read the result
Closed conduit geometry is still visible.
For a circular pipe flowing full, the cross-sectional area is πD²/4 and the hydraulic radius is D/4. Manning's relationship then estimates a mean velocity from the roughness and friction slope; multiplying velocity by area gives discharge capacity.
Governing equation
A = πD²/4, R = D/4, and Q = (1/n) A R2/3 S1/2.
The full-flow Manning form and circular-pipe hydraulic radius are described in FHWA culvert guidance.
Capacity is not a system result.
- Check whether the pipe is actually flowing full and whether the hydraulic grade line, tailwater, air, and surcharge conditions allow that state.
- Include entrance, exit, bend, junction, grate, trash, and barrel losses, plus sediment, blockage, maintenance, and minimum-velocity requirements.
- For partially full flow, solve the circular-segment geometry at the selected depth instead of using A = πD²/4 and R = D/4.