DESIGN OF RIPRAP FOR PROTECTION AGAINST. SCOUR ...

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VOL l6, (No. l) (79) 

ABSTRACT 

THE INDlAN SOCIETY FOR HYDRAULICS 

JOURNAL OF HYDRAULIC ENGINEERING 

DESIGN OF RIPRAP FOR PROTECTION AGAINST. 

SCOUR AROUND BRIDGE PIER 

by 

Bhalerao A. R.t, F.ISH and Garde R. J.1, F.ISH 

From careful study of literature, it is found that various methods have been 

developed for protection of the bed against scour around bridge piers. Use of 

appurtenances has found limitations as regards effectiveness, structural design and 

limited experience in their use.Alayer of riprap(coarse, non-cohesive and non-movable 

material) around the pier enhances the ability of bed material around the pier to resist 

the erosion. On the basis of experimental data collected by theauthor, Worman, Chiew 

and others, a method is proposed for the design of rip rap layerto control scour around 

circular bridge piers. 

INTR~UcTION 

In the recent times, efforts have been directed towards development of methods to. 

reduce or control scour around bridge piers, thereby reducing the cost of bridge pier 

foundation. These methods include (i) modification of upstream face of the pier (ii) 

placing additional appurtenances, and iii) use of vanes, piles etc. These methods are 

found to reduce scour to the extent 20 to 60 percent. However, very few of these 

methods have; been tested on prototype bridges and hence, information about their 

performance, their effect on stability of the pier and cost involved have not been 

reported. 

As indicated by some investigators such as Inglis (1942), Blench (1956), Laursen 

(1956), Hancu (1971), Galay (1987), Worman (1989), Suzuki (1992)and Chiew (1995) 

scour can be effectively controlled by providing a layer or layers of non movable 

riprap around the pier, with or without a filters. Worman (1989) has used this method 

effectively on some bridges. However, there is a necessity for collecting additional 

data in laboratory and field for checking his method or proposing changes in it if 

necessary, using additional data. Hence, this investigation was carried out. As regards 

use of riprap for controlling scour, one has to determine the size, gradation and thickness 

of riprap layer for kno~ characteristics of the pier, flow conditions and bed rnate~aL 

1. Principal, Bharati Vidyapeeth Deemed University College of Engineering, Pune 43 

2. Late Professor (Emeritus), IlT Roorkee, Roorkee .• 

Note: Written discussion of this paper will be open until 30th June 2010. 

ISH JOURNAL OF HYDRAULIC ENGlNEERING; VOL. 16, 20 to, NO.1 

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(80) 

TWO APPROACHES 

DESIGN OF RIPRAP FOR PROTI:CTION AGAINST 


VOL. 16, (No.1) 

The size of non-movable material in riprap can be determined by either critical 

velocity approach or critical shear stress approach. In critical velocity approach, the 

size of riprap (D) can be obtained relating it to average velocity of flow (U), depthof 

flow (Y) and difference in specific weights of sediment and ~ater (f1y s)' if one assumes 

that viscosity is not important. Investigators such as Ishbash (1935), Garde (1970), 

Bonasoundas (1973), Quazi and Peterson (1973), Maynord et al. (1989), Richardson 

et al. (1991), Garde and Kothyari (1995) Parola(1995), Chiew (1995) have suggested 

equations for computation of size of riprap. Most of these equations are for the 

computation of size of riprap in unobstructed flow.Further, it may be noted that when 

water flows, shear or velocity distribution around the bridge pier is affected and 

instantaneous values of these two parameters vary and much greater than time averaged 

values. These two aspects are not considered in these equations. 

Alternately one can specify that /). D in case of riprap layer should be less 

Ys 50 

than a specific value for it to be stable and control scour. Garde and Kothyari (1995) 

recommended this value as 0.03 for riprap in a channel. Here 'to is average shear 

stress in the channels equal to RS. However, in case of bridge pier, local shear around 

the pier is greater than the average shear stress in the channel. Assuming 'to is 

proportional to l.P, experimental evidence indicates that the time averaged local shear 

stress around the pier can be 3 to 5 times the average shear stress in the channel. 

Measurements of shear stress around pier by Hjorth (1975) and Darghi (1987) indicate 

that instantaneous shear around pier can be as high as 11 to 12 times 'to. These two 

facts need to be taken in to account while developing the method for design of riprap. 

