CLEAR WATER SCOUR AT CYLINDRICAL PIERS IN CLAY – SAND MIXTURES

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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











4.11 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






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4.15 Photograph of the scour hole for 7.5 cm pier model (before








4.19 Time-velocity variation for 7.5 cm pier model (before


4.20 Time-velocity variation for 7.5 cm pier model (after mixing

5 % of clay in non-cohesive sediment) 30





4.23 Vertical distribution ofu+ and w+ at vertical section (before


4.24 Vertical distribution ofu+ and w+ at vertical section (after 5


4.25 Vertical distribution ofu+

and w+

at vertical section (after 10



and w+



4.27 Vertical distribution ofuw+

at vertical section (before mixing


4.28 Vertical distribution of uw+ at vertical section (after 5 %




4.30 Vertical distribution of uw+

at vertical section (after 20 %



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4.31 Time-velocity variation for 3.8 cm pier model (before








4.35 Vertical distribution ofu+ and w+ at vertical section (before



and w+




and w+




and w+



4.39 Vertical distribution ofuw+ at vertical section (before mixing











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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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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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(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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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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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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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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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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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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12

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

)


(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

)



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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

)


0

2

4

6

8

10

12

14

16

0 20 40 60 80 100 120 140

Time (min)

Depth(cm

)



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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. 4.11 Photograph of the scour hole for 3.8 cm pier model



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(after 5 % mixing of clay in non-cohesive sediment)




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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



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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)



-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)




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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)



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


-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



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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+


and w+

at vertical section


-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


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


-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+


at vertical section



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-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+


at vertical section


-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+


at vertical section


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)




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-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)



-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)




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-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)



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+


and w+

at vertical section


-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



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-0.2

-0.15

-0.1

-0.05

0

0 0.5 1 1.5u

+, w

+

y+

u+

w+


and w+

at vertical section


-0.2

-0.15

-0.1

-0.05

0

0 0.5 1 1.5u

+, w

+

y+u+

w+


and w+

at vertical section


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+


at vertical section


-0.2

-0.15

-0.1

-0.05

0

-20 0 20 40 60 80 100

uw+

y+


at vertical section



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42

-0.2

-0.15

-0.1

-0.05

0

-20 0 20 40 60 80 100

uw+

y+


at vertical section


-0.2

-0.15

-0.1

-0.05

0

-20 0 20 40 60 80 100

uw+

y+


at vertical section



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43

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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44

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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45

CHAPTER VI

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