Scour Around Bridge Piers

PINSA 64, A, No.4, July 1998, pp. 569-580.e Printed in India

SCOUR AROUND BRIDGE PIERS

R ] GARDE* AND U C KOTHYARI***/NSA Senior Scientist, Central Water and Power Research Station, Khadakwasla, Pune-4'11 024

**Civil Engineering Department, University ofRoorkee, Roorkee (u.P.)(Received 10 June 1997; Revised 05 December 1997; Accepted 04 February 1998)

The paper describes the phenomenon of scour around bridge piers and then enumerates the methOds for its prediction.The scour data from prototype bridges are analysed to comment on the relative accuracy of fo~r methods of s~ourprediction. Brief comments are made on scour ~ound bridg~ piers in clayey bed and gravel-bed nvers. Lastly, vanoustechniques studied for scour control and protectlOn are des,"nbed.

Key W d' Scour in Alluvial Streams; Scour Estimation; Scour Prevention; Scour Protection Devices;or s. Lacey-Inglis Equation; LaurSen-Toch Equation; Melville & Sutherland's Equation; Chitale's

Method; Kothyari-Garde-Ranga Raju's Method

where B1 and D1 are width and depth of flow in theuncontracted section and Bi2 and D2 are thecorresponding values in the contracted section. Thethird type of lowering that takes place around thebridge pier is due to modification of flow structuredue to presence of the pier. Depending on the piershape and free stream condition, an eddy structurecomprising of one or more of the three eddystructures, namely horse-shoe vortex, wave vortexsystem and the trailing vortex system can fonn;this increases the local shear on the bed and causesscour. Typical fonnation of horse-shoe vortex isshown in Fig. 1. Lastly, additional scour can also

The stream bed lowering at the bridge can takeplace due to four primary reasons. If the bridge islocated downstream of a large dam, there is a slowlowering of the bed and reduction of stream slopedue to degradation. Degradation takes place whenthe stream transporting sediment becomesdeficient in sediment supply due to sediment beingstored upstream of the dam. In extreme cases thislowering can be as much as 4 to 6 meters.Secondly, if for reducing the cost of the bridge thestream is contracted by building guide bunds etc.,such contraction can cause additional lowering ofthe stream bed. The depth in the contracted sectionis given by

Introduction

Scour is the local lowering of stream bed elevationwhich takes place in the vicinity or around astructure constructed in flowing water. Scour takesplace around bridge piers, abutments, aroundspurs, jetties and breakwaters due to modificationof flow pattern in such a way as to cause increasein local shear stress. This in tum dislodges thematerial on the stream bed resulting in local scour.In the case of bridges, the estimation of correctdepth of scour below the stream bed is veryimportant since that detennines the depth offoundation. Hubert has stated that since 1950 over500 bridges in USA have failed and that themajority of the failures were related to the scour offoundation material. Such data are not available forthe Indian bridges; however, this has been thematter of concern to the Government of India andsome detailed hydrologic and scour studies havebeen undertaken at selected bridges by theconcerned organisations. This concern about safetyof bridges is primarily due to three reasons whichare: (1) inadequate knowledge about scourphenomenon when the bridges were constructe~;(2) inadequate data on which the design flood waschosen; and (3) increase in the loading on thebridge due to increase in size of trucks, wagons,and their frequency of operation.

Author tor Correspondence:Professor R J Garde, Professor Emeritus, CWPRS, Pune-411 024

DID I=(B/B~O.69 to 0.79,... (1)

570 R J GARDE AND U C KOTHYARI

/~/~~-~/~- ""

_/'/ " 'I/'lsPIER~ ~!

II '

l UJ J 1 Q.~'\'"J :I .,..fII-. $-c '.', _HORSE - SHOE VORTEX,/,) ''( -At! "-'~ I I /---. 1 >

SCOUR AROUND BRIDGE PIERS 571

Table II

The coefficient Ko=

Scour at an angle of inclination ()Scour when the flow is axial

dep~nds on pier shape and () and increases asshown in Table II.

