Supercritical sewer flows at a combining junction: A model study of the Edworthy trunk junction, Calgary, Alberta

An experimental study on supercritical sewer flows at a junction chamber was conducted using a physical model for the Edworthy trunk sewer in Calgary, Alberta. This model study showed that a junction chamber with a small angle of two combining inflows helps to retain kinetic energy of the flow and reduces the water level in the surcharged chamber, thus avoiding the occurrence of fountains. The behavior of the supercritical junction flow in the chamber was investigated. Transition of flow regimes in the chamber was studied and three flow regimes were identified: regime I features shock waves in open-channel flow in the chamber; regime II is characterized by surcharge flow in the chamber and orifice flow in the outlet pipe; and regime III is associated with pipe-full flow in the outlet pipe following the surcharged chamber. It was found that regime II could be the most critical condition for the consideration of water level in a surcharged chamber. A theoretical analysis of regime II flow was carried out, and the degree of submergence in the chamber can be satisfactorily predicted for given inflow conditions. The effects of a rounded entrance of the outlet pipe and the benching in the junction chamber were also evaluated experimentally. Key words: sewer hydraulics, supercritical flow, energy loss, junction manhole.