This line feature class has five subsets to classify data; collector, inline storage, inverted siphon, overflow and tunnel. A collector is a conventional gravity sewer that is designed to the slope and size of the pipe adequate to maintain flow towards the discharge point without surcharging manholes or pressurizing the pipe. An inline storage refers to a number of practices designed to use the storage within the pipe system to detain flows by reducing peak flows. An Inverted Siphon allows sewage to pass under obstructions such as rivers. Unlike the main sewer pipe, the siphon pipes flow under pressure and must have flow velocities greater than 3 ft/s (0.9 m/s) to keep material suspended and several siphons having smaller diameters than the main sewer may be required. Overflows are extra pipeline that allows relief when there are more sewage than the pipe can handle in normal conditions. A tunnel is an underground passageway that has a pipeline within it.