For rectangular piers Ka will be function of ()and lib. Curves between these parameters are "givenby Laursen7

... (2)dsJD= l.4(bID)

indicated that dK -r31 while for sedimenttransporting flow dse-r07 Here dse and dK are thescour depths below general bed level for sedimenttransporting and clear water flows.

The effect of size distribution of the bedmaterial on the scour depth is more significant.When the standard deviation crg of the bed materialis large and the bed material contains somenonmoving sizes for a given discharge, the coarsermaterial would tend to accumulate in the scourhole and inhibit further development of scourdepth. Hence for the same median size, scourdepth will be smaller for material with largergeometric standard deviation crg Here

crg= 1/2(ds/dso-t:dscld'6)and dS4' dso and d 16 are such sizes that 84%, 50%and 16% material is finer than d84 , dso and d 16 sizesrespectively. The percentages 84 and 16 aresuch that for normal or Gaussian distributionds4=(dso+standard deviation) and (d 16 = dso-standarddeviation). If the correction factor Ka is defined as

Equilibrium scour depth for non -K = uniform material .

(j Equilibrium scour depth for uniform

material of the same median size. Ka would depend on ag On the basis of

experimental. data of Raudkivi9 and Kothyari8 thefollowing table is given.

(g) Stratification: Ettema lO and Kothyaris havestudied the effect of stratification of the bedmaterial on scour depth in case of clear waterscour. It is concluded that the stratification, inwhich a relatively thin coarse top layer covers athick fine bottom layer, is the critical condition.Once the top coarse layer is scoured away, scourdepth will rapidly increase.

(h) Effect ofFlow Parameters: Based on certaintheoretical analysis, physical reasoning andanalysis of experimental data, investigators havearrived at the basic flow parameters to which thedimensionless scour depth is related. ThusBreusers et al.'\ Laursen and Toch12, Larrasl3 andEttema10 consider biD as the important 'parameterand hence they related dsJD to biD. Thusaccording to Breusers et al. II

3.772.371.37

7.5 0

1.171.0

Effect oj () on Kt)Jor rectangular pier (/lb=6.0)

e

(e) Effect ofOpening Ratio on Scour Depth: Theopening ratio a is defined as a=(B-b)/B where B. iscentre to centre spacing of the piers and b is thepier width. When b is very small compared to B, ais close to unity and flow around one pier does notaffect that around the other. However, as adecreases, the interference effect becomes morepronounced and scour depth increases; in such acase DsJD or Dsc!d-a-o. Here Dse and DK are scourdepths below the water surface for sedimenttransporting and clear water flows respectively.The analysis of extensive data collected by Gardeet aI. 21 indicate that n=0.30.

(j) Effect ofBed Material Characteristics: In thecase of noncohesive materials, the characteristicsof the bed material that affect the scour depth aresediment density, median size d of the bedmaterial, its standard deviation and stratification.For all practical purposes the density of naturalsediments can be taken as 2.65, a constant value.

As regards the sediment size, Lacey-Inglisapproach (see below) suggests that D se-a- l /6 Sincethe average shear stress on the bed (=yf DS) atwhich bed material moves-known as the criticalshear stress-increases as the sediment sizeincreases, it stands to reason that scour depthshould be affected by the size of the bed material.Hence, for given flow condition, larger than thesediment size d, smaller should be the scour depth.The clear water scour depth should decrease withincrease in sediment size. Analysis of data overlarge range of sediment size by Kothyari8 has

572 R J GARDE AND U C KOTHYARITable III

Variation a/KG with O"g

Cfg 1.0 1.5 2.0 2.40 2.75 3.3 4 7.8

Ka 1.0 0.90 0.75 0.50 0.38 0.25 0.160 0.08

where b is the pier width or diameter.Lacey and InglisJ4 compute Lacey's depth D LQ

for flood discharge Q asDLQ=0.47(Q/j) 1/3 (3)

in metric units and relate scour depth below watersurface Dse to DLQ as

Lacey-Inglis EquationIn the earlier part of this century Lacey analysed

the data from stable irrigation canals flowingthrough loose noncohesive sandy material in Indo-Gangetic plain and obtained the followingequations for depth (or hydraulic radius) DLQ andperimeter (or width) P.

