Scour Hazard Assessment and Bridge

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Scour Hazard Assessment and Bridge

Abutment Instability Analysis

Pezhman Taherei GhazvineiWater Resources Engineering Ph.D. Student

Civil Department, Faculty of Engineering, University Putra Malaysia

e-mail: [email protected]

Thamer Ahmed MohamedProfessor

Department of Civil Engineering, University Putra Malaysiae-mail: [email protected]

Abdul Halim GhazaliProfessor


Bujang Kim HuatProfessor


ABSTRACTSocial and economic detriments are the most important consequences of a bridge failure. Stability

problems of such structures against failure and the depth of the abutments are directly related tothe amount of the adjusted scour. Economy, reliability, and stability have been main concerns

regarding designing abutment bridges enhanced preventing scouring. In this paper a detailed

comparison of the up-to-date works on scour at the bridge abutment are presented including all

possible aspects and scour depth estimation formulas. The experimental data have been collected

from the literature for investigation of prediction method of the abutment local scour depth. It is

found that the scour at bridge is often over-predicted. The Laursen (1963) and Melville (1992)formulas give relatively acceptable prediction of local scour at abutments. These observations are

supported by the statistical tests. Multiple experiments may need more research on full scale

abutment bridge case histories. Additional data and analysis would allow promoting the abutmentbridges design and decreasing the bridges cost.

KEYWORDS: Scour formula, Local scour, Bridge stability, Abutment, Statistical test.

INTRODUCTION

Abutments are structures at the two ends of a bridge which serve the double objectives of

transferring the loads from the superstructure to the foundation bed and giving sidelong support to the

approach embankment. Economy, dependability, and strength are the main reasons of attention to

increase international concerning in a bridge structure without movement joints at the junction of the
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deck on the abutments, named jointless or abutment bridges. If this kind of bridge is constructed on a

waterway, abutments acquit a third function, to protect the embankment against scour of the stream.

Lack of load capacity and bridge scour are the most common reasons of bridge instability. Load

capacity is happened because of an ever-increasing demand for larger volumes of traffic and heavier

trucks and scour occurs because of the erosive action of flowing water, which excavates and carriesaway materials from streambeds and banks bridge foundations by the normal flowing water or flood.

Although it may be greatly affected by the presence of structures encroaching on the channel, scour is

a natural phenomenon caused by the flow of water over an erodible boundary, where the shear stress

generated by the flowing water on the streambed is the basic erosive stress, and the streambed

materials provide the resisting stress against erosion. Scour reaches its equilibrium status when thesetwo stresses get balanced. Study on 503 bridge structures failures in the United States from 1989 to

2000 showed that, the major reason for damage or failure of bridges are those related to scouring at

bridge piers and abutments (Wardhana et al., 2003).

Bridge failure or its' structural damage through the scour leads to serious loss for public safety

and economy. Designing the bridge foundation safely needs an accurate estimation of scour depth,

underestimation may lead to bridge failure while over estimation will lead to excessive constructioncost. In this paper comprehensive review of the up-to-date work on abutment local scour prediction

methods have been evaluated supported by laboratory data analysis and statistical test.

BRIDGES INSTABILITY DUE TO SCOUR

Truss bridge main structures are shown in figure 1. Furthermore Figure 2 shows jointless

(abutment) bridges. In comparison with truss bridges, abutment bridges have many advantages such

as being less expensive in terms of initial cost and long term maintenance.

Scour excavates the bridge structure and structural instability causes bridge failure, whereas scourmay happen at any time. This event predisposed to occur without any previous warning to bridge

structure (Duc et al., 2007). The main parameters which affect scour at bridge contain; floodplain

characteristics, bed materials, channel protection situation, bridge geometry and flow hydraulicsduring the scouring time (Deng et al., 2010). The engineering design of such a hydraulic structure

requires consideration of these factors. As a matter of fact, balancing safety and cost is the mainproblem. Credible predictions of scour depths can assist the design engineers to monitor and correct

the scour problem before any bridge failure or instability. Abutment scour is found to be primarily a

concern for bridges over smaller rivers and streams than for larger rivers, because inadequate design

and monitoring attention has been given to abutment scour at the many small bridges (Ettema et al.,

2004).

