ISSUE NO. 1.
Two-dimensional numerical modelling of flushing processes in water
reservoirs
Nils R. B. Olsen
This study describes a two-dimensional numerical model simulating flushing
of sediments from water reservoirs. The numerical model solves the depth-averaged
Navier-Stokes equations on a two-dimensional grid. A zero-equation turbulence
model is used. The resulting flow field is extrapolated to three dimensions,
and the convection-diffusion equation for the sediment concentration is
solved. A formula for the bed concentration is used as boundary condition,
resulting in a calculation of bed material load. Continuity for the cells
closest to the bed is used to find the bed changes. The pressure field
is used to calculate the location of the water surface. The grid is adaptive
in the vertical direction, and changes according to the calculated water
and bed levels. A porosity model is used to simulate the process when
the water surface drops under the bed level at some locations of the geometry.
The results from the numerical model were compared with data from physical
model studies. The main features of the erosion pattern were reproduced,
and the deviation between the calculated and measured scour volume was
small. This showed that most of the simplifications made in the numerical
model were reasonable. There were deviations between measured and calculated
bed levels in a bend. This was most likely due to secondary currents that
were not possible to model using a two-dimensional approach.
Experimental study on scour rate and channel bed inertia
ZhaoYin Wang
River bed scour is studied in the paper. Two types of scour are identified,
namely local scour and fluvial bed scour. Fluvial bed scour, which occurs
if the flow carries sediment load less than its capacity, can be further
classified according to the causes as flood scour, downstream reservoir
scour, hyperconcentrated flow scour and channelized river scour. The rate
of scour is a key problem for understanding of scour process. More than
100 experiments with clear water flow and 33 experiments with sediment-laden
flows were conducted to investigate the laws of scour rate. A scour rate
formula, the first of its kind, is proposed in the paper, which is proved
valid for unsteady flows. In sediment-laden flow the scour rate reduces
in an exponential law following increase in the ratio of incoming rate
of sediment load to the difference between the capacity and load of the
flow. River bed inertia is a new concept of morphological dynamics which
represents an important property of the river bed. The inertia of various
bed compositions are calculated and the results illustrated that the inertia
increases with sorting coefficient of bed materials. By employing the
bed inertia and the Exner equation the sediment transport rate in the
scour process is theoretically solved. The scour rate under various flow
and incoming sediment load conditions can also be calculated from bed
inertia. Calculated scour rates for natural rivers agree well with the
measured data. A method for calculating maximum scoured depth of river
bed during flood is suggested for hydraulic engineers to design safety
depth of hydraulic works and pipelines across rivers.
Critical tractive stress of non-uniform sediments
P.L. Patel, K.G. Ranga Raju
Experiments on initiation of motion and rate of bed load transport of
different fractions in case of non-uniform sediments are reported. Analysis
of these data as well as those from earlier studies by other investigators
has shown some limitations of the available relationships for critical
tractive stress (CTS) of sediment mixtures. Further analysis of the data
has resulted in a new relationship for CTS in terms of the size of the
particular fraction in question (d ), the geometric mean size (d ) and
the geometric standard deviation (å ) of the mixture. An empirical
relationship has been obtained for the critical tractive stress of the
size d defined as d å .
Three-dimensional Structure of Space-time Correlation on Coherent
Vortices Generated behind Dune Crest
I. Nezu, A. Kadota
Large-scale coherent vortices such as separated vortices and kolk-boils
are often observed and generated behind crests of sand dunes in fluvial
rivers. The kolk-boil vortex greatly affects transport of suspended sediment
which develops up to the free surface. In the present study, both visualisation
by dye-injection technique and turbulence measurements with a laser Doppler
anemometer were conducted in order that the elemental structure of vortices
could be clarified. 3-D space-time structures of the coherent vortices
are discussed by evaluating conventional correlation coefficients and
also by using conditional sampling techniques. Instantaneous velocity-velocity
combinations at different points were simultaneously measured by means
of two sets of laser Doppler anemometers and an automatic traversing system.
Refined physical models of coherent vortices are proposed in the present
study.
Applications of artificial neural networks to the generation of wave
equations from hydraulic data
Yonas B. Dibike, Anthony W. Minns, Michael, B. Abbott
In a new design for computational hydraulic engines, the hydraulic knowledge
itself is encapsulated in forms that are sub-symbolic. In this study,
artificial neural networks are envisaged for this purpose. It is shown
that in the case of simple wave equations the trained ANNs can reinstate
the governing partial differential equations, and so thereby that they
contain the same knowledge, or have the same semantic content. Besides
raising confidence in the capabilities of ANNs in the future engines,
this study opens up another way to mine data for knowledge discovery.
On gradually varied flow profiles in rectangular open-channels
J.R. Dubin
This technical note presents the equation for one-dimensional gradually-varied
steady flow profiles with constant discharge, in a rectangular-section
prismatic channel of constant roughness and small longitudinal slope.
The flow profile differential equation was transformed by means of a
polynomial approximation and then, by an algebraic procedure followed
by the integration of the equation, the solution was obtained.
This solution was applied to a particular case which, through a comparative
analysis, allowed to verify a satisfactory accuracy.