... (4) D LQ=0.47(QIj) 1/3 ... (3)

where Q is the discharge in m3/ S, DLQ and P are inm and f is Lacey's silt factor related to median sizeof the bed material d by the equation

Here f is known as Lacey's silt factor and is givenby eq. 9 (see below). On the other hand, Garde5,Shen et al. 15, Venkatadri et al. 16, Coleman17, andJain 18 show the importance of Froude numberFr=V/vfD. where V is the average velocity offlow. Then dsjD is related as

and

P=4.7S{Q, ... (8)

.. , (5)f=1.76Wl, ... (9)

where K is the constant: Typical equation of thiscategory is U.S. Army Engineers' equation

'" (6)Shen et al. 19 have related .the clear water scour to

the pier Reynolds number Vb/von the assumptionthat the strength of horse-shoe vortex is a functionof Vb/v. Here v is the kinematic viscosity of water.They have proposed the following equation forenveloping curve between dsc and Vb/v.

d being in mm. On the basis of analysis of scourdata on 17 bridges in alluvial rivers in North Indiainglis 14 found that the maximum scour depth belo~water level, Dse is related to computed value of DLQas

Dse=KDLQ, (10)

where K varied from 1.76 to 2.59 with an averageof 2.09. Hence according to Inglis, D

seis given by

the equation

Recent Equations for Prediction ofScour Depth

Below are briefly discussed Lacey-Inglis approachcommonly used in India and a few recentlydeveloped equations.

Generally, the above types of relationships ClI \"valid for cylindrical piers and for piers of othershape and inclination to flow, the dse or dsc valueneeds to be multiplied by K s and Ke. Also theseequations are valid for nearly uniform bedmaterial, where armouring is not pronounced.

dsc=O.000223(Vb/v)O.619, ... (7)

... (4)

When bridge pier foundation is to be designed, thisequation will be used for a flood discharge ofreturn period 50 to 100 years, even though eq. (3)is at best valid for bankful discharge. In the light ofthe variables affecting scour depth mentionedabove, it will be clear that K in eq. (10) shoulddepend on pier shape, sediment size, obliquity offlow etc~ Since these factors are not explicitlytaken into account, Lacey-Inglis method should notbe used outside the range of data on which it isbased.


Laursen-Toch EquationThe equation proposed by Laursen and Tochl2

for prediction ofd~ is

ds/D=1.35(bID)o.70, ... (11)Melville and Sutherland's Equation

Melville and Sutherland2 have proposed amethod for estimating the scoUr at bridge piers.The method is completely based on the analysis ofthe laboratory data. Basically they assumed thatthe largest possible scour depth around the bridgepier is given by

... (12)

This scour depth below the general bed level isreduced by multiplying factors which depend onwhether the scour is clear water scour, depth isshallow and sediment is graded. The multiplyingfactors are determined from the analysis ofexperimental data covering a wide range ofpertinent variables.

Chi/ale's MetholfOThe method proposed by Chitale estimates the

probable maximum scour depth at the bridge pier.Maximum scour depth at the pier dsem is computedas

where Dsem is the maximum anticipated scour depthbelow water surface.Kothyari-Garde-Ranga Raju 's Method

Based on the extensive laboratory data collectedusing uniform and nonuniform' sediments,stratification and steady flows, Kothyari et al. 821have proposed equations for determining clearwater scour depth dsc and equilibrium scour depthdse for steady flows. The analysis was done usingthe mathematical model based on the assumptionof formation of horse-shoe vortex on the upstreamside of the pier. Such a vortex increases the shearstress on the bed and causes scour. Their equationsfor scour depth are

Clear Water Scour:

d (b)0.75(D)0.16[(U2 _U2 )]0,40-l. = 0.66 _ _ c a -0.30b d d (tiysd / PI)

... (15)

where the average critical velocity Uc is given by(U/I(tiysdIP.) =1.20(bld)-O Il(Dld) 0. 16, (16)

Scour Under Sediment Transporting Flow:

Here tiys is the difference in specific weight ofsediment Y5 and water Ys' and Pf:is the mass densityof water.

