CIVILKITengineering toolsOpen partial-flow tool

closed conduits / field note

Why does pipe depth change the Manning result?

A partially full circular pipe is an open channel with changing geometry. The selected depth changes the area and wetted perimeter, so a full-pipe shortcut can produce the wrong result.

Field note / partial-flow pipe hydraulics

Use the circular segment, not the full circle.

For diameter D and flow depth y, the wetted central angle is θ = 2cos−1(1 − 2y/D). That angle defines the circular-segment flow area and wetted perimeter. The hydraulic radius remains the ratio A/P, but both A and P change with depth.

Velocity and discharge do not change identically.

Manning's equation estimates mean velocity from hydraulic radius, slope, and roughness. Discharge is velocity multiplied by area, so it is possible for velocity and capacity to respond differently as the pipe fills. A depth ratio should be kept visible with the result.

What the quick screen cannot prove

  • Whether the selected depth is set by inlet control, outlet control, tailwater, surcharge, or a downstream structure.
  • Pressurised flow, hydraulic grade line, air release, entrance and exit losses, junctions, bends, blockage, sediment, or maintenance state.
  • Whether the selected roughness and slope represent the actual pipe and energy grade line.

Run the partial-flow screen

Part-full circular-pipe relationships are discussed in FHWA HDS-4.