Effect of U-Type Floating Debris Accumulation at Sacrificial Piles on a Local Scour at Bridge Piers

Debris composed of soft and flexible materials (such as grass) accumulates in a horseshoe shape (or U-shape) at sacrificial piles, which can affect the efficacy of these piles in shielding bridge piers against local scour. In the present study, the effect of such accumulation of debris at sacrificial piles on clear-water local scour at a bridge pier located behind the piles was investigated experimentally. Triangular arrangements of sacrificial piles ahead of a circular pier were tested first without and then with different sizes of U-type debris, and the resulting local scour depth was studied. Results showed an up to 91% increase in scour depth at the pier with the debris sizes adopted in this study. The reduction in the protective action of sacrificial piles was great when all sacrificial piles in the group were covered by the debris and the height of the debris at the front was large. However, even in the event of debris jams, sacrificial piles were still able to reduce the extent of local scour at the pier by at least 30% when compared to an unprotected pier. In a debris-free environment, as much as 70% reduction in scour depth was achieved by installing sacrificial piles ahead of the pier. This research recommends the use of sacrificial piles for local scour protection of bridge piers in both debris-free as well as debris-laden flow environments.