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Inverted Siphon

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Crossing Obstacles
Getting water across gaps and valleys
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Inverted Siphon
Crossing valleys with pressure pipe

An inverted siphon takes a channel across a valley by dipping the water down and up through a closed pipe, and it works because water rises to its own level as long as the outlet is lower than the inlet by at least the head lost to friction. The loss is large: 5 L/s through 120 m of 75 mm pipe costs about 2.2 m of fall, as much as 1.1 km of channel at a 0.2% slope, and the pipe at the bottom of a 15 m valley carries about 1.5 bar.

Part of Aqueducts and Channels

An inverted siphon is a pressure pipe laid down one side of a valley and up the other to carry a gravity channel across without a bridge. Despite the name it is not a true siphon: it works by gravity pushing water through a pipe that is always full, and not by suction.

The Siphon Is Paid For In Head

The aqueducts page describes the principle: water rises to its own level, so as long as the outlet is lower than the inlet it works. The pipe must handle the pressure at the bottom, use heavy-walled PVC or HDPE, and have an air release valve at the high point on each side.

The part not to forget is friction. Water pushed through a pipe loses head, and that head must come out of the fall of the channel. Use the Hazen-Williams equation from Pipe Sizing: h = 10.67 × L × Q^1.852 ÷ (C^1.852 × d^4.87), here with C = 140 for plastic.

For 5 L/s through 120 m of 75 mm pipe: Q = 0.005 m³/s, d = 0.075 m. The result is h = 2.24 m, about 2.2 m, and the speed is 0.005 ÷ 0.00442 = 1.13 m/s. With a 90 mm pipe it is 0.92 m.

A channel at 0.2% falls 0.002 m per metre. To lose 2.24 m takes 2.24 ÷ 0.002 = 1,118 m, about 1.1 km, of channel. So the siphon uses up the fall of more than a kilometre of channel, and the downstream channel must start at least this much lower than the upstream one. Entrance and exit losses add to it, and are not included here.

Pressure At The Bottom

Static pressure at the bottom is density × gravity × depth below the inlet water level: for a 15 m valley, 1,000 × 9.81 × 15 = 147,000 Pa = 1.47 bar, about 1.5 bar. A 40 m valley gives 3.9 bar. Choose a pipe class above the static pressure with margin for surges.

What You Need

  • Pressure pipe (heavy-walled PVC or HDPE) with a rated pressure above the bottom pressure
  • An inlet chamber with a screen and a submerged entrance, and an outlet chamber
  • Air release valves at the high points on each side (Air Valves)
  • A drain valve at the lowest point, for flushing sediment
  • Survey data: inlet and outlet heights, and the pipe route down and up

Method: Survey, Size, Lay, Fill, Vent

Step 1: Survey the heights of the inlet and the outlet. The outlet must be lower than the inlet by at least the friction loss.

Step 2: Size the pipe for the flow, keeping the loss within the fall available.

Step 3: Lay the pipe in a trench, bedded and anchored, following the ground, with the high shoulders marked.

Step 4: Build the inlet so the pipe mouth is always under water, with a screen to stop debris.

Step 5: Fill slowly, opening the air valves until water, not air, comes out.

Step 6: Test with water and check for leaks and for air accumulating at the crests.

Common Mistakes

  • Counting only the static drop. Friction uses part of the fall.
  • An inlet not submerged. It draws air and the flow surges.
  • No air release. Trapped air blocks the flow.
  • A pipe class under the bottom pressure. It splits at the lowest point.
  • No drain at the bottom. Silt collects and cannot be flushed.

Safety

Pressure, and chambers that can hold bad air

The pipe at the bottom carries 1.5 bar or more; test with water and bleed the air first, and never test with compressed air, which can shatter plastic pipe. The inlet and outlet chambers are sheltered spaces where air can be short of oxygen and carbon dioxide or hydrogen sulphide can pool, and a person who steps in can collapse in seconds. The commonest fatality pattern is a second person climbing in after the first. Clean from the surface; if someone must enter, ventilate, use a harness and line held by an attendant, and keep a second person outside who pulls the line and never follows. Do not test the air with a flame.

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