As mentioned by Chiew and Melville (1989) the effect of sediment gradation is 

negligible when standard deviation ( (Jg) of riprap is less than 2. Hence, it is desirable 

to have standard deviation ( (Jg) of riprap between 2 and 3. Filter underneath the riprap 

layer is usually required to prevent leaching of base material, which takes place due 

to penetration of turbulence in the riprap layer. This takes place due to penetration of 

turbulence in the riprap layer. However, considering the difficulties in laying the filter 

layer, Worman (1989) has indicated that two or more layers of graded riprap can be 

designee in such a way that provision of filter is not needed. This approach is adopted 

here. 

BRIEF REVIEW 

Generally, the size of riprap is determined by using one of the equations available 

for critical velocity. Worman (1989) has used Ishbash (1935) equation in which 

diameter of riprap D is expressed as function of critical velocity as given below. 

ISH JOURNAL OF HYDRAULIC ENGINEERING, VOL. 16.2010, NO.1

I 

VOL. 16, (No.1) DESIGN OF RIPRAP FOR PROTECTION AGAINST 


u, =O.8sl2g p,;p D 

Hereu =2U. 

c 

On the basis of small-scale experiments, Suzuki (1992) suggested that the thickness 

(T) of the riprap be obtained from Eq. (1). 

where r., and 'to are dimensionless critical shear stress given by (~ 1"~ J and 

r, 50 

~Ys D5~ 

respectively. Kulkarni (1993) has recommended that, thickness of riprap 

(T) can be obtained from the equation. 

U 2 

T=--:--~ 

2 g (:; -IJ (3) 

(81 ) 

(1) 

(2) 

, 

l. • 

j. , 

t 

l. 

r 

I . 

. ':'-:----0.'-' 

v-, :.: -. :.: 

where U is average velocity in unobstructed flow. This equation is to be used for 

protection of bed and banks in the river. On the basis of analysis of the laboratory data 

collected, Worman (1989) has proposed the following equation for thickness of riprap 

as, ... 

"--- -. 

where T is thickness of riprap, p is coefficient of friction for turbulent flow, CD is the 

drag coefficient of particle of bed material and n is porosity the value of which is 

taken as 0.38. Inserting values of p, PS' Pf' Co and n, this equation reduce to 

( U ~6(~) 

2 

) 

gT c DI5 

If (d85 ) > 0.15 

, DI5 

It may be mentioned that in Eqs. (4) and (5), Worman uses U= 2U o where U, 

average velocity in unobstructed channel, in order to account the fact that scouring 

ISH JOURNAL O~HYDRAULIC ENGINEERING, VOL. 16.2010. NO.1 

(4) 

(5) 

~ .-' 

., 

:.. -'~. 

.~:- -." :-: 

." .... - 

.. -.: :.: :. :-,:,:::,:~:",:.- ':::;':~::.::::::':

(82) DESIGN OF RIPRAP FOR PROTECTION AGAINST 


VOL. 16. (No. I) 

velocity near the bridge pier is greater than average velocity in unobstructed flow. 

U 2 

d 

Figure 1shows variation of- 

T 

and D S5 

• when dgs1D is less than 0.1. Onthe basis 

15 g IS 

of short duration tests, Chiew (1995) plotted the graph of UI U c against TID 50 and has 

shown separate regions in which riprap around bridge pier had failed and in intact 

condition. Refer Fig. 2. 

Recently some work has been done by Kothyari, Hager and Oliveto (2007) to 

predict densimetric particle Froude number at incipient scour condition near bridge 

pier as a function of Rld 50 • (dsidI6) and geometry of obstruction. 