The solution obtained here, though rather complex from an algebraic point
of view, is easily programmable; its major merit consists in its contribution
to the mathematical knowledge of flow profiles.
Fluid-dynamic feed-back in shear layer oscillation below a submerged
sluice gate
E.B. Shuy, H.C. Chua
The phenomenon of fluid-dynamic feed-back, which leads to organised periodic
vortex shedding and oscillation of the free shear layer below a submerged
sluice gate, is investigated. The feedback mechanism occurs through the
impingement of the vortices onto a second parallel sluice gate installed
at some distance downstream. The pressure fluctuations at the point of
impingement are fed back upstream along the shear layer to the sensitive
region of flow separation near the lip of the sluice gate. This ampifies
the instability and enhances the vortex formation at the source of instability.
The process of vortex impingement, upstream propagnation of the distubances,
and the incitement of the velocity field at the vortex formation process.
Over the present test range of 30 < Lc/Q 0 < 170, and 2,000 <
ReQ < 10,000, fluid-dynamic feedback was observed to occur within a
Strouhal number range of 0.005 < SQ < 0.025, where ReQ and SQ are
the Reynolds and Strouhal numbers defined in terms of the initial momentum
layer Q 0, and Lc the impingement length. The present results agree well
with those of previous studies, as well as that predicted from linear
stability analysis.
Modeling of flows between two consecutive reverse curves
A. Ghanmi
The objective of this paper is to model the behavior of secondary flows
between two successive and reverse curves. The influence of different
hydraulic and geometric parameters on the secondary current damping was
also analysed. A three-dimensional finite element model has been developed
based on Navier-Stokes equations assuming fluid incompressibility and
hydrostatic pressure. An eddy viscosity formulation based on the mixing
lenght concept has been introduced to correctly reproduce velocity profiles
of turbulent flows. To validate the numerical model,m velocity measurements
have been conducted in the laboratory using two identical 90o curves separated
by a straight reach. The predicted velocities have been compared with
the experimental velocities measured at different channel sections. The
validity of the numerical model formulation was confirmed by the laboratory
work.
ISSUE NO.2.
On the encapsulation of numerical-hydraulic models in artificial
neural network
Y.B. Dibike, D. Solomatine, M.B. Abbott
The optimal control of hydraulic networks often necessitates making a
considerable number of very rapid simulations of flows, such as is not
practical using existing, computationally-demanding, numerical-hydraulic
models. However, the site-specific knowledge and data that is encapsulated
in any such numerical model can be encapsulated in its turn in an artificial
neural network (ANN), and this can provide much faster simulations. In
this study, a number of possible types and configurations of ANNs are
investigated for their suitability to this class of application. When
regarded from a hydroinformatics point of view, this study becomes one
of identifying the most suitable ANN encapsulations of numerical-hydraulic
encapsulations of generic hydraulic knowledge and site-specific data.
Three Dimensional Nonlinear Hydrodynamic Presssures by Earthquakes
on Dam faces with Arbitrary Reservoir Shapes
B-F. Chen, Y-S. Yuan, J-F. Lee
A complete three-dimensional finite difference scheme has been developed
to analyze the earthquake-induced non-linear hydrodynamic pressures on
inclined dam faces with arbitrary reservoir shapes. Both the free surface
waves and the non-linear convective acceleration was included in the analysis.
Numerical experiments have been made to determine the desirable mesh arrangements
and time increments. The computational accuracy were assured by checking
both mass and momentum balance at each time step. The effects of the arbitrary
dam-reservoir system on dam hydrodynamics are systematically studied by
analyzing the dam-reservoir system with various dam shapes, sloped reservoir
banks and uneven reservoir bottoms. For a rectangular reservoir and excited
by x-component ground acceleration, the three-dimensional analysis can
be simplified by a two-dimensional analysis and the results are independent
of the reservoir width. By arbitrary acting direction of ground motion,
the fluid near the abutment is excited both by x-component and z-component
of ground acceleration, the combination of the two-way action results
in the variation of the hydrodynamic pressure distribution on dam face.
The dynamic effects on the hydrodynamic pressure increase as the reservoir
width does, but this increase becomes negligible when the reservoir width
is larger than four times of water depth. The compressibility of water
is important in dam-hydrodynamic analysis, but it is truly problem dependent.
Three-dimensional numerical modelling of bed changes in a sand trap
N.R.B. Olsen, H.M. Kjellesvig
Water and sediment flow was modelled numerically in three dimensions
in a tunnel-type sand trap. The numerical model solved the transient Navier-Stokes
equations with the k-e turbulence model. Simultaneously, the sediment
flow was calculated by solving the transient convection-diffusion equation
for sediment concentration. Multiple sediment sizes were used. Van Rijn's
formula for sediment concentration was used as boundary condition for
the bed. Sediment continuity for the cells close to the bed gave the bed
changes with time. An adaptive grid was used, adjusting to the bed changes.
The processes in the sand trap involved both sedimentation and erosion,
as a large volume of sand were formed and moved through the geometry.
