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ESTIMATION OF BRIDGE PIER SCOUR FOR CLEAR WATER _ LIVE BED SCOUR CONDITION

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ESTIMATION OF BRIDGE PIER SCOUR FOR CLEAR WATER _ LIVE BED SCOUR CONDITION Powered By Docstoc
					   International Journal of Civil Engineering and CIVIL ENGINEERING – 6308
   INTERNATIONAL JOURNAL OF Technology (IJCIET), ISSN 0976 AND
   (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME
                             TECHNOLOGY (IJCIET)

ISSN 0976 – 6308 (Print)
ISSN 0976 – 6316(Online)                                                   IJCIET
Volume 4, Issue 3, May - June (2013), pp. 92-97
© IAEME: www.iaeme.com/ijciet.asp
Journal Impact Factor (2013): 5.3277 (Calculated by GISI)               © IAEME
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        ESTIMATION OF BRIDGE PIER SCOUR FOR CLEAR WATER &
                    LIVE BED SCOUR CONDITION

                                      1
                                       Prof. P.T. Nimbalkar
    Proffesor, Deptt. Of Civil Engineering; Bharati Vidyapeeth Deemed University, College Of
                                    Engineering, Pune-(411043)
                                          2
                                         Mr.Vipin Chandra
        Post Graduate (M.Tech. Hydraulic Engineering) Student at Bharati Vidyapeeth Deemed
                         University, College Of Engineering, Pune-(411043)



   ABSTRACT

           Scour is the Local Lowering of Stream Bed Elevation which takes place in the
   Vicinity or around a Structure Constructed in Flowing water.Scour around Bridge Piers takes
   place due to Modification of Flow Pattern in such a way as to cause Increase in Local Shear
   Stress. For Bridges Estimation of Correct Depth of Scour Below Stream Bed is very
   Important since that determines the Depth of Foundation.Hydraulic Engineers have
   Developed Equations to estimate scour depth with the help of Prototype and Laboratory
   Investigations. For this Study Commonly used Bridge Pier Scour Predictors are Testified
   against Published Laboratory data Obtained from Literature to ascertain which of the
   Predictors produce a reasonable estimate of Scour Depth. The Relative Accuracy of Various
   methods is determined by carrying out Statistical Tests, comparing measured & computed
   Scour Depth graphically &by Computing Percentage Difference in Computed & measured
   Scour Depth .

   Keywords: Local Scour, Laboratory Data, Scour Predictors.

   1)       INTRODUCTION

          Alluvial Streams are Sometimes Partially obstructed by Hydraulic Structures such as
   Spurs, Bridge Piers, Abutments, Guide Banks etc. In some other cases High Velocity Sheets
   of Water from Spillways & Sluice Gates Flow over loose Alluvial Material. In all these cases

                                                92
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME

the Bed Level in the Vicinity of the structures is Lowered as a result of Interaction between
the high Velocity Flow and the Loose Bed & Consequent Modification in the Flow Pattern,
such Local Drop in the Bed Level is known as Local Scour.
        The Knowledge of the Maximum Depth of Scour around such Structures is essential
from the Point of View of Safety of these Structures; Excessive Scour can undermine the
Foundations and lead to the Failure of the Structure. Proper design requires that the
Foundation be taken down to a Level Lower than the Anticipated level of the Scour hole. It
has been Reported that Since 1950 over 500 Bridges have Failed in U.S.A. & Majority of
Failures were Due to Scour of Foundation Material.
Laursen(1963) has defined Scour as Enlargement of a Flow Section by Removal of Material
Comprising the Boundary through the Action of the Fluid in Motion. Such Scour takes place
whenever the rate at which the Sediment is Transported at a point is greater than the rate of
Sediment Supply. Since, in General, the Rate of Sediment Transport Increases with Increase
in Shear Stress for a given Sediment, Scour Results when the Changed Flow Conditions
cause an increase in the Shear Stress on the Bed. Therefore, Analytical Prediction of Local
Scour can be done by First Predicting the Distribution of Shear Stress on a Channel Bed due
to the Introduction of a Structure. As Scour Progress the Shear Stress will Reduce & Scour
will reach its Limit when, at any point, the Shear Stress is Critical or the amount of Sediment
coming in equals the amount of Sediment going out.

AIMS & OBJECTIVES

   1) To Study various Equations given by various Researchers used for Estimation of
      Bridge Pier Scour under Clear Water & Live Bed Scour conditions.
   2) To Check the Validity of these Equations using the Data available from Literature.
   3) To Test the Predictive ability of Selected equations by comparing Computed Scour
      Depth with Observed Scour Depth by using Three- Statistical Tests.

