Effects of collars in reducing local scour depth around bridge abutments under unsteady flow conditions

In this experimental study, semi-circular end collars around semi-circular end abutments and rectangular collars around rectangular abutments were tested in order to investigate the efficiencies of the collars in reducing the local scour depth under unsteady-state clear-water approach flow conditions. Experiments were conducted in a rectangular sediment channel having a sediment pool filled with uniform sand as the bed material. Three different abutment lengths having constant widths were tested under three distinct successive flow intensities that were applied continuously for a duration of 2 h during each experiment. Varying sizes of collars were located at different elevations relative to the bed level. The effect of abutment length, collar width, collar elevation, flow intensity and temporal variation on local scour reduction performances of collars were tested. According to the experimental results, it can be stated that the application of collars around the semi-circular end and rectangular bridge abutments decreases the local scour depth by up to 72% and 51%, respectively. In addition, semi-circular end collars around semi-circular end abutments gave better results in reducing the scour depths than rectangular collars around rectangular abutments. Best collar performances were generally achieved for the largest collar width located around the bed level for semi-circular end abutments and below the bed level for rectangular abutments.