It may be seen that in sediment transportingflows, the scour depth is not dependent onvelocity. It may also be noted that the openingratio a affects the scour depth. When the sedimentis nonuniform, the scour depth is reduced' ascompared to that for uniform sediment.. Thereduction factor Ka is the function of the geometricstandard deviation O'g of the bed material as shownin Table III. Alternatively, when the sediment isnonuniform, effective sediment size, deu be used ineqs 15, 16 and 17 instead of d the median size, theformer being given by

dsem=2.5 b. ... (13)If the bridge is located at a constriction caused byguide bunds, the average depth D2 in the contractedsection is related to that in the uncontracted sectionDlby

... (1)

where Bland B2 are the unobstructed andobstructed widths of the river channel. Thisaverage depth D2 in the contracted section may notbe uniform across the width because ofnonuniform flow distribution and curved entry.Analysis of eight bridges in Indo-Gangetic plainindicated that ratios of maximum depth to theaverage depth D2 varied from 1.2 to 1.67. Hence,Chitale recommends a ratio of 1.7.

.'. Maxim\Ull local depth=1.70 D2

dslb=O.88(bld).67(DId)4fJU -o30

deJd=O.925 O'gO.67

(17)

... (18)

Hence, Dsern=2.5 b+l.70 D2 ... (14) for O'g>1.124.

2009CEZ8203Rectangle





Field data

To assess the relative accuracy of the abovementioned formulae, all available field data onscour around bridge piers were compiled andanalysed. The Indian data on scour around pfer for17 bridges in Indo-Gangetic plain, collected byInglis14, were available. In addition, data collectedby RDSO (Research Designs and StandardsOrganisation22.23.24,2s), Lucknow, on railwaybridges, and some data on scour at bridge piers onGanga canal were also used.

Scour data for 55 bridges in USA published byFroehilch26, six bridges in New Zealand reportedby Melville27, and for five bridges in Canadareported by Neil2s have also been used. Theirsummary is given in Table IV.

In passing, it may be mentioned that not enoughinformation is available on scour measuringequipment, even though the principles and broadcircuitry used in imported equipments are known.There is an urgent need to fabricate the equipmentin Indiaand make it available to user agencies.

Analysis of Field Data for Scour Depth

Using the data mentioned above, four relationshipswere tested for their accuracy of prediction ofscour depth. These were Lacey-Inglis, Laursen-Toch, Melville-Sutherland and Kothyari et al. Thecomparison between observed and predicted scourdepths were plotted for each of the four methods.Typical graph for comparison between theobserved and computed dse or dsc for Kothyari et al.

method is shown in Fig. 2. The results of the fourmethods are compared in Table V.

It can thus be seen that among the four methodstested, namely Lacy-Inglis, Laursen-Toch,Melville-Sutherland and Kothyari et al., themethods by Kothyari et al. and Melville-Sutherland give results of almost the sameaccuracy. These methods are also superior in thatthese take into account the effect of flow depth,velocity, pier size and shape, and the sizedistribution ofbed material on' scour depth.

Some comments can also be made about Lacey-Inglis method. The method is basically empiricaland gives scour depth below the high flood level inthe case of meandering rivers in flood plains insandy materials with sediment size varying from0.2 mm to 0.4 mm. The method, though based on avery limited data from prot0o/Pe bridges, seems togive satisfactory results or oversafe values. Itshould not be used for rivers with cohesive orgravelly bed materials. Further, it is important thatit should be used as was recommended by Inglisi.e., computing DLQ from eq. 3 and then findingDse. Also, since Lacey-Inglis did not independentlyaccount for scour due to nonuniform flowdistribution, contraction and pier geometry andinclination, all these effects are inherent in Lacey'sscour depth.

