This paper presents new achievements in the field of the design of the gravitation and gravitation-underpressure chamber for bottom flushing in the detention chamber immediately after its emptying from the accumulated sewage. Temporary accumulation of the sewage in the detention chamber of the reservoir causes partial sedimentation of the solids suspended in the liquid. The location of the flushing chamber at the maximal level of filling with sewage in the retention chamber substantially improves the effectiveness of flushing of the bottom of the retention chamber with the stream of sewage that is discharged from the flushing chamber. Opportunities for effective flushing of the sludge that remains on the bottom of a detention chamber represents an important operation which is essential for reliable functioning of retention reservoirs within sewerage systems.