DATA & METHODOLOGY

       The Experimental Tables & data are Collected from Research Papers &Literatures
published by Various Authors from their Studies (see references) which are as follows -:

-- SCOUR DEPTH EQUATIONS CONSIDERED IN THE PRESENT STUDY--

       EQUATIONS FOR CLEAR WATER SCOUR & LIVE BED SCOUR BOTH

   a) Richardson’s(1977)




Where K3accounts for Bed Forms ( It is 1.1 for plane Bed & Small Dunes, 1.1 to 1.2 for
Medium Dunes. 1.3 for Large Dunes.)




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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME

   b) Melville’s equation (1997)




Where KI = V/Vc if V/Vc ≤ 1 and KI =1 Otherwise,
Kd =0.57 log (2.24D/ D50) if D/ D50≤ 25 ,Kd= 1 Otherwise,
KyD=2.4 if D/y0 ≤ 0.7 ,, KyD =2     Otherwise.

   c) Jain’s



   d) Hancu’s




   e) Coleman




       EQUATIONS FOR CLEAR WATER SCOUR ONLY

       a) Breusers et al. (1997)

       Presented an Equation that was a Function of V/Vc and y0/D Only.



Where Kv= 0 for V/Vc ≤ 0.5 ,Kv=2 (V/Vc)- 1 for V/Vc ≤ 1, Kv= 1 for V/Vc ≥ 1

       b) Ettema et al (2011)




       c) Laursen and Toch(1956)




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   International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
   (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME

   ANALYSIS OF RESULTS & DISCUSSIONS

            1. Statistical parameters for validation of different equations
                       Three Statistical Tests were carried out to find the Equation which gives the
               minimum error in the computation of Scour Depth. These parameters include Theil’s
               Coefficient (U), Mean Absolute Error (MAE) and Root Mean Sqaure Error (RMSE)
               which are mathematicallydescribed by following equations;

                           U=

                          MAE =

                          RMSE =
          If U=0; for model of Perfect Prediction and U=1 for Unsuccessful Models
   The values of Theil’s Coefficient (U),MeanAboslute Error (MAE) & Root Mean Sqaure
   Error (RMSE) Calculated by different equations for different author’s data are Summarized
   in Table No. (a) & (b).

            a) FOR CLEAR WATER CONDITION

   Following Table is made by Collecting data from the Research Papers of various
   Authors mentioned as following -:

   Table 1-:Data obtained from D.Max Sheppard et al., 2004,
   Table 2-:Data obtained from Jihn- Sung Lai1et al., 2009,
   Table 3,4& 5-:Data Obtained from D.S. Jeng et al., 2006.
  Scour                Theil’s Coefficient;(U)               Mean Absolute Error (M.A.E.)          Root Mean Square Error (R.M.S.E.)
  Depth
Predictors     For      For     For      For      For     For     For      For     For     For     For     For     For     For     For
               Table   Table   Table    Table    Table   Table   Table    Table   Table   Table   Table   Table   Table   Table   Table
                (1)     (2)     (3)      (4)      (5)     (1)     (2)      (3)     (4)     ( 5)    (1)     (2)     (3)     (4)     (5)

Richardson    0.099    0.247   0.129    0.126    0.169   0.205   0.395    0.392   0.350   0.475   0.256   0.336   0.455   0.486   0.537


 Melville     0.281    0.316   0.365    0.086    0.237   0.779   0.866    1.03    0.267   0.535   0.910   0.918   1.566   0.337   0.644


Breusers et   0.276    0.504   0.374    0.292    0.685   0.495   0.500    0.755   0.765   1.119   0.573   0.254   0.955   0.855   1.27
    al.


Ettema et     0.300    0.350   0.207    0.190    0.251   0.960   1.040    0.791   0.668   0.790   1.023   1.25    0.839   0.683   0.931
    al.


Laursen&      0.259    0.342   0.146    0.303    0.290   0.509   0.794    0.516   0.891   0.726   0.699   0.798   0.553   1.30    0.955
 Touch


   Jain       0.296    0.348   0.317    0.315    0.299   0.573   0.784    0.794   0.921   0.740   0.749   0.783   1.09    1.178   1.01


  Hancu       0.284    0.466   0.280    0.260    0.242   0.543   1.533    0.699   0.736   0.578   0.630   2.915   0.914   0.923   0.792


 Coleman      0.136    0.332   0.179    0.193    0.198   0.246   0.423    0.444   0.518   0.477   0.316   0.347   0.575   0.658   0.568




                                                                     95
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME

   b) FOR LIVE BED SCOUR CONDITION

       Following Table is made by Collecting Data from the Research Papers of various
       Authors mentioned as Following

                     Table 6-Data Obtained from D.S. Jeng et al., 2006.

