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CLEAR WATER SCOUR AT CYLINDRICAL PIERS IN
CLAY SAND MIXTURES
Thesis submitted toIndian Institute of Technology, Kharagpur in partial fulfillment of the
requirements for the award of the degree of
Master of TechnologyinHydraulic and Water Resources Engineering
Submitted by,
Mr. Langhi Manojkumar Namdeo
(07CE6108)
Under the guidance of
Prof. Subhasish Dey
Chair Professor, IIT Kharagpur
DEPARTMENT OF CIVIL ENGINEERING
INDIAN INSTITUTE OF TECHNOLOGY
KHARAGPUR- 721302, INDIA
2009
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Dedicated to,My lovely Parents and Friends
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Departmrnt of Civil Engineering,Indian Institute of Technology.Kharagpur-721302
Certificate
This is to certify that the thesis entitled Clear water scour at cylindricalpiers in clay sand mixtures is a bonafeid work carried out by Mr.Manojkumar N. Langhi under my supervision and guidance for the
partial fulfillmet of the requirements for Postgraduate degree of Master
of Technology in Hydraulic and water Resources Engineering during the
academic session 2007-2009 in the Civil Engineering Department, Indian
Institute of Technology, kharagpur, India.
Prof. Subhasish Dey
Department of Civil Engineering
Indian Institue of Technology
Kharagpur
India
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Acknowledgement
I would like to express my heartfelt gratitude to my project supervisor Prof. Subasish
Dey, Chair Professor, Department of Civil Engineering, Indian Institute ofTechnology, Kharagpur, for his invaluable guidance, constant encouragement,
talented and versed advice and helpful suggestions.
I am grateful to Prof.L. S. Ramachandra, Head of the Department, Civil Engineeringand also thankful to all the faculty of Civil engineering department.
I am very much thankful to Mrs. S. Talukdar madam, Head of Laboratory, Indian
Institute of Technology, Kharagpur, for providing necessary facilities during the
research work.
I would like to thank Mr. S. Sarkar, Mr. R. Das, Mr. R. Acharya and Mr. D. Deb for
their co-operation and encouragement during the research work. I am also thankful
to Amol, Anirudha, Avinash, Irfan, Nilesh, Parag, Pinaki, Ramesh, and Santosh for
their cooperation during my project work. I extend my sincere thanks to all, officer,
laboratory staff and my friends, who were very co-operative and always eager to help
me.
I owe a great deal of love, to my parents, my sister, brothers, sister in law and a
friend Sanghu, for their blessing and consistent moral support during my study.
Finally, I bow before the Almighty who has enable me to complete the project work
successfully.
IIT, Kharagpur
Date: . 12. 2009 (Manojkumar N. Langhi)
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CONTENTS
Chapter Description Page No.
List of Tables i
List of Figures ii
List of Symbols v
Abstract vi
I INTRODUCTION 1
1.1. General 1
1.2. Objectives of present investigation 3
II
2.1.2.2.2.3.2.4.
2.5.
2.6.
2.6.1
2.6.2
2.6.3
LITERATURE REVIEW
General
Scour and its classification
Mechanism of local scour
Scour in non-cohesive and cohesive soils
Parameters influencing scour depth at piers
Influence of parameters on scour depth
Approaching flow velocity
Approaching flow depth
Time - variation of scour
4
4
4
4
6
7
8
8
9
10III
3.1.3.2.
3.2.1
3.2.2
3.2.3
3.3.
3.3.1
3.3.2
EXPERIMENTAL SETUP AND
PROCEDURE
General
Experimental setup
Flume
Water supply system
Instrument carriage
Scheme of Experiments
Non-cohesive sediments
The pier model
11
11
11
11
11
12
12
12
12
3.4.
3.4.1
3.4.2
3.4.3
3.4.4
Method of measurements
Discharge
Bed and water levels
Scour depth
Velocity and flow field
12
12
13
13
13
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3.5.
3.5.1
3.5.2
3.5.2
Experimental Procedure
Non-cohesive sediments
After 5 % mixing
After 10 % and 20 % mixing
15
15
16
16IV
4.1.
4.2.
4.2.1
4.2.2
4.3.
4.3.1
4.3.2
4.3.3
4.4
4.4.1
4.4.2
4.4.3
RESULTS AND DISCUSSION
General
Time variation of scour depth
Scour for non-cohesive soil
Scour for mixture of clay and non-cohesive sand
7.5 cm pier model
Time-Velocity variation
Turbulent Intensity
Reynolds stresses
3.8 cm pier model
Time-Velocity variation
Turbulent Intensity
Reynolds stresses
17
17
17
17
18
28
28
31
33
35
35
38
40
V SUMMARY AND CONCLUSIONS 43
VI REFERENCES 45
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i
LIST OF TABLE
Table Title Page No.
4.1 Experimental data of obtaining maximum scour depth for
different percentage of clay for different pier model 19
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ii
LIST OF FIGURES
Figure Title Page
2.1 Flow pattern around bridge piers 6
2.2 Time-variation of clear water and live bed scour after
Chabert and Engeldinger (1956) 10
3.1 Schematic diagram of the experimental set-up 14
4.1 Time-depth variation for 1 cm pier model 19
4.2 Time-depth variation for 2 cm pier model 20
4.3 Time-depth variation for 3.8 cm pier model (before mixing
of clay in non-cohesive sediment) 20
4.4 Time-depth variation for 7.5 cm pier model (before mixingof clay in non-cohesive sediment) 21
4.5 Time-depth variation for 3.8 cm pier model (after mixing 5
% of clay in non-cohesive sediment) 21
4.6 Time-depth variation for 3.8 cm pier model (after mixing 10
% of clay in non-cohesive sediment) 22
4.7 Time-depth variation for 3.8 cm pier model (after mixing 20
% of clay in non-cohesive sediment) 22
4.8 Time-depth variation for 7.5 cm pier model (after mixing 5
% of clay in non-cohesive sediment) 23
4.9 Time-depth variation for 7.5 cm pier model (after mixing 10
% of clay in non-cohesive sediment) 23
4.10 Time-depth variation for 7.5 cm pier model (after mixing 20
% of clay in non-cohesive sediment) 24
4.11 Photograph of the scour hole for 3.8 cm pier model (before
mixing of clay in non-cohesive sediment) 24
4.12 Photograph of the scour hole for 3.8 cm pier model (after 5
% mixing of clay in non-cohesive sediment) 25
4.13 Photograph of the scour hole for 3.8 cm pier model (after 10
% mixing of clay in non-cohesive sediment) 25
4.14 Photograph of the scour hole for 3.8 cm pier model (after 20
% mixing of clay in non-cohesive sediment) 26
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iii
4.15 Photograph of the scour hole for 7.5 cm pier model (before
mixing of clay in non-cohesive sediment) 26
4.16 Photograph of the scour hole for 7.5 cm pier model (after 5
% mixing of clay in non-cohesive sediment) 27
4.17 Photograph of the scour hole for 7.5 cm pier model (after 10
% mixing of clay in non-cohesive sediment) 27
4.18 Photograph of the scour hole for 7.5 cm pier model (after 20
% mixing of clay in non-cohesive sediment) 28
4.19 Time-velocity variation for 7.5 cm pier model (before
mixing of clay in non-cohesive sediment) 29
4.20 Time-velocity variation for 7.5 cm pier model (after mixing
5 % of clay in non-cohesive sediment) 30
4.21 Time-velocity variation for 7.5 cm pier model (after mixing
10 % of clay in non-cohesive sediment) 30
4.22 Time-velocity variation for 7.5 cm pier model (after mixing
20 % of clay in non-cohesive sediment) 31
4.23 Vertical distribution ofu+ and w+ at vertical section (before
mixing of clay in non-cohesive sediment) 32
4.24 Vertical distribution ofu+ and w+ at vertical section (after 5
% mixing of clay in non-cohesive sediment) 32
4.25 Vertical distribution ofu+
and w+
at vertical section (after 10
% mixing of clay in non-cohesive sediment) 33
4.26 Vertical distribution ofu+
and w+
at vertical section (after 20
