Effect of Collar on Time Development of Scouring Around Rectangular Bridge Piers

Collars acts as a barrier to the down flow and prevent its direct impingement to the streambed. In the present work collars with 3 different lengths (5B, 7B and 9B, B=pier width) and constant width equal to 3B were installed around a rectangular pier with aspect ratio equal to 1:3. Also, collars were attached at the streambed level and all tests were conducted at the threshold of bed material motion where maximum depth of scour hole is expected. Piers aligned with the flow and skewed at 10o and 20o to the flow were tested. Long time tests were carried out to study the development of the scour hole around the pier and efficiency of the collar. Based on experiment results with 9B collar, flow could not penetrate below the upstream edge of the collar after 185 hours. Therefore, scouring at the upstream region of the pier will not occur. In this test maximum depth of scouring occurred at the sides of the collar with 60% reduction of local scouring compared with an unprotected pier. With skewed pier performance of the collar on reducing the rate of scouring was decreased. Results of the experiments showed that at skewed piers, after 50 hours depth of scouring reduced about 30% compared with an unprotected pier. In addition the rate of scouring decreased with 9B collar.