Scour in Clayey Soils

When the river bed consists of clayey materialdifferent types of forces act between soil particleswhich resist the dislodgement of particles that

Source

RDSO

Inglis

Upper Ganga Canal

USA

New Zealand

Canada

Table IVSummary offield data

Sediment Flow depth U Pier diameter Scour depthsizedmm m m/s orwidth m below bed

level m

0.43+-1.6+ 1.46-19.11 N.A. 2.33-5.18 2.40-16.25

~r-0.39+ 4.4-18.3 N.A. 3-11.3 7.60*-35.7*

0.18-0.21 0.88-3.00 0.35-1.0 0.68-2.4 1.20-5.87

0.25-90 0.58-19.5 0.46-3.67 0.24-13.0 0.30-7.80

94-230 2.7-3.8 0.87-4.27 0.92-2.4 2.75-4.88

0.50 4.0-7.5 N.A. 1.50 5.30*-9.8*

N.A.-Not Applicable, +Lacey's Regime depth, *Measurement below water surface.

SCOUR AROUND BRIDGE PIERS

10J r-----------------------.575

10'e!

0&II:>IE&IIIn .. 100

.'0

-,1t

-,1O

6

o

o

IfU/~ a A0

0

0 fPo0

cP0

10, .

10J

10

LEGEND', - U S DATA - NEWZE"t.ANOA - U. G CANAL DATA+ - GANGA AT MOkAMEHo - OTHER DATA OF- R 0 SOA - AA"I RIVER OAT .. - iNGLIS OAT.

J10

Comparison ofaccuracy ofprediction ofscour depth bydifferent methods

where F varies between 1.50 and 2.0 as frictionangle decreases from 15 to 50 or less; here C isthe cohesion in kg/cm2

\

Gravel-bed river is that river the bed material ofwhich is usually characterised by relatively largemedian size and large standard deviation. Hence,the bed material consists of material ranging fromvery fine to very coarse particles. It is only duringrelatively large flood that all the particles in the

98

100

100

90

... (19)

85

65

95

SO

79

S9

38

30

Table V

% of Data points falling.within givenerror band

Scour in Gravel-Bed Rivers

f cohesive=F(1+JC)

d. ( COMPUTED) IN mFig. 2 Observed vs computed scour depths by Kothyari et a/.

method .cause scour. These are Van der waa)'s forces,electric surface and other bonding mechanismssuch as hydrogen bond, and chemical cementationbetween particles. Hence scour in clayey materialsis more complex and less understood than the Methodscour in noncohesive sandy material. Unlike in thecase of noncohesive sands, flow conditioIll atwhich clayey material will erode is very difficult to Lacey-Inglispredict because it depends on the type and Laursen-Tochpercentage of clay, quality of water and time.Some investigators have tried to relate the critical Melville-Sutherlandshear or critical velocity to plasticity index, vane Kothyari et al. 86 96 100shear strength and such other properties; but these --::...------------.:....:------=-::..:...-.-attempts are hot very successful. Some basic work these are based on the data from one or twoon scour in cohesive soils has been done by bridges. Hence these methods need furtherMirtskhulava293o One idea that he has introduced verification. According to Namjoshi scour depthis to increase the specific weight of cohesive soils below general bed level ds in cohesive soils doesto account for increased resistance. The increase in not exceed 1.5 b . Kand suggests use of Lacey-the specific weight over the actual specific weight Inglis method with enhanced value of silt factor f.is proportional to its cohesion. He has alsoindicated that when cohesive soil is detached,aggregates of 3 to 5 mm in size come out. Hence, itmay be necessary not to use the actualcharacteristic size of cohesive sediment but thesize of aggregate soil. Because of such difficultiesno rational method is available for estimation ofscour depth around bridge piers in cohesivematerial. Hence further experimental work in thelaboratory is needed under controlled conditions;in addition some field data on scour in clayey soilsneed to be collected.

Namjoshe l and Kand32 have proposed methods forestimation of scour depth in cohesive soils, but


Table VI

The large differences in scour depth predictedemphasize the need for further study of scour ingravel-bed rivers. However, it seems that sinceKothyari et al. 34 and Melville and Sutherland2methods take sediment nonuniformity intoaccount, these methods be used in place of IRC-78-1979 code till additional data on scour ingravel-bed rivers are available.

