Time-dependent Local Scour Around Bridge Piers Under Ice Cover-an Experimental Study

In cold regions,the presence of ice cover in a river changes the flow velocity distribution.An ice cover approximately doubles the wetted perimeter of the river,which increases the flow resistance.The maximum flow velocity under an ice-covered flow condition is shifted closer to the channel bed,which can increase near-bed flow velocity gradient and shear stress,intensifying the local scour process around bridge piers comparing to that under an open flow condition.Excessive scour around bridge piers may cause a collapse of bridges.The prediction of the local scour depth of the bridge pier is a difficult question in the study of bridge.The existing calculation equations are hard to accurately predict the actual scour depth around bridge piers,because of the short flood peaks.Most studies attached importance to the equilibrium scour depth,ignoring the process of the time-dependent local scour depth around bridge piers.In the present study,a series of clear-water experiments were carried out in flume to investigate the time-dependent local scour depth and equilibrium scour depth around a cylindrical pier under both ice-covered and open flow conditions,carried out on the analysis of as water flow depth,approach flow velocity,bridge pier diameter,and sediment median particle size.Results showed that the local scour rate under ice-covered condition is faster than that under open flow condition.The equilibrium scour depth under ice-covered flow condition is 12%more than that under open flow condition.Also,the equilibrium scour time under ice-covered flow condition is 10%more than that under open flow condition.As the pier diameter D becomes larger,the local scour rate becomes larger at the same time as well as the equilibrium scour time.As the flow depth h increases,the equilibrium scour time will increase.As the sediment particle size d₅₀increases,the local scour depth around the bridge pier decreases,as well as the equilibrium scour time.The relationship between the flow intensity and the dimensionless scour depth has been assessed,and the difference between scouring rates of two different cover conditions was studied.An empirical equation for calculating time-dependent local scour depth under ice-covered flow condition was developed.The results of this study can be used as reference for engineering practice.