Half-pipe salvage turns large-diameter pipe, split lengthwise, into ready-made covered or open channel sections: a 300 mm plastic pipe cut in two gives two troughs that each carry about 30-40 L/s when full at a 0.5% slope. Find out what the pipe carried before it came to you, never cut or drill asbestos-cement pipe, and use drinking-water channels only from pipe whose past is known.
Part of Aqueducts and Channels
Half-pipe salvage is reusing large drain pipes, culverts, gutters and similar rounded sections as channel pieces. It gets a smooth, uniform channel without lining work, from material that would otherwise be waste.
A Pipe Cut In Half Is A Smooth Channel
A semicircular trough has a smooth, even surface, so water moves in it without a lining. Pipes of 200-300 mm or larger make useful channel sections, and a salvaged pipe may come in lengths that join end to end. The same idea works with curved roof tiles, large gutters, split drums and corrugated sheet bent into a U.
A Half Pipe Carries More Than Most Supplies Need When Run Part Full
For a semicircle of radius r flowing full: A = π·r² ÷ 2 and the wetted perimeter P = π·r, so R = A ÷ P = r ÷ 2. Manning’s equation (V = R^(2/3) × S^(1/2) ÷ n) then gives, at 0.5% slope and n = 0.011 for smooth plastic (n = 0.015 for a rougher surface like old concrete pipe; Chow 1959):
| Pipe | n | Speed | Flow when full |
|---|---|---|---|
| 200 mm | 0.011 | 0.87 m/s | 14 L/s |
| 200 mm | 0.015 | 0.64 m/s | 10 L/s |
| 300 mm | 0.011 | 1.14 m/s | 40 L/s |
| 300 mm | 0.015 | 0.84 m/s | 30 L/s |
So a 300 mm pipe cut in half carries 30-40 L/s when flowing full at 0.5%. Leave freeboard in practice and run it part full, so the usable flow is less.
What You Need
- Salvaged pipe of known history, of large diameter, without cracks
- A saw that cuts straight along the length, a straightedge and a marker line
- Supports or a trench bed on which each half rests at the right slope
- Sealant or mortar for the joints, and a cover if the water is for drinking
- Dust protection and eye protection for cutting
Method: Identify, Cut, Bed, Join
Step 1: Identify the pipe. What material is it, and what did it carry? Sewer, fuel, chemical and industrial pipe is not for drinking-water channels. If unsure, use it for irrigation only.
Step 2: Mark a straight line along the pipe on both sides and check that the two halves will be equal.
Step 3: Cut along the line with the pipe held still and supported along its whole length so it does not pinch the saw.
Step 4: Bed each half on a firm, level base at the planned slope, open side up.
Step 5: Join the lengths, overlapping each upstream end into the next and sealing the joint.
Step 6: Cover for drinking water (Covered Aqueducts).
Common Mistakes
- A crooked cut. The two halves do not match and leak at the joints.
- Pipe that held chemicals. It can leach into the water.
- Setting the half pipe on an uneven base. It rocks and cracks.
- Joining against the flow. Water runs out of the joint.
- Ignoring a coating (tar, paint) that may be toxic or may flake off.
Safety
Asbestos-cement pipe releases fibres when cut
Old grey cement pipe may contain asbestos. Cutting, drilling or sanding it releases fibres that cause lung disease years later. Do not cut it, break it or use it for a channel; if it is already in the ground, leave it and do not disturb it. Cutting any other pipe produces dust and sharp edges: wear eye protection and a dust mask, and keep clear of the saw. Salvaged metal may carry lead paint or zinc coatings; do not use it for drinking water unless you know it is safe.
What’s Next
- Learn next: Elevated Flumes — carrying a half-pipe over a gap
- Combine with: Lining Materials — alternative smooth surfaces