Evaluation of collar efficiency to prevent scouring around cylindrical bridge piers under live bed condition

The present study was carried out to experimentally investigate the effect of single and double collar on development of local scouring around cylindrical bridge piers under live-bed condition. The single collar with diameter of 3 times the pier diameter placed on the streambed level and double collars one on the streambed level and one on a lower elevation were used. Tests were conducted with different flow intensities equal to 1.4, 2.0, 2.4, 2.8 and 4.0 where flow intensity is defined as the ratio of bed shear velocity to shear velocity of bed material at threshold of motion. One experiment was also carried out with flow intensity of 4.7 at which dunes were washed out and transition flow regime prevailed. The duration of the experiments was long enough to assure complete dune formations and multiple traverses of dunes through the pier location. Results showed that the scour depth fluctuated between a maximum and a minimum value due to the bed features migration. A graph was developed to show the efficiency of single and double collars at different positions and under different flow intensities. With two collars in place, the mean and maximum scour depths at the highest flow intensity, reduced by about 53% and 35% respectively. Efficiency of collar was better in lower flow intensities. Finally, based on the present results, a design table is presented for the elevation of the lower collar based on the flow intensity.