Worked Design Example
A short capacity and part-full check for a DN225 pipe.
The example sizes a DN225 sewer laid at 1% with a Manning roughness of 0.013, then checks the depth and velocity at a low flow. Values are rounded and exclude junction and structure losses.
It demonstrates the two checks that matter on every reach: is the pipe big enough, and is the flow deep and fast enough at the low-flow condition.
Reference table
| Check | Result at 10 L/s | Comment |
|---|---|---|
| Depth of flow | y/D ≈ 0.32 | Well within a normal part-full design depth |
| Flow area | ≈ 0.0109 m² | From the circular segment geometry |
| Velocity | ≈ 0.91 m/s | Compare with the authority's self-cleansing criterion |
| Capacity headroom | 10 of 44.9 L/s | About 22% of full-bore capacity |
Key design considerations
- The low-flow check uses the same diameter and grade as the capacity check; only the discharge changes.
- A flatter grade or a larger diameter would reduce the low-flow velocity, which is where a self-cleansing check fails first.
- Real networks add inflow downstream, so the controlling reach is often the head of the system, not the outlet.
- Structure losses and any backwater are excluded here; a network model would lower the capacity slightly.
Equations
A = πD²/4 = 0.0398 m²
For D = 0.225 m.
R = D/4 = 0.0563 m
Full-pipe hydraulic radius.
Q = (1/0.013)(0.0398)(0.0563)^(2/3)(0.01)^(1/2) ≈ 0.0449 m³/s
Full-bore capacity, about 44.9 L/s.
V = Q/A ≈ 1.13 m/s
Full-bore velocity.
Design checks
- Capacity greater than the design peak flow at the adopted grade.
- Velocity at the check flow above the authority's self-cleansing criterion.
- Gradient consistent with the invert levels shown on the long section.
Standards and authority references
- WSA 02 (current edition)
- Authority self-cleansing criteria
Confirm the current edition, amendment and any local supplement before applying any of the above.
Limitations and assumptions
- Results are rounded and illustrative; recompute for the actual diameter, grade and roughness.
- Assumes clean pipe at the stated roughness and no sediment bed.
Related calculators
This reference is general engineering information. Confirm requirements against WSA 02, the current Australian Standards and the local water authority's supplement before use in design, and obtain project-specific advice for the conditions at hand.