An Experimental Study of Local Scour Around Circular Bridge Pier

8/10/2019 An Experimental Study of Local Scour Around Circular Bridge Pier

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International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol:13 No:01 23

134901-2525-IJCEE-IJENS February 2013 IJENS I J E N S

Abstract Study of sour bridge piers is extremely importantfor the safe design of the piers and other hydraulic structures.

An experimental investigation of local scour around circular

bridge piers in sand is presented. The principal objective of this

study is to carry out a much longer duration Tests to evaluate

the time development of the local scour at cylindrical pier in

addition to the evaluation of the effectiveness of a pier shape

and different flow rates on the depth of local scour.

This study describes the variation of scour depths that may

occur at bridge piers. There has been a difficulty in estimation

of accurate scour depth, which includes similitude aspects of

laboratory experiments on scour at bridge piers, complicate the

development of reliable scour-estimation relationships. In a

practical sense, the difficulties imply that estimation

relationship can only be of approximate accuracy. Experimental

investigations have been studied to examine the maximum

depth of scour and its pattern along longitudinal as well as in

transverse directions.

It was found that the scour depth increases with time. In

addition, the maximum depth of scour is dependent on both time

as well as flow rate, it was noticed that maximum depth of scour

was increasing with increase of flow and time as well. However

results presented here are encouraging and are very much inthe agreement with the previous studies related to scour at

bridge piers.

I ndex Term-- Bridge pier, Scour, Time dependence,

Equilibrium scour depth, Sediment transport.

1. INTRODUCTION

Scouring may be defined as the removal of material around

piers, abutments, spur dikes, and embankments caus ed by flow

acceleration and turbulence near bridge sub-structural

elements and embankments.

Scour is the removal of sediment around or near structures

located in flowing water. It means the lowering of the riverbed

level by water erosions such that there is a tendency to expose

the foundat ions of a bridge. It is the result of the erosive action

of flowing water, excavating and carrying away material from

1Assistant Pro fessor, Civil Engineering Department , King Saud

University, Riyadh, KSA; phone: 0049501214635; fax: +49014677008;

e-mail:[email protected]

the bed and banks of streams and from around the piers and

abutments of bridges. Such scour around pier and pile

supported structures and abutments can result in structural

collapse and loss of life and property. The amount of this

reduction below an assumed natured level is termed scour

depth.

Pier scour is the greatest single cause of bridge failures.

With the prospect of more severe and more frequent floods

due to climate change, reducing the risk of bridge failure is

becoming increasingly important. Scour is a worldwidephenomenon and of great concern es pecially to civil engineers.

Any structure placed in a river, whether of natural or human

origin, will tend to promote scour and deposition due to a

sudden change in the flow direction or high velocity flow.

Scouring has long been acknowledged as a severe hazard to

the performance of bridge piers.

The type of local scour is classified according to the mode

of sediment transport in the approaching flow. They are clear

water scour and live bed scour. Clear water scour occurs when

sediment is removed from the scour hole but not supplied by

the approaching flow; while live bed scour occurs when there

is a general sediment t ransport by the approaching flow [1].A large amount of literature has been published on the local

scour at and around a bridge pier. The total scour at a river

crossing consists of three components that, in general, can be

added together [2]. They include general scour, contraction

scour, and local scour. Cheremisinoff et al. [3] on the other

hand divided scour into two major types, namely general scour

and localized scour.

Local scour at pier site has been subjected to many

investigations throughout the world and only very limited

success has been achieved by the attempts to model scour

computationally, and physical model remains the principal tool

employed for studying the scour at the bridges and the site ofother hydraulic s tructures [4].

Tamer et al. [4] presented in his research that the flow depth

and velocity have an appreciable effect on the local scour and

the data from the physical model showed that doubling the

flow depth will result in more than 200% increase in the scour

depth. It is necessary to involve the hydraulic engineers in the

design s tage for bridges to take care of hydraulic effects of the

flow on these bridges. Many methods were proposed for

estimating the local scour around piers at the bridge site, but

these methods were based mainly on the data collected from

An Experimental Study of Local Scour Around

Circular Bridge Pier in Sand Soil

Ibrahim H. Elsebaie1
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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physical models and field data need to be collected to verify

these methods [4].

Richardson [5] indicated in his study that bridge

foundations should be designed to withstand the effects of

scour without failing for the worst conditions resulting from

floods equal to the 100-year flood, or a smaller flood if it will

cause scour depths deeper than the 100-year flood. Bridge

foundations should be checked to ensure that they would not

fail due to scour resulting from the occurrence of a super flood

in order of magnitude of a 500-year flood [5].

Oscar [6] presented an experimental investigation on time

variation of three-dimensional scour-hole geometry at a circular

pier in sand. Time-dependent scour-hole geometry has been

measured by a new high-resolution non-intrusive method.

