Plastic sheeting laid over a smooth, stone-free channel bed cuts seepage to under 1% and goes in quickly, but it lasts only 3-10 years and breaks down in sunlight, so cover it with soil or gravel and anchor its edges. A 100 m run of the standard channel needs about 165 m² of sheet, and a plastic-lined channel is slippery for anything that falls in.
Part of Aqueducts and Channels
Plastic lining is the use of salvaged or purchased plastic sheeting as a waterproof skin in a channel. It is the quickest lining to install and the least durable.
Plastic Stops The Water, But Not The Sun Or The Stones
The aqueducts page rates plastic sheeting at 3-10 years, low to moderate cost, under 1% seepage, best for quick temporary lining. Plastic is watertight, but its enemies are ultraviolet light, which makes it brittle, and sharp objects, which puncture it. A cover of soil or gravel on top deals with both. The same sheeting is used to line a pit (Plastic Liner).
What You Need
- Heavy plastic sheeting (pond liner or layered tarp), with a known history if the water is for drinking
- A smooth, stone-free channel bed, padded with sand, fine soil or cloth
- Waterproof tape or heat welding for seams, with overlaps of 30 cm as for pit liners
- Soil or gravel to cover the lining
- Anchor trenches along both edges
Method: Smooth, Pad, Lay, Seam, Cover
Step 1: Trim the channel smooth and remove every stone and root.
Step 2: Pad the surface with fine soil, sand or cloth.
Step 3: Lay the sheet from the downstream end so the overlaps shed water forward, with the sheet loose enough to follow the shape.
Step 4: Seam with tape or heat welding, overlapping by 30 cm.
Step 5: Anchor the edges in shallow trenches along both banks and backfill them.
Step 6: Cover the lining with a layer of soil or gravel, which also protects it from sunlight.
Worked Example
The standard section dug 0.30 m deep has a lined perimeter of 0.30 + 2 × 0.30 × √2 = 1.15 m. Allow 0.25 m beyond each bank for the anchor trench (an example allowance): 1.15 + 0.50 = 1.65 m of sheet width. Per 100 m: 1.65 × 100 = 165 m². With under 1% loss, a 5 L/s intake delivers over 5 × 0.99 = 4.95 L/s.
Which Plastics Are Fit For Drinking Water
Plastic can leach, and heat and sunlight speed it up. Do not line a drinking-water channel with sheeting that held fuel, pesticide, fertiliser or chemicals, or with roofing membrane of unknown composition. For a drinking-water channel the aqueducts page recommends lining and covering it; if you cannot establish what the plastic is, use it only for irrigation or livestock water.
Common Mistakes
- Leaving the sheet uncovered. Sunlight destroys it within a few years.
- Sharp stones under it. A single puncture drains the channel.
- Edges not anchored. Wind and flow lift it.
- Overlaps against the flow. Water gets under the seams.
- No way out for people or animals. See the safety note.
Safety
A plastic-lined channel is a trap
Wet plastic is very slippery, and a child or animal that falls in cannot climb out. Provide steps or rough strips at intervals on the banks, cover the channel near paths and settlements, and fence it where people pass. Heat-welding seams gives off fumes and can ignite the plastic; do it in the open air, away from anything that burns.
What’s Next
- Learn next: Lining Materials — comparing the options
- Combine with: Sediment Management — silt on a plastic lining is hard to clean without tearing it