Author(s): Mohammad Badali Mashahir; Amir Reza Zarrati; Ebrahim Mokallaf
Linked Author(s): Amir Reza Zarrati
Keywords: Scouring; Bridge pier; Collar; Shear stress; Rectangular pier; Circular pier 1
Abstract: Application of a collar around the bridge piers is an effective countermeasure for local scouring. Installation of a collar at the streambed level protects the riverbed against the direct impact of down flow produced at the upstream face of the pier and therefore reduces the scour depth. In addition, existence of a collar reduces the rate of scouring considerably. In the present work effect of bed shear stress on development of scouring around bridge piers protected by a collar was examined. Experiments were conducted with two different bridge pier sections; i) circular and ii) rectangular. Width of the collars used in this study was 3 times the pier diameter or width and collars were installed at the streambed level. In these experiments, flow intensity that is the ratio of bed shear velocity to the critical shear velocity, calculated from the shields’ diagram, was 0. 85, 0. 9, 0. 923, 0. 95 and 0. 97. In addition, data for flow intensity equal to 0. 7 was used from a pervious study in this investigation. Results of this work showed that the bed shear stress affects the performance of a collar considerably. Results of tests with flow intensity of 0. 90 with rectangular pier indicated that until 150 hours no scouring was observed at the rectangular pier perimeter. However, with flow intensity of 0. 95, after 17 hours scouring began at the upstream face of the rectangular pier. Similarly, rate of scouring increased considerably with increasing flow intensity from 0. 7 to 0. 9 around the circular pier.
Year: 2007