% mixing of clay in non-cohesive sediment) 33
4.27 Vertical distribution ofuw+
at vertical section (before mixing
of clay in non-cohesive sediment) 34
4.28 Vertical distribution of uw+ at vertical section (after 5 %
mixing of clay in non-cohesive sediment) 34
4.29 Vertical distribution of uw+ at vertical section (after 10 %
mixing of clay in non-cohesive sediment) 35
4.30 Vertical distribution of uw+
at vertical section (after 20 %
mixing of clay in non-cohesive sediment) 35
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iv
4.31 Time-velocity variation for 3.8 cm pier model (before
mixing of clay in non-cohesive sediment) 36
4.32 Time-velocity variation for 3.8 cm pier model (after mixing
5 % of clay in non-cohesive sediment) 37
4.33 Time-velocity variation for 3.8 cm pier model (after mixing
10 % of clay in non-cohesive sediment) 37
4.34 Time-velocity variation for 3.8 cm pier model (after mixing
20 % of clay in non-cohesive sediment) 38
4.35 Vertical distribution ofu+ and w+ at vertical section (before
mixing of clay in non-cohesive sediment) 39
4.36 Vertical distribution ofu+
and w+
at vertical section (after 5
% mixing of clay in non-cohesive sediment) 39
4.37 Vertical distribution ofu+
and w+
at vertical section (after 10
% mixing of clay in non-cohesive sediment) 40
4.38 Vertical distribution ofu+
and w+
at vertical section (after 20
% mixing of clay in non-cohesive sediment) 40
4.39 Vertical distribution ofuw+ at vertical section (before mixing
of clay in non-cohesive sediment) 41
4.40 Vertical distribution of uw+ at vertical section (after 5 %
mixing of clay in non-cohesive sediment) 41
4.41 Vertical distribution of uw+
at vertical section (after 10 %
mixing of clay in non-cohesive sediment) 42
4.42 Vertical distribution of uw+
at vertical section (after 20 %
mixing of clay in non-cohesive sediment) 42
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v
LIST OF SYMBOLS
Particular Description
d50a - Median diameter of sediment particles
H - Approaching flow depth (L)
l - Transverse length of abutments (L)
U - Average approaching flow velocity (LT-1
)
Ua - 0.8Uca (LT-1)
Uc - Critical velocity for sediment particles (LT-1)
Uca - Critical velocity for armor particle size d50a(LT-1
)
u* - Shear velocity of approaching flow (LT-1
)
u*c - Critical shear velocity of bed sediment (LT-1
)
u - Fluctuating component of streamwise velocity (LT-1
)
w - Fluctuating component of vertical velocity (LT-1
)
u+ - Normalized streamwise turbulent intensity component (M0L0T0)
w+ - Normalized vertical turbulent intensity component (M
0L
0T
0)
y+ - Normalized vertical depth (L
0)
uw+ - Normalized Reynolds stresses (M
0L
0T
0)
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vi
ABSTRACT
Scour holes created by three-dimensional flow of water around bridge piers
are a major cause of failure of bridge pier foundations. An evaluation of the effects of
scouring around bridge piers forms necessary step in bridge design. The problem of
scouring at cylindrical pier model on non-cohesive sand and on a bed containig
different percentage of clay in non-cohesive sand was investigated experimentally.
All the experiments were performed in a 12 m long, 0.6 m wide and 0.71 m deep
horizontal flume. Non-cohesive sand of diameter 0.15 mm, different percentage of
clay such as 5, 10 and 20 % and different pier models were used in the experimental
runs.
The time-averaged velocity components, turbulent intensity components,
vertical depth components and Reynold stresses within the scoured bed were taken by
the Acoustic Doppler Velocimeter (ADV) at the upstream side of two different
cylindrical pier models. Four pier size of diameter 7.5 cm, 3.8 cm, 2 cm and 1 cm
were considered for depth measurement in initial set of experimental runs for non-
cohesive sand. In such bed condition velocity measurements were performed only for
7.5 cm and 3.8 cm pier model. For further sets of experimental runs, thoroughly
mixed clay content of 5 %, 10 % and 20 % in non cohesive sand were used for depth
and velocity measurements in the vicinity of 7.5 cm and 3.8 cm pier model.
An experimental result have shown that the time required to attain maximum
constant scour depth in non-cohesive sand is less and therefore, low maximum
constant scour depth was obtained due to increment of clay content in non-cohesive
sand. The volume of scour hole at the upstream of the pier model was decreased with
increased in clay content and the flow velocity in the scour hole of non-cohesive sand
with higher clay content was also got reduced. Due to flow separation, pronounced
bulges were observed in the vertical distribution of normalized streamwise turbulentintensity component and Reynolds stresses, while spike was observed near the bed for
turbulent intensity components because of the shuddering effect of the primary vortex.
Keywords: Pier models; three-dimensional flow; scour.
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1
CHAPTER I
INTRODUCTION
1.1 General
An alluvial river bed is subjected to continuous change. Flowing water erodes,
transports and deposits sediment in the river, altering its bed elevation and adjusting
its boundaries. Changes in bed elevation may be due to natural causes or by the
activities of man which lead to changes of river bed or river geometry. Scour around
bridge piers is just one example of the many different effects resulting from human
interference with the river.
Scour holes created by flow of water past bridge piers are a major cause of
failure of bridge pier foundations. Failure of bridges due to such scour at theirfoundation is a common occurrence and each year a colossal amount is spent to
repair, reconstruct or replace bridges whose foundations have been under-cut by the
scouring action of stream flow. In the year 1947 the considerable bridge losses in the
State of Iowa were in large measure responsible for the determination of the Iowa
State Highway Commission to sponsor an intensive study of the problem with the
goal of evolving means for predicting probable scour depths (Laursen et al. 1956). As
of 1995 it was estimated that approximately 84 percent of the 575,000 bridges in theNational Bridge Inventory are built over waterways (Richardson et al. 1995). Of these
bridges, approximately 121,000 are considered to be scour susceptible and of those
121,000, approximately 13,000 are considered to be scour critical (Jones 1993). A
study completed by the Transportation Research Board in 1984 estimates that an
average of 150 bridges in the United States fail each year due to sediment transport
and local scouring of piers or abutments (Davis 1984). Between the years 1985 and
1987, a total of 90 bridges were destroyed in New York, Pennsylvania, Virginia and
West Virginia due to either pier or abutment failure. In 1994 the state of Georgia
experienced over 500 bridge failures due to scour caused by Hurricane Alberto (Jones
2002).
It is apparent that failures of bridges have brought significant life and financial
losses. To ensure public safety and minimize the losses of bridge failures, more
extensive studies on scour at bridge crossings are necessary. In particular,
comprehensive studies deciphering the mechanisms themselves which initiate scour
should be at the forefront of any current or future research. Until these initiating
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2
mechanisms are well understood, the potential for scour around bridge support
structures could prove to be a major concern for bridge design engineers.
