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Evaluation of the effect of collar on controlling scouring around Oblong pier in a 180 degree bend.

Introduction

Scour is defined as the erosion of stream bed sediment around an obstruction in a flow field. More over, scouring is one of the main factors of financial losses and deaths and the following are some of the examples: scouring was the main cause of destruction of Scholarie Creek bridge in New York (1987) which Killed 10 people. In 1989, the destruction of us 51 bridge on the Hatchie river in Tennesee Killed 9 people. In another instance pointed out by Lagasse et al., [1], the bridges on the northern and southern borders of California were destroyed by flood, Bridges human deaths, every year millions of dollars are spent for replacing and maintaining bridges as well as indirect expenses for transporting equipments [1]. In a study by American Federal Road Beurea, [2] reported that loss of money was around 100 million dollars because of bridge scouring[2].

Melville and Coleman [3] found that scouring caused by rivers lead to 36-million--dollar loss in New Zealand [3]. In Iran, too, many bridges break down because of scouring and many financial and life losses occur every year. Statistics show that, despite of improvements in technology of bridge construction, their destruction has increased. There for, investigating this phenomenon seems necessary. Research Hypotheses and Dimensional Analysis:

ds = f (b, W, [theta], y, B, [S.sub.0], V, g, [d.sub.50], R, [[rho].sub.s], [PHI], [rho], [mu], h, L, t)

The above parameters are as follows:

ds: Scouring depth

L: Pier length

h: Ogival position regarding bed surface

[theta]: Pier angle with Ogival

B: Channel width

V: Flow velocity

[d.sub.50]: Diameter of which 50% of bed materials are smaller

R: Central radius of channel bend

[[rho].sub.s]: Bed materials special mass

[PHI]: Bed sand internal friction angle

[rho]: Fluid special mass

[mu]: Fluid viscosity

b: Pier width

w: Ogival width

y: Flow depth

[S.sub.0]: Bottom slope

g: Gravity acceleration

t: Time

Using Buckingham theory, equation (1-4) was made dimensionless.

Fr is flow Froude number, Re is flow Reynolds number.

ds/b = f(W/b, y/b, B/b, Fr, [d.sub.50]/b, R/[lambda], Re, [Re.sub.*], h/b, L/b, [theta]/[lambda], [PHI]/[lambda], [S.sub.0], t/[t.sub.e])

After removing the constant parameters, the equation is as follows:

ds/b = f(W/b, h/b, t/[t.sub.e])

Material and Methods

To study the amount of scouring around bridge piers with rectangular sections and a triangular Ogival in river bends, a physical model was prepared. The experiments were Conducted in a circular plaxiglass flume with central angle of 180 degree, central radius of R = 2.8m and with of B=0.6m (R/B=4.7). The direct channel length was 9.1 m long which was attached to a channel with a 180-degree bend. This circular channel was attached to a control gate and a discharge tank through another direct channel which was 5.5 meters long (figure 1).

[FIGURE 1 OMITTED]

Following the recommendation made by [4], to avoid the effects of channel walls on point scouring in all the experiments, the pier diameter should not be more than 10 percent of the channel width [4]. Consequently, a 6-centimeter metal pier was used and its wall was a 12-centimeter plaxiglass sheet (figure 2). As [5] suggest, to avoid formation of average-diameter sand, sands bigger than 0.7 mm should be used [5]. Also, to remove the effects of sediments on scouring depth compared to pier diameter to the average, sands should be less than 50 [4]. In the same line, [5] proposed a 25-30 relation. Considering these facts, a layer of natural river sand with an average diameter of 1.4 mm and standard deviation coefficient of 1.3 was selected and used in a 15-centimeter thick layer for the experiments. As, to eradicate the effects of roughness, water depth should be more than 20mm and in all the experiments was constantly 12 cm [5]. Because point scouring was studied in clear water situation, to avoid erosion and sediments movement upstream of the pier, average flow velocity should be less than critical velocity (u<uc). In all the experiments the proportion of shear velocity to critical sheer velocity was 0.93. the necessary discharge was measured by a 60-degree triangular weir at the beginning of the flume. To determine the equilibrium time of the experiments, a prolonged experiment was conducted lasting for 6 hours on a bridge pier in a discharge of 27 litters per second in the 60-degree position. It was seen that during the first 2 hours, approximately 98 percent of scouring occurs. Therefore, the equilibrium time was set 2 hours in all the experiments.

As the beginning of each experiment. The channel bed was leveled under a constant slop by a moveable leveler. Then the pier was installed in 60-degree position. After the pump was started, the end gate was closed and then clear water slowly flow in the channel to avoid ripples and disturbance in the bed. The time for saturation of the channel was 20-30 minutes. After ensuring that the sediments were soaked after a few minutes, the pump was started with a low discharge and gradually it was set at the necessary discharge using the main valve of the pipe leading to the stilling basin. After ward, through exact and simultaneous setting of the downstream valve and the gate, the flow depth was 12 cm and the necessary discharge was obtained. After 2 hours, the pump was shutdown and the end gate was shut to slightly drain the water in the channel not to affect the topography of the bed. To determine the time of scouring development, the maximum scouring depth upstream of the pier was exactly measured by a point gauge. A after the equilibrium time, the end gat was gradually opened to slowly release the water from the channel. Then the topography of the bed around the pier at different positions and discharges was measured. To exactly study the occurred charges in the bed, The measurement points were set at 2 cm a long the width and a long the length with regard to the 2-centimeter positions.

