More Reading

For more reading on Manning's Formula and storm drain system design, see "Storm Drain System Design (4) - Horizontal Pipe Sizing".

Gravity Pipeline Design

Manning Formula:

The popular empirical equation for calculating gravity flow is Manning Formula.

Calculating Flow:

For open channels and conduits other than circular, use hydraulic radius, R, directly in Manning Formula:

Calculator No.1 - Flow in Partial Full Pipes:

Flow in partial full pipes can be calculated with the following tables by entering n, D, h/D and S directly, where n is Manning roughness coefficient, h/D is the ratio of wate depth to pipe diameter, D is pipe diameter in ft and S is slope. Other hydraulic elements are also listed for reference.

Calculator No.1 Flow/velocity in gravity piping (n, h/D and S are pre-entered for convenience)
nD (mm)h/D (0-1) S (m/m)

Calculator No.2 - Diameter in Full Pipes:

Diameter required for pipes flowing in full can be calculated with Calculator No.2 by entering n, Q and S directly. This is convenient for calculating storm drain in plumbing engineering when flow is known.

Calculator No.2 Diameter of gravity piping flowing in full
nQ (L/s)S (m/m)

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