MAXIMUM LOCAL SCOUR DEPTH VARIATION AT BRIDGE PIERS

Gye-Woon Choi¹  and  Sang Jin Ahn²

¹Associate Professor, Dept. of Civil and Environmental System Engineering,

Univ. of Inchon, Inchon, Korea, +82-32-770-8467(Tel), +82-32-762-7683(Fax),

E-mail: mgyewoon@lion.inchon.ac.kr

²Professor, Dept. of Civil Eng., Chung Buk National Univ., Cheong Ju,

 Korea, +82-43-261-2380(Tel), +82-43-261-2199 (Fax),

E-mail: hydrosys@trut.chungbuk.ac.kr

Abstract:In this paper, the maximum local scour depth variation by interaction between bridge piers was examined through the experiments having five piers in the non-cohesive bed material in the experimental flume. In the experiments, the maximum distance of 25 times of the pier length and the maximum distortion width of 8 times of the pier width were utilized. Through the experimental studies, it was indicated that the local scour depths are severely affected when the interaction between adjacent bridge piers exists. In the low flow, which can be characterized as low Froude number, the maximum local scour depth is obtained when the piers are installed in the straight line in the flow direction without any distortion.  However, in the high flow, which can be characterized as high Froude number, the maximum local scour depth is obtained when the piers are installed with some distortion widths from the flow direction.  The local scour depth by interaction between bridge piers is changed depending upon the Floude numbers, the distance between piers, and the distortion width between adjacent bridge piers. The maximum scour depth increment of about 60% when the interaction between bridge piers exists is obtained compared to the single pier. Also, because the scour patterns are affected by the time variation, the maximum scour can be indicated in the different piers by passing time.  It can be suggested that the maximum scour depth at bridge piers predicted by applying the existing equations should be increased in the case of interaction between bridge piers. 

Keywords : local scour, non-cohesive bed material, experimental flume, flow direction

1    INTRODUCTION

If a new bridge in the river in the urban area is constructed, the hydraulic condition and geographical changes in the river are severely indicated.

The water depth flocculation and velocity increment are the main indications of the hydraulic condition changes and the variation of the bed configuration is the main indication of the geographical changes.

In the previous days, the researches for forecasting the scour at bridge piers are mainly focused on the case of construction of a single bridge in the river.  The results were usually obtained through the analysis of the experimental data.  However, in the rivers such as Han river runs through the downtown in Seoul, Korea, the bridges are successively constructed for the purpose of the transportation and the delivery of the industrial production.  These bridges can be constructed with the very short distance because recently the road is being frequently constructed between the existing roads.  In these cases, the different bed configuration patterns compared to that in a single pier are observed.

In this paper, the local scour depth variations at piers, which is affected by the bed configuration change depending upon the interaction between bridge piers are examined through the experiments.

2    EXPERIMENTAL EQUIPMENT AND METHODS

The experiments were conducted in the experimental flume which consists with three parts such as pump and distribution pipe system, storage and distribution tanks in the upstream and downstream, and the rectangular channel of 12 m.  The slope of maximum 2% can be obtained by adjusting the channel.  Five elliptical piers are used to investigate the effect by the successive bridges piers as shown in Fig. 1.

Table 1    Boundary conditions for experiments

The longitudinal distance between bridges and the distortion width between piers in the cross sectional directions are shown as C and B in Fig. 1 and Table 1. In Fig. 1 and Table 1, L and d indicate the length and width of the pier, respectively.

Fig. 1    Installation of the experimental piers in the flume

3    ANALYSIS OF THE EXPERIMENTAL DATA

About 100 figures for analyzing the increment of the scour by interaction between bridge pires were obtained. Fig. 2 indicates the maximum scour depth changes at five piers by increasing Froude Numbers. 

The distance of 5 times as much as the pier length is utilized. As shown in the Fig. 2(a), when the distortion width is not existed, the maximum scour is occurred in the Pier No.1, which is installed in the upper position. However, as shown in the Fig. 2(b), when the distortion width exists, the maximum scour depth is occurred in the piers which was installed in the lower position.