Methods of Scour Control and Prevention

Since taking the bridge pier sufficiently deep intothe bed to tak-e care of anticipated maximum scourdepth and the grip length requirement is quiteexpensive, some attempts have been made toreduce the scour either by some modification ofthe pier, or some addition to it, and/or byincreasing the ability of the bed to resist the scour.These methods are briefly discussed below. It may,however, be mentioned that, to. the authors'knowledge only a couple of methods discussedbelow have been used in prototype bridges, andhence their feasibility from the point of view ofstructural design, construction and economy need

3.125 m2.142m1.520 m4.800 m

Method

RaudkiviKothyari et at. 34IRC CodeMelville and Sutherland

earlier. Thus one can consider scour to occur withthe original bed material in place without thepresence of armour layer.

The methods proposed by Kothyari et al. 8 andMelville and Sutherland2 take into account theeffect of sediment nonuniformity and henceannouring effect indirectly. Therefore, it isrecommended that these methods be used in placeof Lacey-Inglis method using q and /=24.However, to study the relative accuracy of thesemethods there is need to collect scour data fromgravel-bed river which are not available at present.To intlicate the large variation in scour depth onecan see the results obtained for a hypotheticalproblem solved by Garde and Kothyari34 with thefollowing data:

U=2.5 mis, D=2.80 m, Diameter of circular pipeb=2.5 m, a=almost equal to unity, 8=0, dso=45mm, crg=2.125

... (21)

bed material move; as the discharge reduces thecoarse particles, which cannot be moved "by theflow, accumulate on the bed surface and form alayer of nonmovable particles known as ~rmourlayer or paving. For low discharge there is nosediment transport since the original material isoverlain by annour layer. The standard deviationof the top layer is usually much smaller than thatof underlying original material. The top layerthickness if> one to two times the largest size in thebed material.

When the bridge pier is constructed in such astrata and the discharge is sufficiently large, thescour development would progress. During suchdevelopment, the coarser particles wouldaccumulate in the scour hole and partly inhibitfurther development of the scour. Ultimately theaccumulated coarser material would stop furtherscour and the scour depth obtained would be lpuchsmaller than that in uniform material of the samedso.

The IRC-78-1979 code recommends that scourdepth in gravel-bed rivers can be estimated usingLacey-Inglis approach involving dischargeintensity q m3Ism namely

DLq=1.33(q21f)1/3 ... (20)

and silt factor of 24. Here q is the discharge perunit width of channel and DLq is depth of flowcalculated using q. In this connection, it may bestated that no field data have been published tosupport this contention In view of the fact that bedmaterial size of the gravel bed rivers varies over awide range. Published data of gravel bed riversindicate the depth relationship.

see Hey and Heritage33 Here b varies between 0.33and 0.49 and c.between - 0.03 and - 0.12. This isdifferent from Lacey's eq. (3). In addition such amethod does not take into account the effect of pierwidth and its shape.

Bridge foundation are normally designed for aflood of 50-year return period whereas the averageannual flood has a return period of 2.33 yrs. Hence,at such a high discharge all the available sedimentsizes in the bed would move and would thusdestroy the armour layer or pavement formed


WSDELTA WINGLIKE PLATE

Fig. 6 Delta-wing-like triangularplate -Fig. 5 Pier collar

Tanaka and Yan037, Chiew8 and others havetested slots in cylindrical piers (Fig. 4). With theoptimum dimension and location of the slot in thepier in the direction of flow the scour ratio was0.85 to 0.70 and reduction in width of scour holefrom 0 to 25 per cent. The slot near the watersurface reduces the effective depth of flow whereasthe slot near the bed causes the jet issuingdownstream. This jet deflects the downward flowin front of the pier and reduces the scour. It is feltthat keeping such a slot in the pier may createstructural problems and may endanger the safety ofthe bridge.

