Ain Shams Engineering Journal (2016) 7, 1045–1052

Ain Shams University

Ain Shams Engineering Journal

www.elsevier.com/locate/asejwww.sciencedirect.com

CIVIL ENGINEERING

Temporal variation of clear-water scour at

compound Abutments

* Corresponding author at: Department of Civil Engineering,

Estahban Branch, Islamic Azad University, Estahban, Iran.E-mail addresses: redac02@usm.my (A.Ab. Ghani), reza564@gmail.

com (R. Mohammadpour).

Peer review under responsibility of Ain Shams University.

Production and hosting by Elsevier

http://dx.doi.org/10.1016/j.asej.2015.07.0052090-4479 � 2015 Ain Shams University. Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Aminuddin Ab. Ghani a, Reza Mohammadpour b,c,*

aREDAC, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Penang, MalaysiabDepartment of Civil Engineering, Estahban Branch, Islamic Azad University, Estahban, IrancYoung Researchers and Elite Club, Estahban Branch, Islamic Azad University, Estahban, Iran

Received 6 February 2015; revised 8 July 2015; accepted 12 July 2015Available online 24 August 2015

KEYWORDS

Local scour;

Compound abutment;

Compound pier;

Bridge foundation;

Time-dependent scour;

Scour mechanism

Abstract Most of actual abutments in rivers are built on foundation, while there is limited number

of study available on the effects of the foundation on the local scour. In this study, temporal vari-

ation of local scour around compound abutment was investigated experimentally under clear-water

conditions. The results showed that a suitable level of foundation is able to decrease the scour depth

and increase scour time during the flood events. The trend of temporal scour depth at compound

pier and abutment is similar. The scour depth develops to top of foundation quickly, and then

the foundation postpones the scour development (lag–time). Duration of lag–time depends on

the foundation level, velocity ratio (U/Uc) and foundation dimension. This study highlights that

proper design of foundation level increases duration of scouring and provides enough time to treat

bridge foundation after the flood events.� 2015 Ain Shams University. Production and hosting by Elsevier B.V. This is an open access article under

the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

The local scour around piers and abutments is a huge prob-lem in the collapse of bridges. It is a complicated problem

that has been investigated significantly in recent years [1]. Inthe vicinity of the abutment/pier, the local scour is caused

by the development of high shear stress due to three-dimensional separation of the boundary layer [2,3]. Anaccurate prediction of scour depth influences the economic

aspects of bridges. Overestimation of the scour depth resultsin designing a deep foundation level and thus leads to anuneconomical design of bridges. However, the underestima-tion of scour depth would result in shallow foundation and

provide a chance to expose the foundation to the flowwhich is definitely dangerous for bridge safety [4].Temporal variation of local scour at pier/abutment can pro-

vide an exact prediction of scour depth, particularly duringthe flood. Therefore, design of foundation level based ontemporal scour depth is more accurate than that for equilib-

rium scour depth [5].The time variation of local scour at abutment is an essential

aspect of the hydraulic engineering [1]. Recently, the

Figure 1 Compound vertical-wall abutment (Bu is extension of

foundation upstream of the short abutment).

1046 A.Ab. Ghani, R. Mohammadpour

significance of the temporal scour evolution has been furtheremphasized rather than the equilibrium scour depth [2,6–14].

Most investigations have been carried out for abutments withuniform cross section. Similarly, most scour depth equations inthe literature focus on local scour for this kind of abutment[15–23].

However, due to geotechnical and financial reasons,actual abutments or piers are built on foundations withor without pile groups [24–27]. Such abutments/piers are

identified as compound abutments/piers [7]. In spite of usingthe compound abutment, there is limited information on theeffects of the foundation or pile cap on the local scour [28].

Only a few studies have investigated the effect of founda-tion geometry on the pier scour such as Chabert andEngeldinger [29], Jones et al. [30], Fotherby and Jones

[31], Melville and Raudkivi [32], Coleman [25], and Ataie-Ashtiani et al. [27]. Furthermore, limited studies are avail-able for time variation of scour at these kinds of piers[32,33]. If the effect of foundation was considered into vari-

ation of local scour, the conservative approach would beunnecessary and abutment scour may be reduced by properdesign of the foundation. Therefore, an attempt for investi-

gation of time variation of local scour at compound abut-ment is deemed very necessary.

