An open channel is a dug trench with a flat bottom and sloping sides that carries water downhill by gravity alone on a gentle slope of 0.1–0.5%, a fall of 1-5 m per kilometre. It is the simplest and cheapest way to move water over distance, and it suits irrigation, but an unlined channel loses 20-40% of its water to seepage and an open one is unsafe for drinking water.
Part of Aqueducts and Channels
An open channel is a trench dug to a steady gentle slope so that water flows along it by gravity. It needs no pump, no pipe and few materials, and is the oldest way to bring water to land and settlements.
A Steady Gentle Slope Does The Work
Water in a channel flows downhill, and the slope sets how fast. A channel that is too steep erodes its own bed. One that is too flat does not flow. For an earth channel the right slope is 0.1-0.5%, a drop of 1-5 m in every 1,000 m. For a lined channel, 0.5-2% (Optimal Gradient).
A channel can be a bare ditch (Earth Channels), a ditch with a lining (Lined Channels), or a ditch with a cover (Covered Aqueducts). Where the route meets a gap or valley, see Crossing Obstacles.
What You Need
- A survey method to hold the slope: an A-frame level or a water tube level
- Digging tools and labour
- A clear route from the water source to where it is needed, falling the whole way
- A plan for sediment at the start and along the route (Sediment Management)
Method: Survey, Stake, Dig, Check
Step 1: Survey the route and stake it at a constant slope.
Step 2: Dig a trapezoid section: flat bottom, sloping sides (Channel Cross Section).
Step 3: Leave freeboard, 10-15 cm of wall above the water line, so rain or a partial blockage does not overtop it.
Step 4: Let water in slowly and walk the line, looking for scour, pools and leaks.
Step 5: Maintain it: clear silt and plants (Vegetation Control).
Worked Example
A 1 km channel at 0.2% falls 1,000 × 0.002 = 2 m. Marking stakes every 50 m and lowering each by 10 cm from the one before gives the bed line.
If it loses 20-40% to seepage and takes 5 L/s from the source, it delivers 5 × (1 − 0.2) = 4 L/s down to 5 × (1 − 0.4) = 3 L/s, assuming the loss applies over the whole run.
Common Mistakes
- An uneven grade. Pools form in flat spots and erosion in steep ones.
- No freeboard. The channel overtops.
- Using an open channel for drinking water. Leaves, animals and bird droppings get in; line and cover any drinking-water channel.
- No settling basin at the intake. Silt fills the channel.
What’s Next
- Learn next: Channel Cross Section — sizing the trench
- Combine with: Channel Construction — more on building channels, ditches and canals