             Scour Depth        Theil’s          Mean           Root Mean
              Equation         Coefficient      Abolute        Square Error
                                  (U)         Error (MAE)        (RMSE)

                             For Table (6)    For Table (6)    For Table (6)
                                                                    0.755
             Richardson           0.204            0.647

                                  0.223            1.394            1.420
             Melville

                                  0.406            1.570            3.674
             Jain
                                                   1.238            2.048
             Hancu                0.479
                                                   0.779            0.756
             Coleman              0.272


        For Both the Tables i.e. a & b; The Minimum Values (Best Prediction) of Statistical
Parameters; U, MAE and RMSE are mentioned in BOLD UNDERLINED for different
equations,indicate more appropriate results for Scour Depth calculations. As per the
calculations of the Statistical Parameters, in case of Live Bed scour & Clear Water Scour
conditions reveals that in general, Richardson Method produce more reasonable estimate of
Scour Depth as compared to other methods given by various Authors.

CONCLUSION

       Following Conclusions are derived from Three Statistical Parameters i.e. Theil’s
Coefficient (U), Mean Absolute Error (M.A.E.) & Root Mean Square Error (RMSE).
       Eight Commonly used equations namely by Richardson, Melville, Breusers et al.,
Ettema et al., Laursen& Touch, Jain, Hancu, & Coleman for estimation of Scour Depth in
Clear Water conditions were selected & validated using Experimental Data of various
Authors. The study shows that The Richardson Formula gives a reasonable estimate of
Local Scour depth.
       In case of Live Bed Scour conditions; Five Commonly used equations namely by
Richardson, Melville, , Jain, Hancu, & Coleman for estimation of Scour Depth were selected
& validated using Experimental Data of various authors. The study shows that The
Richardson Formula only gives the reasonable estimate of Local Scour Depth.




                                             96
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME

Notations

D= Pier diameter
dse= Equilibrium Scour depth
dse/D = Dimensionless Equilibrium Scour depth.
       = Scour Depth Obtained from Field Observations.
       = Scour Depth Predicted from the Application of the Selected Scour Equation.
d50= Sediment Size for Which 50% of the Sediment is Finer.
Kd = Sediment Size Factor.
Kyd= Flow Depth Adjustment Factor
KI= V/Vc
K3= Bed form Adjustment Factor.
F = Froude Number of Incoming Flow Velocity.
Frc = Froude Number of Critical Velocity.
g= Gravitational force.
V= Approaching Flow Velocity.
Vc = Critical approaching flow velocity.
U= Theil’s Coefficient.
MAE = Mean Absolute Error.
RMSE = Root Mean Square Error.

REFERENCES

1)B.W.Melville & A.J Sutherland (J. Of Hydraulic Engg. 1988.114:1210-1226) “ Design
Method For Local Scour At Bridge Piers”.
2)D.Max Sheppard,M.ASCE1,MufeedOdeh M.ASCE2& Tom Glasser(J. Of Hydraulic
Engg.October,2004.130:957-963; ASCE/OCTOBER 2004/959) “ Large Scale Clear–Water Local
Pier Scour Experiments”.
3)D.S. Jeng; S.M. Bateni; E. Lockett (2006; The University Of Sydney,Deptt. Of Civil
Engineering,Enviornmental Fluids/Winds Group) “Neural Network Assessment For Scour Depth
Around Bridge Piers- Research Report No. R855”.
4)Jau-Yau Lu, M.ASCE1; Zhong-Zhi Shi2; Jian-Hao Hong3;Jun-Ji Lee; Ph.D.4; &Rajkumar V.
Raikar (Journal Of Hydraulic Engineering 2011.137:45-56; ASCE/JANUARY 2011 ) “Temporal
Variation Of Scour Depth at Non-Uniform Cylindrical Piers.
5)Jihn- Sung Lai1; Wen-Yi Chang2; & Chin Lien Yen, , F.ASCE3 (Journal Of Hydraulic
Engg.2009.135:609-614; ASCE/JULY 2009) “ Maximum Local Scour Depth at Bridge Piers
Under Unsteady Flow”.
6) R.J. Garde; K.G. RangaRaju (2000, New Age Publications); Mechanics Of Sediment
Transportation & Alluvial Stream Problems”.
7)Seung Oh Lee1& Terry W. Sturm, M.ASCE2 (Journal Of Hydraulic Engg.2009.135:793-802) “
Effect Of Sediment Size Scaling On Physical Modelling Of Bridge Pier Scour ”.
8)Subhasish Dey1,Sujit K. Bose2,&Ghandikota L.N. Sastry3. (J. Of Hydraulic Engg.
1995.121.869-876) “Clear Water Scour at Circular Piers: A Model”.
9)Thamer Ahmed Mohamed; MegatJohari M.M. Noor; Abdul HalimGhazali&Bujang B.K. Huat
(Am. J. Environ. Sci 1 (2):119-125,2005) “Validation Of Some Bridge Pier Scour Formulae
Using Field & Laboratory Data”.
10)Wen- Yi Chang1; Jihn – Sung Lai2 ;& Chin Lien Yen, F.ASCE3 (Journal Of Hydraulic Engg.
2004.130:905-913; ASCE/SEPTEMBER 2004) “ Evolution Of Scour Depth at Circular Bridge
Piers ”.

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