In this study, temporal variation of local scour at com-

pound abutment was investigated experimentally under clear-water conditions. The top of foundation was located belowthe initial bed at three different positions which were chosenwith respect to sediment bed. Another objective of this

research was to determine similarity and difference betweentemporal variations of scour depth at compound abutmentand compound pier.

Table 1 Abutment-geometry characteristics of the present study.

Expe. no. Abutment type Foundation

Lf (cm) Bf (cm)

AB-I Uniform – –

AB-II Uniform – –

AB-III Uniform – –

FA21 Compound 5.5 11.0

FA33 Compound 9.0 18.0

FA42 Compound 12.0 24.0

FA43 Compound 12.0 24.0

2. Experimental setup

Three uniform and four compound abutments (Fig. 1) withvertical-wall shape and short length were chosen for tests

(Table 1). Melville [15] reported that abutment with a ratioof length (L) to flow depth (y) smaller than one (L/y< 1)can be considered as short length abutment. A flume with rect-

angular cross section having the dimension of 6.0 m long,0.6 m wide, and 0.6 m deep was chosen for all experiments.In this flume, regarding abutment length (Table 1), the effectof contraction scour is negligible. The flume was equipped with

a 0.25 m deep sand recess through the channel. A number ofexperiments were initially conducted to make sure a fullydeveloped boundary layer was obtained, and the test section

was chosen at X= 3.5 m from inlet of flume. The flow velocitywas measured using the Streamflow current meter. It has theability to measure velocities as low as 5.0 cm/s. Verification

of the flows was made using a weir located at the downstreamend of the channel. It was found that the flow differences werewithin 1%. Since the maximum scour depth occurs at the nose

of abutment, a camera and ruler were installed inside the trans-parent abutment to measure the scour depth at the nose of theabutment in different time. Uniform sediment withd50 = 0.60 mm and geometric standard deviation, rD = 1.20

was used in all tests. To maintain the clear water conditions,the flow velocity was set close to the critical velocity of sedi-ment (U/Uc between 0.94 and 1), where Uc was estimated using

the Shields diagram and expressions given by Melville andColeman [7]. It was observed that the ripples depth is smalland negligible. The topography of the scour hole after each

run was measured using a point gauge with an accuracy of±1 mm.

Since development of the scour hole never stops [34], three

long-time experiments were conducted to determine therequired duration of experiments with AB-I (�50 h), AB-II(�67 h) and AB-III (�84 h).

The results show that more than 90% of maximum scour

depth can be obtained after 42 h. After this time, the variationof scour depth was observed to be less than 1 mm for each timeinterval of 7 h (Fig. 2). However, this criterion was used as

semi-equilibrium time for uniform and complex abutments.As shown in Table 2, different duration was observed for dif-ferent location of the foundation.

3. Local scour at compound abutment

To investigate the effects of foundation level (Z) on a com-

pound abutment, three cases were considered based on the

Abutment Length ratio (L/Lf)

Bu (cm) L (cm) B (cm)

– 4.0 8.0 –

– 5.5 11.0 –

– 7.0 14.0 –

1.5 4.0 8.0 0.73

2.0 7.0 14.0 0.78

6.5 5.5 11.0 0.46

5.0 7.0 14.0 0.58

Figure 3 Three cases for compound abutment below the initial

bed.

Table 2 Summary of experimental result at uniform and compound abutment.

