The invention relates to a method for rinsing a waste water channel (1) in a section (2) that is installed underneath an obstacle (3), particularly for rinsing a section that is designed as a culvert (10). According to said method, the waste water flowing into the waste water channel is contained by way of a containment device (14) that is arranged in a lower culvert head (6) or in a lower channel section (9) such that when the containment device is in a closed position, the waste water is contained up to at least one upper culvert head (4), and a channel section (9) located downstream of the containment device is dry. The contained waste water is released by moving the containment device, and the section to be rinsed is rinsed by means of pressure drop rinsing. On a unit for rinsing the channel operating according to the method, means (27, 28, 29) for detecting the fill level in the waste water channel and a pivotable and/or linearly displaceable rinsing shield (15) are provided.