstream of each of the existing barrages on the Nile river in Egypt to secure its stability, which might have been affected after the construction of the Aswan High Dam. The present paper is intended to investigate the characteristics of seepage flow beneath two structures with an intermediate filter. The downstream structure (the proposed weir) has a slopping middle apron and two flat aprons in the upstream and downstream sides and is also provided with upstream and downstream cutoffs. The upstream structure (the existing barrage) has upstream and downstream cutoffs. A conformal mapping technique is used to solve the problem. Equations to calculate the uplift pressure distribution acting on both the existing barrage and the suggested weir are obtained; also equations to calculate the values of exit gradient along the intermediate filter and the downstream bed are derived. The seepage flows which seep into and/or drained from the intermediate filter are estimated. The analytical results are verified using experimental measurements performed on electrical analogue model and a very good agreement is noticed. A computer program is designed to compute the seepage flow, the uplift pressure and the exit gradients.



ISSUE NO. 6



Large scale flow structures and mixing processes in shallow flows
Gerhard H. Jirka
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Shallow turbulent flows occurring in wide rivers, estuaries, lakes or coastal regions, as well as the atmosphere, are readily susceptible to transverse disturbances that lead to two-dimensional coherent structures. the shallow jet, the shallow wake and the shallow mixing layer are examples of such flow patterns. three types of generation mechanisms are proposed for these flows; namely, topographical forcing, internal transverse shear instabilities, and secondary instabilities of the base flow. the large-scale coherent structures greatly influence the mixing and transport of pollutants that are released into such flows. they may also play an important role in transverse momentum exchange that controls friction in wide channel flows.

Simulation of shallow transverse shear flow by generalized second moment method
V. H. Chu, W. Altai
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A numerical scheme known as the second moment method is generalized to simulate advection and diffusion processes in a Lagrangian frame without projecting the data onto the Eulerian grid. Solutions of the the K-e turbulence model equations, and the advection/diffusion equation of the dye, are obtained by this fully Lagrangian method in the simulations of a shallow transverse shear flow where the advection process in certain region of the flow is dominant over the turbulent diffusion. The results are compared with the data available from a recent series of experimental investigations on the shallow shear flows. The Lagrangian method is positive definite, mass conserving and unconditionally stable. The successful simulations in both the advection and diffusion dominant regions of the flow ascertain the robustness of the method.

Radially spreading buoyant flows
Gregory A. Lawrence, Michael R. Maclatchy
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The nature of the radially spreading flow that forms after a vertical buoyant jet impinges on the surface has been investigated. Previous studies have provided conflicting conclusions as to whether or not an internal hydraulic jump forms in the radially spreading flow. Experiments by various investigators have been examined. The radial flow initially behaves like a buoyant surface jet whose thickness increases linearly due to the entrainment of ambient fluid into the jet via shear instabilities. Eventually, buoyancy forces dominate and these instabilities collapse leaving a radial buoyant plume of almost constant thickness. Visualisations of the experiments provide no compelling reason to postulate the existence of an internal hydraulic jump.

Entrainment studies towards the preservation (containment) of 'freshwaters' in the saline environment
Richard Burrows, Kamil H.M. Ali
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Flexible floating bottomless containment has recently been considered as a potential means of keeping freshwater separated from seawater in an estuary or coastal area. Speculative applications by the authors for marine located waste stabilisation ponds and for water quality management in salt intruded estuarial impoundments are cited herein.Viability assessment for such schemes calls for the study of, amongst other environmental disturbance factors, entrainment across the density-stratified interface between the freshwater and the saltwater flowing underneath. Experiments conducted with a horizontal wall-jet of saltwater discharging beneath a freshwater body retained by a pair of baffles, to create underflow in a two-layered stratified system, are the focus of attention in this paper. Results have been appraised against findings from the literature and implications to the proposed practical applications are discussed. In so doing, the form and structure of the interface between two stratified fluids has been investigated together with the rate of entrainment created by shear flow of the heavier saltwater underlayer.


Laboratory model studies of flushing of trapped saltwater from a blocked tidal estuary
Michael J. Coates, Yakun Guo and Peter A. Davies
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Results are presented from a series of laboratory model studies of the flushing of saline water from a partially- or fully-closed estuary. Experiments have been carried out to determine quantitatively the response of the trapped saline volume to fresh water flushing discharges Q for different values of the estuary bed slope á and the density difference (ñ)0 between the saline and fresh water. The trapped saline water forms a wedge within the estuary and for maintained steady discharges, flow visualisation and density profile data confirm that its response to the imposition of the freshwater purging flow occurs in two stages, namely (i) an initial phase characterised by intense shear-induced mixing at the nose of the wedge and (ii) a relatively quiescent second phase where the mixing is significantly reduced and the wedge is forced relatively slowly down and along the bed slope. Scalings based upon simple energy balance considerations are shown to be successful in (i) describing the time-dependent wedge behaviour and (ii) quantifying the proportion of input kinetic energy converted into increasing the potential energy of the wedge/river system. Measurements show that the asymptotic value of the energy conversion factor increases with increasing value of the river Froude number Fr0 at small values of Fr0, thereafter reaching a maximum value and a gradual decrease at the highest values of Fr0. Dimensional analysis considerations indicate that the normalised, time-dependent wedge position (xw)3(g )0/q2 can be represented empirically by a power-law relationship of the form (xw)[(g )0/q2]1/3 = C {(t)[(g )0 2/q]1/3}n, where the proportionality coefficient C is a function of both Fr0 and the slope angle á and the exponent n has a value of 0.24. Successful attempts are made to relate the model data to existing field observations from a microtidal estuary. Experiments with multiple, intermittent periodic flushing flows confirm the importance of the starting phase of each flushing event for the timedependent behaviour of the saline wedge after reaching equilibrium in the intervals between such events. For the parameter ranges investigated and for otherwise-identical external conditions, no significant differences are found in the position of the wedge between cases of sequential multiple flushing flows and steady single discharges of the same total duration.