A physical model test was carried out to verify the numerical model. Bed
levels were measured at a longitudinal profile and a cross-section. The
results from the physical and numerical model compared well.
A theoretical analysis of discontinuous flow with mobile bed.
J. Sieben
The non-linear mathematical model for depth-averaged flow allows for
discontinuous solutions that correspond with the dynamic behaviour of
rapid changes in flow. Such discontinuities, that can fundamentally affect
the predicted morphological response of a river, are analyzed theoretically
and numerically. Hereto, the entropy conditions as defined by Lax and
the Rankine-Hugoniot or shock relations are used to analyze propagation
rates and stability. Apart from sub-critical and supercritical flows,
a transition regime can be identified for flows with mobile beds.
First-Order Estimation of Stochastic Parameters of a Sediment Transport
Model
F-C. Wu, H.W. Shen
First-order approximation techniques for estimating stochastic parameters
of a sediment transport model are presented. The non-homogeneous compound
Poisson model of Shen-Todorovic eliminating certain idealized assumptions
to describe the movement of sediment in natural streams is a revision
of the earlier homogeneous model of Einstein-Hubbell-Sayre. However, the
complexity of the non-homogeneous model and the difficulty in determining
the model parameters has limited its application. The proposed approximation
techniques employ the first-order Taylor expansions, with respect to a
selected temporal or spatial point by a finite difference, of the cumulative
probability distribution function (CDF) of particle displacements. The
first-order expansions are divided by the original CDF for further simplification.
The simplified forward- and backward-expansions are numerically solved
as a system to evaluate the parameter at the specified point. The non-homogeneous
parameters are pursued with successive applications of this procedure
to various points. An example of sediment infiltration into the gravel
column is provided showing the procedures of parameter estimation and
the verification of results. Temporal and spatial variations of the parameters
are also discussed.
High concentration granular shear flow
W.C. Mih
An equation for high concentration granular flow is presented. The equation
contains viscous and impact coefficients, which have been determined.
The viscous coefficient is a constant. The impact coefficient has been
correlated to properties of the solid and fluid. The equation agrees reasonably
well with several sets of experiments by different investigators covering
a wide range of granular flows.
Concentration field of multiple circular turbulent jets
J.E. Hodgson, A.K. Moawad, N. Rajaratnam,
The Reichardt hypothesis on the lateral transport of momentum has been
used previously to predict the velocity field of multiple turbulent jets.
In this paper, based on an extension of the Reichardt hypothesis to the
lateral transport of pollutant, a method has been developed to predict
the concentration field of multiple circular turbulent jets discharged
into a large stagnant ambient. Experimental observations from one experiment
with three parallel jets support the theoretical predictions.
Layer-Averaged Modeling of Two-Dimensional Turbidity Currents with
a Dissipative-Galerkin Finite Element Method Part II: Sensitivity Analysis
and Experimental Verification
S-U. Choi
Choi (1998) proposed a finite element model for the simulation of turbidity
currents spreading two-dimensionally in deep ambient water. The dissipative-Galerkin
formulation was used for the hyperbolic feature of the governing equations.
Accurate front tracking was accomplished by employing the deforming grid
generation technique. In the present paper, mass conservative property
of the two-dimensional numerical method is tested in the case of a buoyancy-conserving
saline current. Sensitivity analyses are also performed to see the model
response to such parameters as total buoyancy flux, inlet velocity, flow
resistance coefficient, slope, and sediment particle. Finally, laboratory
experiments are conducted by generating saline density currents for the
verification of the numerical model. The experimental data are obtained
with an emphasis on bulk characteristics of the density current such as
propagating pattern, longitudinal and lateral spreading distances, and
opening angle. The observed result appears to be in a good agreement with
the computed solution, which suggests that the developed numerical model
is capable of simulating the turbidity current propagating on a slope
with no lateral restriction.
ISSUE NO. 3,
Riprap at bridge piers
C. A. Duarte, J. A. Sáinz.
The influence that protective riprap have on the scour holes that develop
near bridge piers has been studied in clear water conditions with circular
and rectangular piers. Non-dimensional graphs, relating the characteristic
dimensions of the scour holes with the riprap elevation above bed level
and its width, are presented. Also, the necessary minimum width of riprap,
deduced from tests, is compared to criteria proposed by various authors.
Grain and Form Resistance in Gravel-Bed Rivers
Robert G. Millar
Grain and form resistances for bankfull and near bankfull flows have
been determined for 176 gravel reaches using data compiled from several
published sources. Partitioning grain and form resistance is based on
the original definition of ks adopted by Nikuradse (1933). Grain resistance
is calculated with the Keulegan equation assuming a grain roughness height
equal to D50. This defines a lower bound to the observed flow resistance.
Form resistance, which includes grain protrusion, pebble clusters, dunes,
bars and pool-riffle sequences, is shown to be significant at bankfull
flow. Computed form resistance comprises up to 90% of the total. No predictive
relation for form resistance has yet been developed. The results have
implications for river restoration efforts that include the reintroduction
of pool-riffle sequences into degraded or channelised rivers, and indicate
that constructed pool-riffle sequences would have a significant effect
on flood levels.
