As well known, a periodical cleansing of sewers results necessary in order to avoid or reduce many unpleasant consequences due to the presence of sediments, such as the solid re-suspension during rain events and the reduction of the sewer flow capacity. For this purpose, different systems based on flushing techniques have been adopted in sewers. In the present paper, the results of an analysis on the hydraulics of a flushing device, the Hydrass gate, is presented. In particular, an experimental investigation on a scale model under steady conditions was carried out in order to determine the behaviour of the device during the flushing phase and outflow relations were derived for the different outflow conditions. A numerical model was finally set up for testing the relations under unsteady conditions, using for validation the experimental data of a previous measurement campaign carried out in a sewer reach of the city of Lyon, France.