This talk described the implementation of a numerical tool named Itzï for the modelling of floods in megacities by coupling an open source GIS surface flow model and an urban drainage model. To the best of our knowledge, this is the first time the coupling of a damped partial inertia numerical model with a drainage model is implemented and tested using a real event. The numerical tool employs dynamic datasets in space and time and the case study corresponds to the city of Kolkata, India. The ability of this open-source software to represent drainage operation and surface flooding in a megacity is demonstrated, paving the road towards its use in other large urban areas of the world.