Scour protection around vertical-wall bridge abutments with collars

The cause of local scour around a bridge abutment is mainly the presence of spiraling vortex flow structure called the principal vortex. A rigid surface around an abutment with a collar can arrest the growth of the vortex, and therefore, reduces the scour depths around the abutment compared to the case without a collar. This paper presents an experimental investigation related to the collars of different sizes placed around vertical-wall bridge abutments at various elevations in an erodible bed flume that consists of two different types of almost uniform sand of (d50)1 = 1.48 mm and (d50)2 = 0.90 mm to show the effect of collars on the scour depths around the abutments. A series of experiments were conducted at the laboratory under clear-water conditions at flow intensity of U/Uc = 0.90, where U is the average velocity of the approach flow and Uc is the critical flow velocity for the sediment motion. The maximum scour depths forming around the abutments were determined at the end of each experiment conducted with and without collar for a time period of 6 hours. Experimental results showed that, a collar placed on the abutment at or below the bed level is able to reduce the maximum scour depth around the abutment by 70–100% as a function of the collar and abutment sizes in comparison to that of an unprotected abutment. The optimum location of a collar on a vertical-wall abutment, which will result in the maximum percentage reduction in the maximum scour depth, is generally below the bed level and can be expressed as a function of the dimensionless abutment length and collar width. It was also observed that although the abutment itself was protected, the collars of larger sizes shift the scour region away towards downstream from the abutment, with scour depths smaller than the maximum scour depth at the abutment for the case of no collar. The experimental observations and analysis presented in this study established that the optimum dimensionless location of the collar on the abutment which yields the maximum scour reduction was not affected by the sediment having finer diameter. Therefore, abutment-collar arrangements could be an appropriate alternative solution to riprap protection that is widely used in practice in countering scour problems at bridge abutments.