An elevated flume is a supported trough of wood, bamboo or sheet metal set on posts that carries an open channel across a gap, and it is practical for spans of up to 20-30 m; beyond that an inverted siphon is usually easier. Hold the same slope across the span: at 0.2% a 30 m span falls only 6 cm, and a 30 cm wide trough with 20 cm of water holds about 1,800 kg of it, which every post must carry.
Part of Aqueducts and Channels
An elevated flume is a channel lifted on a framework so it can cross a gully, a stream or a low area at the same height as the line on either side. It keeps the water open to the air and the slope unchanged.
A Flume Is A Small Bridge That Carries Water
The trough does the same job as the channel on either side, and the supports do the job of a bridge. The weight is the problem: water is heavy, and a flume full of it puts a lot of load on a few posts.
The aqueducts page gives practical spans of 20-30 m. Longer than that, the posts and bracing become a large structure, and an inverted siphon is usually easier.
What You Need
- Trough material: planks, bamboo (Bamboo Aqueduct), sheet metal, or half pipes (Half-Pipe Salvage)
- Posts and cross-bracing, founded on firm ground clear of flood level
- Sealing for joints: overlaps laid in the direction of flow
- A level to set each support to the slope (Water Tube Level)
- A cover or a screen, if the water is for drinking
Method: Set The Posts, Hold The Slope, Join The Trough
Step 1: Survey the span and set the heights of both ends to match the channel on each side.
Step 2: Plant the posts firmly and brace them against sideways loads.
Step 3: Set the supports at the falling heights the slope needs.
Step 4: Lay the trough on the supports, with each upstream length overlapping into the next, and fix it so it cannot shift.
Step 5: Test with water, watching for leaks, sag and movement.
Step 6: Inspect after storms and floods, and for rot.
Worked Example
A 30 m span at 0.2%: drop = 30 × 0.002 = 0.06 m = 6 cm over the entire span. A sag of a few centimetres in the middle can wipe out the fall and leave a pool, so the trough must be stiff or well supported.
A 30 cm wide trough holding 20 cm of water has 0.30 × 0.20 = 0.06 m² of water section, so 0.06 m³ per metre, which weighs 60 kg per metre. Over 30 m: 60 × 30 = 1,800 kg of water, a weight of about 1,800 × 9.81 = 17,700 N, or 17.7 kN, before the weight of the trough and posts. Each post carries half the water of the spans on either side of it. The page gives no post spacing, and none is claimed here; the spacing is set by the stiffness of the trough and has to be proved by loading it.
Common Mistakes
- Trough sagging between posts. Water pools and the grade is lost.
- Joints overlapped against the flow. They leak.
- Posts in a flood path. The first flood takes the flume.
- No bracing. The structure racks sideways.
- Forgetting the load. A flume full of water is much heavier than an empty one.
Safety
A full flume is a heavy load overhead
A 30 m flume holds about 1.8 tonnes of water, and a post failure drops it. Do not stand or work beneath a charged flume or let people or animals shelter under it. Do not walk on the trough; it is not a footbridge. Build and test it empty first, and fill it slowly while watching the supports.
What’s Next
- Learn next: Inverted Siphon — when the gap is too wide for a flume
- Combine with: Vegetation Control — roots and rot in the supports