A theoretical analysis on armouring of river beds.
J. Sieben
In the modelling of an armouring river bed, usually numerical models
are required. To enable a quick analysis of different parameters a simplified
analytical model is suggested. The limiting assumptions of this model
are uniformity in conditions of flow and uniformity in conditions of morphology.
Comparison with experiments indicate a fair correspondence between the
observations and the predictions of changes and equilibrium conditions.
A Two-dimensional Computational Model to Simulate Suspended Sediment
Transport and Bed Changes
Reinaldo García-Martínez, Iván Saavedra C., Beatriz
Febres de Power, Eduardo Valera, Carlos Villoria
This article presents a two-dimensional finite element mathematical model
to simulate suspended sediment transport in coastal regions. Sediment
transport is formulated in terms of a hydrodynamic model plus a convection-diffusion
equation with source and sink terms representing the erosion and deposition
processes. Test problems treating the evolution of a hypothetical dredged
trench in a channel, perpendicular to the main flow direction, are used
to evaluate the sensitivity of the model to empirical parameters.
The model was calibrated, validated and later applied to predict the circulation
patterns and bottom bed evolution in Puerto Miranda oil terminal, located
in the Maracaibo Strait, Venezuela. Results show that the model is able
to predict the bed evolution of the dredged access channels to the terminal.
Notwithstanding the complexity of the sediment transport phenomena that
occur in dredged trenches, the relatively simple model proposed herein
is capable of giving useful results in practical problems.
Large-eddy simulation of flow in a rectangular open channel
J.shi, T.G. Thomas, J.J.R.Williams
Turbulent flow in a narrow open channel is investigated using the large-eddy
simulation (LES) technique in which the surface is allowed to freely deform.
A relatively large Reynolds number of 90,400 (defined as 4RUmean/) is
used in the simulation and it is shown that good resolution is required
in order to reproduce the effects of the turbulence-driven secondary currents.
The simulated results are in good agreement with experimental measurements
and previous calculations.
Large flow structures in a turbulent open channel flow
Aldo Tamburrino, John S. Gulliver
The outer region flow structure in an open channel flow is studied by
means of flow visualization and velocity measurements. The structure can
be thought of as formed by large streamwise vortices scaling with the
flow depth, creating upwelling and downwelling motions at the free surface.
Associated with the downwelling motion induced by these vortices are regions
with higher longitudinal velocity on the free surface. Similarly, "boils"
and eddies with a vertical axis are detected in the zones corresponding
to the upwelling. Three temporal components are used to analyze velocity
measurements: a temporal mean value, a component associated with slow
fluctuations, and a third component associated with fast fluctuations.
It is shown that the contribution of fast fluctuations to the turbulent
intensities is important near the wall, at z+ < 1000. Slow fluctuations
are the primary contributors in the remainder of the boundary layer’s
outer region. It is proposed that the large streamwise vortices, a manifestation
of the largest turbulence scales, are a major contributor to slow fluctuations.
A geometrical method for computing the distribution of boundary shear
stress across irregular straight open channels
Saeed R. Khodashenas and André Paquier
A method called Merged Perpendicular Method (M.P.M.)has been developed
to compute the distribution of boundary shear stress across irregular
straight channels. In a concave angle of a cross section, computed shear
stress is lower than in a convex angle. Comparison with experimental data
has shown that the new method calculates more accurate shear stress than
other geometrical methods.
Finite difference scheme for longitudinal dispersion in open channels
Z. Ahmad, U.C. Kothyari, K.G. Ranga Raju
The available analytical and numerical solutions of the equation for
longitudinal dispersion in open channels are limited to uniform flows.
Presented in this paper is a solution technique based on combined operator
approach where advection and diffusion processes of longitudinal dispersion
equation are treated concurrently in non-uniform flows. A variable size
spatial grid based finite difference solution of the advection process
has been obtained by developing a variable spatial grid so that the root
of the trajectory of the concentration characteristic passes through the
computational nodes. For solution of the diffusion process, Crank-Nicholson
scheme has been used. To eliminate the possibility of numerical oscillations,
weighting coefficient has been introduced to the pollutant concentration
in the time stepping. Proof-of-the-concept tests have been made using
the existing numerical and analytical solutions as the basis. The model
has been extended by incorporating in it the one-dimensional grid search
method for determination of DL values using observed temporal variation
of concentration (C-t curves) at two or more stations. Finally, a procedure
of computing the C-t curves at downstream locations is presented in the
paper.
Application of an acoustic particle flux profiler in particle-laden
open-channel flow
Shen C. and U. Lemmin
An acoustic particle flux profiler (APFP) has been used to non-intrusively
and simultaneously profile the instantaneous concentration and the 2-D
velocity vector in particle-laden open-channel flow. A temporal resolution
of and a spatial resolution of have been achieved by carefully choosing
the APFP parameters. The performance of the APFP has been investigated
in two flows under capacity charge conditions. The measured results for
the mean profiles of velocity, velocity variance, Reynolds stress, concentration,
particle flux, moment flux, diffusion coefficients, etc., are compared
with the theoretical or semi-empirical formulas. Good agreement has been
found. The fluctuation fields of velocity and concentration are highly
correlated. Two kinds of coherent structures, the ejection and the inrush,
are found to be the dominant structures in the outer region which are
responsible for particle entrainment, resuspension and deposition in a
particle-laden flow.
