Reducing local scour at bridge piers using an upstream subsidiary triangular pillar

The objective of this experimental study was to examine the effect of an upstream subsidiary triangular pillar on the development of local scour at a single circular bridge pier. The triangular pillar was placed such that its apex faced the approach flow and its base length was equal to the pier diameter (D) in all experiments. The experiments were conducted to investigate the effect of the spacing between the base of the subsidiary pillar and the bridge pier (S) and the apex angle (α) of the triangular pillar on the dimensions of the scour hole and its temporal development at different Froude numbers. The experiments were conducted in a rectangular flume under clear-water scour conditions at Froude number ranging between 0.1 and 0.3. Relative spacing (S/D) of 0, 0.5, 1, 1.5, 2, and 3, as well as apex angle (α) of 60°, 90° and 120° were tested. The results show that the maximum scour depth at the pier could be minimized using three different combinations of spacing and apex angle. The highest reduction in the maximum scour depth (d _sm) of approximately 28 % was possible using S/D of 3 and α of 90° compared to the pier-alone case. Overall the performance of the 90° apex angle pillar surpassed the other two angles.