Cases Expe. Z (cm) y (cm) Time (h) U (cm/s) U/Uc L/y Z/L dse/L

I AB-I – 11.10 50.90 27.6 0.96 0.36 – 1.68

I AB-II – 11.00 66.70 28.6 0.98 0.50 – 1.73

I AB-III – 9.70 84.70 26.5 0.95 0.72 – 1.68

II FA21 0.00 10.80 48.20 28.3 0.99 0.37 0.00 1.75

II FA33 1.00 10.90 35.60 27.2 0.95 0.64 0.14 1.73

II FA42 1.00 12.00 37.70 28.4 0.97 0.46 0.20 1.58

II FA43 1.00 10.80 51.90 28.7 0.95 0.65 0.14 1.66

II FA21 1.50 11.10 50.00 27.6 0.96 0.36 0.38 1.50

II FA33 3.00 13.00 48.40 27.7 0.94 0.54 0.43 1.93

II FA42 3.00 13.10 42.90 27.7 0.94 0.42 0.55 1.98

II FA43 3.00 11.30 37.50 28.9 0.95 0.62 0.43 2.07

III FA21 3.50 11.10 41.70 27.7 0.96 0.36 0.88 0.88

III FA21 5.00 10.50 35.60 29.7 0.98 0.38 1.25 1.25

II FA33 5.00 10.60 38.60 28.5 0.99 0.66 0.71 1.47

II FA42 5.00 10.20 48.40 29.4 0.99 0.54 0.91 1.33

II FA43 5.00 10.80 44.00 28.0 0.97 0.65 0.71 2.01

II FA33 7.00 11.50 47.00 28.3 0.97 0.61 1.00 1.07

III FA42 7.00 12.00 50.60 29.4 0.99 0.46 1.27 1.27

III FA43 7.00 10.90 57.20 28.0 0.97 0.64 1.00 1.00

III FA33 8.00 11.30 57.00 27.4 0.96 0.62 1.14 1.14

III FA42 8.00 12.00 53.40 28.1 0.98 0.46 1.45 1.45

III FA43 8.00 11.10 58.30 28.3 0.96 0.63 1.14 1.14

III FA33 9.00 10.90 55.70 27.8 0.95 0.64 1.29 1.29

III FA43 9.00 11.30 58.50 27.8 0.95 0.62 1.29 1.29

Figure 2 Temporal variation of scour depth at uniform abutment in terms of (a) centimeter; (b) percentage dsdsmax

� 100� �

.

Temporal variation of clear-water scour 1047

foundation level (Fig. 3). In Case I, the foundation level waslocated below the scour hole. In Case II, the scour depthreaches to top of foundation, and the principal vortices in

front of the abutment were weakened by the foundation. In

Case III, the foundation exposes to the scour hole. In all cases,the foundation level (Z) was measured regarding initial bed

level.Case I can be observed for an abutment with small dimen-

sions and when the level of foundation was located under the

equilibrium scour depth (dse). In this case, the compound abut-ment is similar to uniform abutment with a maximum scourdepth of 2L [15]. In Case II, the scour depth reaches to foun-

dation level (Z) and remains constant at this level. In this case,the foundation would not be exposed to the scour hole, and thedepth of scour is approximately equal to foundation level(ds � Z) [35]. This case can be observed for foundation level

in the range of 1 < Z/L< 2. In Case III, the foundationexposes to the scour hole, and the foundation size (Lf) influ-ences the scour hole. This condition was observed for 0 6 Z/

L < 1 [35].

Figure 4 Temporal variation of scour depth for experiment

FA42.

1048 A.Ab. Ghani, R. Mohammadpour

4. Scour mechanism at compound Abutment

The trend of local scour at compound abutment is time depen-

dent. To investigate the effects of foundation elevation (Z) ontemporal variation of local scour around abutment, differentvalues of Z were chosen in this study. The experimental data

and flow condition are given in Table 2. In this table, ABand FA were selected for uniform and compound abutment,respectively.

The time variation of the local scour at FA42 with different

values of Z is shown in Fig. 4. Generally, the scour depth isdeveloped to top of foundation quickly and extension of foun-dation in front of abutment protects the bed and postpones the

scour development as a lag–time. For example, in FA42 withZ= 3.0 cm, the scour depth reaches the top of foundationafter 30 min and is fixed at this level for approximately

400 min (7 h). The results indicated that the lag–time forFA42 with Z= 1.0 cm is just 80 min. The scour hole isextended in parallel to and in front of the abutment duringlag–time. In addition, development of local scour in parallel

to abutment (in the flow direction) is faster than that in frontof abutment (upstream).

