Alexandria Engineering Journal (2015) 54, 197–203

HO ST E D BY

Alexandria University

Alexandria Engineering Journal

www.elsevier.com/locate/aejwww.sciencedirect.com

ORIGINAL ARTICLE

Experimental investigation of local scour around

multi-vents bridge piers

* Corresponding author. Tel.: +20 966 551 195 776.

E-mail address: Yasser_eng1997@zu.edu.eg (Y.A. Mohammed).

Peer review under responsibility of Faculty of Engineering, Alexandria

University.

http://dx.doi.org/10.1016/j.aej.2015.03.0041110-0168 ª 2015 Faculty of Engineering, Alexandria 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/).

Yasser A. Mohammed a,*, Yasser K. Saleh b, Abdel-Azim M. Ali c

a Water Eng. Dept., Faculty of Engineering, Zagazig University, Zagazig, Egyptb Higher Technological Institute, 10th of Ramadan City, Egyptc Hydraulic Research Institute, Egypt

Received 6 April 2011; revised 23 July 2014; accepted 8 March 2015Available online 3 April 2015

KEYWORDS

Bridge pier;

Local scour;

Collar;

Sacrificial pile;

Current deflector;

Hydraulic structures

Abstract The harmful effect of local scour around bridge piers and abutments can induce high

maintenance costs or even bridge collapse resulting in the disturbance of traffic and possibly human

losses. In the present research, an experimental study was carried out to investigate local scour

around multi-vents bridge supports. Different methods of scour-countermeasures were applied to

minimize and control the formed scour around bridge supports. It was found that, using collar

around piers, current deflectors and sacrificial pile upstream piers reduced local scour depth by

more 90%.ª 2015 Faculty of Engineering, Alexandria 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

Scour is the most serious waterway problem at which greaterthan 60% of the reported failures can be attributed to it [26].There are many reports about bridges failure around the

world due to scouring [10,15]. The mechanism of scouringaround a bridge pier is very complex and has been reportedby various investigators [9,14] and [5]. Abdelhafiz [3] studiedthe effect of pier shape and spacing on energy loss and scour

around bridge piers. It was found that the maximum scouroccurs upstream of the pier and the depth of scour holeincreases as tail Froude number increases. Fotherby and

Jones [8] used collars around bridge pier for reducing and

controlling local scour depth. The authors recognized the

potential usage of both a collar and a footing for scour reduc-tion. Experimental studies were applied using piers providedwith slots and collars under different flow conditions byKumar [12]. Kumar et al. [13] studied the efficiency of slots

with different lengths and angles of attack. It was concludedthat a slot can be effective in reducing scour, particularly if itextends into the bed, and that the slot is practically ineffective

if the approach flow has a high obliquity with respect to theslot. Zarrati et al. [27] worked on the application of a collarto control the scouring around rectangular bridge piers hav-

ing a rounded nose. It was found that collar effectivenessimproves as the collar becomes wider and as the level atwhich it is positioned on the pier becomes lower. Reductionof local scour in the vicinity of circular bridge pier groups

using collars and riprap was studied by Zarrati et al. [28].The effect of triangular collar around bridge pier was studiedexperimentally by Mohamed et al. [18]. Different shapes and

Figure 1 Sketch for experimental model details.

Figure 2 Relative scour depth Ds/yt versus tail Froude number Ft for, (i) Central pier (C.P) and side pier (S.P) case of multi-vents bridge,

(ii) case of two vents bridge.

Figure 3 Scour profiles in case of C.P at different Froude numbers (Ft).

198 Y.A. Mohammed et al.

Figure 4 Scour profiles in case of S.P at different Froude numbers (Ft).

Figure 5 Scour contour maps around multi-vents bridge piers

for (a) Ft = 0.34, (b) Ft = 0.42, and (c) Ft = 0.53.

