This paper is devoted to 3D modelling at the city level from data sources considered as open. The open data presented in this paper enable free usage, modifications, and sharing by anyone for any purpose. The main motivation was to verify feasibility of a 3D visualization of floods purely based on open technologies and data. The presented state-of-the-art analysis comprises the evaluation of available 3D open data sources, including formats, Web-based technologies, and software used for visualizations of 3D models. A pilot Web application visualizing floods was developed to verify the applicability of discovered data sources. 3D visualizations of terrain models, 3D buildings, flood areas, flood walls and other related information are available in a pilot application for a selected part of the city of Prague. The management of different types of input data, the design of interactive functionality including navigation aids, and actual limitations and opportunities for future development are discussed in detail at the end.