         (a) C=5L, B=0d                      (b) C=5L, B=6d

Fig. 2    Maximum scour depth variation in the different distortion widths and Froude numbers

In the Fig. 2, “d” indicates the pier width and 0d and 6d are the distortion widths between bridge piers. Fig. 3 indicates the maximum scour depth increment rate in case of the interaction between bridge piers in Froude number of 0.6. The increment rates are given by the maximum scour depths at bridge piers compared to the maximum scour depth in a single pier.  From the Fig. 3(a), the maximum scour depth increment rate of about 60% is obtained at pier No. 4. 

(a) C=5L, Fr=0.6                   (b) C=10L, Fr=0.6

Fig. 3    Scour depth increment rate by increasing pier numbers

Fig. 4 indicates the maximum scour depth variation by changing Froude numbers in the different pier distance.The scour depths are usually increased by increasing Froude numbers even though the increment rates of the maximum scour depths are comparatively small in the high Froude numbers.

(a) C=5L, Pier No.1                  (b) C=10L, Pier No.1

Fig. 4    Maximum scour depth variation by changing Froude numbers

The maximum scour depth compared to the pier width does not exceed 3.0 even though the maximum scour depths are occurred in the different piers by changing the distance between piers.

Fig. 5 shows the maximum scour width variation in the difference pier by changing the distance between bridge piers. 

(a) C=5L, Fr=0.6                      (b) C=10L, Fr=0.6

Fig.5    Maximum scour width variation by changing the distance between bridge pires

The experiments were conducted in the Froude number of 0.6.  The distances between bridge piers were changed from 5L to 10L. The maximum scour width compared to the maximum scour depth does not exceed 1.6 in the experiments. The maximum pier width to the maximum pier depth is indicated in the different piers by changing the distance between bridge piers.

(a) C=5L, Fr=0.6                          (b) C=10L, Fr=0.6

Fig. 6    Maximum scour depth variation by changing the distortion width

Fig. 6 shows the maximum scour depth variation by changing the distortion width in Froude number of 0.6.  The distances between bridge piers are 5-25 times as much as the length of the bridge pier.  Fig. 6(a) and Fig. 6(b) show that the maximum scour depth is occurred at the distorted piers and the scour depth is decreased by increasing the distortion width when the distance between bridge piers is relatively short (C=5L, 10L).

4    CONCLUSIONS

To find out the local scour depth variation by interaction between bridge piers, 100 experiments were conducted.  The experimental conditions were obtained based upon the variation of the distance between bridge piers, the distortion width between adjacent bridge piers and Froude number.

In this paper, the following conclusions were obtained based upon the analysis of the results of the experiments.

First, the local scour depth at a pier, when the interaction between bridge piers exists, is severely affected by the scours at the other piers.

Second, when the interaction between bridge piers exists, the maximum scour depth is increased by about 60% compared to that of a single pier.  However, the increasing rate is changed depending upon the pier distance and the distorted width in the cross sectional direction.

Third, in the low flow, which can be characterized as the low Froude number, the maximum scour depth is obtained when the piers are installed in the straight line in the flow direction.  However, by increasing Froude number the maximum scour depth is obtained when the piers are installed with some distortion to the flow direction.

Fourth, it can be inferred that the existing equations to predict the maximum scour depth at bridge piers should be adjusted when the interaction between bridge piers exists.

References

Breusers, H. N. C., Nicollet, G., Shen, H. W., (1977) “Local Scour around  Cylindrical Piers”, J. of Hydraulic Research, pp. 221-252.

Raudkivi, A. J., Ettema, R., (1983) “Clear Water Scour at Cylindrical Piers”, J. of Hydraulic Engineering, ASCE, Vol. 109, pp. 338-350.

Richardson, E. V. and Richardson, J. R., (1990), “Bridge Scour”, Draft, Colorado State University, Fort Collins, Co.