In combined sewer systems suitable locations are selected where only a fraction of the maximum discharge is allowed to continue towards the sewage treatment station. To this purpose, overflow structures are used. In this paper a storm overflow structure constituted by a sideweir and a bottom opening is proposed. It is particularly suitable for subcritical approach flows. A hydraulic design procedure is described and, based on systematic observations and by using governing flow equations, the main hydraulic features are specified. The results reveal that the proposed overflow structure has advantageous characteristics with reference to efficiency, reliability, and maintenance.