EXPERIMENTAL PROGRAMME 

Keeping in view' the information available, the problem posed for the study was to 

develop a method to determine size and thickness of the riprap, which willprotect the . 

bed around the bridge pier from scour. For the bed material and riprap size chosen, 

three types of experiments were conducted in 0.30 m wide, 0.60 m deep and 10 m 

long tilting flUII1ein the Fluid Mechanics Laboratory of CivilEngineering Department 

of Bharati Vidyapeeth University. Experiments were related following conditions 

/gT 

1 

0.8 

0:6 

0...• 

0.2 

0 

fi ~ 

/ 0 

/V 

~ 

, 

, 

, 

O· 0.05 0.1 0.15 

FIG. 1 THICKNESS OF RIPRAP - WORKMAN RELATIONSHIP 

ViDc 

1 

0.8 

0.6' 

004 

0.2 

0 

F

I 

II 

VOL. 16, (No.1) DESIGN OF RIPRAP FOR PROTECfION AGAINST 

(83) 


., 

1. Incipient Scour of bed material around the pier 

2. Incipient scour of riprap of different size and thickness 

3. Scour with riprap protection. 

Over 150 runs were conducted using circular pier of 50 mrn diameter under clear 

water condition. Table 1 gives standard deviation, size of bed and riprap material 

tested in the experiments. The circular ring with engraved marking in nun was used to 

lay riprap of appropriate thickness and flush with original bed level, Bed material of 

predetermined thickness was removed from scoured area, weighed and riprap of same 

weight was then slowly added to the scour hole and levelled to the undisturbed bed 

level. 

In addition to the data collected in the present study, data collected by Knight 

(1975), Dey (1995), Chiew (1995), Melville (1997),ha:ve also been used for 

determining DIU c for incipient scour and those by Worman (1989) and Chiew (1995) 

for size and thickness of riprap layer. Worman (1989) had used three bed materials of 

median diameter 0.17,0.36 and 0.78 mm and five ripraps of size varying from 8 to 48 

mm. Depth of the flow varied from 300 to 400 mm whereas Chiew (1995) used bed 

material of mean size 0.96 mm and three riprap of size 2.60, 4 and 4.85 mm. 

TABLE-l 

CHARACTERISTICS OF BED AND RIPRAP MATERIAL 

(pRESENT STUDy) 

ANALYSIS OF DATA 

Bed Material Riprap Material 

d so (mm) 0'& Dso(mm) O'g 

0.20 2.45 1 1.37 

0.27 2.69 2. 1.90 . 

0.36 1.28 3 2.36 

0.40 2.53 4 2.50 

0.50 2.63 5 2.66 

0.68 2.73 8 1.58 

Limiting values ofUfUcfor Incipient Scour 

The critical velocity at which sediment of a given size will just move in an 

un~bstructed uniform flow was obtained by combining Shields' and Yalin-Karman 

relationship with Karman-Prandl's equation of Diu. in hydro-dynamically smooth 

and rough channels. Analyzing the generated data, following type of equation was 

obtained. 

(:;::X~:~~~~P:~::;:;'~~1::r~7:??·:·;·'·~:·?::;·::;·:·; 

>::. :::;:.:-:;~:: ;.::::~:::::. :':":"::~,, ::.' ~=:' . .'::'"'::'~: 

ISH JOURNAL OF HYDRAULIC ENGINEERING, VOL 16,2010, NO. I 

:.:;.:.;:.:~-y.:..:>~.?::~.:':;::::;:::::;:.::.:.:...;:::, ..... :..... : .. :.,,: ::.:':.::.::,::.. ::: ::.:::.: ... :;::::.... :.::::'::: ::::-, :::;: .. :.: '.: -:: : .::.: 

..>:-....~ .':.~-~".~: ~;~:~:.':' :'~..-::;