Due to the overall complexity of field conditions there is no generally
accepted principle for the prediction of scour around bridge piers and abutments have
evolved from field experience alone. The flow of individual streams exhibits amultiple variation, and great inequality exists among different rivers. The alignment,
cross section, discharge, and slope of a stream must all be correlated with the scour
phenomenon, and this in turn must be correlated with the characteristics of the bed
material ranging from clays and fine silts to gravels and boulders. Finally, the effect
of the shape of the obstruction itself - the pier or abutment - must be assessed. Since
several of these factors are likely to vary with time to some degree, and since the
scour phenomenon as well is inherently unsteady, sorting out the influence of each of
the various factors is virtually impossible from field evidence alone.
An analytical approach is equally difficult. If an obstruction, such as a pier, is
placed in a stream, the flow pattern in the vicinity of that obstruction will be modified.
Because the capacity for the transport of sediment is a function of the flow, the
transport-capacity pattern will also be modified. In any area where, as a result of the
modified pattern, the capacity for transport out of the area is greater than the rate at
which material is supplied to the area, scour will occur. Conversely, where the
transport capacity is less than the rate of supply, deposition will occur. The resultant
changes in the stream bed will further modify the flow pattern - and the capacity
pattern - until equilibrium between capacity and supply is again achieved at every
point on the stream bed. An analytic solution would have to combine a prediction of
the flow pattern and a description of the local transport capacity of the flow. Although
an approximation of the flow pattern might be attempted, a comparable solution for
the capacity is not yet possible.
The experimental approach has been tried in the past with limited success,
usually because the goal was restricted to a particular installation or to some special
phase of the general problem. The earliest report on a laboratory study which has
done by Engels at Dresden, Germany, in 1894. In that report it described that study
reference is made to an earlier one in France in 1873 by Durand-Claye. Neither these
early experiments nor subsequent studies done in later period by various investigators
in various countries have been sufficiently general to obtain the desired result - a
means of predicting scour in the field. However, considerable investigations on pier
scour have been carried out further and a reliable design method is now available
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3
(Melville and Sutherland 1988). These all the investigations pertain to scour around
piers founded in cohesionless sediment. Study on the problem of local scour around
bridge piers in cohesive sediments is still in its intial stage. Unlike in the case of non-
cohesive sediments, the flow condition at which cohesive material gets eroded is
difficult to predict as it depends upon a variety of factors such as the type andpercentage of clay content present, stage of compaction or consolidation etc. Further,
only limited study has been carried out on the temporal variation of scour depth
around bridge piers founded in cohesive sediments.
1.2 Objectives of present investigation
The aim of the present investigation is to study experimentally the flow field,
influence of different parameters on equilibrium scour depth, time variation of scour
depth at cylindrical piers under clear water scour condition. The main objectives ofthe study are as follows:
Investigation of the three-dimensional turbulent flow fields in the vicinity oftwo different cylindrical pier models placed on non-cohesive sand and on a
bed containing different percentage of clay in non-cohesive sand.
Determination of time-variation of scour depth for various bed conditionsaround different cylindrical pier models.
Determination of time-velocity variation for various bed conditions aroundtwo different cylindrical pier models.
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4
CHAPTER II
LITERATURE REVIEW
2.1 General
In this Chapter, a comprehensive review of the investigations on local scour atbridge pier is presented. Scour and its classification, scouring mechanism, parameters
affecting scour depth and time-variation of scour are discussed in successive section.
2.2 Scour and its classification
Scour is a natural phenomenon of lowering the level of riverbeds by the
erosive action of flowing stream. Scour is classified into two types, general scour and
local scour. General scour in the river occurs due to change in the characteristics of
river while local scour develops near the structure due to modification of the flowfield as a result of obstruction to the flow by the structures. On the basis of time taken
for scour development, general scour can be categorized as short-term scour and long-
term scour. Short-term general scour develops during a single or several closely
spaced floods. It may occur due to convergence of flow, a shift in the channel thalweg
or braids within the channel, and bed-form migration. On the other hand, the long-
term general scour is the general aggradation or degradation of streambed elevation
due to natural (e.g. channel straightening, volcanic activities, and climate change) and
human causes (e.g. channel alterations, streambed mining, dam/reservoir construction,
and land-use changes).
Local scour is classified as clear-water scour and live-bed scour. Clear-water
scour occurs when the sediment is removed from the scour hole but not supplied by
the approaching stream. In contrast, the live-bed scour occurs when the scour hole is
continuously fed with the sediment by the approaching stream.
2.3 Mechanism of local scourThe boundary layer in the flow past a bridge element undergoes a three-
dimensional separation. The dominant feature of the flow about a pier is the system of
vortices which develops. The most important of these are the horseshoe vortex and the
wake-vortex system. Laursen and Toch (1956) described the formation of horseshoe
vortex. At the nose of the pier the approach flow velocity goes to zero. Since the flow
velocity decreases from a maximum at the free surface to zero at the bed, the
stagnation pressure decreases with distance from the water surface and this pressure
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5
difference drives the flow. Therefore, separation occurs at the upstream face of pier
and shear layer rolls up along the obstruction to form a vortex system in front of the
element which is swept downstream by the river flow. Viewed from the top, this
vortex system has the characteristic shape of a horseshoe and thus called a horseshoe
vortex. The horseshoe vortex results from a concentration by the pier of vorticityalready present in the approaching flow. However, the wake-vortex system is
generated by the pier itself (figure 2.1). The formation of the horseshoe vortex and the
associated downflow around the bridge element results in increased shear stress and
hence a local increase in sediment transport capacity of the flow. This leads to the
development of a deep hole (scour hole) around the bridge element, which in turn,
changes the flow pattern causing a reduction in shear stress by the flow thus reducing
its sediment transport capacity. The temporal variation of scour and the maximum
depth of scour at bridge elements therefore mainly depend on the characteristics of
flow, pier and river-bed material. The formation of the horseshoe vortex and the
associated downflow cause scour at different elements of a bridge such as pier,
abutment and spur dike. The mechanism of scour around bridge piers has been studied
by Melville (1975), Kothyari et al. (1992a & b), Dey (1995), Dey et al. (1995), Dey
(1999), Horst (2004) whereas, studies on the mechanism of scour around abutments
and spur dikes have been studied by Kothyari et al. (2001), Barbhuiya (2003), Dey et
al. (2004 & 2005), Barbhuiya et al. (2004a & b).
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6
Flow
Pier
Surface Roller Wake Vortex
Downflow
Scour hole
Sediment bed Horseshoe vortex
Fig.2.1 Flow pattern around bridge piers
2.4 Scour in non-cohesive and cohesive soils
Non-cohesive soil consists of the bed material ranging from very fine to very
coarse. When bridge pier is constructed in such a strata and the discharge is
sufficiently large, the scour development would progress. For non-cohesive sediment,
the submerged density of the soil and gravity forces provides the main resistance to
erosion. During scour development, the coarser particles would accumulate in the
scour hole and partly inhibit further development of the scour. Ultimately the
accumulated coarser material would stop further scour and the scour depth obtained
would be much smaller than that in uniform material.
The mechanism of cohesive material scour is fundamentally different from
scouring of alluvial non-cohesive materials. The process involves not only the
balancing of flow induced shear stresses and the shear strength of soils to withstand
scour, but also the chemical and physical bonding of individual particles and the
properties of the eroding fluid. Hence scour in cohesive materials is more complex
and less understood than the scour in non-cohesive sandy material. It is believed that
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7
scour in cohesive soils occurs when the fluid shear is sufficient to overcome the
tensile strength of the bed material and the submerged unit weight of the soil. Very
little work has been carried out on the basic mechanism involved on the scouring of
cohesive soils. One reason could be complexity of the problem; the physico-chemical
aspects and the resistance to scour in cohesive soils, particularly, governed by widevariations in the sediment properties.
