An earth channel is an unlined ditch built on a 0.1–0.5% slope that loses 20–40% of its water to seepage depending on the soil, so a 5 L/s intake delivers 3-4 L/s at the end. That is acceptable for irrigation and not acceptable for drinking water, because of the contamination risk; build it with compacted banks, a steady grade and freeboard of 10-15 cm, and watch for scour and leaks.
Part of Aqueducts and Channels
An earth channel is a ditch dug in the ground with no lining, shaped to a trapezoidal section. It is the cheapest channel to make, and the one that loses the most water and needs the most upkeep.
Seepage Is The Price Of A Channel With No Lining
Water soaks through the bed and sides in proportion to how open the soil is. The aqueducts page puts the loss at 20-40% for unlined earth channels, depending on soil type, and says it is acceptable for irrigation and not for drinking supply. Another page in this wiki (Building Gravity-Fed Aqueducts) gives 20-50%; both are broad ranges, so measure your own channel (Flow Comparison). The figures do not say over what length the loss is counted, so treat them as the loss over a whole run.
Seepage also marks where problems begin: a green strip, a wet bank or a boggy patch beside the channel shows where water is leaving, and it can undermine the bank.
What You Need
- Survey equipment to hold 0.1-0.5% (Water Tube Level)
- Digging tools, a rammer or heavy tamper for the banks, and a template for the section
- Stakes and string along the line
- A way to check the soil: feel it. Sand drains fast; clay holds water.
- A plan for spoil: a bank on the low side, not a heap on the channel’s edge
Method: Stake, Dig, Compact, Test
Step 1: Stake the line at a constant slope (Optimal Gradient for the safe slope for your soil).
Step 2: Dig to the section. Flat bottom, sloping sides: 1:1 in firm soil, 2:1 in sandy soil (Channel Cross Section).
Step 3: Place the spoil on the downhill side and compact it in layers into a bank; this adds freeboard of 10-15 cm.
Step 4: Trim the bed and sides smooth to the template; check the grade every 10 m.
Step 5: Let water in slowly, a little at a time, so the soil wets and settles instead of scouring.
Step 6: Walk it and fix scour, slumps and leaks. Where seepage is serious, line it (Lining Materials).
Worked Example
A 5 L/s intake at 20% loss: 5 × 0.80 = 4 L/s delivered. At 40% loss: 5 × 0.60 = 3 L/s. So an irrigator who needs 4 L/s at the field must take 4 ÷ 0.60 = 6.7 L/s at the intake in the worst soil, and 4 ÷ 0.80 = 5 L/s in the best, assuming the percentages hold. The extra intake is the cost of an unlined channel; a lining that cuts loss to 2-5% costs materials but saves water.
Diagnosing Problems
| Symptom | Likely cause |
|---|---|
| Pool in a flat spot | Grade dips; re-survey and regrade |
| Scour in a steep stretch | Slope above the soil limit; flatten or line |
| Wet green strip on the bank | Seepage; compact or line |
| Bank slumps | Side slope too steep for the soil |
| Overtopping | No freeboard, or a blockage |
Common Mistakes
- Using an earth channel for drinking water. Leaves, animals and runoff contaminate it.
- Loose, uncompacted banks. They erode and slump.
- A heap of spoil on the edge. It slides back into the channel.
- Filling it quickly. Dry soil scours before it wets.
What’s Next
- Learn next: Lined Channels — cutting the seepage
- Combine with: Vegetation Control — keeping the channel clear