Figure 1:Truss Bridge.


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Figure 2:Jointless (abutment) Bridge.

Excavating and carrying away material from the bed and banks of streams is named general scour

(Richardson et al., 2001). The long-term or short-term scour can be recognized by the time taken for

general scour development (Bruce W. Melville et al., 2000). The structures increase the local flow

velocities and turbulence level, and depending on their spaces, can give rise to vortices that exert

increased erosive forces on the adjacent bed and remove the sediment material in the surroundings of

the bridge piers or abutments. This kind scour is called local scour (May et al., 2002). The total depthof scour which includes general and localized scour is called total scour. Localized scour can occur as

either clear-water scour or live-bed scour. Clear water scour occurs in relatively low flows when the

bed material upstream of the scour area is at rest. At the bridge area, bed material is removed and

transported away but there is no deposit of material from upstream simultaneously (Maddison, 2012).

Live bed scour occurs by the continuous erosion and deposition when there is general sediment

transport during periods of flooding (Bruce W. Melville, et al., 2000). Contraction scour occurs as a

result of the constriction of a waterway, and/or a natural means or human adjustment of the

floodplain. The overall effect of this phenomenon is the lowering of the channel bed (Alabi, 2006).

An overview of the scour distribution around bridge abutments is showed graphically in figure 3.

According to the result of the perfect laboratory experiments applying rectangular channels and

uniform sediment, it was conducted that the accuracy of scour depth estimate is less than the

measured scour depth of the field or laboratory conditions (Hong, 2005).

Figure 3: Concepts in local and contraction scour at bridge abutment.


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ABUTMENT LOCAL SCOUR

An Overview of Abutment Local Scour Prediction

Abutments are characterized considering of abutment form, structure, and layout. These

characteristics along with channel morphology and sediment characteristics, lead to the three regions

of abutment scour as shown in figure 4. Abutment layout and flow field, along with the erodibility of

sediment and soil at bridge site may cause the deepest scour to occur at any, or all, of three locations.

Scour condition 1 is located in the main channel near abutment; scour condition 2 occurs in a short

distance downstream of the abutment; and scour condition 3 is located at the abutment itself.

Maximum scour depth at these locations can be different and occurs at different rates, in conformity

with soil and flow-field characteristics.

It was predicated that the obstruction ratio effect on the time development of abutment local scour

depth may not be large; however, important effect can be observed (Ballio et al., 2009). The

dimensional analyzing of abutment scour in compound channels for bankline and setback abutments

showed that the maximum scour depth was approximately 10 times the unconstricted floodplaindepth, but this value may be reduced by large main channels with very rough floodplains, larger

sediment sizes and less backwater or lower floodplain velocities (Sturm, 2006). It was detected that

the maximum scour depths occurred around the upstream corner of the vertical-wall abutments

(Yanmaz et al., 2007a). Increasing approach flow depth increases the scour depth (Dey & Raikar,

2005). If the abutment length decreases and the width of the channel be constant, abutment scour

depth approaches to zero (Ettema et al., 2010). Increasing the relative abutment length, increases the

distance between the abutment and the point of maximum abutment scour depth (Cardoso et al.,

2010).

Figure 4: Three regions of scour in abutment bridges (Yorozuya, 2005)


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Equation (1), listed in Table 1, was suggested for simple predicting local abutment scour in clear

water condition (Liu et al., 1961). The scour process was investigated considering local and general

characteristic values and contraction ratio between obstructed cross section and the width of the flume

in long duration scour laboratory tests around bridge abutments in clear water flow. The results

showed that there was a positive correlation between contraction ratio and the depth of the scour andthe bridge pier model was not applicable directly to prediction of scour depth in the bridge abutment

(Ballio et al., 2001). Local scour depth development was analyzed in a several experiments at

vertical-wall abutment with different lengths of abutment in clear-water flow and uniform sediments.