Experimental results provide information for a quantitative

definition of the different scour phases, namely initial,

development, stabilisation and equilibrium phase. The

obtained data from the study can be used in improving bridge

scour monitoring and testing results of numerical simulations

[6].

Jau-Yau Lu et al. [7] conducted a research for proposing a

semiempirical model to estimate the temporal development of

scour depth at cylindrical piers with unexposed foundations.

Acylindrical pier with a foundation is considered as

nonuniform pier. The simulated results obtained from the

proposed model are in good agreement with the present

experiment results with the experimental data. He concluded in

his study that model agrees satisfactory with experimental

data.

Qiping [8] indicated in his recent s tudy that scour prediction

methods developed in the laboratories and the scour equations

based on laboratory data did not always produce reasonable

results for field conditions. Recent research indicates that

laboratory invest igations often oversimplify or ignore many of

the complexities of the flow fields around the bridge piers.

Patrick D. A. [9] conducted a research; the use of collars for

reducing the effects of local scour at a bridge pier is presented

together with the time aspect of the scour development. The

adoption of a collar is based on the concept that its existence

will sufficiently inhibit and/or deflect the local scour

mechanisms so as to reduce the local scour immediately

adjacent to the p ier. The overall objective of the research is to

study the temporal development of the scour for a pier fitted

with a collar and a pier without a collar.

Many researchers have conducted various studies to

predict the maximum depth and diameter of scour hole. An

attempt has been made to review few previous studies related

to scour ( [10]. Scour has been the major concern for safety of

marine and hydraulic structures. A large number of hydraulic

structures failed as the local scour progresses which gradually

undermines the foundations . It is important to control the local

scour depth at downstream of hydraulic structures to ensure

safety of these structures [10].

Recent study by Guney et al. [11] showed that the Local

scours around bridge piers influence their stabilities and play a

key role in bridge failures. In his study local scours around

bridge piers resulting from uns teady flow was measured. Sabita

and Maiti [12] performed study in the field of Local scour

around a cylindrical pier in a channel with an erodible bed or

natural bed . It was concluded that the highly unsteady

complex flow field around a circular pier produces scour hole

mainly for the presence of vortices. The main mechanism that

drives the formation and evolution of the scour hole around

bridge pier is horse shoe vortex motion.

Failure of bridges due to local scour has motivated many

investigators to explore the causes of scouring and to predict

maximum scour depth at bridge piers [13 &14].

In this work, an experimental study was conducted to

investigate the effect of the pier shape, discharge and time on

the main scour hole dimensions. Also, the maximum depth of

scour and its pattern along the longitudinal as well as in

transverse directions were investigated.

2.

EXPRIMENTALSETUP

2.1 Experiment Apparatus

Experiments were conducted in a rectangular transparent

glass flume in the hydraulics laboratory, College of

Engineering, King Saud University. The overall length of flume

was measured to be 9.45 m. This length includes inlet, outlet

and the working section. The length of flume was found to be

sufficient to provide stable flow conditions in the flume. The

flume was 45 cm deep with a bed width of approximately 30 cm.

The flume was constructed on an adjustable steel frame, 1.3 m

above the laboratory floor. The flume is provided with two

controlling gates, one vertical gate upstream of the workingsection and a tailgate downstream of the flume. Water into the

channel was supplied from a sump tank constructed below the

floor level of the laboratory. Centrifugal pump, having a

maximum capacity of 27 l/s, serve the purpose of lifting water

and supplying it to the channel. Discharge was measured by a

V- notch fitted at the end of the flume. Two point-gauge

mounted on a sliding aluminum frame was utilized to measure

surface elevations at upstream and downstream of the pier.

2.2 Experimental Procedure

A cylindrical pier is placed at the middle of the channel

section of the flume. The cylindrical shape of the pier is similarto the circular bridge pier. The wooden circular pier of 5 cm

diameter was fixed on the flume bed at 2.5 m from the upstream.

The bed was leveled thoroughly with the sand and initial level

(elevation) of s and bed was taken with the sliding point gauge

prior to the start of flow in the channel. All the levels of bed

with different time intervals were taken with the same moving

gauge installed at upstream as well as down stream sep arately.

Every time runs were started by allowing the water to flow over

horizontal bed with a defined flow rate.


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For every run a different discharge was maintained and

the water was allowed to flow for successive time periods of 5,

10, 15, 30, 60 and 120 minutes. After each and every defined

time interval the elevation of the sand bed was gauged with the

same moving gauge. Scour depth measurements were taken

along three directions namely X1 (in the center line of

pier/longitudinal), X2 in the same longitudinal direction but

near to channel boundary and parallel to center line of pier,

variation of scour depth was also taken in transverse

direction(Y), as shown in Fig. 1. The arrangement of the pier in

the flume is shown in Fig. 2 (a and b).

