What is an inverted siphon?
An inverted siphon, also called a sag pipe (in Dutch zinker), is a pipe that passes beneath an obstacle, such as a ditch, canal, river or sunken road, and comes back up on the other side. The pipe forms a U shape: it dips down, runs under the obstacle and rises again.
How it works
An inverted siphon is always full of water. Because the water level on the inlet side is higher than on the outlet side, the difference in head pushes the water through the pipe. This is the principle of communicating vessels: no pump is needed as long as the inlet is higher than the outlet.
Difference from a culvert
- A culvert is a pipe or box that lets water pass under a road or embankment, usually in a fairly straight line and at roughly the level of the watercourse.
- An inverted siphon clearly drops down and rises again. It is used where a straight passage will not fit, for example where a gravity sewer has to cross a watercourse that lies deeper than the sewer itself.
Applications
- Sewers: a gravity sewer crossing a canal or river.
- Water management: leading water from one watercourse to another beneath a crossing watercourse.
- Utilities: water mains and gas pipes are also taken under waterways, often by directional drilling.
Practical points
- Silting: at the low point the flow slows down, so sand and silt can settle. Inverted siphons therefore need regular flushing or cleaning.
- Flow velocity: the design must ensure enough velocity in the pipe to carry solids through.
- Design and consent: crossing a watercourse needs permission from the water authority. The design is work for a specialist.
Related terms
- Culvert
- Sewer
- Drain trap
- Stoneware pipe
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