Analyzing the records demonstrated that dimensionless time to equilibrium scour can be explained as

a relation between flow intensity and the length of the abutment (Coleman et al., 2003). The data forclear water and uniform sediment in compound channel were analyzed to develop a scour prediction

model based on the functional relationship that is shown in equation (10) by Kouchakzadeh (1996).

3D flow field in scour hole under clear-water condition at a 45 wing-wall abutment was measured by

conducting experiments in a laboratory flume.

The profile of turbulence intensity and the average of time velocity elements, Reynolds stress and

turbulent kinetic energy at different horizontally angle planes was captured. Plotting the contours ofthe vortices showed that inside the scour hole the positive vortices is compressed, although at the

surface of the abutment wall the negative vortices can be seen (Dey et al., 2010). The relation

between abutment geometry, scour-hole and turbulence elements and solid structures was

investigated in a laboratory experiments with three different geometrical phases. According to erosion

processes these three phases are flat bed formation (FB), the logarithmic scour formation (LS) and the

equilibrium scour formation (ES). Study of time-averaged coherent construction in these experiments

showed that in the last two phases (LS and ES) the development of the scour-hole increased the

vortex complexity. It was recounted that the formulas based on the average of the shear stress at the

wall border and critical Shield motion for sediment were not suitable for representation the local

erosion characteristics (Bressan et al., 2011). The scouring processes under clear-water scour

condition were studied using non-uniform and uniform sediments with different sizes of vertical

abutment walls and 45 wing wall. The time variation of scour depth was computed by numerical

solving of the different equations that are based on the conception of the sediment conservation.

According to the results, several parameters that affect the non-dimensional depth of the

equilibrium scour were the ratio of the length of the abutment and sediment diameter, the ratio of the

depth of the flow and the length of the abutment and the excess abutment Froud number (F). Theequations No. 11-13 were obtained to compute the equilibrium scour depth in different abutment

shapes by Dey (2005). Based on the equation (No. 14) purposed by Oliveto et al., 150 laboratory

experiments were conducted to study the effects of sloping abutment and unsteady flow on scour

development process. Results showed when the threshold Froud number is larger than 0.60 the

proposed formula computed a confident scour depth, meanwhile the formula were restricted to a

larger amount of threshold Froud number, F= 1.20 (Oliveto et al., 2005). In order to investigate thetime-dependent variation of scour hole shapes with size and location of maximum scour depth, series

of experiments were conducted with uniform sediment under clear water condition. Scour contoursand variation of scour depth around vertical-wall abutment were measured temporally. It was

observed if the length of the abutment decreases the horizontal distance between the sediment

deposition location and maximum scour depth decreases. Also the maximum scour depths were took

place at the upstream surface of the abutment. An empirical formula for time-dependent surface area

and volume of the scour holes was extended as equation No. (15), (Yanmaz et al., 2007b). Some well-

known equations on abutment local scour, in clear water condition, are provided in Table 1.


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Abutment Local Scour Data and Statistical Test

The laboratory abutment local scour data under clear-water conditions used in this study were

obtained from literature review (Dey & Barbhuiya, 2005), including 100 abutment local scour depth

data. Statistical tests were conducted to evaluate the predicted scour depths at abutment location forthe physical model using the Liu (1961), Laursen (1963), Melville (1992, 1997), Kouchakzadeh

(1996), Olevieto (2002) and Yanmaz (2007) methods. The predicted abutment local scour depths that

were obtained from the selected formulas of table 1 and laboratories experimental data were all used

in computing the parameters of the statistical tests. The statistical tests include the Regression,

Correlation, Mean Absolute Error(MAE), Root Mean Square Error (RMSE) and the Theils

coefficient(U), that mathematically are described by Equations (16-18). In order to evaluate the

prediction accuracy of each equation, the scattered points of observed and computed abutment local

scour depths have be compared with the line of perfect agreement.