Thomas39, Ettema40, Chiew8, and Haghighat41

have . experimented with circular collar ofappropriate diameter placed around the circularpier at a certain elevation above or below the bed,(Fig. 5). The optimum diameter of collar is foundto be 3b while location above the bed is 0.2 D. Forthis condition the scour ratio would be 0.80, whilefor a collar of 6 b diameter this ratio would be0.45. Visual observations have shown that thecollar of adequate diameter inhibits the growth ofhorse-shoe vortex and prevents it from reachingthe bed; as a result the scour is reduced. Ettema40

studied the reduction in scour when collar wasplaced on or below the bed; such a collar wouldprovide nonerodible surface but will not the inhibitgrowth of horse-shoe vortex.

Gupta and Gangadharaiah42 experimented withthe delta-wing-like triangular plate placed just infront of the pier as shown in Fig. 6. The twovortices released on the two sides of the triangularplate are in opposite direction to the horse-shoevortex and hence the scour around the pier isreduced. The devices experimented with by Leviand Luna43 are shown in Fig. 7. These included anobstacle, a plate of small height and a group ofpiles placed in front of the pier. Among the three

~--3b

Fig. 3 Pier with cassion

wsYs,- 0 u0 ~ 0i/. ~/ ~/

Fig. 4 Slot in piers

ttl be evaluated further before these methods canbe used in the field with confidence.

o~

both under otherwise identical conditions. Scourratio Sr for caisson varies from 0.30 to 0.50, seeChabert and Engeldinger4 and Jones et aJ. 36.

W.S.

Pier ModificationProvision of the caisson or well having the

diameter three times the diameter of the pier isrecommended by Chabert and Engeldinger'" Shenand Schnieders, and Jones et a/. 36 While Shen andSchneiders have suggested the use of a lip,Charbert and Engeldinger suggested that the top ofcaisson be at bll depth below the general bedlevel. The top surface of caisson protects the bedfrom scouring action of the horse-shoe vortex andthus reduces scour. The caisson top should bebetween 0 and 2.4 fib, see Fig. 3 for definition ofy

The efficiency of such a device can bequantified by the scour ratio Sr defined as

Sr = scour with devicescour without device


PIER fCOLLAR...

-.., .--....---. ,,

+--I

,/1

Fig.9 Vanes tested by Odgaard and Wang

Riprap Protection

Protecting the river bed and banks prone toerosion by large size nonmovable stones (calledriprap) is an age old practice. Riprap blanket beingflexible, is not weakened by slight movement orlowering of the bed. If 'to is the average shear stresson the bed in Nlm2, the size of nonmovable stonearound the pier is given by 'tj120 m. If such stonesare placed on finer bed material, the fine materialunderneath may get washed. For this reason propergradation of armour layer is needed. Otherwise afilter needs to be provided underneath the riprap.Limited experience about riprap protectionunderlain by properly designed filter has indicatedthat it is rather difficult to place relatively thinlayers of filter under deep water which is flowing.Hence, efforts have been made to provide riprapProtection without filters. This has been done by

47 .Worman47 According to Worman a geometncstandard deviation of 2 can be assumed for riprapand d.IS can be determined. The thickness of riprapT at the scour hole is given by

W.S

o

>- ~r~s,

---

devices tested, the vertical plate with btfb=2, s/b=2and tiD 0.30 to 0.40 seems to be a better device.For such case the scour ratio was 0.30. Vittaleta/. 44 have replaced the solid cylindrical pier ofdiameter b by a group of three small piers ofdiameter 0.302 b each, and placed at an angularspacing of 1200 This was found to be effective inreducing the scour. The scour ratio obtained was0.60 (Fig. 8). They also tested the scour reducing

--.......... ----+-

0'302 b

Fig. 8 Pier group tested by He

where UI=twice the flow velocity in the river, ds8Sis such a size of river bed material that 85 per centof the material is finer than this size, and daiS issuch a size of armour layer that 15 per centmaterial is finer than this size. Worman has statedd Id should be less than or equal to (j.l O.585' 115 .Twenty bridges in Sweden have been prOVIdedwith riprap protection according to the abovemethod and according to Worman no significantscour is reported. With this design criteria, no filteri needed underneath the armour layer.

