Many financial and living expenses are caused annually due to the destruction of bridges in the flood events. Studies show that changes in bridges' geometry can lead to changes in the time to reach maximum scouring. The purpose of this study was to investigate the time variation of scour depth at the bridge pier. For this purpose, a laboratory flume with a length of 14 meters, a width of 1.5 meters and a height of 0.7 meters was used. Four different forms of the slot were created on the rectangular pier including two rectangular (vertical and horizontal), square and diamond slots. Experiments were carried out at three levels: above the bed, on the bed and under the bed at four flow rates of 21.2, 25.6, 29 and 32 l/s in sediments with a mean diameter of 0.5 mm. The results of this study showed that the scour rate is higher in the first minutes of the start of the test. Over time, the intensity of changes is reduced. increasing the depth and volume of the scour hole, decreases the intensity of the changes. Moreover, there is a delay between approaching the dimensionless scour number to a certain degree and next changes in this parameter, which indicates that in a flood event, performing emergency measures could be effective. In addition, at a given time, placing slot at the top of bed increased the scouring dimensionless number by twice rather than placing under the bed.