It is concluded that the recently developed APFP is a powerful non-intrusive
instrument for sediment transport studies in open-channel flow and that
it works well under capacity charge conditions where other instruments
have difficulties. It provides rich information on the details of flow
structures and of particle-turbulence interactions.
ISSUE NO. 4,
Application of an acoustic particle flux profiler in particle-laden
open-channel flow
Shen C. and U. Lemmin
An acoustic particle flux profiler (APFP) has been used to non-intrusively
and simultaneously profile the instantaneous concentration and the 2-D
velocity vector in particle-laden open-channel flow. A temporal resolution
of and a spatial resolution of have been achieved by carefully choosing
the APFP parameters. The performance of the APFP has been investigated
in two flows under capacity charge conditions. The measured results for
the mean profiles of velocity, velocity variance, Reynolds stress, concentration,
particle flux, moment flux, diffusion coefficients, etc., are compared
with the theoretical or semi-empirical formulas. Good agreement has been
found. The fluctuation fields of velocity and concentration are highly
correlated. Two kinds of coherent structures, the ejection and the inrush,
are found to be the dominant structures in the outer region which are
responsible for particle entrainment, resuspension and deposition in a
particle-laden flow.
It is concluded that the recently developed APFP is a powerful non-intrusive
instrument for sediment transport studies in open-channel flow and that
it works well under capacity charge conditions where other instruments
have difficulties. It provides rich information on the details of flow
structures and of particle-turbulence interactions.
Application Of Artificial Neural Networks To The Simulation Of A
Two Dimensional Flow
Y.B. Dibike, M.B. Abbott
The practice of numerical simulation of flows and other processes occurring
in water has now matured into an established and efficient part of hydraulics.
At the same time, however, the models themselves often become very extended.
In many situations, given the divergence between the response-time requirements
and the computational-time requirements of numerical models, the need
arises to reduce the time needed to simulate the impact of given input
events on hydraulics systems. In this study the possibility of using systems
composed of agents consisting only of artificial neural networks (ANNs)
as modelling tools for the simulation of tidal flow in a two-dimensional
flow field is investigated. In particular this involves the modelling
of a process that evolves in time and the ANNs themselves function as
non-linear dynamic systems that effectively reproduce the behaviour of
the fluid at any one place and any one time from the behavior at other
places at earlier times. Different types of ANN-agent architectures are
investigated in order to asses their ability and relative performance
in encapsulating the site-specific knowledge and data necessary to reproduce
the temporal sequence of states observed in a modelled area.
On the oscillating characteristics of hydraulic jumps
M. Mossa
This paper investigates oscillating characteristics and cyclic mechanisms
in several hydraulic jumps, with experimental study made of the hydraulic
jumps under two periodically repeated flow conditions. The analysis shows
that the vortex roll-up process is linked to fluctuations of the longitudinal
location of the jump toe. The paper includes comments on those few works
in literature which deal with the subject. Measurements were made of the
time interval during which each type of hydraulic jump was present along
with the surface profile elevations downstream of the roller. Results,
oscillating characteristics and cyclic mechanisms are in agreement with
the new concept of turbulence. Indeed, it has become increasingly evident
that the organized periodic motion is superimposed on a background of
random turbulence, with many flows containing eddies, vortices whose description
is more influential than previously thought.
An investigation of the suitability of two-dimensional mathematical
models for predicting sand deposition in dredged trenches across estuaries
C. T. Mead
Two-dimensional-horizontal and two-dimensional-vertical mathematical
models of flow and suspended sand transport are used in typical engineering
predictions of deposition in a dredged trench across an estuary. The results
of the models are compared, and it is found that the sedimentation predictions
are fundamentally dependent on the specification of mobile bed sand, and
that the predictions of the two types of model are qualitatively different.
This provides insight into the benefits of both modelling techniques,
and indicates a need for further model development.
Velocity and turbulence distributions in combined wave-current flows
over a rippled bed
F. Marin
This paper describes an experimental programme carried out in a laboratory
channel, to investigate the velocity distributions over a rippled bed
in combined wave-current flows. In this study, waves propagate against
a turbulent current. Velocity measurements were made with a laser-Doppler
anemometer. The Nikuradse roughness length ks of the bed has been estimated
from the traditional logarithmic velocity profile for current alone. The
most significant effect of the superposition of waves on a current is
the increase of the apparent roughness of the bed with increasing wave
height. This effect is well described by Sleath’s [49] model. As
for rough flat beds, the turbulence intensity over ripples varies inversely
with height at sufficiently large distance from the bed for wave-dominated
flows.
