The hydraulics of supercritical flow across manholes in sewers is explored using systematic experimentation. Due to the expansion at the manhole entrance an in-manhole wave is generated. Further, at the downstream manhole end, flows with a sufficiently large filling ratio impinge on the wall and lead to a so-called swell. In addition, due to shock wave generation in the downstream sewer, a sewer wave is generated. The heights and locations of these three waves were determined in terms of basic hydraulic quantities. More importantly, the capacity of the manhole and the downstream sewer under wave action was quantified. It was found that in order for free surface flow to be maintained the common design standard for sewers with a supercritical approach flow have to be revised. These implications have to be accounted for in future designs.