As shown in Fig. 5, the scour hole in the foundation

nose is enlarged in the area, and it is somewhat deeper thanother parts at the upstream side. The deepest depth at theupstream of abutment gradually develops around the sides

of the foundation to create a shallow groove parallel tothe foundation. Subsequently, the depth of the scour aheadof the foundation is more increased due to the formation of

a vortex at upstream of the foundation. This vortexenlarges and deepens the scour at the corner of the founda-tion, and the sediments besides the foundation are carriedout to downstream of the abutment. Although, the founda-

tion postpones development of scour depth firstly, but if thefoundation exposes to the scour hole, the vortex in front ofexposed foundation increases the scour depth. The lag–time

(latency) directly depends on the foundation and abutmentdimensions (L, Lf) and foundation level. The rate of sedi-ment transport decreases with increasing the scour hole

dimension, and it will be stopped approximately close tothe equilibrium scour depth.

5. Temporal variation of local scour at compound abutment

As shown in Fig. 4, for an infinite value of Z, the top offoundation is below the equilibrium scour depth, and the scour

hole at compound abutment is similar to uniform abutment.For FA42 with Z = 8 cm (Z/L = 1.45) and Z = 7 cm(Z/L = 1.27), the ratio of Z/L is in range of 1.0 < Z/L 6 2,

then the foundation stops development of scour depth as anobstacle in front of abutment. In these abutments, the maxi-mum scour depth is equal to the foundation level (Case II).For instance, the maximum scour depth at FA 42 with

Z= 7 cm (Z/L = 1.27) is equal to 7.0 cm while this valuefor uniform abutment of AB-II with same abutment dimen-sions is around 10.0 cm. Therefore, a proper design of founda-

tion level decreases the sour depth at compound abutment.For Z = 5 cm (Z/L = 0.91), Z= 3 cm (Z/L = 0.55) and

Z= 1 cm (Z/L = 0.18), the value of Z/L is located in range

of 0 6 Z/L < 1. In these conditions, the foundation exposesto scour hole and influences both scour depth and scour time(Case III). For example, for compound abutment of FA42 with

Z= 5.0 cm, the scour depth reaches to 5.0 cm after 100 min,remains at this level for a long time (600 min or 25 h) and isequal to 5.0 cm after 600 min (25 h). However, for uniformabutment AB-II with similar dimension, the scour depth

increases rapidly after 100 min and reaches to around 7.0 cmafter 600 min. Furthermore, due to effect of foundation, themaximum scour depth at compound abutment (7.5 cm) is less

than uniform abutment (9.5 cm). This reduction of scour depthand time-lag are very useful to prevent the failure of bridgeespecially in the flood events that bridges are the main struc-

tures in transportation. Generally, the peak of flood may notbe long-lasting to develop equilibrium scour depth and theflood may be stopped within lag–time. Therefore, the lag–time

postpones the maximum scour depth and provides an opportu-nity to repair the bridge foundation after the flood events.Fig. 6 shows the topography of scour hole around compoundabutment of FA-42 with Z= 1.0 cm.

As mentioned, if the foundation exposes to the scour hole,another vortex is produced by foundation which increases thescour depth. As shown in Fig. 7 the scour depth at the com-

pound abutment of FA43 with Z= 3.0 and 5.0 cm is biggerthan uniform abutment of AB-II. The results illustrated thatunsuitable design of foundation level increases the scour depth

which is dangerous for bridge safety.Similar trends can be observed for compound abutments of

FA21 and FA33 in Figs. 8 and 9, respectively. In all tests, thescour depth was measured at the nose of compound abutment.