Investigation of local scour around multi-vents bridge piers 199

dimensions of collar around bridge pier were examinedexperimentally by Abdel-Aal and Mohamed [2]. Kothyariand Kumar [11] proposed a new mathematical model forcomputation of temporal variation of scour depth at circular

compound bridge piers. EL-Ghorab [6] studied experimen-tally the local scour depth around bridge piers provided withslots. The effect of pile cap thickness on the temporal evolu-

tion of the maximum scour depth and the development ofequilibrium conditions was investigated by Ferraro et al.[7]. Sheppard et al. [24] performed an evaluation of the exist-

ing equations for the local scour at bridge piers. Nordila et al.[20] studied the local scour depth at wide bridge piers. Inaddition, laboratory studies of sacrificial piles for pier scour

protection are reported by Shen et al. [23], Chang andKarim [4], Paice and Hey [21], and Singh et al. [25]. Thesestudies showed up to 50% scour reduction due to the pres-ence of the sacrificial piles. Laboratory studies on the use

of sacrificial piles as a pier scour countermeasure are reportedby Melville and Hadfield [16]. This study aimed to controland reduce the local scour around multi-vents bridge piers

using collar, current deflectors that fixed over collar andmobile bed, and sacrificial pile upstream piers.

2. Experimental work

The experimental work was carried out in a re-circulatingchannel with 6 m length, 20 cm depth and 60 cm width.

This study is carried out in the hydraulic laboratory of col-lege of engineering, Zagazig University. The median sandsize (D50) is 1.50 mm, and the geometric mean (D85/D15) is

1.41. The experimental work was conducted under theclear-water condition. For each test of the experimental pro-gram, the sand was leveled along the entire length of flumeusing a wooden screed with the same width as the flume.

The sand level was checked in randomly points with a pointgauge. The flume was slowly filled with water to the requireddepth. The pump was then turned on and its speed increased

slowly until the desired flow rate was achieved, after that thetailgate was adjusted to get the required water depth. At theend of the test the pump was turned off and the flume was

drained slowly without disturbing the scour topography.Discharge was measured using pre-calibrated office meterfixed on a feeding pipe. Flow depths range from 35 mm to70 mm with discharge of 6.5 L/s. The time duration for each

experiment was taken as 12 h, at this time the scour depthreaches more than 90% of the equilibrium scour depth thatbased on preliminary experiments. The scour profile was

recorded with a point gauge with 0.1 mm accuracy. PerspexPier with width 3.0 cm, and 40 cm length was installed inthe channel centerline. Tow half piers at two sides of the

flume were fixed to represent a case of multi-vents bridge.

Figure 6 The relationship between Ds/yt and Ft for C.P with different scour countermeasures.

Figure 7 The relationship between Ds/yt and Ft for S.P with different scour countermeasures.

200 Y.A. Mohammed et al.

The nose and tail of the piers have a rounded shape. Thewidth of the pier was chosen so that the effect of flume side

walls and sediment size on the depth of scour hole wasnegligible, [22]. Rectangular collar width of 3.0 times the pierwidth was tested. The current deflector (C.D) over collar

with height equals 0.33 times pier width, length equals 1.5times pier width and with angle equals 0.23p, was tested,(optimal dimensions of C.D over collar by [19]. In addition,

C.D over mobile bed (height over bed equals 0.33 times pierwidth, length equals 1.5 times pier width and parallel to theflume centerline) was tested in existence of collar and C.Dover it, (Fig. 1). Finally, the tested models were provided

with sacrificial pile with diameter 0.33 pier width, and differ-ent positions upstream piers, (Xp = 2, 3, 4, 5 times the pierwidth).

3. Analysis of experimental results

3.1. Scour around bridge piers

Fig. 2 shows the relationship between the maximum relative

scour depth (Ds/yt) and tail Froude number (Ft) for central pier(C.P) and side pier (S.P). For the multi-vents bridge, it wasfound that, Ds/yt increases as Ft increases and vice versa. Inaddition, the maximum relative scour depth at the C.P is

higher than that created at the S.P, at which the maximum flowvelocity occurs at the channel centerline and it reduces to itsminimum limits at the boundary. In addition, results of local

scour around two vents bridge were plotted in the same figure,Melville and Chiew [17] and Abdel-Aal and Mohamed [2]. It

Figure 8 Scour contour maps around bridge pier at Ft = 0.53

for: (a) collar, (b) collar with C.D over it, and (c) collar with C.D

over rigid and mobile beds.