Concluding Remarks

.The critical review of available literature andanalysis of ,prototype scour data around bridge


upstream side of the pier which will scour materialthere. and deposit it in the scour hole of the pier;and (iv) provide armour layer of adequatethickness and appropriate size distribution whichwould inhibit scour.

Among the various devices, collar, vanes, andarmour layer seem promising. Field studies need tobe conducted in India to gain experience abouttheir usage and. cost effectiveness. Lastly, there isan urgent need to review codal provisions forestimation of scour, in view of available additionalinformation.

v

y

NotationsPier width or pier diameter

, Channel widthSediment sizeSize of armour coat or riprap materialCharacteristic size of bed materials; also scourdepth below bed levelClear water scour depth below bed levelScour depth below bed level in sedimenttransporting flowAverage depth of flowLacey depth computed using the equation with QLacey depth computed using the equation with qD+dscD+dseLacey's still factor (=1.76..Jd)Froude number (=U/vgD)Coefficient of proportionality between Dse and DLQShape coefficient of pierCoefficient to take into account effect of sedimentnonunifortnity on scourObliquity coefficientPier lengthDischarge per unit width of channelDichargeChannel slopeThickness of riprapAverage shear stress on the bedAverage velocity of flowLocal maximum average velocityCritical velocity for sedimentShear velocity (=...JtjPr)Difference in elevation between river bed and topsurface of caissonOpening ratio (=(B-b)/B)Specific weights of water and sedimentMass density of fluidAngle between axis of pier and the flow directionKinematic viscosity of the fluidGeometric standard deviation of

sedimen~~(ds/ds(JH-dso +d I6)Subscript 16, 50, 84 Sediment size such that 16, 50or 84 per cent of material is finer than thecorresponding size.

~/qQsT

a

Yf Ys~B

DDLQDLQDscDsefFrKK.Ka

piers have brought out certain major observations.During the past four or five decades a number ofequations have been developed' for predicting thescour depth. Many of these are based on limitedlaboratory data and a few on the basis of limitedfield data. These studies have brought out theeffect of flow conditions, pier diameter and itsshape, sediment size and its nonuniformity and thenature of flow (clear water or sedimenttransporting) on scour.

There are difficulties of getting properinstruments for measuring transient bed level inthe scour hole and maximum scour depth inprototype bridges. Such equipment thoughavailable abroad is not available and used in India.

When available scour data in sandy beds areanalysed using methods of Lacey-Inglis, Laursen-Toch, Melville-Sutherland, and Kothyari et al., it isfound that the methods proposed by Melville-Sutherland2 and Kothyari et al. 34 give more or lessthe same accuracy. Further, these two methodstake into account all the factors affecting scouraround bridge piers. Hence these are superior tothe otherlt methods. It is also concluded that Lacey-Ingli& method should be used for sand bed rivers inprecisely the same manner as recommended byInglis. This should not be used for rivers withclayey or gravel bed. Not enough infonnation isavailable on scour around bridge piers in clayeymaterial. The phenomenon being very complexfurther laboratory studies under controlledconditions and field studies on measurement ofscour are needed. In the case ofgravel-bed riversthe provisions of IRe code seem arbitrary. Themethods of Melville and Sutherland2, and Kothyariet al. 34 seem more logical for prediction of scour in Togravel-bed rivers and should be used. Yet there are Vno field data available to comment on the relative' VIaccuracy of prediction by these methods. Hence ~cefforts need to be made to collect scour data ingravel-bed rivers.

Several devices have been tested which wouldreduce scour at bridge piers or inhibit itsdevelopment. These work on the following.principles: (i) prevent formation or reduce:effectiveness of horse-shoe vortex; (ii) developcirculatory flow near the bed in the directionopposite to that of horse-shoe vortex to reduce ornullify its effect; (iii) provide device on the


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Documents

Scour Around Bridge Piers