| Editorial |
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| by WILLI H. HAGER |
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Vol: 44 / Issue: 6
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| Bed roughness induced entrainment in a high Richardson number underflow |
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| by ROCÍO LUZ FERNÁNDEZ AND JÖRG IMBERGER |
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Vol: 44 / Issue: 6
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| Field data, collected in a shallow underflow into a medium-sized reservoir, show that the dimensionless entrainment, E , is non-negligible at high Richardson numbers. For such underflow, the field data demonstrate that E is mainly dependent on the stress generated at the bottom, directly determined by the relative bed roughness and the properties of the current. The flow is shown to quickly reach a condition of normal flow where Ri is constant; the value of Ri being a strong function of the bottom slope. An entrainment law is presented, in terms of both Richardson number and bottom drag coefficient, written as E= E(Ri, CD) , that shows good agreement with the values of E measured in the field. |
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| 1-D modelling of conveyance, boundary shear and sediment transport in overbank flow |
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| by S. ATABAY AND D. W. KNIGHT |
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Vol: 44 / Issue: 6
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| Several sets of experimental data on conveyance capacity, the division of flow between a main river channel and its floodplains, boundary shear, and sediment transport in overbank flow are presented. The strengths and weaknesses of various 1-D modelling approaches are highlighted. The coherence method (COHM) of Ackers [J. Hydraul. Res. 31 (1993) 509] and the weighted divided channel method (WDCM) of Lambert and Myers [Proc. Inst. Civil Engineers Water Maritime Energy 130 (1998) 84] are shown to be two useful 1-D methods for dealing with overbank flows. They are relatively simple to apply, and give not only the stage–discharge relationship, but also the division of flow between the main channel and the floodplains. Their general validity has been extended by testing the two methods against data sets other than those used in their original formulation. A modified version of the WDCM is presented that gives better predictions for homogeneously roughened channels. It is shown that the WDCM does not give good predictions for heterogeneously roughened compound channels. A comparison is made between the COHM and WDCM concerning their relative accuracy and ease of application for conveyance and boundary shear calculations. Finally, the COHM is applied to some mobile boundary compound channel data and shows that once the total and zonal channel discharges are calculated correctly, the sediment transport concentration may be predicted reasonably well for high discharges but not so well for low discharges and shallow floodplain depths, unless the boundary shear stress on the main channel bed is specified or calculated. |
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| An explicit two-dimensional finite element model to simulate short- and long-term bed evolution in alluvial rivers |
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| by R. GARCÍA-MARTINEZ, R. ESPINOZA, E. VALERA AND M. GONZÁLEZ |
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Vol: 44 / Issue: 6
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| This paper presents a two-dimensional model to solve the vertically averaged shallow water and sediment transport equations for short- and long-term simulations. The model implements a four-time-step explicit scheme that permits longer time steps than previous explicit finite element models. Also, an algorithm is incorporated that decreases considerably computer times for long-term simulations while preserving the advantages of explicit schemes. The model results compare very well with analytical solutions for simplified one- and two-dimensional problems. A 5-year simulation for the Apure River in Venezuela, demonstrate the ability of the model to predict long-term evolution for large rivers in a computational efficient manner. The resulting bed evolution patterns compare well to the observed behavior of control structures used in similar applications. |
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| Discharge prediction in compound channels by end depth method |
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| by SUBHASISH DEY AND MARTIN F. LAMBERT |
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Vol: 44 / Issue: 6
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| Experimental observations showed that in a compound channel, the free surface elevation transverse to the flow direction in the main channel is different to that in the floodplain in the vicinity of the end section. The non-horizontal transverse free surface profile (sagging at the main channel zone) is due to the differential accelerated flow velocities in the main channel and the floodplains. In this paper, free overfall in compound channels is theoretically modeled, considering vertical subdivisions of flow area into three zones: main channel, left floodplain, and right floodplain. The Froude number in a compound channel should be estimated on the basis of the flow in subdivided zones rather than for the whole cross-section. The finding that the critical depth in the main channel and the floodplains do not coincide has important implications for the determination of the Froude number in compound channels. Applying a momentum equation based on the Boussinesq approximation, equations of end depth that are related to the discharge are developed separately for the subdivided zones. A method to estimate the discharge from the end depths in the main channel and the floodplain is presented through computational steps. The streamwise flow profiles in the main channel and the floodplain, upstream of the end section, are computed using the streamline curvature at free surface. Experiments were conducted in three different shaped symmetrical compound channels to verify the model. The results, obtained using the model, agree satisfactorily with the experimental data. |
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| Computation of flow velocity in rough channels |
