A lined channel has its bed and sides faced with stone, concrete or clay so it loses under 5% of its water to seepage, resists erosion, and can run at a steeper 0.5–2% slope. The smooth lining also makes the water fast: in a 30 cm wide channel with 15 cm of water, a concrete lining at 2% runs at about 1.9-2.2 m/s, so bends need extra freeboard and gentle curves.
Part of Aqueducts and Channels
A lined channel is an open channel whose bed and sides are faced with a durable, smoother material. The lining stops water leaking out and soil being washed away, which makes the channel steeper, cleaner and longer lasting.
A Lining Trades Cost For Water, Cleanliness And Slope
The aqueducts page contrasts the two:
- Unlined earth channels lose 20-40% of their water to seepage.
- Lined channels lose less than 5%, and are much easier to keep clean.
- Lined channels can run at 0.5-2% because the lining resists erosion, against 0.1-0.5% for earth.
The options, and which to pick, are covered in Lining Materials: clay, stone set in mortar (Stone Aqueduct), concrete and plastic.
A Smooth, Steep Channel Runs Fast
With a smooth lining, water meets little resistance. Using Manning’s equation (n = 0.013-0.015 for concrete with a trowel finish, per Chow 1959) for a 30 cm bottom, 1:1 sides and 15 cm depth:
| Slope | Speed, n = 0.015 | Speed, n = 0.013 |
|---|---|---|
| 0.5% | 0.97 m/s | 1.12 m/s |
| 1% | 1.37 m/s | 1.58 m/s |
| 2% | 1.94 m/s | 2.24 m/s |
So at 2% the speed is about 1.9-2.2 m/s.
The Froude number, Fr = V ÷ √(g × A ÷ T) where A is the flow area and T the surface width, tells you the character of the flow. At 0.5% it is 0.92-1.06, near 1. At 1% it is 1.30-1.51 and at 2% 1.84-2.13. Above 1 the flow is supercritical: fast and shallow, it does not slow for a bend, but runs up the outside wall, and it can jump suddenly where the slope flattens. Keep curves gentle and freeboard generous on steep lined runs, and put drops or a stilling pool before a flat stretch.
What You Need
- Lining material and tools for the chosen type
- A prepared, trimmed channel to the design section
- Survey equipment to hold the grade
- Time for curing if cement is used (Curing)
Method: Dig, Trim, Line, Cure, Test
Step 1: Dig the channel to the section with extra room for the thickness of the lining.
Step 2: Trim and compact the bed and sides.
Step 3: Place the lining from the bottom up, in sections that you can finish and cure.
Step 4: Leave joints where the lining can move without cracking, and fill them.
Step 5: Cure and test with water, watching for leaks and for flow at bends.
Worked Example
A 5 L/s intake: with 5% loss it delivers 5 × 0.95 = 4.75 L/s, against 3-4 L/s from an unlined earth channel (loss 20-40%). For irrigation, that is 0.75-1.75 L/s more water for the same intake.
Common Mistakes
- Lining a bad grade. The lining makes the faults permanent.
- Skipping curing. The lining cracks.
- A sharp bend on a steep run. Fast water rides over the outside wall.
- Forgetting that a lined channel is slippery. People or animals that fall in cannot climb out.
What’s Next
- Learn next: Lining Materials — choosing among the options
- Combine with: Covered Aqueducts — adding a cover for drinking water