The city of Osijek has got a combined sewerage system. It is the North sewer main, constructed at the beginning of the twentieth century that constitutes the major part of the existing sewerage system. The original concept of a sewer system has been subjected to adjustments and extended on multiple occasions The South sewer main, however, has been constructed in the time frame of the last fifty years, with the final objective of transporting wastewater to the central wastewater treatment plant, whose realisation is still pending. Great contribution to the future development is recognized in establishing the high quality mathematical model for urban drainage. Unfortunately, its application and the upgrading are not making adequate progress. Nevertheless, the ongoing activities involve spatial observations of rainfall events. The objective of this paper is to present option for system management, based on examples of a combined sewer overflow. EPA SWMM5.0 model is applied as a tool and presentation includes a drainage simulation, together with management of overflows and resulting options. The paper emphasizes the necessity for further runoff measurement inside the sewerage system.