Investigation of turbulence flow and sediment entrainment around a bridge pier

Local scour around the obstacles such as bridge pier is the result of complex interaction between turbulent flow and sediment particles at the mobile bed. The entrainment of sediment particles from the bed is stochastic in nature and it is strongly influenced by instantaneous shear stresses of the bursting process. The focus of this study is the investigation of turbulent flow and analysis of contribution of each bursting events to the Reynolds shear stress to find the dominant bursting events and also the flow structure around the circular bridge pier. The velocities around the bridge pier were measured in three dimensions using an Acoustic Doppler Velocimeter. These velocities were measured at different positions around the bridge pier and at different depths. Quadrant analysis is used to recognize the susceptible regions for sediment entrainment and deposition. According to quadrant analysis sedimentation is the dominant effect in the scour hole whereas at higher levels the erosion force becomes more important. In downstream of the pier, sediment particles are put in suspension and transported downstream due to sweep. Our results indicate that the secondary currents are more dominant in downstream of the pier, as compared to the upstream of the pier. Consequently the maximum velocity in downstream of the pier takes place in a location closer to the bed. In upstream of the pier the stream-wise component of velocity becomes positive for which the universal log-law turns out to be valid.