Inverted siphons carry flow under obstructions such as sanitary sewers, water mains, or other structure or utilities that may be in the path of the storm drain line. Siphons should be used only where avoidance or adjustment of the obstruction is not practical. Figure 10-19 shows a profile of an inverted siphon. A minimum flow velocity of 3 fps (1 m/s) is recommended to keep sediment suspended.The conduit size through the inverted siphon used in a storm drain system should be the same size as either the approaching or exiting conduit. In no case should the size be smaller than the smallest of the approaching or exiting conduit.Because inverted siphons include slopes of zero and adverse values, the head losses through the structure must be accounted for. The sources of the losses can be friction, bends, junctions, and transitions. See Chapter 6, . Alternative means of avoiding the obstruction may be necessary if losses are unacceptable. Maintenance access should be provided at either or both ends of the inverted siphon as indicated in Figure 10-19.