Two-Phase Formulation of Suspended Sediment Transport
B. P. Greimann1, M. Muste, F M. Holly Jr.
Using a two-phase formulation, the vertical and horizontal momentum equations
for sediment are used to obtain the concentration and velocity profiles
of a dilute suspension of particles in a 2D uniform flow. Assuming the
form of the vertical turbulent intensities and dilute concentrations of
sediment, one can solve the equations analytically and compare them with
experimental data. No empirical coefficients in the model are tuned to
match individual experiments, for which the experimental data cover a
large range of particle sizes and densities. The models are shown to accurately
predict two experimentally observed but theoretically unexplained phenomena:
the increased diffusive flux of large particles, and the measurable velocity
lag of particles. The increased diffusion of large particles is shown
to originate from the added diffusive nature of the sediment’s Reynolds
stresses. The horizontal velocity lag of particles is due to an induced
velocity, termed the drift velocity, resulting from the correlation of
particle concentration with areas of low horizontal velocity fluid.
Mechanism And Conditions For Change In Channel Pattern
Z.-Q. Deng, I. P. Singh
Using the principle of maximum entropy and field data, this paper derives
the mechanism and conditions for change in the channel pattern of alluvial
rivers. Theoretical analyses indicate (i) the mechanism of formation of
different channel patterns in a river system is one of maximizing entropy
of the system under different local flow and boundary conditions, and
(ii) the river pattern with sinuosity S = * / 2 1.57 conforms to the principle
of maximum entropy. The equations governing the change in a channel pattern
are derived and thresholds, as the conditions of channel pattern changes,
determined, using theory and field data from 70 alluvial rivers having
different channel patterns. The result of this study suggests that river
management and training works should help rivers achieve and maintain
a sinuosity S close to 1.57 and the stability criterion >0.2.
Key words: Alluvial rivers, channel pattern change, threshold condition,
mechanism, principle of maximum entropy, sinuosity, stability criterion.
On the behaviour of advected plumes and thermals
K. L. Pun, m. J. Davidson
The results of an experimental investigation into the behaviour of advected
plumes are presented. Measurements of tracer concentration are made using
laser-induced fluorescence and image processing techniques. These measurements
provide the basis for studying the bulk properties and fluctuation statistics
of the flows. The experimental results show that the appropriate characteristic
velocity for predicting the centreline location of an advected plume is
the cross-sectional average velocity and not the centerline velocity (which
has been utilised for this purpose in the past). The reason for this is
discussed in the context of the large scale turbulent structures evident
in the flow. In addition, it is shown that reasonably accurate predictions
of the bulk properties of advected plumes can be made by assuming the
behaviour of an advected plume is similar to that of a plume discharged
in a still ambient fluid (a still plume), which is simply advected by
the ambient current. Notable differences can be found in the fluctuation
statistics of still and advected plumes. Detachment of tracer from the
plume is observed, but it does not have any significant impact on the
bulk properties of the flow. In the context of the advected plume experimental
results the behaviour of an advected thermal is discussed, with particular
reference to the appropriate characteristic velocity for defining its
path. The data set also enables us to locate the transition from an advected
plume to an advected thermal with some confidence.
Key words: advected, plumes, thermals, velocity scales, experimental
data
ISSUE NO. 5
Pipeline Start-Up With Entrapped Air
J. Izquierdo, V.S. Fuertes, E. Cabrera, P.L. Iglesias, J. García
Serra
A mathematical model for the assessment of the pressure head maxima that
air pockets within a pipeline can originate on start-up is presented.
This model is based on a general model addressing the filling of a pipeline
with several air pockets published by the authors. Here the simulation
of the operation of a discharge valve in order to control the peak pressure
following the pump start-up is included. Also, in order to correctly model
reverse flow through the pump use is made of the dimensionless Suter curves.
Water movement is modelled through rigid column theory and air - water
straight cross section moving interfaces are considered. Because of the
huge pressure values that the very rapid compression of the air can cause,
and in order to avoid pipe breaks, practical engineers must lend great
attention to this problem.
Variable Parameter Muskingum-Cunge Methods For Flood Routing In A
Compound Channel
X. Tang, D. Knight, P. Samuels
This paper investigates the properties of the Variable Parameter Muskingum-Cunge
method (VPMC) for flood routing, using several hypothetical flood hydrographs
in a prismatic compound channel with significant floodplains. Two variants
of the VPMC method (MVPMC3, VPMC4-1) are tested and these tests show that
VPMC4-1 is relatively better. However, both schemes still suffer, to different
degree, a loss of outflow volume which depends on bed slope and roughness
of the floodplains. Furthermore, a well-known initial leading edge ‘dip’
occurs under certain conditions, and a less well-known phenomenon, referred
to as trailing edge ‘oscillations’, is found to occur on the
recession stage of the outflow hydrograph in steep channels. These oscillations
become more serious as the roughness of the floodplains increases, but
gradually disappear with decreasing bed slope. These oscillations are
a consequence of the variation in the convective wave speed in a compound
channel and have, to the Authors’ knowledge, not been reported before
in the literature on flood routing. A condition for selecting appropriate
space and time steps in order to eliminate both ‘dip’ and ‘oscillations’
is obtained. A scheme with the routing parameters (c and D) modified to
take account for the effect of the longitudinal hydrostatic pressure term
is compared with an earlier VPMC method and shown to exhibit an improvement
in terms of volume loss. Two empirical relationships to estimate the percentage
of volume loss for a given bed slope are presented. Finally, different
approaches for predicting the c~Q relationship in the VPMC method are
shown to have some effect on the outflow hydrographs, particularly for
compound channels with mild bed slopes.