Many investigators have studied the scour phenomenon in coarse-grained soils
while scouring in cohesive material was studied by Partheniades (1965), Kamphuis
and Hall (1983), Briaud et al. (1999), Rambabu et al. (2003). From previous research
Rambabu et al. (2003) concluded that the rate of erosion in cohesive soil is dependent
on many parameters such as induced shear stress, moisture content and density of the
soil type, shear strength of the soil, type of clay and its adsorbed complex,
temperature etc. Whereas, according to Molinas et al. (1998a) cohesive materials,
once eroded, remain in suspension. As a result, the phenomenon identified as clear-
water local scour in non-cohesive materials always prevails. Along with eroding fluid
properties, the scour process in cohesive soils is strongly affected by the amount of
cohesive material present in the soil mixture as well as the types of mineral clay,
initial water content, soil shear strength, and compaction of the clay. Hence by the
knowledge gained in the past in cohesive material scour Molinas et al. used two
different types of clay mixtures and studied local scour around abutments and
analyzed the effects of compaction, initial water content, soil shear strength, and the
approach flow conditions on abutment scour. Molinas et al. (1998b) studied pier scour
in montmorillonite clay soils and along with analyzing the effects of compaction, soil
shear strength, and the approach flow conditions on pier scour in unsaturated cohesive
soils and influence of initial water content of saturated clay on pier scour they
developed scour prediction equations in unsaturated and saturated cohesive soils to
quantify the scour which may occur around circular piers.
2.5 Parameters influencing scour depth at piers
Scour at piers is influenced by various parameters (Breusers et al. 1977), which
are grouped as follows:
Parameters relating to the pier: Size, shape, spacing, number and orientationwith respect to the approaching flow direction.
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8
Parameters relating to the bed sediment: Median size, particle size distribution,mass density, angle of repose, cohesiveness.
Parameters relating to the approaching flow condition: Approaching flowvelocity, approaching flow depth, shear velocity and roughness.
Parameters relating to the fluid: Mass density, viscosity, gravitationalacceleration and temperature (may not be important in scour problems).
Parameters relating to the time: Time of scouring for an evolving scour hole. Parameters relating to the unsteadiness: Passage of flood wave in rivers and
waves in marine environment.
2.6 Influence of parameters on scour depth
2.6.1 Approaching flow velocity
The depth of the local scour hole is closely related to the undisturbed approachflow velocity. The idea about the effect of approach flow velocity on local scour
depth under live-bed conditions have changed over the years. Early researchers
related the relative scour depth (normalized by the flow depth) to the Froude number.
Most of the conclusions drawn that for a given flow depth, the scour depth increase
indefinitely, either at an increasing or a decreasing rate, with increasing velocities.
The numerous equations relating normalized scour depth and Froude number are
summarized by Melville (1975). Kandasamy (1989) showed that the scour depthincreases with increase in flow depth due to incorporation of the flow Froude number.
It is generally recognized that the shear velocity*
u is an important parameter
not only in distinguishing clear water scour from the live bed scour but also in
representing the erosive power of the flowing stream for a given sediment size. Clear
water scour occurs for the approaching flow velocity up to the critical velocityc
U for
bed sediments, that is / 1c
U U ; while live bed scour occurs when / 1c
U U .For
nonuniform sediments, Melville and Sutherland (1988) defined an armor velocity aU ,
which marks the transition from clear to live bed conditions for a sediment-
transporting flow and is equivalent toc
Ufor uniform sediments. Thus, for nonuniform
sediments, live bed conditions prevail when / 1a
U U . However, if / 1a
U U ,
armoring of the bed occurs as scour proceeds and clear water conditions exist. Dongol
(1994) conducted an extensive series of experiments to study the effect of approaching
flow velocity on scour depth at vertical-wall, wing-wall and spill-through abutments
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under live bed conditions in uniform and nonuniform sediments. His results are
complimentary to the studies of Chiew (1984) and Baker (1986) for live bed scour at
bridge piers in uniform and nonuniform sediments, respectively.
Chabert and Engeldinger (1956) stated that as the approach flow velocity
exceeds the critical velocity for sediment entrainment, the scour depth decreases toabout 10 % less than the maximum scour depth at the critical velocity and thereafter,
an increase in the velocity has no effect on the local scour depth. However, it was
recognized that under clear water conditions, the maximum scour depth occurs when
cU U . This scour depth is called the threshold peak. For / 1
cU U , that is under
live bed conditions, scour depth initially decreases with increase in approaching flow
velocity reaching a minimum value and then increases again toward a second
maximum. The second maximum occurs at about the transitional flatbed stage ofsediment transport on the channel bed and is termed the live bed peak.
2.6.2 Approaching flow depth
According to Laursen (1952), the approaching flow depth H is an important
factor to determine scour depth. Experimental results of Kandasamy (1989) indicate
that for a constant value of the shear velocity ratio* *
/c
u u , the maximum scour depth
increases with the increase in approaching flow depth. It was also observed that the
maximum scour depth increases at a decreasing rate with increase in approaching flow
depth. According to Kandasamy (1989), for shallow flow depths, the scour depth
increases proportionally with H, but is independent of l. On the other hand, for
intermediate flow depths, the scour depth depends on both Hand l. However, Melville
(1992) distinguished short and long abutments. He concluded that for short abutments
( / 1l H ), the scour depth is independent of flow depth; and for long abutments
( / 25l H ), the scour depth is dependent on flow depth. However, most abutments
are neither long nor short, as a result of which the scour depth is influenced by both H
and l.
There is a consensus that the maximum scour depth increases at a decreasing
rate with increase in approaching flow depth and there exists a limiting depth
corresponding to which the maximum scour depth is independent of the flow depth.
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2.6.3 Time - variation of scour
Figure 2.2 shows the schematic diagram describing the time-variation of scour
depth at cylindrical pier after Chabert and Engeldinger (1956). Time to reach
equilibrium scour depth varies widely, ranging from a day to a fortnight. Anderson
(1963) stated By virtue of the logarithmic character of the development of the scourregion with time, a practical equilibrium is reached after a relatively short time, after
which the increase in the depth and extent of scour becomes virtually imperceptible.
Rouse (1965), however, stated that scour is an ever-increasing phenomenon and there
is no real equilibrium scour depth. Some of the researchers thought that the variation
of scour depth with time is logarithmic but, few researchers proposed an exponential
time-variation of scour; while Bresuers (1967) and Cunha (1975) gave a power law
distribution. (see Barbhuiya 2003). General consensus is that the equilibrium scour
depth at pier is attained asymptotically.
Fig. 2.2 Time-variation of clear water and live bed scour after Chabert and
Engeldinger (1956)
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CHAPTER III
EXPERIMENTAL SETUP AND PROCEDURE
3.1 General
Experiments were carried out in Hydraulic and Water resources EngineeringLaboratory of the Indian Institute of Technology, Kharagpur, India. The details of the
experimental setup, scheme of experiments, experimental procedures and method of
measurements are given in this Chapter.
