Influence of Semicircular Collar Diameter and Its Alignment on Scour Depth and Flow Pattern around Bridge Abutment

Reducing scouring is inevitable to prevent the destruction of hydraulic structures on the water flows. In this research, the effect of semicircular collar on the reduction of scour depth around the abutment was investigated with the aim of identifying the flow pattern changes around this structure. Experiments were carried out under clear water conditions. Semicircular collars were examined on semicircular abutments in two sizes 1.5L and 2L (L is abutment length against the flow) and at three different levels relative to the bed; bed alignment, 0.2L below and 0.2 above the bed. The results showed that the existence of the collar, in addition to reducing the final scour depth, caused a delay in the scouring process. This effect has also been increased by increasing the size of the collar. In addition, the position of collars with the same size can improve collar performance and efficiency on the design cost. Based on the results of the experiments, the collar with 2L size and under the bed showed better performance and reduced the final scour depth by 58% compared to the control abutment. Also, according to the experiments, the position of the collar under the bed showed better performance. By investigating the flow pattern around the abutment in conditions with and without collar, it was found that the collar reduces the flow velocity in different directions, especially in the upstream of the abutment. Also, its effect on downflow reduces the strength of the vortices and changes the reciprocating behavior and displacement of the vortices. So that the presence of collar has reduced the maximum downflow velocity at the upstream abutment by 39%.