Local scour depth at a bridge pier protected by a collar in steady and unsteady flow

Scour hole development around a circular bridge pier protected by a collar under steady and unsteady flow conditions is thoroughly investigated. At the first stage, formation of a scour hole around the collar under different steady flow conditions was studied. Results showed that flow intensity, which is defined as upstream undisturbed bed shear velocity divided by bed material critical shear velocity (u_*/u_*c), is the major factor affecting collar efficiency in postponing scour depth at the pier nose. Decreasing u_*/u_*c from 0·99 to 0·8 increased the time taken to undermine the collar by about 26 times. At the second stage, formation of a scour hole around the collar under different hydrographs was studied. The range of u_*/u_*c in a hydrograph and its characteristics were found to be the most important factors for collar efficiency. Finally, empirical equations were developed to calculate the time taken to undermine the collar and for scour depth development.