3.2 Experimental Setup
3.2.1 Flume
Experiments were performed in a horizontal, re-circulating flume with a
rectangular cross-section 12 m in length, 0.6 m in width and 0.71 m deep. At the testsection, the side walls of the flume were made of transparent glasses. At the inlet
section of the flume concrete stilling basin was provided through which water enters
into the flume. The stilling basin consisted of one perforated baffle wall and two
vertical steel screens covering the full cross section for damping the flow turbulence
and waves. An adjustable tailgate was installed at the downstream end of the flume to
control the flow depth. The location of test section was made in such a way that the
flow became fully developed before it reaches the test section. The sediment recess
consisted of rectangular box made up of perspex sheet 12 mm in thickness with a
dimension as 0.85 m length, 0.60 m width and inner depth of 0.165 m. The test
section was located 3.5 m from the flume entrance. On the upstream side of sediment
recess, the false floor of height 0.177 m were constructed above the original bed level
of the flume in such a way that it allows water to pass uniformly over the test section
without causing its turbulent characteristics over the sediment particles, while on the
downstream side, the false floor of same height and 0.8 m in length was constructed.
Five small holes were provided at the bottom of the downstream wall of the sediment
recess to drain out the water from the sediment bed. Provision was also made to trap
the washed-out sediment particles at the downstream side of floor by constructing
barrier wall near tailgate with same height of the false floor as shown in fig 3.1.
3.2.2 Water Supply System
The flume was connected to the water supply system comprised of a constant
head reservoir about a height of 4 m above the ground level, an inlet tank, a large
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underground reservoir and the pumps. The water was pumped to the constant head
reservoir and supplied to the inlet tank through the valve fitted at their junction. A
calibrated V-notch weir was fitted at the inlet tank through which water entered into
the flume via stilling basin.
3.2.3 Instrument carriage
An instrument carriage comprised of a main unit, which travelled on the rails
in the longitudinal direction and the auxiliary unit, which carried the instruments, such
as point gage, ADV probe etc. travelled in the transverse direction.
3.3 Scheme of Experiments
3.3.1 Non-cohesive Sediments
The Indian Standard sieves were used for the preparation of sediment samples.
The data of the sieve analysis were plotted to draw particle size distribution curves.
From the curve the mean diameter of sample was selected to be 0.15 mm. This was
used as base material to which clay was added in different proportions for further
experimental runs.
3.3.2 The pier model
The experiments were performed using four different types of perspex sheet
pipes with diameters, 7.5 cm, 3.8 cm, 2 cm and 1 cm to symbolize a small scale model
of a bridge pier. All the four type of piers were used for the experiments in non-
cohesive sediment. However, for the experiments in the mixture of sand-clay only two
models (7.5 cm and 3.8 cm) were used.
3.4 Method of measurements
3.4.1 Discharge
The discharge into the flume was regulated by a valve fitted at the junction of
constant head reservoir and inlet tank and decided using calibrated V-notch fitted at
the inlet tank. The V-notch was calibrated and calibration equation was used for the
measurement of discharge Q, given as a function of head of water H above the sill
level of the V-notch as
159.29174.0 HQ 3.1
The water level in the inlet tank was measured using the vernier point gage
with an accuracy of 0.1 mm.
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3.4.2 Bed and Water Levels
Instrument carriage carrying a vernier point gage with an accuracy of 0.1 mm
was used for the measurement of bed level and water surface level above the bed. The
water surface level above the bed was adjusted by using tailgate at the downstream
side of the flume.
3.4.3 Scour Depth
The maximum scour depth near the pier for all the experimental runs were
measured using a vernier point gage with an accuracy of 0.1 mm.
3.4.4 Velocity and Flow Field
The Vectrino Velocimeter was used for the measurement of instantaneous
three-dimensional component of velocity. The Vectrino velocimeter operated on a
pulse-to-pulse coherent Doppler shift to provide instantaneous three-dimensional
velocity components at a rate of 50 Hz. The acoustic sensor comprised with
transmitting transducer and receiving transducers. The receiving transducers were
mounted on short arms around the transmitting transducer at 1200
azimuth intervals.
The transmitting transducers emitted acoustic beams with a frequency of 10MHz. The
beams travelling through the water arrived at the measuring point which is 5 cm
below the transducer, where they were reflected by the ambient particles within the
flow being received by the receiving transducers. The processing module performed
the digital signal processing required to measure the Doppler shift. A real-time
display of the data in graphical and tabular forms was provided by the data acquisition
software. There was no requirement of seeding of the flow during experiments, as the
signal-noise ratio (SNR) was in the range of 12 to 16. Because of the interference due
to echoes from the flume bed, the receiving signal might be disturbed near the bed,
which may result in inaccurate velocity measurement. The measurement by the
Vectrino probe was not possible in the zone located 5 cm below the free surface. Aspecial carriage structure was made to facilitate the movement of Vectrino
Velocimeter along different radial line with respect to pier centreline. Sampling rate
and sampling volume adopted for present experiment was 100 Hz and 2.5 cm3
respectively.
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Perforated baffles
Outlet Sediment recess Pier model False floor Screens From inlet tank
Tailgate Sediment trap
0.177 m
0.5 m 0.85 m 3.5 m
(Dimension not in scale)
Fig. 3.1 Schematic diagram of the experimental set-up
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3.5 Experimental Procedure
The experiments were carried out in four different parts. In the first part of the
experiments non-cohesive sediment was used.
3.5.1 Non-cohesive sediments
Initially, non-cohesive sediments of mean diameter 0.15 mm were used to fill
the sediment recess around the pier model of 7.5 cm. The preparatory work for the
experimental runs involved the following subtasks
1. Prior to the commencement of the experimental runs, pier model was placed inthe middle of sediment recess. The bed was properly levelled using planner
and final bed level was checked using a point gage.
2. On the upstream and the downstream side of the pier in the sediment recess,armor layer of desired thickness and 0.15 m wide was placed.
3. At the time of actual runs, in order to avoid the undesirable scour, whichotherwise would happen by the action of sheet flow with inadequate flow
depth, the flume was first slowly filled with the water by a pipe at a low rate at
the downstream side. Once the water level of desirable height was reached, the
experimental runs were started by adjusting the inflow rate and maintaining
the required flow depth within a flume by a downstream gate.
4.
The runs were taken for a maximum period of 2 hours, to ensure that themaximum scour depth was obtained.
5. To avoid the partial filling of scour hole by the sediments while draining outthe water from the flume, the water was first drained out by opening a valve in
the upstream end of the flume and adjusting the tailgate so that a minimum
flow velocity occurred at the sediment recess. Finally, water was drained out
very slowly by opening the holes at the bottom of downstream walls of the
sediment recess, sediment trap and downstream gate.
6. For further runs in non-cohesive sediments 3.8 cm, 2 cm and 1 cm pier modelwas placed successively in the sediment recess and procedure from 2 to 5 was
followed.
7. In all the experiments, two runs for 7.5 cm and 3.8 cm pier model wereperformed. Initial run carried out for depth measurement and second run
conducted for velocity measurement. Both depth and velocity were measured
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at the upstream of pier. For 2 cm and 1 cm pier model only depth
measurement were taken at the upstream of the pier.
3.5.2 After 5 % mixing
In the second and further part of the experiments two pier models (7.5 cm and
3.8 cm) were used. The non-cohesive sand was dried completely and clay soil of 5 %
by weight was mixed in it thoroughly. Procedure from 1 to 5 was followed by using
5 % clay and sand mixture to measure the depth and velocity at the upstream side of
the pier. At each run care was taken to maintain 5 % of clay soil in non-cohesive
sediments.
