In the present study, the flow field around a circular pier is investigated with experimental measurements and numerical simulations.The transient flow patterns during erosion are studied in detail.The results show that the traditional equations of particle motion are not perfect for the calculation of the sand motion under this complex flow situation.The scouring agents,such as turbulent intensity,the fluctuating pressure and the vertical pressure gradient,having many effects on the sand motion with the increasing scouring depth,need to be considered in modifying the traditional model.