A channel crosses a gap or valley by either of two methods: an elevated flume, a supported trough on posts that is practical for spans of 20-30 m, or an inverted siphon, a pressure pipe that dips down and up the far side and is usually easier beyond that. A siphon crossing a valley 15 m deep puts about 1.5 bar on the pipe at the bottom, so the pipe must be strong enough for it.
Part of Aqueducts and Channels
Crossing obstacles is the problem of taking a gravity channel over a gully, valley or other gap without losing its slope. The two standard answers are to carry the channel across on supports or to put the water into a pipe that follows the ground.
The Channel Must Keep Its Slope Across The Gap
A channel holds a gentle slope all the way. When the ground drops away, the channel either stays up at the same height on a structure, or the water goes down and up through a closed pipe. There is also a third option, going round the head of the gully along the contour, which costs extra length and extra channel but needs no structure; compare the extra length with the cost of the crossing before deciding.
| Option | Works when | Page |
|---|---|---|
| Elevated flume | The gap is up to about 20-30 m | Elevated Flumes |
| Inverted siphon | The gap is wider or deeper, and the exit is lower than the entry | Inverted Siphon |
| Go round the contour | The detour is short | Open Channels |
What You Need
- A survey of the gap: width, depth and heights of both sides (Water Tube Level)
- The design flow of the channel
- Materials: timber, bamboo or sheet metal for a flume; pressure pipe for a siphon
- An allowance for the head lost in the crossing
Method: Survey, Compare, Choose
Step 1: Measure the gap and the heights of both sides.
Step 2: Check the exit is lower than the entry, by enough to pay for friction in a siphon.
Step 3: Compare a flume (lots of structure, no pressure), a siphon (little structure, pressure) and a detour (extra length).
Step 4: Choose the simplest that works and build it with the same care as the rest of the line.
Worked Example
Pressure at the bottom of a siphon is the weight of the water column above it. A valley 15 m deep: 1,000 × 9.81 × 15 = 147,000 Pa = 1.47 bar, about 1.5 bar. A 40 m deep valley puts 392,000 Pa = 3.9 bar on the pipe. The pipe class must exceed the pressure with margin.
Common Mistakes
- Building a flume that is too long. The posts and trough become the whole project.
- A siphon with the exit higher than the entry. It will not run.
- Forgetting friction. A siphon uses up head (Inverted Siphon).
- Ignoring flood paths. A flume low in a gully is swept away.
What’s Next
- Learn next: Inverted Siphon — crossing under pressure
- Combine with: Elevated Flumes — crossing on supports