One of the most common strategies for sewer cleaning is to generate flushing flows using flushing gates to store water in its upstream sewer pipe. Therefore it is important to obtain the flow information of the flushing waves and their eroding effects. In this study, the flow characteristics of the flushing wave and the flushing effect were investigated by a transient flow calculation using a commercial computational fluid dynamics code. The values of bottom shear stress were obtained and the effect of several factors were discussed. The water depth and the slope were related to the release rate of the storage volume, while the flushing volume determined the flushing distance at long sewer distances. The initial downstream water level was found to dramatically reduce the flushing effect. Equations based on the storage depth were developed to estimate the flushing effect, and suggestions for the installation and operation of flushing gates were provided.