RESULTS

Figures 5-11 show scattergrams for the predicted and measured abutment local scour depthsobtained from the application of the selected formulas and laboratory experiments, respectively. It

appears that the Melville (1992) and Laursen (1963) formulas give reasonable prediction, while the

Yanmaz (2007), Kouchakzadeh (1996), Liu (1961), and Oliveto (2002) formulas appear to over-

predict and the Melville (1997) formula appears to under-predict the abutment local scour depth. This

is perhaps to be expected for the fact that the formulas are obtained from experimental studies using

laboratory flumes with rectangular cross section and have flat immovable walls, while most of the

natural channels are non-rectangular with instable and rough bed and banks. Moreover, flow

distribution through the natural channel is non-uniform. Under-predictions were less common than

over-predictions, but occurred in some of the methods. This observation is supported by the statistical

tests conducted on selected formulas as shown in Table 2.

Figure 5: Comparison Between Measured AbutmentLocal Scour Depths Obtained from Experiments andComputed Scour Depths Using Liu (1961) Formula.

Figure 6: Comparison Between Measured AbutmentLocal Scour Depths Obtained from Experiments and

Computed Scour Depths Using Laursen (1963) Formula.


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Figure 7: Comparison Between Measured AbutmentLocal Scour Depths Obtained from Experiments andComputed Scour Depths Using Melville (1992) Formula.

Figure 8: Comparison Between Measured Abutment

Local Scour Depths Obtained from Experiments andComputed Scour Depths Using Melville (1997) Formula.


Computed Scour Depths Using Kouchakzadeh (1996)Formula.


Computed Scour Depths Using Oliveto (2002) Formula.


Computed Scour Depths Using Yanmaz (2007) Formula.


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CONCLUSION

A comprehensive review of the up-to-date investigations on abutment local scour depth is

introduced in this paper. Different type and location of scour in abutments bridges and various

approaches conducted for predicting local scour depth at abutment were presented. Laboratory studiesdesigned for investigation of abutment bridge scour were collected and summarized. The

experimental data for predicting the abutment local scour as a function of variables characterizing the

flow, abutments and sediments collected from the literature.

Statistical and graphical comparison between the predicted and observed depth of abutment local

scour from experimental data showed that for predicting the local scour depth at abutments the

Laursen (1963) and Melville (1992) formulas give comparatively preferable prediction, while the Liu

(1961), Kouchakzadeh (1996), Yanmaz (2007) and Oliveto (2002) formulas appear to over-predict

and Melville (1997) formula appears to under-predict the abutment local scour depth.

It was observed that most of the formulas give an over prediction local scour depth at bridge

abutments, especially when compared with the recorded scour at the abutment site. This statement

was supported by the statistical tests, i.e., when the Theils coefficient, U, Mean Absolute Error(MAE), and Root Mean Square Error (RMSE) of each of the selected formulas were compared.

According to the literature review and statistical analyzes, if the channel and bridge geometry

condition change, then most empirical equations for predicting abutment local scour depth may not be

appropriate.

In laboratory model studies, internal flow characteristics do not truly represent prototype bridge

abutment scouring in the river in view of large-scale distortion of models. Most of the recent methods

on scour prediction have the limitations of the laboratory conditions based on the method

assumptions, the simplified test geometry conditions, the flume tests and the numerical simulation

method. Although the review showed many equations have been developed to predict bridge

abutment scour, and a large database for flume measurements is now available, the problem of scour

at bridge abutment has remained little understood.

The main deficiency of prior studies is that they do not consider local scour development at

abutments related with contraction bridge zone. Hence, multiple experiments may need more researchon full scale abutment bridge case histories. Hence, multiple experiments may need more research on

full scale abutment bridge case histories. Additional data and analysis would allow promoting the

abutment bridges design and decreasing the bridges cost. Accurate prediction of total scour depth in

an abutment bridge is crucial in bridges' stability designing. Underestimation can result in loss of life

and structural collapse while overestimation can cause large financial losses on construction of asingle bridge.