3.5.3 After 10 % and 20 % mixing
In the case of 10 % and 20 % of mixing, the non-cohesive sediment was dried
completely and 10 % and 20 % of clay soil by weight was added thoroughly in non-
cohesive sediments, respectively. For both cases procedure from 1 to 5 above, were
followed for measurement of depth and velocity at the upstream side of the pier. The
care was also taken to maintain the required percentage of clay soil in respective run.
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CHAPTER IV
RESULTS AND DISCUSSION
4.1 General
The experiments were conducted in four different sets. In each set two runs for
two pier models were performed. First run carried out for depth measurement and
according to such measurement and initial bed condition velocity measurements took
place in the second run. Velocity measurements were taken during the formation of scour
hole. Both the depth and velocity were measured for a period of 2 hours to ensure that the
maximum constant scour depth could be reached. Non-cohesive sediment of mean
diameter 0.15 mm was used in initial set of experimental runs. Clay soil of percentage 5,
10 and 20 was thoroughly mixed in non-cohesive sediment for further sets of
experimental runs. Data collected in each run was used to plot Time-Depth variation,
Time-Velocity variation, Turbulent Intensity and Reynolds stresses discussed further in
this chapter.
4.2 Time variation of scour depth
4.2.1 Scour for non-cohesive soil
Depth measurements were taken as an initial measurement in each set of
experimental runs to study the behaviour of scour at the upstream side of the pier model.
For first set, non-cohesive sediment of mean diameter 0.15 mm was used for four
different types of pier model, such as 1 cm, 2 cm, 3.8 cm and 7.5 cm. Time variation of
scour depth for these pier models is shown in figures 4.1 - 4.4. These entire scour profiles
showed that during the initial periods of scouring the pick-up rate was very high (for
about 20 minutes) but it decreased and gradually become asymptotic to the time axis in
the final periods. This was because of the horseshoe vortex. The particles at the base of
the cylinder are removed due to fluid-induced forces under the combined effect of bed
shear stress, turbulent agitation, and oscillation of the horseshoe vortex (Dey, 1996). Atthe initial periods of scouring, due to small dimensions of the scour hole the size of the
horseshoe vortex was small. Consequently, the high bed shear stress developed beneath
the vortex which caused rapid dislodgement of the sediment particles. Hence, there was
rapid increment of profile in short period of time. As scour hole increased with time, the
size of the horseshoe vortex also increased and therefore its strength decreased. Thus, the
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bed shear stress induced by the vortex gradually decreased which resulted into the
process of sediment pick-up to proceed at decreasing rate. Therefore, the profile changed
its trend (after about 20 minutes) and gradually increased till the maximum constant
scour depth could be reached.
4.2.2 Scour for mixture of clay and non-cohesive sedimentThe main cause of scour occur in clayey soil is due to different types of forces act
between soil particles which resist the dislodgement of particles. These are Van der
Waals forces, electric surface and other bonding mechanisms such as hydrogen bond,
and chemical cementation between particles. Hence scour in clayey materials is more
complex and less understood than the scour in non-cohesive sandy material (Garde et al.,
1998). Therefore for next three sets of experimental runs different percentage of clay in
non-cohesive sediment was used. Clay content of 5 %, 10 % and 20 % were mixed
thoroughly in non-cohesive sediment and respective run carried out for depth
measurement. The plot of depth variation with time for all these runs is shown in figures
4.3 4.10. Time-variation of scour depth for all these experimental run showed that
initially sediment pick-up rate was very high but it decreased and gradually become
asymptotic to the time axis in final periods as in case of non-cohesive sediment. The
major difference between a non-cohesive and a cohesive sediment scour is that the
erodibility for a fully consolidated, cohesive clay material is much less than that of sand
(Hsu, FWRRC Annual Technical Report 2006). Thus, increment of clay content in non-
cohesive sediment caused less scour depth compared to full non-cohesive sediment. As
shown in figures 4.84.10,although the initial rate of scouring was high for all runs the
maximum depth attained for non-cohesive sediment was maximum as compared to the
other runs which carried out by mixing different clay content given in the table 4.1 as
well as the volume of scour hole around the pier model decreased with increased in clay
content, shown in figures 4.11 4.18. Even run (not given in the table) were conducted
for velocity measurement. For all the experimental runs discharge, depth over the bed andtime for scouring process were kept constant. The same result was observed for both 7.5
cm and 3.8 cm pier model.
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Table 4.1 Experimental data of obtaining maximum scour depth for different percentage
of clay for different pier model
Run
Clay content in non-
cohesive sediment
(%)
Pier model
diameter (cm)
Maximum scour depth attained
after 2 hours (cm)
1 0 7.5 15.5
3 0 3.8 8.9
5 5 7.5 14.8
7 5 3.8 8.7
9 10 7.5 14.5
11 10 3.8 7.8
13 20 7.5 13.65
15 20 3.8 7.5
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 20 40 60 80 100 120 140
Time (min)
Depth(cm)
Fig. 4.1 Time-depth variation for 1 cm pier model
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0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 20 40 60 80 100 120 140
Time (min)
Depth(cm
)
Fig. 4.2 Time-depth variation for 2 cm pier model
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120 140
Time (min)
Depth(cm)
Fig. 4.3 Time-depth variation for 3.8 cm pier model
(before mixing of clay in non-cohesive sediment)
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0
2
4
6
8
10
12
14
16
0 20 40 60 80 100 120 140
Time (min)
Depth(cm
)
Fig. 4.4 Time-depth variation for 7.5 cm pier model(before mixing of clay in non-cohesive sediment)
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120 140
Time (min)
Depth(cm
)
Fig. 4.5 Time-depth variation for 3.8 cm pier model
(after mixing 5 % of clay in non-cohesive sediment)
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0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120 140
Time (min)
Depth(cm
)
Fig. 4.6 Time-depth variation for 3.8 cm pier model(after mixing 10 % of clay in non-cohesive sediment)
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120 140
Time (min)
Depth(cm
)
Fig. 4.7 Time-depth variation for 3.8 cm pier model(after mixing 20 % of clay in non-cohesive sediment)
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0
2
4
6
8
10
12
14
16
0 20 40 60 80 100 120 140
Time (min)
Depth(cm
)
Fig. 4.8 Time-depth variation for 7.5 cm pier model(after mixing 5 % of clay in non-cohesive sediment)
0
2
4
6
8
10
12
14
16
0 20 40 60 80 100 120 140
Time (min)
Depth(cm
)
Fig. 4.9 Time-depth variation for 7.5 cm pier model(after mixing 10 % of clay in non-cohesive sediment)
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0
2
4
6
8
10
12
14
16
0 20 40 60 80 100 120 140
Time (min)
Depth(cm
)
Fig. 4.10 Time-depth variation for 7.5 cm pier model
(after mixing 20 % of clay in non-cohesive sediment)
Fig. 4.11 Photograph of the scour hole for 3.8 cm pier model
(before mixing of clay in non-cohesive sediment)
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Fig. 4.12 Photograph of the scour hole for 3.8 cm pier model
(after 5 % mixing of clay in non-cohesive sediment)
Fig. 4.13 Photograph of the scour hole for 3.8 cm pier model
(after 10 % mixing of clay in non-cohesive sediment)
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Fig. 4.14 Photograph of the scour hole for 3.8 cm pier model
(after 20 % mixing of clay in non-cohesive sediment)
Fig. 4.15 Photograph of the scour hole for 7.5 cm pier model
(before mixing of clay in non-cohesive sediment)
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Fig. 4.16 Photograph of the scour hole for 7.5 cm pier model
(after 5 % mixing of clay in non-cohesive sediment)
Fig. 4.17 Photograph of the scour hole for 7.5 cm pier model
(after 10 % mixing of clay in non-cohesive sediment)
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Fig. 4.18 Photograph of the scour hole for 7.5 cm pier model
(after 20 % mixing of clay in non-cohesive sediment)
4.3 7.5 cm pier model
4.3.1 Time-Velocity variation
Velocity measurements for all experimental runs were taken as secondarymeasurement during the formation of scour hole for both the pier models. It was observed
earlier that, the obstruction of the flowing stream by a bridge pier caused a three-
dimensional separation of flow, as it travels by the side of the pier, forming a vortex flow
field around the pier which moved downstream (Dey et al., 1995; Dey, 1995). Thus,
streamwise velocity changed its direction near the bed. In the present experimental runs,
the vertical distributions of time averaged streamwise velocity component for this pier
model is plotted. Almost similar pattern was observed for all the profile depicted in the
figures 4.19 4.22. Due to flow separation negative streamwise velocity was observed
during the formation of scour hole upstream of the pier as well as almost constant
negative velocity was observed during the formation of scour hole. Higher negative
velocity was observed for such type of pier model.