Fig. 1. Circular bridge pier

Fig. 2. (a & b) Arrangement of the cylindrical pier in the flume

2.3 Sediment Material

The bed material was a mixture of sand with grain sizes ranging

between 0.075 and 2.00 mm. The s and was filled in the working

section of the channel up to a layer of approximately 11 cm in

thickness. The variation of size of bed material (san d) has been

presented in table I.

TABLE I

SIEVE ANALYSIS

Sieve

No.Dia (mm)

Weight

ret.% Retained % Passing

# 10 2 0 0 100

# 16 1.18 78.8 15.8 84.2

# 20 0.850 466.6 93.3 6.7

# 40 0.425 499 99.8 0.2

# 60 0.250 499.2 99.8 0.2

# 100 0.150 0 0 0

# 200 0.075 0 0 0

3. RESULTS AND DISCUSSIONS

The variation in the depth of scouring along the channel

and in the vicinity of the pier can be directly observed through

a scale attached at different sections on the Plexiglass each at

10 cm interval and at the pier face. The Scour pattern at bridge

pier and the Scour in the vicinity of pier are shown in Fig. (3

and 4).

Fig. 3. (a & b): Scour patt ern at bridge pier

b

(a)

b

a


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Fig. 4. (a& b): Scour in the vicinity o f pier

The results obtained show the variation of scour depth with

distance X1, X2 in longitudinal direction and with y in

transverse direction respectively, as shown in Fig. (5-7) for one

of the flow rates used. In this set of run the flow rate was

maintained at flow rates range between 5.82 and 16.88

liter/second. The observations obtained from all runs with

different flow rates in the longitudinal directions X1 & X2

showed that the depth of scour is comparatively higher in

ups tream while it is less in down stream side of the pier. It was

found that the maximum depth of scour was attained

approximately after one hour of run and there was a very little

increase in scour depth in next one hour of run. Scour depth

variation in the longitudinal direction X1was gauged in the

center line of pier while scour depth variations in the

longitudinal direction X2, for different time intervals (starting

from 5, 15, 30, 60 & up to 120 minutes ), was gauged near to the

channel boundary. As evident from the graphs of X1 series

that the level of the sand bed was bit higher than the initial

elevation of bed in the down stream of pier which was

subsequently reducing with the higher time interval, the reason

behind it is the deposition of eroded material from the vicinity

of upstream of the pier. The variation of scour depth with

distance Y in transverse d irection was found to be almost same

towards both the sides of pier. Centre line of pier is located at

X1= 32.5 cm. SeriesI data were recorded by putting switch

on and off for flow with every reading while series II data

were recorded in continuation of flow.

Fig. 5. Variation of scour depth with longitudinal direction (X1)

Fig. 6. Variation of scour depth with longitudinal direction (X2)

Fig. 7. Variation o f scour depth with t ransverse direction (Y)

All the results depicted here show an increase in maximum

depth of scour with the increase of flow rate in longitudinal as

well as in transverse direction. The variation of scour depthwith distance Y in transverse d irection was found to be almost

like same trend towards both side of the pier but maximum

depth of scour was noticed to be little more compared to

results presented with smaller discharge.

The maximum scour depth around the cylindrical pier is

measured for different flow rates and the experimental results

are presented in graphical form to predict the equilibrium scour

depth around the cylindrical pier. The variation of depth of

scour hole is presented in Fig. (8 and 9). It is observed that as

the depth of flow increases the scour hole depth increases but

a

b


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rate of increment is not linear. It is clear that the maximum

depth of scour is dependent on both time as well as flow rate.

Also, it was observed that as increasing the flow in the

channel initial depth of scour hole is increasing. As time

increases saturation comes and the equilibrium scour depth

reaches.

Fig. 8. Variation of maximum scour depth with t ime

Fig. 9. Variation of maximum scour depth with t ime

The results obtained by this study confirm the results obtained

by [6, 7, 9, 13], where it was obs erved that the maximum depth

of s cour is highly dependent on the experimental duration. The

depth of the scour hole increases as the duration of the

increased flow that initiates the scour increases. The extent of

scour observed at the pier also increases as the duration of the

tests increases. It was found by Patrick [9] that the temporal

development of the scour hole at the pier was dependent onwhether or not the pier was fitted with a collar placed at the

bed level.