Investigation of local scour around multi-vents bridge piers 201

was found that the scour depth formed around pier in case oftwo vents bridge, is higher than that created around the multi-vents bridge piers, under the same flow conditions. Scour pro-

files at different Froude numbers were presented in Figs. 3 and4 for C.P and S.P, respectively. It was found that, the localscour depth created at piers for both C.P and S.P increasesas the tail Froude number increases. Fig. 5a–c presents scour

contour maps for different Ft, it was observed that the scourhole dimensions increase as the Ft increases. Moreover, forall scour contour maps, the main scour hole forms upstream

bridge piers. The deposition was formed downstream of scour

hole within the pier length in case of low Froude numbers.Otherwise it formed downstream of bridge pier.

3.2. Collar, current deflectors, and sacrificial pile

The effect of collar, as a protective scour plate, was investi-gated on scour characteristics around bridge pier. Collar

attached to bridge pier at some level usually close to the mobilebed was found to be the optimum position [1]. Fig. 6 shows theeffect of fixing rectangular collar (Bo = 3, relative width of

rectangular collar) around C.P on local scour depth. The pres-ence of rectangular collar around C.P reduced scour depth byat least 65%, compared to no collar case. For the previous

case, it was observed that, scour holes were formed at thetwo sides of bridge piers in case of high values of Froude num-bers. So, Straight C.D (current deflector) (LC.D = 1.5b,hC.D = 0.33b and h = 0.23p) was fixed over the rectangular

collar (Fig. 1), to control and minimize the formed scour holes.In addition, second row of current deflector (C.D) was used formore controlling the local scour depth. This second row of cur-

rent deflector was fixed parallel to pier centerline over themobile bed with the same dimensions like the first one. Itwas investigated that, the current deflectors over collar and

mobile bed reduce the scour depth by approximately 10%.At which the current deflectors were considered good toolsfor distributing the flow velocity uniformly along the lateraldirection of channel at bridge piers. The same results for S.P

were obtained, as shown in Fig. 7. Scour contour maps forthe multi-vents bridge pier were shown in Fig. 8. It was foundthat, the presence of current deflectors over mobile bed and

collar leads to reduce the scour hole dimensions around C.Pand S.P compared to the other cases, i.e. the case of no collarand the case of collar with one row of current deflector over

the collar.Sacrificial piles are placed upstream of bridge pier for the

purpose of protecting it from local scour. The piles, which

themselves may be subject to substantial scour, protect the pierfrom scour by deflecting the high-velocity flow and creating awake region behind them, Melville and Hadfield [16]. Thesepiles could be used as a support to aqueduct, and gas or water

pipes. Effect of Sacrificial pile (pile diameter equals 0.33 pierwidth) at different spacing (Xp = 2, 3, 4 and 5 times pierwidth) upstream the pier nose on the local scour depth created

around bridge piers, was studied. Fig. 9 shows the relationshipbetween the maximum relative scour depth at Froude numberequals 0.53, and different scour countermeasures (i.e. No-col-

lar case, one row of current deflector over collar, additionalcurrent deflector over mobile bed, and sacrificial pile at differ-ent positions from pier nose). This figure presents all scourcountermeasures used through the present study compared

to the no-collar case. It is obvious that the collars aroundthe multi-vents bridge piers provided with current deflectorson the mobile bed and collar are considered good tools to

minimize the local scour depth. In addition, Sacrificial protec-tive piles located upstream of the bridge piers at Xp = 4, pro-duce minimum relative scour depth around multi-vents bridge

piers compared to the other positions (Xp = 2, 3, and 5).Finally, the scour reduction using the combination of previousscour countermeasures (collar, current deflectors, and sacrifi-

cial pile) was 91%.

Figure 9 Ds/yt versus different scour countermeasures, at Ft = 0.53.

202 Y.A. Mohammed et al.

4. Conclusion

In the present work, the scour around piers through multi-vents bridge was studied experimentally. The present researchshows that using collar around multi-vents bridge piers leads

to reduce the local scour depth by 65%. In addition, the cur-rent deflectors over the mobile bed and collar share to reducethe local scour depth by additional 10%. These current deflec-

tors have the ability to minimize and control the dimensions ofside scour holes around multi-vents bridge piers. The protec-tive sacrificial pile upstream of bridge pier, collar and current

deflectors reduce the local scour depth at bridge piers by more90% compared to the unprotected bridge piers.

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