Experimental investigation of time-dependent clear water scour around bridge pier due to a trapezoidal hydrograph

Local scours around bridge piers influence their stabilities and play a key role in bridge failures. In this study, the local scours around a bridge pier resulting from unsteady flow are investigated. The experiments are carried out in a rectangular flume 80 cm wide and 18.6 m long by using uniform sediment with median diameter of 3.5 mm and geometric standard deviation of 1.4. The unsteady flow is generated by means of a trapezoidal hydrograph. The bridge pier with circular cross section has a diameter of 8 cm. The flow depth is monitored by ultrasonic sensors and the flow rate is measured by electromagnetic flow meter. The temporal variations of scour depth are measured indirectly, by placing Ultrasonic Velocity Profiler (UVP) transducers downward vertically. Three transducers of 4 MHz are located around the bridge pier. The local scours due to input hydrograph are obtained by registering the distances from the tip of the transducer to the top level of the sediment layer in the course of unsteady flow experiments. The temporal evolution of local scours as well as the final depths and configurations of the local scours are obtained. The experimental findings are interpreted in the light of present classical knowledge. This study constitutes the first part of the researches planned to investigate the time dependent local scours due to various trapezoidal hydrographs. Keywords: trapezoidal hydrograph, bridge pier, local scour, UVP, temporal evolution © ASCE 2012