Vortex drop shafts (VDSs) are special drop structures typically adopted for elevation differences larger than about 5 m (Gisonni and Hager 2012). In few cases (Volkart 1984; Bruschin and Mouchet 1985) they were utilized also to combine various sewer collectors. This uncommon concept was adopted to realize the new VDS operating in Cossonay, Switzerland. A physical model was opportunely built and tested to analyze the main hydraulic features of the entire structure. The present study provides the preliminary results focusing on the flow falling along the vertical shaft and entering the outlet structure, composed of a dissipation chamber and a tailwater tunnel.