Canal lining: how much water an unlined channel loses

Conveyance losses in unlined earth channels are one of the largest and least visible inefficiencies in agricultural water use. Depending on soil, channel length and how long water sits in the section, a substantial share of what is pumped in never reaches the field. In sandy and gravelly ground — which describes most of the UAE — the share is at the high end.

Where the water goes

Three mechanisms:

• Seepage through the bed and banks. The dominant loss, and the one lining eliminates almost entirely.
• Evaporation from the open surface. Significant in summer, and unaffected by lining — only covering or piping addresses it.
• Operational losses — overtopping, leaking structures, tail-end spill. Fixed by operation and by structures, not by membrane.

Because seepage is the largest and the most fixable, it is where lining money goes furthest.

Lining options compared

The composite option is worth understanding properly. Concrete alone is not a reliable barrier — it cracks, and cracks conduct. Concrete over a geomembrane gives you a barrier that does not crack and a wearing surface that protects it. On any channel where lining life matters, that is the arrangement to specify.

Design points that matter

• Side slopes. Exposed membrane on slopes steeper than about 1V:1.5H tends to slide, especially under any soil cover. Either flatten the slope or use textured sheet.
• Anchor trenches. Continuous along both banks, with intermediate anchoring on long slopes so the sheet cannot creep downward over time.
• Structures. Offtakes, division boxes, headwalls and drops are all terminations. Each gets a batten strip, sealant and an extrusion-welded detail. These are the leak points on a lined channel.
• Velocity. Above a certain flow velocity an exposed membrane is at risk from debris and abrasion, and needs concrete, concrete-filled geocell or rip-rap over a heavy protection geotextile.
• Access crossings. If vehicles or animals cross the channel, build a designed crossing. They will cross whether or not you build one.

The failure mode people forget

A lined channel is at its most vulnerable when it is empty. If groundwater is high, or if rain has infiltrated behind the lining, the pressure beneath the membrane is no longer balanced by water above it — and the liner floats. Once a section lifts, it folds, and folds tear.

The fix is designed in, not retrofitted: relief vents through the membrane, or a drainage layer beneath it discharging to a low point. On any site with a seasonal water table, this belongs in the design from day one.

Working out whether it pays

The calculation is simpler than it looks:

1. Measure flow entering the section and flow leaving it, on a day with no offtakes running.
2. The difference, less an estimate for evaporation from the open surface area, is seepage.
3. Multiply by the hours the channel runs per year to get annual loss.
4. Multiply by your cost per cubic metre — pumping energy, tanker cost, or the value of the crop that water grows.
5. Compare against the lining cost over its design life.

On long conveyance channels in permeable ground, the payback is frequently a small number of seasons. And unlike most agricultural investments, it keeps paying with no operating cost.

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