A covered aqueduct is a lined channel roofed with flat stones, concrete slabs or wooden planks, which keeps out leaves, animals and bird droppings, reduces evaporation by 80-90% and prevents algae growth. Leave access points every 50-100 m for inspection and cleaning, and for longer runs over flat ground use a closed pipe instead: a 75 mm HDPE pipe carries enough for 20-30 people with just 5 m of head.
Part of Aqueducts and Channels
A covered aqueduct is a channel with a roof over it. The cover separates drinking water from the surface environment, at the cost of making the inside harder to see and reach.
Cover Stops What Falls In, And Light That Grows Algae
An open channel collects whatever falls or walks into it: leaves, droppings, dead animals, runoff. A cover blocks all of it, shades the water so algae do not grow, and cuts evaporation by 80-90%, according to the aqueducts page. For a drinking-water channel the aqueducts page gives “line and cover” as the rule.
The simplest cover is flat stones, concrete slabs or wooden planks laid across the top of a lined channel. Wood rots; stone and concrete last. Over long, flat stretches, a closed pipe can replace the channel altogether: a 75 mm HDPE pipe carries enough water for 20-30 people with just 5 m of head (Pipe Sizing shows how to check it for your length).
What You Need
- A lined channel to put the cover on (Lined Channels)
- Cover material: stone, slabs or planks, wide enough to rest on both banks and strong enough to carry whatever will cross
- Access covers that lift out, every 50-100 m
- A small vent or gap, screened with mesh
- A rope, harness and attendant for anyone who has to go inside
Method: Line, Cover, Leave Access
Step 1: Line the channel and test it with water first.
Step 2: Lay the cover across the top, resting on the channel walls or on a ledge, so it cannot fall in.
Step 3: Seal the joints between slabs against surface water and insects, but not so tightly that the cover cannot be lifted at the access points.
Step 4: Place an access point every 50-100 m, with a cover that can be lifted by two people and put back.
Step 5: Mark the route so nobody drives a cart over a cover that cannot take it.
Worked Example
A 1 km channel with access every 50-100 m has 1,000 ÷ 100 = 10 to 1,000 ÷ 50 = 20 access points. Each is a place to clear leaves and silt, and each is also a place a person might go in. A closed pipe has none of these, but also nothing you can reach into when it blocks.
Common Mistakes
- Covering an unlined channel. Seepage and roots remain; line it too.
- No access points. The channel silts up and cannot be cleaned.
- Planks resting on soft edges. They collapse.
- Open access hatches left off. People and animals fall in.
Safety
A covered channel can hold bad air, and the rescuer dies second
A roofed channel or its access chamber can be short of oxygen, and may collect hydrogen sulphide or carbon dioxide, which is heavier than air and pools in low places, particularly where silt and leaves are rotting. Do not climb in. Clean from the access point with long-handled tools. If someone must go in, ventilate at two openings, use a harness tied to a line held by an attendant, and keep a second person outside who pulls the line and never follows. The commonest fatality pattern is a second person entering after the first collapses. Do not test the air with a flame.
What’s Next
- Learn next: Stone Aqueduct — durable, mortared construction
- Combine with: Sediment Management — keeping the covered channel clean