4. CONCLUSION

Local scour monitoring is very important to avoid major

damages that may occur. An experimental investigation on time

variation of three-dimensional scour-hole geometry at a circular

pier in sand has been presented. The experimental results

provided information for a quantitative definition of the

different scour phases, namely initial, development,

stabilisation and equilibrium phase. Performing the sequence

of experiments and analyzing the results presented here in this

study for local scour at bridge piers, following conclusions can

be drawn:

The depth of scour increases with time, however it was

found that the rate of increase of scour depth was

decreas ing for a longer time interval. Rate of flow does

affect the depth of scour; scour depth was more withhigher flow rate.

Maximum scour depth was observed to occur at the

upstream of the pier. The maximum depth of scour is

dependent on both time as well as flow rate, it was

noticed that maximum depth of scour was increasing

with increase of flow and time as well.

It is observed that the coarse portion of the sediment is

deposited at downstream zone of the pier. However

scour hole dimensions in the transverse direction was

found to be almost same.

Result indicates that scour may take a relatively long time

to reach an asymptotic state.

The presented data can be used in improving bridge

scour monitoring and testing results of numerical

simulations. Thus, it is clear from the study that the scour

at bridge pier is very important for design of protection

works and hence sufficient provisions should be made

during designs against expected scour at bridge piers.

ACKNOWLEDGMENT

The experiments for this study were carried out in the

Hydraulics Laboratory of the College of Engineering, King

Saud University, Kingdom of Saudi Arabia. The author is

grateful for the support provided by the Laboratory staff.

REFERENCES[1] Dey, S., Bose Sujit K. and Sastry, L. N., Clear water scour at

circular piers: A model, , Journal of Hydraulic Engineering,

1995; 121(12), 869-876.

[2] Richardson, E.V. and Davies, S.R., Evaluating scour at bridges.

Rep. No. FHWAIP- 90-017 (HEC 18), Federal

Administration, U.S. Department of Transportation,

Washington, D.C., 1995.

[3] Cheremisinoff et al., Hydraulic mechanics 2. Civil Engineering

Practice, Technomic Publishing Company, Inc., Lancaster,

Pennsylvania, U.S.A., 1987; 780 p.

[4] Thamer A. M., Megat J. M., Mohd N., Abdul Halim G.,

Badronnisa Y.and Katayon S., Physical Modeling of Local

Scouring around Bridge Piers in Erodable Bed J. King Saud

Univ., Vol. 19, Eng. Sci. (2), 2007; 195-207.

[5] Richardson, E.V., "Instruments t o Measure and Monitor Bridge

Scour" , First International Conference on Scour of

foundations, ICSF-1, College Stat ion , T exas, 17-20 Nov, Vol.

2, 2002; 993-1007.


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[6] Oscar Link, Time Scale of Scour around a Cylindrical Pier in

Sand and Gravel, Third Chinese-German Joint Symposium on

Coastal and Ocean Engineering National Cheng Kung

University, Tainan November 8-16, 2006.

[7] Jau-Yau Lu, M. Asce, Zhong-Zhi Shi, Jian-Hao Hong, Jun-Ji

Lee, and Rajkumar V. Raikar, Temporal Variation of Scour

Depth at Nonuniform Cylidrical Piers, Journal of Hydraulic

Engineering , 2011.

[8] Qiping Y., Numerical Investigations of Scale Effects on Local

Scour around A Bbridge Pier. A Thesis submitted to the

Department of Civil and Environmental Engineering In

part ial fulfillment of th e Requiremen ts for the degree of

Master of Science, 2005.

[9] Pat rick D. A.., Time Development of Local Sscour at A

Bridge Pier Fitted with A Collar. A Thesis Submitted to the

College of Graduate Studies and Research in Partial Fulfillment

of the Requirements for the Degree of Master of Science in

the Department of Civil and Geological Engineering

University of Saskatchewan Saskatoon, Saskatchewan,

Canada, 2006 .

[10]Padmini K. and Asis M., Local Scour Around Hydraulic

Structures, International Journal of Recent Trends in

Engineering, Vol. 1, No. 6, May 2009.[11]Guney M.S., Aksoy A.O. and Bombar G., Experimental Study

of Local Scour Versus Time Around Circular Bridge Pier, 6th

Interna tional Advanced Technologies Sym posium (IATS11),

16-18 May 2011, Elaz, Turkey

[12]Sabita Madhvi Singh #, P . R. Maiti * Local Scouring around a

Circular Pier in Open ChannelInternational Journal of

Emerging Technology and Advanced Engineering Website:

www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 5, May

2012)

[13]Melville, B.W. and Chiew, Y.M., T ime scale for local scour at

bridge piers. Journal of Hydraulic Engineering, ASCE, 1999;

125(1): 59-65.

[14]Ting et al., Flume tests for scour in clay at circular piers.

Journal of Hydraulic Engineering, ASCE, 2001; 127(11): 969-

978.

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An Experimental Study of Local Scour Around Circular Bridge Pier