Local Scour and Sediment Deposition at Bridge Piers during Floods

Abstract

The local scour and sediment deposition at a bridge pier during flood waves is analyzed to investigate the effects of different flow and sediment regimes (regulated and unregulated discharges with or without excess sediment supply). Concurrent field measurements of scour and streamflow were performed during 6 days at the Rapel Bridge, over the Rapel River (mean annual discharge, QMA=172 m3/s), located in Central Chile (71°44’9” W, 33°56’22” S). During the measurements, river discharge was regulated by the operation of a hydropower plant, located 24 km upstream of the bridge, which follows a daily hydropeaking scheme. A simple mathematical model of scour and deposition is proposed, and field measurements are used to estimate optimal model parameters and to evaluate model performance. The model was applied to pre- and postdam scenarios to compare expected scour caused by a natural flow regime and by hydropeaking considering different excess sediment supply. Results show that the ultrasonic scour sensor is reliable for real scour monitoring under the presented field conditions. A single and easy-to-perform measurement of scour evolution during one flood was enough for estimation of optimal model parameters. The calibrated model reproduced measured scour and deposition in a verification case with high precision, i.e., root mean square error, RMSE=0.023 m and Nash-Sutcliffe efficiency, NSE=0.96. The model application showed that scour and deposition are very sensitive to the excess sediment supply. After two years, scour in the predam scenario resulted higher than in the postdam scenario when equilibrium conditions or no sediment deposition occurred. In case of equilibrium conditions and excess sediment supply deposition occurred during the falling limb of floods producing important refilling of the scour hole. However, floods’ high peak-discharges and excess sediment supply produced high scour depths of comparable magnitude as those after the two years hydrograph, which occurred only briefly around the peak discharges before sediment deposition, illustrating the complex interactions between flow and sediment in time, with important consequences for monitoring of bridge pier scour in the field and for forensic analyses.