f.J~~~~~~~~i::S*/:'~~~':':''i:;;:;::~·:::;:;:);:j;:

~~~'"...':.,;,s.':"":';:~~~"'- .•.. " :..-~....:...:-.- ..#- 

•••••• '" .". :.;~'"__= 

...•~ .. _. ·_'·-'::~=--_'~'''''':~''''~':''''_'~''':'''·'·''-'/:-J';.'~.r~ .. 

o t 

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IS 

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n 

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II 

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e 

r 

n 

e 

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s 

f 

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VOL. 16, (No. I) DESIGN OF RIPRAP FOR PROTECTION AGAINST 


.\ TABLE-3 

UlU c AND CORRESPONDING SHEAR AT PIER 

S.N Name of the Investigator UIU 't p = M 't oc 

c 

Shear Stress by velocity Measurement 

I Nicollet (1977) 0.42 5 

Circular PierRounded nosed Pier 0.50-0.65 3 

2 Lee Jong (1973) 

Circular pier without 0.40 6.25 

attachment Round nosed and 0.50 4.00 

Rectangular 

3 Bressure and Roudkivi (1977) 0.5 4 

4 Chiew (1995) 0.3 11 

5 Melville (1999) 0.34 8 

6 Dey (l993) 0.475 4 

7 Present 0.438 5 

Shear Stress by Preston Measurement 

8 Hjorth (1975) - 12 

9 Melville (1975) - 3.5 

10 Darghi (1987) - 3.5 

Studies of Einstein and E1. Sarnni (l949), Gessler (1967) and Little Mayer (l972) 

have shown that the lift as well as shear at the bed fluctuates in turbulent flow, and 

follows Gaussian distribution as an approximation with standard deviation «J') in 

dimensionless form varying from 0.45 to 0.57. Therefore, the maximum shear stress 

near the pier can be 'tpmax = (Tp + 3 x 0.45Tp) = 2.35'tp. Here, value of o assumed is 

0.45. Therefore, riprap layer can be disturbed when 'tpmax-2.35 (5'[) = 12 'to' 

Patel and Raga Raju (1999) in their analysis of critical shear stress of non uniform 

material have recommended the use of characteristic size of bed material D a' which 

is given by D g x o g instead ofD so to for account non- uniformity of sediments. Here, 

D g isgeometric median size of riprap and og geometric standard deviation. Therefore, 

to prevent scour around the bridge pier , the size of the riprap material D can be 

g 

calculated as, 

D = 12'[0 

a flys t. co 

.... "'; .-: ,".":-." ."::. . ., ' .. 

" 

ISH JOURNAL OF HYDRAULIC ENGINEERING, VOL. 16,2010, NO.1 

(85) 

(8) 

l,if; 

. 

i, 

.. 

I,

i I 

1 

(86) 

where n, =Dg xDg 

DESIGN OF RIPRAP FOR PROTECTION AGAINST 


VOL. 16, (No.1) 

As an approximation, for Gaussian distribution Dg and D(J assumed as D50 and D84 

of rip rap mixture respectively. Further, Patel and Ranga Raju (1999) have given 't. co 

as a function of ag' This relationship is used to obtain 't. co for corresponding given 

magnitude of ag of the material used by Worman, Chiew and in present study.For the 

determination of size of riprap, one can choose the magnitude of ag and substitute the 

corresponding value of 't.c(J in Eq. (9) and can find average size of riprap. 

Size of the riprap calculated using Eq. (9) for Worman data was found to be 50 to 

96 percent higher in comparison with size of the riprap used in his experiments. 

Worman suggested use ofIshbash equation for the determination of size of the riprap 

taking local velocity (u) around the pier as twice the average velocity in unobstructed 

flow (u=2U). Size of the riprap computed using Ishbash equation (with u= 2U) for the 

data collected by Worman was also found to be 25 to 50 percent higher than those of 

used in his experiments. 

Further, data collected by Worman and Chiew were analyzed for computation of 

non-dimensional critical shear stress ('t.J for incipient scour of riprap and it was 

found that average value of 't. c as 0.00174 and 0.013 respectively. In the present 

study nms were also conducted for incipient scour of riprap and average value of 

non-dimensional critical shear stress (-r.J for riprap of given size was found to be 

0.0088. Further, in the present study 41 runs were observed either with no scour or 

with negligible scour. The non-dimensional critical shear stress (-r.J in these scour 

runs was found to vary from 0.003 to 0.09 giving an average value of 0.028. Table 4 

gives these details. 

TABLE-4 

NON-DIMENSIONAL CRITICAL SHEAR STRESS 

S.No. Name of the Investigator t., (J'c 

