Urban floods are frequently provoked by the failure of drainage systems. Sometimes, the malfunctioning of drainage systems arises from uncertain or complicated hydraulic conditions developing into sewer manholes. Junction manholes represent basic elements for sewers as they enable two or more upstream branches to merge into a single downstream branch. Initial researches mainly investigated the hydraulic behaviour of subcritical junctions with constant branch diameters and an even invert. However, the junction of supercritical flows is the most complex process, mainly due to the likely occurrence of shock waves, breakdowns or choking flows. Furthermore, the sewers laying is commonly characterized by the branch tops alignment. Bottom offsets thus control the manhole inlets when upstream branch heights differ from the downstream one. In this regard, a physical model investigation was carried out at the Laboratory of Hydraulic Constructions (LCH) of the Ecole Polytechnique Fédérale de Lausanne (EPFL). The model reproduced a 45◦ junction manhole with aligned up and downstream branch tops and variable upstream branch diameters. The experimental tests focused on the scenario with supercritical flows entering the junction manhole. First outcomes in terms of flow behaviour and shock waves formation are described.