Modelling and assessment of water quality indicators in a semi-enclosed shallow bay
Jianhua Tao and Qingxue Li, Roger A. Falconer and Binliang Lin
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Bohai Bay is a large, semi-enclosed shallow water basin located along the western region of the Bohai Sea in the north-eastern part of China. As a result of rapid economic development in the surrounding coastal hinterland, the water quality characteristics within Bohai Bay have been of growing concern in recent years. Details are given herein of an aquatic environmental assessment study undertaken to investigate the hydrodynamic and transport processes within the Bay using an integrated numerical modelling approach and field measurements. 3-D numerical model was used to simulate the tide flow and 2-D model was deployed with moving boundary to simulate the water quality. In order to improve on the water quality characteristics in the near-shore coastal zone, an ocean disposal plan was also considered, with various scenarios being simulated using the 2-D model. From an analysis of data collected from the field measurements, it was found that both chemical and phytoplankton indicators were important in the assessment of the overall water quality of the Bay. The occurrence of red tides was also found to be closely linked to the composition of phytoplankton community.

On the head-discharge relation of a ''duckbill'' elastomer check valve
Joseph H.W. Lee, David L. Wilkinson and Ian R. Wood
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Duckbill-shaped elastomer check valves are often installed on wastewater effluent diffusion systems and stormwater outfalls to prevent backflow or sea water intrusion. Unlike fixed diameter ports, the jet velocity varies nonlinearly with port discharge flow. A new theory to predict the hydraulic characteristics of a duckbill elastomer check valve (DBV) is presented, and it provides a theoretical basis for the correlation of all DBV hydraulic data.
It is shown that a DBV can be considered as a smooth converging nozzle. By modelling the duckbill valve as a two-dimensional rubber membrane, a theory is developed to relate the DBV material properties and flow conditions to the valve deformation. The valve opening area is predicted as a function of the driving pressure head; a linear head-discharge relation is predicted. The predictions are in excellent agreement with experimental data. The derived head-discharge relation can be readily incorporated into a diffuser internal hydraulics code to determine the variation of discharge flow and jet velocity along a multiport diffuser fitted with duckbill check valves.


Turbulent structures in partly vegetated open-channel flows with LDA and PIV measurements
Iehisa Nezu, Kouki Onitsuka
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The effects of vegetation growing in rivers in and at the edge of the water on turbulent structures and the associated sediment transport are not clarified yet, in spite of their importance for hydraulics and river engineering. In this study, turbulence measurements of open-channel flows with a vegetated zone at a half channel width were conducted by making use of both, a laser Doppler anemometer(LDA) and the particle-image velocimetry(PIV). It was found that the intensities of the secondary currents and the turbulent energy increase with an increase of the Froude number. The turbulence is advected spanwisely near the free surface by the secondary currents, which are generated by the anisotropy of turbulence. The horizontal vortices near the free surface are generated by the velocity inflectional instability, which increases with an increase of Froude number and the vegetation density. The bed (wall) shear stress in the vegetated zone was calculated by the Reynolds equation with the aid of the empirical drag coefficient.

On the motion of suspension thermals and particle swarms
J. Bühler, D. A. Papantoniou
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When a load dense particulate material is dumped from a barge, it descends in the water body as an irregularly shaped particle cloud. Provided that the particles are sufficienly small to be kept in suspension initially, the motion of the cloud is similar to that of a thermal of warm air rising in the atmosphere. Both types of buoyant clouds increase in size and slow down as they progress. For suspension clouds this thermal stage ends when their velocity has decreased to a value which is close to the individual settling velocity of the particles in a calm fluid, and the particles start falling out through the lower fringes of the cloud. After this transition is completed, a smooth, bowl-shaped particle swarm develops. An important difference between these two flow regimes is that in the thermal stage the fluid inside the cloud moves in unison with the particles, whilst the interstitial fluid in the swarm stage remains nearly motionless as the particles rain through it. In this paper we propose a relation for the growth rate and velocity of thermals which is also applicable in the swarm stage. The resulting description thus covers the entire range of flows which are of interest for barge dumping. Experimental results are presented which allow a determination of the relevant flow constants in the swarm stage.

The nature of rough-wall oscillatory boundary layers
R.V. Krstic, H.J.S. Fernando
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The nature of an oscillatory (turbulent) boundary layer over a roughwall was studied experimentally by oscillating a flat plate in an otherwise quiescent fluid. The velocity field was measured with respect to a fixed (laboratory) coordinate system and was converted to that relative to a coordinate system fixed to the plate, thus allowing to make inferences on oscillatory flows over rough surfaces such as the coastal ocean boundary layer. The flow visualization revealed that the boundary layer is fed with dipole-like vortex structures generated due to flow separation at roughness elements. The boundary-layer thickness was found to scale with the extent to which these vortex structures travel away from the wall. Enhanced turbulent intensities as well as vertical fluxes of horizontal momentum were observed in the boundary layer at phases conducive for vorticity generation (i.e. in the proximity of maximum flow velocities). At high Reynolds numbers, the turbulent length-scale measurements agreed well with a parameterization proposed by Grant and Madsen (1979). Eddy diffusivities based on Reynolds and total stresses, however, did not agree with available models and showed wide variability over an oscillating cycle.