1 Worman (1989) 0.00174 1.18 -1.38 

2 Cfiiew (1995) 0.136 1.25 -1.27 

3 Present- runs for incipient scour of riprap 0.0088 1.36-2.67 

(analysis) 

4 Present - zero scour run with riprap 0.028 1.36 - 2.67 

ISH JOURNAL OF HYDRAULIC ENGINEERING, VOL. 16,2010, NO.1 

v-, 

......... 

-, , ::...... " 

..... ;"...'- ", :.::.:::: . 

(9) 

I I 

i 

I iI 

I

..... 

. . .. 

..... -.. - - ..'~ , .•................• 

VOL. 16,(No.1) DESIGN OF RIPRAP FOR PROTECTION AGAINST 


From this table, it is evident that Worman's method over predicts the size by about 

5 to 8 times, and Chiew's method gives 2 times larger size than that of observed in the 

experiments, where as in case of data collected in the present study it is 1.6 times 

(average) larger in comparison with observed size of rip rap. 

ANALYSIS OF RIPRAP TmCKNESS 

For studying the effectiveness of riprap in reduction of scour, the parameter C. = 

C a /C b has been calculated. Here C b is the value of constant obtained ill Kothyari et al. 

(1992) equation for scour in clear water studies, for non-uniform base material (Eq. 

(10».. . 

(87) 

(10) 

C is value of C when riprap was used and some scour was observed. 

~ 

The ratio of C/C b called C. takes in to account the effect ofU, Y, opening ratio a 

and /1.Ys on scour and hence, it should be function of D,= dsJDso and T. = T/3cr. Dso 

related to rip rap layer and bed material size only. Thickness of riprap layer can be non 

dimensionalised by maximum size of the riprap material, which can be expressed as 

3. Dso. Anew term therefore, introduced and expressed as T. =T/3cr a D so ' where T 

is the thickness of the riprap layer, cr is the standard deviation of riprap mixture 

a . 

given by ~D84 /D 16 and Dso is median size ofriprap mixture. lfthe sizesin riprap are 

distributed normally, 99.73 percent values will be within the range ofD so± 3c •. Hence, 

3cra Dso is as good as maximum size of the riprap mixture when D100 is not known. 

The experimental data having eight ranges of D. starting from 0.045 and 0.78 

were plotted as C/C b Vs T. for respective range ofDiand the equation between them 

is obtained as 

C a _ C = 0.5 D~·98 T.- 2 . 50 

C b - • (ll) 

By assuming that at a value of C. as low as 0.05, riprap around the bridge pier will 

be stable. Hence, this equation can be solved for with this value for determining the 

thickness of riprap. Data collected by Worman (1989) and Chiew (1995) are used for 

the comparison of the thickness of riprap layer computed using Eq. (10). Figure 3 

shows the thickness of riprap layer used by these investigators in their experiments 

for zero scour condition and thickness computed using Eq. (11). The plot shows 86% 

of Worman's data points and 68 % ofChiew's data points fall within the error band of 

. '\ 

ISH JOURNAL OF HYDR,AULIC ENGlNEERING, VOL. 16,2010, NO. 1_ 

':.' 

- 

'-"'.- 

,:.: :.: 

.:: . 

;;.....

l~':"'2C":;'§;{k)!"i;,,:dX'6B'~k;X~J/%#iiMtiiiW;i;l1~li.~1:i1t{l~'i;';';~Jf.~ 

>:~ 

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s: .. 

., 

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: 

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":.: 

f:- 

"':'.": 

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VOL 16.(No, 1) 

-v 

± 50 %. Data collected in no scour runs of present study are also plotted in the same 

figure. It is observed that the 75% of data collected in present study fall in the error 

band of ± 50 %. 

Thickness of riprap provided should also be economical. Blench (1957) suggested 

that thickness of riprap for bed protection may be three times the largest size of the 

stone in a mixture (DI~. Figure 4 shows the comparison between TlDloo observed 

and computed. 

If Eq. (11) is used to compute the thickness of riprap for Worman's and present 

data. thickness of riprapcomputed isfound to be orderone to three times D 100; however. 

it is four to five times that of D IOO in case of data collected by Chiew, Thickness of 

riprap layer using Worman and present data are found appropriate. however, it is 

slightly higher in case Chiew's data. In Worman's data the flow conditions, the 

characteristics of riprap, bed material and thickness of riprap used were pertaining to 