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In successive experimental runs, clay content was increased in non-cohesive
sediment and velocity measurements were taken at the upstream side of the pier model. It
was observed that, the volume of scour hole upstream of the pier decreased with the
increment of clay content. Consequently, size of horseshoe vortex became small.
Therefore, the flow velocity in the scour hole of sediment with higher clay content was
lower than the lower or without clay content in non-cohesive sediment. Hence, the time
averaged streamwise velocity profileexhibited higher constant negative velocity for a run
performed after mixing 20 % of clay content in non-cohesive sediment than the run
conducted after mixing 10 %, 5 % and 0 % of clay content in non-cohesive sediment in
succession.
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0 20 40 60 80 100 120 140Time (min)
Velocity(m/s)
Fig. 4.19 Time-velocity variation for 7.5 cm pier model
(before mixing of clay in non-cohesive sediment)
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-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0 20 40 60 80 100 120 140
Time (min)
Velocity(m
/s)
Fig. 4.20 Time-velocity variation for 7.5 cm pier model
(after mixing 5 % of clay in non-cohesive sediment)
-0.2
-0.15
-0.1
-0.05
0
0.050.1
0.15
0.2
0.25
0.3
0 20 40 60 80 100 120 140
Time (min)
Velocity(m/s)
Fig. 4.21 Time-velocity variation for 7.5 cm pier model
(after mixing 10 % of clay in non-cohesive sediment)
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-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0 20 40 60 80 100 120 140
Time (min)
Velocity(m/s)
Fig. 4.22 Time-velocity variation for 7.5 cm pier model
(after mixing 20 % of clay in non-cohesive sediment)
4.3.2 Turbulent Intensity
The vertical distribution of normalized streamwise turbulent intensity component
u+ [=
0.52
/u U ], where u is the fluctuation ofU] at the upstream of pier model is
illustrated in the figures 4.23 4.26. It was observed that the streamwise turbulent
intensityvaries little in the zone fory+
< -0.15 having distribution more or less linear. On
the other hand, in the zone fory+
> -0.15, where the reversal of flow occurred, normalized
streamwise turbulent intensity component increases towards the scoured bed, but it
reduced in the vicinity of the bed. A most significant feature of the distribution is the
pronounced bulges immediately above y+
= -0.16 line, due to flow separation inside the
scour hole. Near the scoured bed at the upstream side of pier model the spike was
observed. These are due to the shuddering effect of the horseshoe vortex. The
experimental runs conducted using different percentage of clay in non-cohesive sediment
showed similar pattern of turbulent intensity component.
The vertical distribution of normalized vertical turbulent intensity component w+
in the scour hole at the upstream side of the pier model is shown in the same figures 4.23
4.26. The distribution pattern of w+ is almost similar to that of u
+. However, it is
apparent that w+do not show spike near the scour bed, as there is no shuddering effect of
horseshoe vortex in the vertical direction. For all runs performed in different
experimental conditions, same distribution pattern ofw+was observed.
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-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
0 0.5 1 1.5 2 2.5u
+, w
+
y+
u+
w+
Fig. 4.23 Vertical distribution ofu+
and w+
at vertical section
(before mixing of clay in non-cohesive sediment)
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
0 0.5 1 1.5 2 2.5u
+, w
+
y+
u+
w+
Fig. 4.24 Vertical distribution ofu
+and w
+at vertical section
(after 5 % mixing of clay in non-cohesive sediment)
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-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
0 0.5 1 1.5 2 2.5u
+, w
+
y+
u+
w+
Fig. 4.25 Vertical distribution ofu+
and w+
at vertical section
(after 10 % mixing of clay in non-cohesive sediment)
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
0 0.5 1 1.5 2 2.5u
+, w
+
y+
u+
w+
Fig. 4.26 Vertical distribution ofu
+and w
+at vertical section
(after 20 % mixing of clay in non-cohesive sediment)
4.3.3 Reynolds stresses
Figures 4.27 4.30 represents the vertical distributions of normalized Reynolds
stresses uw+
(= 2*
/u w u ) at upstream section of pier model for different experimental
condition. Reynolds stresses shows the distinguishable swell immediately below y+
= 0
line, inside the scour hole, as a result of flow separation. However, in the vicinity of the
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34
scoured bed it reduces drastically. Also normalized Reynolds stresses changed its sign
due to reversal of flow near the scoured bed. Similar pattern of observation was also
made for all conditions of experimental runs.
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
-20 0 20 40 60 80 100uw+
y+
Fig. 4.27 Vertical distribution ofuw+
at vertical section
(before mixing of clay in non-cohesive sediment)
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
-20 0 20 40 60 80 100
uw+
y+
Fig. 4.28 Vertical distribution ofuw+
at vertical section
(after 5 % mixing of clay in non-cohesive sediment)
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35
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
-20 0 20 40 60 80 100
uw+
y+
Fig. 4.29 Vertical distribution ofuw+
at vertical section
(after 10 % mixing of clay in non-cohesive sediment)
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
-20 0 20 40 60 80 100
uw+
y+
Fig. 4.30 Vertical distribution ofuw+
at vertical section
(after 20 % mixing of clay in non-cohesive sediment)
4.4 3.8 cm pier model
4.4.1 Time-Velocity variation
The vertical distribution of time averaged streamwise velocity component is
shown infigures 4.314.34. Almost similar distribution pattern was observed for all the
profile. The phenomenon of three-dimensional separation of flow caused by the bridge
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36
pier which obstructs the flowing stream as it travels by the side of the pier formed a
vortex flow field around the pier which moved downstream. Thus, streamwise velocity
changed its direction near the bed. Due to flow separation negative streamwise velocity
observed during the formation of scour hole upstream of the pier is shown in figures.
Almost constant negative velocity was observed during the formation of scour hole. This
is more apparent in the profile plotted for such pier model than the profile plotted for 7.5
cm pier model.
For further experimental runs clay content was increased in non-cohesive
sediment and velocity measurements were taken at the upstream side of the pier model.
In this case also the volume of scour hole around the pier decreased with the increment of
clay content. Consequently, size of horseshoe vortex became small. Therefore, the flow
velocity in the scour hole of sediment with higher clay content was lower than the lower
or without clay content in non-cohesive sediment. Thus, because of the flow, which was
more contained in the scour hole the time averaged streamwise velocity profile illustrated
infigureexhibited higher constant negative velocity for a run performed after mixing 20
% of clay content in non-cohesive sediment than the run conducted after mixing 10 %, 5
% and 0 % of clay content in non-cohesive sediment in succession.
