Due to the three-dimensional flow pattern interacting with bed material, the process of local scour around bridge piers is complex. Many former studies, including experimental and numerical, have been carried out to develop relationship for estimation of maximum scour depth under steady flow condition. Due to the this fact that in fluvial rivers, significant transport of bed materials often takes place during peak-flow discharge in a flood event, but a few studies available for assessing the scour depth under unsteady flow condition. The previous results show that the maximum scour depth occurring under a flood hydrograph can be much smaller than the calculated value using peak flow discharge. In this study, the evolution of scour depth during the period of hydrograph with special attention to the rising limb has been simulated by SSIIM program. The main ability of SSIIM compared to other CFD programs is simulation of sediment transport with moveable bed in complex geometry. As for input boundary condition, the stepwise discharge hydrograph was used. The comparison of results with experimental data shows the capability of numerical simulation for assessing the maximum scour depth under unsteady flow condition especially in rising limb of hydrograph. The results also show that during the rising period of hydrograph, the scour depth increases steadily and more than 80% of final scour depth is occur at the rising limb of hydrograph.