6. Time variation of local scour at compound abutments and

piers

To compare temporal variation of scour depth at compoundpier and abutment, the collected data for compound pier givenby Melville and Raudkivi [32] were compared with present

study. Fig. 10 shows a comparison between compoundabutment of FA42 with Z = 1.0 cm (Table 1) and compoundpier with D= 5.0 cm, D* = 7.0 cm and Z= 3.0 cm, where D

and D* are pier and foundation diameter, respectively. Thedimension ratio of pier (D/D* = 0.71) is close to abutment(L/Lf = 0.78). The results indicate that the trend of temporal

scour at the pier and abutment is quite similar. In both cases,

Figure 5 Development of local scour hole for experiment FA42, Z = 3.0 cm (a) after 30 min; (b) after 7 h.

Figure 6 Scour hole at compound abutment for experiment

FA-42, Z = 1.0 cm (a) side view; (b) topography.

Figure 7 Temporal variation of scour depth for experiment

FA43.

Figure 8 Temporal variation of scour depth for experiment

FA21.

Temporal variation of clear-water scour 1049

the local scour develops to top of foundation and stops in thislevel for a certain time (lag–time), and then it increases withincreasing time until equilibrium depth. The lag–time occursat foundation level and it is different for pier and abutment.

The dimensions affect the maximum scour depth. As shownin Fig. 10, maximum scour depth at abutment withL = 7.0 cm is bigger than pier with D= 5.0 cm.

To investigate the effect of dimension on local scour, twomore comparisons were conducted with same foundation leveland different dimension in Figs. 11 and 12. In Fig. 11, com-

pound abutment of FA42 with L/Lf = 0.46 (Table 1) wascompared to compound pier with D/D* = 0.37. Both dimen-sions of abutment and foundation at abutment are bigger than

those at pier. The foundation level was located at the same

level of Z= 3.0 cm. The scour depth is developed toZ= 3.0 cm (top of foundation) and stopped at this level forseveral hours. The lag–time is around 100 min (�1.7 h) and

400 min (�6.7 h) for pier and abutment, respectively. A bigvalue of lag–time at abutment can be due to the large size offoundation (Lf = 12.0 cm) in comparison with pier

(D* = 8.1 cm). During the lag–time, the sediment around

Figure 9 Temporal variation of scour depth for experiment

FA33.Figure 12 Comparison between temporal scour at compound

abutment and pier with Z = 5.0 cm.

Figure 13 Comparison between temporal scour at compound

abutment and piers with different ratio of U/Uc.

Figure 11 Comparison between temporal scour at compound

abutment and abutment with Z= 3.0 cm.

Figure 10 Comparison between temporal scour at the pier and

abutment with different foundation level.

1050 A.Ab. Ghani, R. Mohammadpour

foundation is removed by the flow, and the foundation exposesto the scour hole. Then, the scour depth at compound abut-ment increases rapidly and reaches to approximately 11.0 cm

after around 2000 min (33 h). However, the scour depth atcompound pier increases with time and reaches to 4.0 cm at

the approximately same time. A big difference between scourdepth at compound pier and scour depth at compound abut-

ment may be due to large size of abutment and its foundation.It can be concluded that the dimension is a main parameter todevelop the local scour at compound pier/abutment.

Same pier and abutment were compared in Fig. 12 withZ= 5.0 cm. The local scour at both pier and abutmentincreases with time and reaches to top of foundation at

Z= 5.0 cm. Due to small size of the pier, principal vortex infront of the pier is weaker than abutment. Therefore, the scourdepth at compound pier is confined by top of foundation at thelevel of 5.0 cm. A large size of abutment and foundation pro-

duces the strong principal vortex in front of abutment, and thescour depth increases after lag–time to around Z = 8.0 cm. Acomparison between Figs. 11 and 12 indicated that a suitable

design for foundation level considerably decreased the localscour around both compound pier and abutment. Therefore,in addition to dimension, the foundation level has a significant

effect on temporal variation of local scour at compound pierand abutment.

