Inverted Siphons

An inverted siphon or depressed sewer is a sewer that runs full under gravity flow at a pressure above atmosphere in the sewer. Special feature of inverted siphon is that its profile is depressed below the hydraulic grade line. This is very useful when a sewer line has to be laid across a stream, a highway cut, or any other similar obstruction. When the profile laid below the ground, that portion of the sewer is known as inverted siphon (or, a depressed sewer) (Figure 12).

               Figure 12 : An Inverted Siphon Carrying Gravity Flow under an Obstacle Along the Route

It is obvious that this section of sewer lies below the hydraulic grade line, flowing full and under pressure. In order to maintain appropriately higher velocities to disallow solids settling down in the sewer pipe, generally two or three different sizes of parallel pipes are provided to carry the minimum, average and peak flows. Since the siphon is subject to pressure, while flowing, ductile iron pipes or concrete encasement is provided in order to prevent leakage. The siphon may be constructed as a U with vertical or inclined legs.

True siphons are also used in sewerage practices depending upon the ground profile (topography). A true siphon is a sewer that flows full with the flow line above the hydraulic grade line, the pressure in the sewer being less than atmospheric.