Turbulent Flow In Open Channels With Smooth And Rough Flood Plains
D. Sofialidis, P. Prinos
The effect of flood plain roughness in compound open channel flow is
studied numerically with a low-Reynolds k-ù model, which is non-linear
and therefore capable of predicting the turbulence anisotropy and the
turbulence-driven secondary currents. The simulation of rough walls is
achieved through a simple boundary condition for ù at the top of
the roughness. Free surface modelling is also included, based on an empirical
approach. For relative roughness greater than unity the lateral shear
layer that develops at the main channel-flood plain interface is intensified.
The velocities follow the logarithmic law in the interaction region for
high and low relative depths with smooth and rough flood plains. Secondary
currents and turbulent shear stresses are reproduced well by the model
when compared with available measurements and are found greater in the
interaction region as relative roughness increases. Turbulent intensities
are captured sufficiently with a slight underestimation of u? near the
channel bed. Turbulence is enhanced at the interface when the flood plain
bed is rough and remains unaffected in the rest of the flow. The free
surface simulation is proved capable of reproducing the depression of
the maximum velocity below the free surface which becomes more pronounced
with increasing relative roughness.
Energy Loss Due To Secondary Flow And Turbulence In Meandering Channels
With Overbank Flows
K. Shione, Y. Muto, D.W. Knight, A.F.L. Hyde
An investigation of energy losses due to boundary friction, secondary
flow, turbulence, expansions and contractions in meandering compound channels
with overbank flow is described. The compound meandering channel was divided
into three sub-areas, namely the main channel below the bankfull level,
the meander belt width above the bankfull level and a region outside the
meander belt above the bankfull level, and turbulence data obtained by
a Laser Doppler anemometer system. The energy loss due to the shear stress
on the horizontal plane at the bankfull level was estimated using the
measured Reynolds stresses and sectional averaged velocity, and the energy
loss due to secondary flow below the bankfull level was then estimated.
Both energy losses were found to make a significant contribution to the
total energy loss in the lower layer for shallow overbank flow. The energy
losses due to the contraction and expansion in the meander belt were evaluated
and found to be significant component of the total energy loss in the
upper layer for high overbank flow, but less so for shallow overbank flow.
The energy losses due to bed friction and shear at the bank full level
were found to be significant for shallow overbank flow.
Incipient Sediment Motion With Upward Seepage
Nian-Sheng Cheng, Yee-Meng Chiew
This study investigates the effect of upward bed seepage on the critical
condition of incipient sediment motion in open channel flow both analytically
and experimentally. The critical condition was derived by analyzing the
forces acting on a sediment particle lying on a permeable horizontal bed
subjected to seepage. The ratio of the critical shear velocity with seepage
to that without seepage depends on the ratio of the hydraulic gradient
of seepage to its critical value under the quick condition. Experimental
results concerning incipient motion of cohesionless uniform sediments
in open channel flow show that for a particular size of sediment, the
critical shear velocity decreases with increasing seepage velocity. All
measurements generally support the theoretically derived expression of
the critical shear velocity in the presence of an upward seepage.
The Role Of Near Wall Turbulent Structures On Sediment Transport
P.Sechet, B. Le Guennec
In this paper, we present the results of an experimental investigation
aiming at understanding the interaction between near-wall coherent structures
(or burst) and bedload transport in an open channel flow. This experiment
associated Laser Doppler Anemometer measurement of the instantaneous velocity
near the wall with real time measurement of sand particle trajectories
on the smooth end of a hydraulic flume. We will first give a description
of the bursting phenomenon and we will present the experimental process.
Then, we will give some details about the specific signal processing used
in this work to detect coherent structures in the velocity signal. To
conclude we will show that some characteristic scales of the particle
motion are commensurate with those of the burst.
Analysis And Formulation Of Flow Through Combined V-Notch-Gate Device
Abdulaziz A. Alhamid
A combined flow over V-notch weir and below contracted rectangular gate
was studied and analyzed. The study covered both free and submerged gate
flow conditions, under different weir-gate dimensions. These dimensions
include notch angle, notch height, gate width and gate height. The experimental
data were analyzed and the roles of the different flow and weir-gate parameters
are discussed. Based on dimensional analysis and using non-linear regression
analysis, discharge equation was developed for both free and submerged
gate flows. The developed equation consists of parameters for gate effect,
weir effect and interference of the two devices. Equation showed good
agreement with the experimental data and covers a wide range of weir-gate
parameters and gate flow conditions.
ISSUE NO. 6
Flushing Sediment Through Reservoirs
H.W. Shen
To remove reservoir sediment accumulation for the sustaining of the useful
life of reservoir has received increased attention due to the difficulty
of constructing new dams. This article is to review current status on
the flushing sediment through reservoirs and also to stress the needs
of incorporating the risk analysis for the planning of flushing sediment
operation through dams.
