Supercritical flows in sewer systems occur as a result of topographical conditions seen in mountainous cities. This type of flow can be enhanced by the plastic pipes that are now used in sewer systems, due to their low wall roughness. It is therefore important to understand the hydraulic behaviour of the sewer system's junction manholes, which is usually characterized by shockwaves. The present study focuses on symmetrical manholes that, despite having supercritical flow, have the same diameters as those found in conventional subcritical manholes. Two physical models were employed in order to carry out systematic experimentation and to explore entry flow combinations. The approach flow filling ratios varied from 20% to 85%, and several flow conditions were tested in order to identify prominent flow characteristics. The independent variables used to analyse wave patterns include Froude number, velocities, and entry flow filling ratios; the dependent variables include shockwaves location, length and height. The study also offers some recommendations regarding symmetric junction manhole design.