The velocity ratio (U/Uc) is another parameter whichdirectly affects on time variation of local scour at the com-

pound abutment and pier. In Fig. 13, temporal scour depthat the compound abutment of FA21 (Table 1) is compared

Temporal variation of clear-water scour 1051

to the compound piers of S5 and S3 given by Lu et al. [33]. Thepiers with similar dimension of D = 3.5, D* = 5.0 cm and D/D* = 0.7 are located at Z= 2.5 cm. However, the velocity

ratio at S3 (0.9) is higher than S5 (0.8). As shown in Fig. 13,the scour depth reaches to top of foundation at S3 and S5 afteraround 6.0 min and 70 min, respectively. However, this time is

around 7 min for abutment FA 21 with L/Lf = 0.73,Z= 1.5 cm and U/Uc = 0.96, and it is close to those at S3.Low difference between pier and abutment with different size

can be due to approximately same velocity ratio.On the other hand, the lag–time at S5 with low value of U/

Uc (0.8) is more longer (900 min) than S3 with U/Uc = 0.9(70 min). It can be concluded that the lag–time depends on

the velocity ratio, and it decreases with increasing U/Uc.Furthermore, the velocity ratio also influences the scour depthdevelopment after lag–time. The maximum scour depth at

compound pier/abutment increases with increasing velocityratio. For instant, the equilibrium scour depth at S3 (7.8 cm)is bigger than that at S5 (3.4 cm). The results indicated that

the velocity ratio is another significant parameter on time vari-ation of local scour at compound pier and abutment.However, using riprap and gravel with big size decreases veloc-

ity ratio [36], and it can be suggested as a practical solution forreduction of local scour at compound pier and abutment.

7. Conclusions

In this study, temporal variation of local scour at compoundabutment is investigated experimentally under the clear waterconditions. The compound abutments included a rectangular

abutment founded on a larger rectangular foundation. Thetop surface foundation was located below the initial bed atthree different positions with respect to the general level of

the channel bed. In all experiments, the scour depth is devel-oped to top of foundation quickly, and then the foundationpostpones the scour development for a certain time (lag–time).

Duration of lag–time is depended on the dimension of abut-ment, foundation size and the foundation level. A comparisonbetween the compound abutment and pier indicated that the

trend of temporal scour depth at abutments is quite similarwith piers. Furthermore, the foundation level, dimension andvelocity ratio (U/Uc) are main parameters, which influencethe maximum scour depth and lag–time. This study highlights

that a proper design of foundation level decreases the scourdepth and increases the duration of scouring. Furthermore,the lag–time provides an opportunity to repair the bridge foun-

dation after the flood events.

Acknowledgments

All experiments were conducted at the REDAC PhysicalModelling Laboratory, Universiti Sains Malaysia. The authors

would like to thank Universiti Sains Malaysia for the financialsupport under the RU GA grant (1001/REDAC/8022009).

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Prof. Dr. Aminuddin Ab Ghani is a lecturer at

the Department of Civil Engineering,

Universiti Sains Malaysia (USM) since

17th January 1994. He is presently the Deputy

Director of River Engineering and Urban

Drainage Research Centre (REDAC). His

main research interests are Open Channel

Hydraulics, Sediment Transport, and

Sustainable Urban Drainage Systems

(SUDS). He has published over 100 articles in

journal and conference proceedings at inter-

national and local levels. He is also appointed

as referee for international and national

journals including Urban Water Journal,

IWA Journal of Water Science and

Technology, ASCE Journal of Hydraulic

Engineering, ASCE Journal of Irrigation and

Drainage, IAHR Journal of River Basin

Management, and ICE UK Water

Management Journal.

Dr. Reza Mohammadpour is a lecturer at the

Department of Civil Engineering, Islamic

Azad University – Estahban branch since

September 2001. His main research interests

are Local scour, Open Channel Hydraulics,

Sediment Transport, water pollution and

water quality. He has published more than 30

articles in journal and conference proceedings

at international and local levels. He is also

appointed as referee for international and

national journals including ASCE Journal of

Hydraulic Engineering, Geomate journal, in

Journal of Water Resource Management,

Journal of Water Science and Technology,

Caspian Journal of Applied Science Research

journal, Journal of Water Resource and

Hydraulic Engineering (JWRHE).