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APPENDICES

Table 1:Equations for uniform abutment local scour under clear water conditions

Reference Equation Applicability

Liu et al.

(1961)

= 12.5 (1) Clear-water condition

Laursen (1963)

= 2.75 .

. 1 (2) Clear-water scour at anabutment encroaching in to

the main channel

= 1.89. (3) At the threshold condition

Melville(1992)

= (4) Clear-water condition

= (5)

= 2 (6) = 2(). for1 25 = 2; (7) = 2 for < 1 = 10 (8) = 10 for > 25 Melville (1997) = (9) Clear-water conditionKouchakzadeh

(1996)

= 13.5. .. (10)

Clear-water condition,

uniform sediment in

compound channelF = Ug y

Oliveto et al.

(2002) = = 0,068 . (11)

Clear-water condition

z = (hb

) for

rectangular abutmentN= 1.25 for verticalabutment= (d d) T = t t t = z ( g ) g = ( )

Dey (2005)

= 8.689... (12) Clear-water condition forsemicircular abutment = 7.281... (13) Clear-water condition forverticall-wall abutment

= 8.319.

.

. (14) Clear-water condition for

45 wing-wall abutment

Yanmaz et al.

(2007) = 0.25.( ).(). (15)

Clear-water condition,

uniform sedimentT = t d(gd). L = ( ) Mean Absolute Error (MAE) is a quantity used to measure how close predictions are to the

eventual outcomes. The mean absolute error is given by;


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MAE= || (16)Root Mean Square Error(RMSE) is a frequently used measure of the differences between values

predicted by a model or an estimator and the values actually observed. RMSE is a good measure

of accuracy and defined as the square root of the mean square error:

RMSE= (17)Theils coefficient,, is a measure of nominal association and is define by following equation:

= ()() () () (18)where, is the error in the predicted abutment local scour depth for event of the record from theapplication of the formulas and is number of records. U is Theils coefficient ( 0 for model ofperfect prediction and

1for unsuccessful model).

(d)is scour depth obtained from

experiments or laboratory observation and (d) is the corresponding predicted scour obtained fromthe application of the selected scour formulas. The smaller values of MAE, RMSE and obtainedfrom equations (1-15) indicate an effective prediction.

Table 2: Summary of the Statistical Tests on the Selected Formulas (Table 1)

Scour Equation

Regression CorrelationMean Absolute

Error, MAE

Root Mean

SquareError,

RMSE

Theils

coefficient,

U

Computed

Data

Computed

Data

Lab.

Data

Computed

Data

Computed

Data

Computed

Data

Liu et al. (1961) 0.142 0.576

0.

048

0.204 0.187 0.674

Laursen (1963) 0.614 0.686 0.120 0.044 0.282

Melville (1992) 0.573 0.611 0.214 0.048 0.247

Melville (1997) 3.463 0.596 0.123 0.048 3.411

Kouchakzadeh

(1996)0.049 0.227 0.199 0.253 0.696

Oliveto et al. (2002) 0.227 0.825 0.127 0.151 0.587

Yanmaz et al.

(2007)-0.002

-0.0100.993 0.344 0.808

No cases of negative scour were documented in the application of the equations for any of the

methods. Although negative scour could be explained as sediment accumulation, but could also beexplained as outside the range of conditions for the equations were designed and indicated that the

equations are not applicable for the given conditions. In these comparison, no satisfactory correlation

was found between observed abutment local scour and predicted abutment local scour from the two

prediction methods, but some of the individual methods applied in this study showed comparatively

preferable correlation with observed abutment local scour depths; like, Laursen (1963) and Melville(1992).


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ACKNOWLEDGEMENTS

The first author is grateful for Prof. Thamer Ahmed Mohameds useful comments to write this

article. Also the authors are extremely thankful from the prior researcher mentioned in this article,

because of their efforts to develop the understanding of the scour condition by performingexperimental investigations.

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