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| by DONATELLA TERMINIANDMASSIMO GRECO |
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Vol: 44 / Issue: 6
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| Accurate estimate of flow-velocity profile is of crucial importance both for scientific purposes and for solving numerous engineering problems that include, among others, sediment transport, contaminant transport, flow resistance evaluation. This paper presents a new empirical equation to represent the vertical velocity profile. The proposed equation is essentially a modified form of the well-known logarithm law of the wall and contains three parameters having a clear physical meaning. The applicability of the equation and its accuracy assessment for different hydraulic conditions, including non-uniform conditions, is verified by using experimental data obtained by different sources. The values of the mean square errors determined by the best-fitting of the law to the selected experimental data, are compared with those obtained by applying other equations proposed in literature. The comparison highlights that the proposed law allows the reproduction of all the hydraulic conditions examined maintaining an acceptable value of the mean square error. |
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| Depth-integrated flow modelling taking into account bottom curvature |
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| by B.J. DEWALS, S. ERPICUM, P. ARCHAMBEAU, S. DETREMBLEUR AND M. PIROTTON |
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Vol: 44 / Issue: 6
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| Successfully modelling flows over a spillway and on strongly vertically curved bottoms is a challenge for any depth-integrated model. This type of computation requires the use of axes properly inclined along the mean flow direction in the vertical plane and a modelling of curvature effects. The proposed generalized model performs such computations by means of suitable curvilinear coordinates in the vertical plane, leading to a fully integrated approach. This means that the flows in the upstream reservoir, on the spillway, in the stilling basin and in the downstream river reach are all handled in a single simulation. The velocity profile is generalized in comparison with the uniform one usually assumed in the classical shallow water equations. The pressure distribution is modified as a function of the bottom curvature and is thus not purely hydrostatic. Representative test cases, as well as the application of the extended model to the design of a large hydraulic structure in Belgium, lead to satisfactory validation results |
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| Experimental analysis of the segregation of dry avalanches and implications for debris flows |
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| by BARBARA ZANUTTIGH AND ALICE DI PAOLO |
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Vol: 44 / Issue: 6
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| The objectives of this contribution are to analyse and quantify the segregation process of dry granular avalanches, both parallel and perpendicular to the flow. Experiments were carried out using two dry granular mixtures, a steep chute of variable length and a collecting bin divided by transversal and longitudinal sectors, in which the flowing material falls down at the chute outlet. The free-falling of the flow in the segregator bin, the filling in time of the bin itself, the grain-size composition of the bin content with varying the distance from the chute outlet and axis are examined by image analysis and by sieving each bin cell. Segregation is very effective and rapid along flow direction whereas appears to be substantially negligible perpendicularly to the flow. The rapidity of flow segregation can be directly related to flow depth, whereas effects of mixture grain-size composition seems not appreciable. The longitudinal grain-size composition profile in time is reconstructed, showing coarser particles concentrated at the flow front and on the flow surface and fine fractions prevailing at the tail and at the bottom. Possible extension of these results to debris flows are discussed considering scale problems and comparing the results with other available experimental works and with field observations of natural events. |
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| Oblique impingement of circular water jets on a plane boundary |
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| by A JALIL AND N. RAJARATNAM |
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Vol: 44 / Issue: 6
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| This paper presents the results of an experimental study on the oblique impingement of turbulent water jets of diameter d equal to 50.8, 101.6 and 152.4 mm, placed above a horizontal plate at vertical distances varying from 45 to 255 mm and at impingement angles of θ with the plate equal to 15º, 30º , 45º, 60º and 75 , with the velocity of the jet in the approximate range of 3–10 m/s and the jet Reynolds number varying from 380,000 to 720,000. The deflected jet was mostly in the forward direction with very little backward flow for θ < 45º , which increased to an estimated value of about 5–10% for larger angles of impingement. The forward flow in the deflected jet or stream was in the supercritical state. The thickness profile of the deflected jet in the centerplane was similar for all the experiments if plotted in terms of a suitable scale which was either the thickness at the normalized distance from the impingement point x/d =1.0 or 5.0, for x/d up to about 20. These scales in terms of the jet diameter varied mainly with the angle of jet impingement. The transverse thickness profiles in the deflected jet were also found to be similar. The velocity profiles in the boundary layer of the deflected jet were similar and its thickness d remained constant for x/d up to about 7.5 as in the case of stagnation flows. The boundary shear stress t0 appeared to increase first with the distance from the impingement point and then decrease as in the case of the impingement of submerged impinging jets |
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| A simple mathematical model to predict the particle size distribution of the armour layer |