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0 20 40 60 80 100 120 140
Time (min)
Velocity(m/s)
Fig. 4.31 Time-velocity variation for 3.8 cm pier model
(before mixing of clay in non-cohesive sediment)
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37
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0 20 40 60 80 100 120 140Time (min)
Velocity(m/s)
Fig. 4.32 Time-velocity variation for 3.8 cm pier model
(after mixing 5 % of clay in non-cohesive sediment)
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 20 40 60 80 100 120 140Time (min)
Velocity(m/s)
Fig. 4.33 Time-velocity variation for 3.8 cm pier model
(after mixing 10 % of clay in non-cohesive sediment)
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38
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0 20 40 60 80 100 120 140
Time (min)
Velocity(m/s)
Fig. 4.34 Time-velocity variation for 3.8 cm pier model
(after mixing 20 % of clay in non-cohesive sediment)
4.4.2 Turbulent Intensity
The vertical distribution of normalized streamwise turbulent intensity component
u+
[= 0.5
2/u U ], where u is the fluctuation ofU] at the upstream side of the pier
model depicted infigures 4.354.38.Due to the reversal of flow occur for the zone y+
>
-0.04, normalized streamwise turbulent intensity component u+ increases towards the
scoured bed and reduced in the vicinity of the bed. A distinguishable feature of
pronounced bulges was observed above y+
> -0.04 line, due to separation of flow inside
the scour hole. Because of the shuddering effect of the horseshoe vortices less distinct
spike as compared to 7.5 cm pier model was observed near the scour bed at the upstream
side of the 3.8 cm pier model. Same vertical distribution of normalized streamwise
turbulent intensity component u+
and pronounced bulges were observed for runs
conducted using different percentage of clay in non-cohesive sediment.
The vertical distribution of normalized vertical turbulent intensity component w+
in the scour hole at the upstream side of the pier model is also illustrated in the same
figures 4.354.38. As in this case, there was no shuddering effect of horseshoe vortex in
the vertical direction no spike was observed near the scour bed for w+but this pier model
also depicted almost similar distribution pattern ofw+
to that u+. Every run conducted in
different experimental condition containing different percentage of clay content in non-
cohesive sediment for such pier model shows similar result.
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39
-0.2
-0.15
-0.1
-0.05
0
0 0.5 1 1.5u
+, w
+
y+
u+
w+
Fig. 4.35 Vertical distribution ofu+
and w+
at vertical section
(before mixing of clay in non-cohesive sediment)
-0.2
-0.15
-0.1
-0.05
0
0 0.5 1 1.5u
+, w
+
y+
u+
w+
Fig. 4.36 Vertical distribution ofu+ and w+ at vertical section
(after 5 % mixing of clay in non-cohesive sediment)
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40
-0.2
-0.15
-0.1
-0.05
0
0 0.5 1 1.5u
+, w
+
y+
u+
w+
Fig. 4.37 Vertical distribution ofu+
and w+
at vertical section
(after 10 % mixing of clay in non-cohesive sediment)
-0.2
-0.15
-0.1
-0.05
0
0 0.5 1 1.5u
+, w
+
y+u+
w+
Fig. 4.38 Vertical distribution ofu+
and w+
at vertical section
(after 20 % mixing of clay in non-cohesive sediment)
4.4.3 Reynolds stresses
Figures 4.39 4.42 exhibits the vertical distributions of normalized Reynolds
stresses at the upstream section of the pier. In this case also, inside the scour hole,
distinguishable bulges was observed immediately below y+
= 0 line, as a result of flow
separation. In the vicinity of the scoured bed Reynolds stresses changed its sign due to
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41
reversal nature of flow and became small near the scoured bed. Similar distribution
pattern was observed for all experimental runs performed in different experimental
conditions.
-0.2
-0.15
-0.1
-0.05
0
-20 0 20 40 60 80 100uw+
y+
Fig. 4.39 Vertical distribution ofuw+
at vertical section
(before mixing of clay in non-cohesive sediment)
-0.2
-0.15
-0.1
-0.05
0
-20 0 20 40 60 80 100
uw+
y+
Fig. 4.40 Vertical distribution ofuw+
at vertical section
(after 5 % mixing of clay in non-cohesive sediment)
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42
-0.2
-0.15
-0.1
-0.05
0
-20 0 20 40 60 80 100
uw+
y+
Fig. 4.41 Vertical distribution ofuw+
at vertical section
(after 10 % mixing of clay in non-cohesive sediment)
-0.2
-0.15
-0.1
-0.05
0
-20 0 20 40 60 80 100
uw+
y+
Fig. 4.42 Vertical distribution ofuw+
at vertical section
(after 20 % mixing of clay in non-cohesive sediment)
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CHAPTER V
SUMMARY AND CONCLUSIONS
The experiments were conducted in four different sets using different
percentage of clay content in non-cohesive sand such as 20 %, 10 %, 5 % and without
clay (0 %). Depth and velocity measurements were taken at the upstream side of the
pier models by using Acoustic Doppler Velocimeter (ADV) in the laboratory flume.
Measurements were taken in the vicinity of the pier model for a maximum period of
two hours. Four pier size of diameter 7.5 cm, 3.8 cm, 2 cm and 1 cm were considered
for depth measurement in initial set of experimental runs containing non-cohesive
sand of mean diameter 0.15 mm. In such bed condition, velocity measurements were
performed for 7.5 cm and 3.8 cm pier model. For further sets of experimental runs,
thoroughly mixed clay content of 5 %, 10 % and 20 % in non- cohesive sand were
used for depth and velocity measurements in the vicinity of 7.5 cm and 3.8 cm pier
model.
Data captured in each run was used for the development of relationship
between time and depth and time and velocity as well as turbulent intensity in
streamwise and vertical direction and Reynolds stresses were plotted, which leads to
the following conclusions:
1. The time scale required to attain the maximum constant depth is importantparameter. The time required to attain maximum constant depth in non-
cohesive sand is less compared to the bed containing even small clay content
in non-cohesive sand. Therefore, low maximum constant depth was obtained
due to increment of clay content in non-cohesive sand compared to full non-
cohesive sand bed condition.
2.
The state of equilibrium provides the most important step toward simplifyingthe erosion problem from an engineering point of view because the maximum
equilibrium scour can be estimated as the most conservative design. Thus,
predicting the maximum constant scour is the most fundamental step to study
a scour problem.
3. The volume of scour hole at the upstream of the pier model was decreasedwith increase in clay content. As the flow is more contained in the scour hole,
the flow velocity in the scour hole of non-cohesive sand with higher clay
content was lower than the lower or without clay content.
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4. The vertical distribution of normalized streamwise turbulent intensitycomponent and Reynolds stresses for both type of pier model (7.5 cm and 3.8
cm) showed distinguishable features of the pronounced bulges and were
reduced in the vicinity of the scoured bed.
5.
The distribution pattern of vertical velocity componentw+
is almost similar tothat of streamwise velocity component u+ except the spike near the scour bed,
as there was no shuddering effect of primary vortex in the vertical direction.
6. The data captured for different conditions of experimental runs would beuseful for the development of mathematical models of flow field in a scour
hole at bridge pier. Thus, the accurate estimation of scour depth would be
possible in different bed conditions using flow field model.
7. Using similitude modelling (geometric/ dynamic/ kinematic) obtained resultscan be extended for real world situation.
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CHAPTER VI
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