the observed "no scour" condition. In Chiew's data, the flow conditions given were 

intended for intact andfailed conditions of riprap. Average of flow data corresponding 

to intact and failed condition has been used in the present analysis to verify the Eq. 

(11). Theflow conditions pertaining to condition ofriprap described as intact. observed 

in his experiments may not be corresponding to the true Uno scour" condition. This 

could be possible reason for computed thickness of riprap found higher than observed. 

Line of Agreement. 

1000 

'.' tOO 

',' 

.., 

-- 

~ 

I •• 

... 

to 

r 

I- 

, , , 

, , 

, , 

, 

.' 

, , 

Line of Agreement 

, 

, , 

, , , , , , 

, , , 

, , 

, , , 

, 

~' _;)1 l"- I"'/Cl 

" B-' A, 

i-" ,6' •• 

,.0 •• 

,.111 

0 

1 10 100 1000 

AG. 3 THICKNESS OF RIPRAP (PRESENT METHOD) 

ISH JOURNAL OF HYDRAUUC ENGINEERING. VOL. 16, 2010. NO, 1

'. 1) 

ne 

or 

xi 

Ie 

d 

It 

. ., 

f 

VOL. 16, (No.1) 

DESIGN OF RIPRAP FOR PROTECTION AGAINST 


Incipient Scour of Bed and Riprap Material 

The value of UfU c near the circular bridge pier in present study varied from 0.3 to 

0.65 and authors recommends it as 0.438. When bed material around the circular 

bridge pier starts just moving, the value of UfU c is 0.438. 

Assuming the 'to alP, and corresponding fluctuations in the shear/velocity around 

bridge pier, authors found that maximum value of instantaneous shear stress near the 

bridge pier is about 12 'to. Size of the riprap can be calculated using Eq. (9) with (Jg of 

riprap between 2 to 3. 

1· 

,," 

10010- 

, 

, 

, 

, 

, 

+lS t>~p 

, .' 

, 

, , 

"I



ACKNOWLEDGEMENTS 

The authors are thankful to the reviewers for their constructive comments. 

REFERENCES 

VOL. 16, (No.1) 

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Chiew, Y. M. (1995). Mechanics of Riprap Failure at Bridge Piers. JHE, ASCE, Vol. 

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Darghi, B. (1990). Controlling Mechanism of Local Scour. JHE,ASCE, Vol. 116,No. 

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. Dey, S. (1997). Local Scour at Piers Part I Review of Development of Research. 

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Einstein, H. A. and El Samni, S. A. (1949). Hydrodynamic Forces on a Rough W~ll. 

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Garde, R. 1. (1970). Initiation of Motion on Hydro Dynamically Rough Surface Critical 

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Garde, R. J. and Ranga Raju, K. G (2000). Mechanics of Sediment Transport and 

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ISH JOURNAL OF HYDRAUUC ENGINEERING, VOL. 16,2010, NO.1

. I) VOL. 16. (No. 1) DESIGN OF RlPRAP FOR PROTECIlON ,AGAINST 

SCOUR AROUND BRIDGE PIER' 

:5'. 

1- 

l. 

). 

I. 

I 

! 

i 

i 

I 

. . . 

. . . -. ~ . . 

_.-. ..- 

Knight, D. W. (1975). A Laboratory Study of Local Scour and Bridge Piers. Proc. 

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Kothyari, Ll.C; Hager, W. H. and Oliveto, G. (2007). GeneralisedApproachJorClear 

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Auckland. 

Little, W. C. and Mayer, P. G. (1972). The Role oj Sediment Gradation on Channel 

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0672. 

Maynord, S. T., Ruff, J. F. and Abt, S. R. 1(1989). Riprap Design. JHE, VoL 115,No. 

7, pp. 937-949. 

Melville, B. W. (1997). Local Scour at Bridge Sites. Report No. 117, School of 

Engineering, University of Auckland. 

Parola, A. C. (1993). Stability oj Riprap at Bridge Piers. JHE, ASCE, VoL 119, No. 

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Sediments. JHR, IAHR, Vol. 37, No.1, pp 39-58. 

Quazi, M. E., Peterson, A. W. (1973). A Methodfor Bridge Pier Riprap Design. Proc. 

FIrst Canadian Hydraulic Conference, Edmont, Canada, pp. 96-106. 

Richardson, E. v., Harrison, L. J. and Davis, S. R. (1991). Evaluating Scour at Bridges. 

Rep. No. FHWA-IP-90-0l7HESI8, Federal Highway Administration (FHWA), 

~ashington, D.C. 

Worman, A. (1989). Riprap Protection without Filter Layers. JHE, ASCE, Vol. 115, 

No. 12, pp. 1615-1630. 

NOTATIONS 

a. angle between axis of the pier and approach flow 

~ constant used by Worman 

Ys specific weight of sediment 

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