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| by R. J. GARDE, ADITYA SAHAY AND SHEKHAR BHATNAGAR |
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Vol: 44 / Issue: 6
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| A simplified method to predict the particle size distribution of the armour coat in the case of parallel degradation has been proposed. Using all available data, the particle size distribution of the armour layer was found to follow a normal probability distribution. A mathematical formulation to predict the median size of the armour coat has also been given. Some of the earlier methods for the prediction of the same have been studied and their accuracy has been compared to that of the proposed method. The time evolution of the river bed armouring process has also been investigated and the findings discussed. |
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| Prediction of downpull on closing high head gates |
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| by ISMAIL AYDIN, ILKER T. TELCI AND ONUR DUNDAR |
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Vol: 44 / Issue: 6
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| Downpull on tunnel gates installed in the intake structure of a hydroelectric power plant was studied experimentally using a hydraulic model. The pressure distribution on the gate lip surface was measured, and the lip downpull was evaluated by surface-area integration of the measured pressure distribution. An easy to use lip downpull coefficient was defined as a function of the lip angle and gate opening. The lip downpull coefficient function is linked to a one-dimensional mathematical model of unsteady flow in the intake-penstock system. The model is based on the integral energy and continuity equations. Overflow through the gate spacings is also included in the model to compute the water level in the gate shaft and to evaluate the downpull component on the top face of the gate. Time-dependent calculation of the total downpull force acting on a closing gate is exemplified. The total downpull is also measured by the direct weighing method for fixed and closing gates. Predictions of the mathematical model compare favorably with the downpull obtained from the direct weighing method. |
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| Resistance coefficient in a smooth concentric annular pipe |
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| by MICHELE MOSSA |
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Vol: 44 / Issue: 6
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| This paper reports an experimental study aimed at determining turbulence and resistance coefficient characteristics of the flowfield in smooth concentric annular conduits. The measurements of velocity field were performed with a laser Doppler anemometer system in the flow region between the inner wall and the radius of maximum velocity (inner region). Velocity profiles along a vertical line normal to the walls are represented in terms of law of the wall. The maximum mean local velocity occurs at positions in agreement with literature formulae. For different distances from the inner wall micro and integral scales of turbulence were calculated. The results show that in flows with high Reynolds numbers small eddies tend to assume greater frequencies and that the time interval over which fluctuating velocity is correlated with itself becomes smaller. A statistical description of turbulence is also reported. The expression of the inner resistance coefficient was obtained by integrating the velocity law. The calculated resistance coefficients are in fair agreement with the experimental ones. Both are always greater than the corresponding values for circular conduits. |
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| Prediction of contaminant plumes (shapes, spatial moments and macrodispersion) in aquifers with insufficient geological information |
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| by AMRO M. M. ELFEKI |
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Vol: 44 / Issue: 6
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| The first Macrodispersion Experiment (MADE1) at Columbus Air Force Base in northern Mississippi is utilized to perform numerical simulations of solute transport in an aquifer. The purpose is to illustrate the capability of the coupled Markov chain (CMC) model in delineating the complex geometrical configuration at the site for solute transport simulations under the lack of geological information. The CMC model is also used to study the effect of reducing geological information on the transport predictions in terms of plume configuration, first and second spatial moments and macrodispersion. The results show the power of the coupled Markov chain methodology in delineating aquifer heterogeneity at the site. Conditional simulations on 16, 9 and 6 boreholes show reasonably the same plume behavior in terms of average longitudinal and vertical extensions, especially in the far-field; 9 boreholes seem to provide practically acceptable results in terms of the global plume shape. This indicates more reliability on the use of the CMC model for subsurface characterization. Comparison of CMC model results, in terms of aquifer characterization, with the model used by Eggleston and Rojstaczer [Water Resour. Res. 34 (1998) 2155] (polynomial regression trend, Kalman filter trend, hydrofacies trend and Kriging) shows that the CMC model behaves better. The CMC model conditioned on 9 and 16 boreholes with mid-range conductivities for each lithology captures both the plume shape and the observed plume spatial moments at the MADE site. |
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| Discussion: An experimental investigation of jet flow on a stepped chute |
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| by M.K. EL-KAMASH, M.R. LOEWEN and N. RAJARATNAM, Journal of Hydraulic Research, Volume 43, 2005, Issue 2, pp. 31–43 |
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Vol: 44 / Issue: 6
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| MASAYUKI TAKAHASHI, YOUICHIYASUDA ANDIWAO OHTSU |
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| Papers Published in Associated Journals |
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| by JHR |
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Vol: 44 / Issue: 6
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