Local scouring in the vicinity of bridge piers which are implemented in the natural streams is the subsequent of 3D complex flow patterns. During the local scour process the accelerated flow develops the local scour depth and if it is not predicted correctly the bridge failure occurs. Local scouring around bridge piers is a strictly time dependent phenomenon and subsequently the evolution of local scour depth under the unsteady flow condition is essential for both structure safety and ecological issues. In the present study, development of local scour depth around a circular pier under time varying flows of the three different types of step-wise hydrographs has been simulated by using a fully 3D numerical program SSIIM along with applying a superposition scheme. Results show good agreement between predicted numerical results and experimental data whereas the error rates of the maximum simulated local scour depths were less than 10%.(Esmaeili et al. 2009). When this undesirable event occurs, a scour hole develops around bridge piers as, in general, the local scour depth increases with increasing flow (Dehghani et al. 2012). If the severe scour finally exposes the pier foundation in the scour hole, the safety and stability of the bridge will be in danger. Therefore, there is a great need to develop good prediction methods for pier scour evolution especially under the time varying flows of the flood hydrograph.