Supercritical Flow in Sewer Manholes

The breakdown of supercritical flow in sewer manholes may be dangerous when designed according to current practice. This issue is particularly relevant for drainage systems conveying discharge in both stormwater and combined sewers. Based on detailed hydraulic experimentation, the main features of three manhole types were observed, including the through-flow, the bend and the junction manholes. Using systematic observations and a hydraulic approach, the large data sets allowed presentation of a modified and generalized design for such hydraulic structures, thereby accounting for Froude similitude. Both the hydraulic flow patterns along with design guidelines are presented to allow for a safe and straightforward design of manholes in these sewer networks. The design relates particularly to freeboard requirements and to the discharge capacity of sewer manholes. It is demonstrated that the dimensionless discharge capacity of the junction and the through-flow manholes is roughly twice and three times larger than for bend manholes, respectively.