Rapid urbanization is increasing the demand for public transport in Switzerland’s major cities. The new m3 metro line in Lausanne is planned to circulate by 2030. Its construction will require a modification of the underground flow section of the Flon River vaulting at the Flon metro station. The Flon River drains a natural watershed, as well as the combined sewage network of the city of Lausanne. Two prior modifications of the vaulting geometry at the Flon station were carried out to accommodate the infrastructure of the m2 and LEB metro lines. The research aim was to assess the proposed vaulting geometry underneath the m3 by means of a hybrid hydraulic modelling approach combining a numerical and physical model in order to evaluate the new capacity limit of the system. A design discharge of 90 m3/s was defined, corresponding to a 100-year return period. A 3D numerical model at prototype scale was developed in the commercial software packages ANSYS Fluent and Flow-3D to simulate a multiphase flow. The physical model was built at a reduced scale of 1:20 based on Froude similarity. Taking into account the precission of the measurements on the physical model and the accuracy of the numerical results, they all lye withing the assumed limitations. Reagrding the engineering project, the results indicate that the design discharge cannot be maintained under free-surface flow conditions with the proposed vaulting geometry. The main limitation for the validation of the results is the lack of in-situ stage-discharge data.