Effect of submergence of sacrificial piles on local scour reduction at a bridge pier under U-type debris jam conditions

A group of non-submerged sacrificial piles placed in front of a bridge pier have a propensity to trap a significant volume of floating debris during river floods, which affects their effectiveness as a scour countermeasure. This problem can be avoided by using short piles such that the tips of the piles lie below the free water surface, also known as submerged piles. However, it will cause the floating debris to accumulate at the pier itself. The current study attempts to ascertain whether or not such trapping of debris with submerged piles is preferable to that by emergent piles from the perspective of local scour protection of the bridge pier. Laboratory flume tests were conducted under uniform, subcritical, steady, and clear-water flow conditions. The ideal submergence value was ascertained by examining submerged sacrificial piles with varying spans (or lengths) with respect to the flow depth. The local scour reduction efficacy of these submerged piles was then compared with non-submerged piles, first without and subsequently with U-type (horseshoe-shaped) debris jams. Results revealed that submerged sacrificial piles spanning 50-60 % of the flow depth provide optimum protection to the pier against local scour, and this was slightly higher than that offered by emergent piles in an identical layout. In addition, submerged sacrificial piles with ideal submergence protected the pier better against local scour than emergent piles under Utype debris jam conditions.