Inverted siphon

An inverted siphon (depressed sewer) is a sewer section which dips below the hydraulic gradient line to pass under an obstruction such as a stream, a railway, a road or a depression, and then rises to the original level. It runs full under pressure; it is not a true siphon.

Circumstances for providing it

• To cross a river, canal, stream or drainage channel below the bed.
• To pass under railway lines, highways, tunnels or underground utilities.
• To cross a deep valley or depression where the ground drops below the sewer's gradient and a bridge would be costly.

Purposes served

1. Lets the sewer cross an obstruction without pumping, using only the head available.
2. Avoids the cost of an aqueduct or bridge.
3. Keeps the sewage flowing by gravity with only a small loss of head.

Construction

 Inlet                          Outlet
 chamber                        chamber
  __                              __
 |  |-\                        /-|  |-->
 |  |  \  Ground level         /  |  |
 ----    \______________/ ------
         depressed sewer (below stream)

• The sewer is made of cast iron or steel (or RCC encased in concrete) pipes, laid below the obstruction.
• An inlet chamber and an outlet chamber are built at both ends. The inlet chamber has an overflow, and sluice gates or stop planks so that one of the pipes can be shut.
• Usually two or more pipes (dry weather pipe and wet weather pipe) are provided, so that the velocity in the dry weather flow is not less than 0.9-1.0 m/s, which prevents silting.
• The pipes are also provided with cleaning access (manholes and washouts).
• The head loss is equal to $\dfrac{v^2}{2g}$ plus friction, and the levels are fixed so that the loss is available.