[FIGURE 2 OMITTED]

Results and Discussion

The result of the above graphs reveal that at different diameters of Ogival, the installation of the Ogival at 0.1 b height under the bed compared with on the bed position, caused the least amount of scouring because on the bed Ogival, scouring stared and rapidly developed with on the horse shoe vortex in front of and under the pier at the initial moments of the experiment. Moreover, with the installation of the Ogival with different diameters at 0.5 and 1b heights under the bed, the scouring depth increased compared with the height of 0.1b under the bed. Investigating the volume of scouring hole and sedimentation pile at different positions and discharges (graph 1).

[GRAPHIC 1 OMITTED]

Graph (2) demonstrates the percentage of scouring depth decrease at different positions for different Ogivals for a pier with no protective structure. Based on this graph, it can be concluded that with the increase in the Ogival diameter, the percentage of scouring depth decrease grew. Also, it can be seen that there is a high discrepancy between the percent age of scouring depth decrease at 1/5 b Ogival and other Ogivals indicating the minor effect of this Ogival on controlling scouring.

At 0.1b position under the bed which the least scouring amount occurred, with the increase in Ogival diameter, the control of scouring was averagely 48.4 percent at 1.5b Ogival which has the least controlling of scouring, 56.06 percent at the 2b Ogival, 68.18 percent at the 2.5b Ogival, 75.75 percent at the 3b Ogival which has the highest controlling of scouring with highest amounts at 3b,2.5b, 2b, and 1.5b Ogivals respectively. The reason explaining this fact is that with the increase in Ogival diameter, a high volume of the goes down ward and does not allow it to hit the bed which results in weakening of shoe horse vortexes and ultimately stops scouring depth increase.

[GRAPHIC 2 OMITTED]

[d.sub.s]/b = a[(W/b).sup.[beta]] [(h+c/b).sup.d] Ln[(t+e/[t.sub.e]).sup.f]

In the above equation, a, b, c, d, e, f are constant experiment coefficients which are obtained through the results.

Therefore, equation 4 can be written as follows:

[d.sub.s]/b = 1.45[(W/b).sup.1.24] [(h+4.5/b).sup.0.085] Ln[(t+[t.sub.e]/[t.sub.e]).sup.0.185]

And the related regression is 0.95 (graph 3).

[GRAPHIC 3 OMITTED]

In all experiments, after setting the discharge and depth, some vortexes formed immediately around pier and scouring started at a rapid pace. With the formation of scouring hole, sediments move downstream. After some time, the sediments risen from the scouring hole reach an area in which the effects of the pier and the vortexes formed behind the pier are minor.

In this situation, the sediments moved from the scouring depth move down stream with the secondary flow and two or more cracks from around the pier (figure 3).

[FIGURE 3 OMITTED]

Conclusion:

Conclusion this experimented study, investigated the scouring development time around a rectangular pier with triangular and four different Ogivals. Installed at 60-degree bend. Effective variables in this study were the installation position and the size of the Ogivals. Results showed that maximum scouring depth occurred with the Ogival (w=1.5b, Ogival width ) at h=1b position under the bed and the minimum scouring depth occurred with the Ogival (w=1.5b) at h=0.1b position under the bed. Investigating maximum scouring depth at different positions shows that the highest scouring amounts occurred at positions 1b under the bed, on the bed, and 0.1b under the bed respectively.

Refrences

[1.] Lagasse, P.F., L.W. Zevenbergen, J.D. Schall and P.E. Clopper, 2001. Bridge scour and stream instability countermeasures: Experience, selection and design guidelines. FHWA. No. 23, 2nd ed., U.S. Department of Transportation, Washington, D.C., pp: 65.

[2.] Alabi, P.D., 2006. Time development of local scour at bridge pier fitted with a coller. MS thesis, University of Saskatchewan, Canada, pp: 121.

[3.] Melville, B.W. and A.J. Raudkivi, 1996. Effect of foundation geometry on bridge pier scour. Journal of Hydraulic Engineering, ASCE, 114 (10): 203-209.

[4.] Chiew, Y.M. and B.W. Melville, 1987. Local Scour around Bridge Piers. Journal of Hydraulic Research, 25 (1): 15-26.

[5.] Ettema, R., 1980. Scour at Bridge Piers. Report No. 216, University Of Aucland, School Of Engineering, New Zealand, pp: 60.

(1) M. Heidarnejad, (2) R. Ghasemi and (3) A. Masjedi

(1,2) Department of Irrigation, Science and Research Branch, Islamic Azad University, Khouzestan, Iran.

(3) Department of Irrigation, Islamic Azad University, Ahvaz Branch, Iran.

Corresponding Author

Mohammad Heidarnejad, Department of Irrigation, Science and Research Branch, Islamic Azad University, Khouzestan, Iran.

E-mail: m.heidarnejad@srbiau.ac.ir
Table1: Percentage of the effect of Ogival on
decreasing scouring based on size and position.

Collar place\ Bed h/b = h/b = h/b =
Size collar level 0.1 b 0.5 b 1

W/b=1.5 34 48 30 19
W/b=2.0 48 56 37 22
W/b=2.5 63 68 48 28
W/b=3.0 71 76 56 38
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Title Annotation:Original Article
Author:Heidarnejad, M.; R.Ghasemi; Masjedi, A.
Publication:Advances in Environmental Biology
Article Type:Report
Geographic Code:1USA
Date:Nov 1, 2011
Words:1920
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