Extreme weather events with heavy rainfall are frequent features of climate change. Housing development and paving of green spaces are increasing the run off volume entering the sewer networks, often beyond their capacities. The polluted excess water either causes fl ooding of urban areas or it is released directly into local waters, known as combined sewer overfl ow (CSO). Both scenarios cause severe threats to public health and to aquatic life in our rivers and lakes. We consider these kinds of CSOs as unacceptable in a modern society. To prevent this, it is necessary to build either storm water retention basins or deep storage tunnels.