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In natural water ways, piers or any obstruction in channels will obstruct the flow and cause an increase in water levels upstream of the channel. The amount of backwater caused by piers depends mainly on their geometric shape, their position in the stream, the flow rate and the amount of channel blockage. So SMS, Surface water modeling system and RMA2, Resource Management Associate two dimensional finite element models were used to study open channel flows passing through bridge piers. And also the changing of the upstream backwater value and velocity was analyzed. To investigate the influence of piers, two series of hydrodynamic numerical models, one row of piers in three different shapes and two rows of quadrate piers in different spacing along the flow direction were used during the simulations.The result shows that the variation of both water level and velocity are related to the location. In general if only one row of pier in the channel, the water level rises and the velocity decreases in the upstream. While in the downstream, the water level falls and the velocity increases. It can also see from the study that quadrate piers have the largest amount of variation in upstream water surface elevation and velocity increasing area. The amount of variation in circular pier has the smallest among the three kinds of piers in the same scale. If two rows of the piers in the channel, between the two piers, the water surface level usually drops firstly and then increases again. The amount of drop water level behind the first pier and the amount of increase water level in front of the second pier are dependent on the distance between the two piers. When the distance is over 50 times breadth of the pier, there is no more effect each other.