Live-Bed Scour at Bridge Piers in a Lightweight Polystyrene Bed

Experiments on scour at bridge piers were carried out in a sediment recirculating flume with a bed of polystyrene particles. The focus of the research reported in this paper was given to live-bed experiments. All experiments were run until equilibrium conditions with flow velocities u ranging from 0.8–8.5 times the critical velocity for the incipient motion of sediment particles uc. Maximum scour depth was continuously measured with an endoscopic camera from inside the pier, which was constructed by a plexiglass cylinder. Bed load transport with dunes was identified as the main transport mode for 1<u/uc<4. For ratios of u/uc≥4 entrainment into suspension without development of bedforms dominated. Results show that scour rapidly reach equilibrium in the bed of polystyrene pellets. In the clear-water experiments equilibrium scour depth remained constant in time. In the bed load region the fluctuation of scour depth corresponded to the dune migration through the scour hole, while in the suspended load region the fluctuation of scour depth was negligible and was superposed by general erosion. For both clear-water and live-bed conditions maximum equilibrium scour depth continuously increased with flow intensity u/uc. Maximum scour depth was in accordance with a logarithmic curve and reached values up to 3.3 times the pier diameter for u/uc=8.5. Extrapolation of the obtained results for an estimation of equilibrium scour depth in a lifelike worst-case scenario is discussed.