Reduction of local scour around a circular bridge pier using the collars and sacrificial piles in non-uniform sediment

In the present study, circular and octagonal collars together with sacrificial piles are tested for their efficiency as protection devices against local scour in the non-uniform sand bed of geometric standard deviation 2.29 and median grain size 1.18 mm. To examine the effect of width and elevation on scour depth, experiments are conducted with circular and octagonal collars of widths $1.5b$ and $2b$ placed on the bed and $0.25y$ above the bed where $b$ is the pier diameter and $y$ is the flow depth. The octagonal and circular collars of width 2b placed in flush with the bed achieved maximum scour reductions of 78.33 % and 75 %, respectively. The efficiency of the collars decreased when their width is reduced and when they are placed above the bed. Among the sacrificial piles in transverse and staggered arrangements, a maximum scour reduction of 55 % is observed when the piles are placed transverse to the flow at a distance of 2b from the pier center. The efficiency of the sacrificial piles decreased when they are moved away from the pier center. A maximum scour reduction of 86.67 % is seen for the combination of the octagonal collar of width $2b$ on the bed and piles in the transverse arrangement. A new empirical equation is proposed for temporal and equilibrium scour for collar around the pier, including the armor effects. The sensitivity analysis revealed that $b_c/b$ is the most sensitive parameter followed by $H/y$, $k_s$ and $T_c$ where where $b_c$ is the collar width, $H$ is the location of the collar from the water surface, $k_s$ is the shape factor and $T_c$ is dimensionless time.