CIVILKITengineering toolsOpen pipe-flow tool

closed conduits / field note

What does a full-pipe Manning capacity assume?

A full-flow capacity is useful when the internal diameter, slope, roughness, and flow state are explicit. It is not the same as proving that a drainage system will convey that flow in service.

Field note / pipe hydraulics

Full flow fixes the geometry.

For a circular pipe flowing full, the area is A = πD²/4 and the hydraulic radius is R = D/4. Manning's equation uses those properties with the roughness coefficient and friction slope to estimate mean velocity.

Roughness and slope are model inputs.

The selected roughness should represent the internal pipe boundary, joints, lining, ageing, sediment, and maintenance state. The slope should be the friction or energy slope used for the capacity estimate, not automatically the site ground slope.

What the quick screen cannot prove

  • Partial-flow depth, surcharge, inlet control, outlet control, tailwater, air entrapment, or pressurised conditions.
  • Entrance, exit, bend, junction, grate, trash, sediment, blockage, and other system losses.
  • Whether the selected pipe meets local drainage, self-cleansing, velocity, freeboard, flood-risk, or maintenance criteria.

Run the full-pipe screen

The full-flow relationship and circular-pipe radius are also shown in FHWA culvert guidance.