Free bars in rivers
M. Tubino, R. Repetto, G. Zolezzi
In the paper we review some recent work on the mechanics of formation
and development of river bars. The emphasis is placed on the instability
process which leads to the spontaneous development of bars in almost straight
reaches of alluvial rivers. A three dimensional formulation of the problem
is presented along with a discussion on the relevant closure relationships.
Results of linear and non linear theories for free bars under bedload
dominated conditions are summarised. Furthermore, account is given on
the effect on bar instability induced by suspended load, grain sorting
and width variations. Some as yet unpublished results are also presented.
Fluvial processes in streams with vegetation
T. Tsujimoto,
Recently, vegetation is a key of river management where environmental
aspects should be taken care of as well as the safety against flood and
the water resources utilization. Then, the management of fluvial processes
related to vegetation is inevitably important, and fluvial hydraulics
and hydraulics of flow with vegetation must support it. These two fields
have been recently developed obviously, and coupling them must provide
us answers to various emerging problems in new river engineering.
In this paper, after an explanation how to treat the flow with vegetation,
2D analysis of fluvial process related to flow with vegetation are introduced
with typical processes. Particular interests are paid on the topics of
fluvial process related to flow with vegetation which concerns with changes
of river landscape.
Regime Channels In Cohesionless Alluvium
M. S. Yalin, A. M. Ferreira Da Silva
The meaning of the regime channels is explained and the prominent ways
of their investigation are outlined. A particular attention is given to
the determination of regime channels by the current "rational"
methods: the present paper is the authors’ contribution to this approach.
The computational method suggested herein rests on the regime channel
formation criterion, which is revealed on the basis of the Second Law
of Thermodynamics and the Gibbs’ equation. The (usual) consideration
of the transport rate formula is replaced by the authors’ regime
width relation; thus the need for the beforehand knowledge of the transport
rate is eliminated. The validity of the relations derived is tested by
comparing them with the field and laboratory data of various sources.
Progress In The Modeling Of Alluvial Fans
G. Parker
Alluvial fans are fan-shaped zones of sediment deposition associated
with a transition from an erosional or transportational upland to a depositional
basin. The agents of formation of alluvial fans may be debris flows, channelized
fluvial flows or unchannelized sheet flows. The debris flows may range
from dense ones carrying boulders to relatively thin mud flows. The fluvial
flows may be in the meandering, wandering, anastomosing or braided configuration,
and may be transitional to unchannelized sheet flows or mud flows. Here
a description of the many manifestations of alluvial fans is presented.
The factors controlling fan development are discussed. Engineering problems
associated with fans are introduced. Progress in the experimental, theoretical
and numerical modeling of fans is reviewed. Finally, suggestions for future
research are presented.
Hydraulic Networks In Nature
A. Rinaldo
The analysis of natural hydraulic networks over a wide range of scales,
allowed by modern digital mapping technology, reveals extraordinary diversity
of natural forms, and yet deep regularity and symmetry, regardless of
geology, climate, vegetation or exposed lithology. River networks are
indeed a paradigm of scale-invariant, or fractal, forms ubiquitous in
nature, whereas tidal networks bear the both the signatures of scaling
processes and of different dynamic origin and susceptibility to environmental
factors. Here I review a subjective choice of linked results, suggesting
that the dynamic origin of the recurrence of similar network forms in
nature relates to a common mechanics of growth and stabilization of open,
dissipative systems with many degrees of freedom.
Floodplain sedimentation along extended river reaches
P. Narinesingh, G.J. Klaassen, D. Ludikhuize
Large scale changes in floodplain geometry, as contemplated for the Rhine
River as part of combined re-naturalisation and flood control projects
in The Netherlands, will change the rate of floodplain sedimentation.
To investigate the seriousness of the anticipated increase, a modelling
approach was developed to simulate floodplain sedimentation along extended
river reaches with a series of floodplains alternatingly located on both
sides of the main channel. The floodplains are schematised as sedimentation
basins, which function as sinks for the suspended sediment, which is conveyed
to the floodplains during floods via convective transport from the main
channel. To assess the deposition of fine sediments in a floodplain, the
method of Chen (1975) is used. Via the repetitive use of the continuity
equation for fine sediments, a longitudinal reduction in sediment concentration
is found. The modelling approach was verified using suspended load data
from the IJssel River, one of the distributaries of the Rhine River in
The Netherlands, during flood. Using the developed approach the local
and the downstream effect of e.g. lowering the floodplain levels with
several meters could be determined.
Malpasset dam-break revisited with two-dimensional computations
J.-M. Hervouet, A. Petitjean
Malpasset dam-break occurred in the south of France in 1959 and caused
421 casualties. This unique example of total failure of an arch dam makes
it a case of utmost interest for testing and validating software.
In the past, a scale model and a one-dimensional computation have been
used to reproduce the accident. This paper presents two-dimensional computations
of Malpasset dam-break performed with TELEMAC-2D. They provide an improvement
on one-dimensional computations when there are sharp bends in the river
and when the flood spreads in the coastal area.
The study concludes that 2D simulations of flood-waves are already possible
on domains with a length of some 10's of kilometres.
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