Aqueducts and Channels
Why This Matters
Your water source is rarely right next to where people live. Moving water 500m, 2km, even 10km without any pumping is entirely possible — civilizations have done it for thousands of years with nothing but gravity and a consistent slope. An open channel is the simplest infrastructure your community can build, and it transforms which land is habitable.
Grade: The Most Important Number
A channel that’s too steep erodes its own bed. A channel that’s too flat doesn’t flow. The optimal gradient for an earthen channel is 0.1% to 0.5% — that’s a drop of 1-5m for every 1,000m of channel length.
For a lined (stone/concrete) channel, you can go steeper: 0.5% to 2%, because the lining resists erosion.
Surveying the Route
You need to maintain a consistent, gentle downhill slope across potentially rough terrain. Two tools make this possible:
A-frame level: Build two equal-length poles (2m each) joined at the top with a crossbar. Hang a plumb line from the apex. Mark where the plumb line crosses the crossbar when the legs are on level ground. To survey: place one leg at your starting point, pivot the other until the plumb line hits the level mark. That point is at the same elevation. Step forward and repeat.
Water tube level: Fill a clear plastic tube (10-20m long) with water. Hold both ends up — water finds its own level. When the water line matches at both ends, those two points are at the same height. Use this to mark level points along your route, then add the calculated drop.
Tip
For a 1km channel at 0.2% grade, you need a total drop of 2m. Mark level points every 50m along the route, then lower each stake by 10cm compared to the previous one. That’s your channel bed.
Open Channels
The simplest option. Dig a trench, shape it, and water flows.
Cross section: A trapezoidal shape (flat bottom, angled sides) is most efficient and stable. For a community supply carrying 2-5 L/sec:
- Bottom width: 30cm
- Side slopes: 1:1 (45 degrees) in firm soil, 2:1 in sandy soil
- Water depth: 15-20cm
- Freeboard (wall above water line): 10-15cm
Unlined earth channels lose 20-40% of water to seepage, depending on soil type. Acceptable for irrigation. Not acceptable for drinking water supply — too much contamination risk.
Lined channels lose less than 5% to seepage and are much easier to keep clean.
Lining Options
| Material | Lifespan | Cost | Seepage Loss | Best For |
|---|---|---|---|---|
| Puddled clay | 5-15 years | Free | 10-15% | Irrigation, clay-rich areas |
| Flat stones + mortar | 20-50 years | Moderate | 2-5% | Permanent drinking water supply |
| Concrete (3cm) | 30-50+ years | High | <2% | Main supply channels |
| Plastic sheeting | 3-10 years | Low-moderate | <1% | Quick temporary lining |
Puddled clay lining: Spread wet clay 10-15cm thick on the channel bed and sides. Compact it by trampling or pounding until it’s dense and smooth. Let it cure for a few days before running water. Cracks in dry weather — keep it wet or cover with a thin gravel layer.
Stone lining: Lay flat stones on the bed and sides, pointed side down. Fill gaps and cover with cement mortar (1:3 cement:sand). Smooth the surface.
Covered Aqueducts
Covering your channel prevents contamination (leaves, animals, bird droppings), reduces evaporation by 80-90%, and prevents algae growth.
Simplest method: lay flat stones, concrete slabs, or wooden planks across the top of a lined channel. Leave access points every 50-100m for inspection and cleaning.
For longer distances over flat terrain: use a closed pipe instead. A 75mm HDPE pipe carries enough water for 20-30 people with just 5m of head.
Crossing Obstacles
Inverted siphon: When your channel route crosses a valley, dip the water down and back up through a closed pipe. Water rises to its own level, so as long as the outlet is lower than the inlet, it works. The pipe must be strong enough to handle the pressure at the bottom of the dip. Use heavy-walled PVC or HDPE. Install an air release valve at the high point on each side.
Elevated flume: Build a supported trough (wooden, bamboo, or sheet metal) on posts across the gap. Practical for spans up to 20-30m. Beyond that, an inverted siphon is usually easier.
Settling Basins
Install a settling basin at the channel intake and every 500-1000m along the route. This is simply a wider, deeper section (1m x 1m x 0.5m deep) where water slows down and sediment drops out. Include a drain pipe at the bottom for periodic flushing.
Common Mistakes
| Mistake | Consequence | Prevention |
|---|---|---|
| Inconsistent grade | Pools form in flat spots, erosion in steep spots | Survey carefully, check grade every 10m |
| No freeboard | Water overtops walls in rain or when debris partially blocks channel | Always build walls 10-15cm above design water level |
| Unlined channel for drinking water | Contamination, excessive water loss, animal access | Line and cover any drinking water channel |
| No settling basins | Sediment fills channel, reduces capacity | Install basins every 500-1000m with drain plugs |
| Tree roots near channel | Roots crack lining, block flow | Clear all trees within 3m of channel, inspect annually |
What’s Next
- Water Distribution — getting channel water to individual users
- Flood Control and Drainage — managing excess water
- Cisterns and Rainwater Storage — storing channel water for dry periods