Calculate self-cleansing velocity in a sewer pipe with Manning’s equation from diameter, slope and pipe material, and check it against a minimum velocity.

Default 1 is a just-full gravity-flow reference, not pressurized flow.

Typical Manning n for clean pipe; aged or sliming pipe is rougher. Use your design value if you have one.

Optional settings
Blank uses the sewer type’s minimum.

Blank omits the shear criterion.

Typical 10 ft/s (3.048 m/s) limit; use your project’s value.

Self Cleansing Velocity Formula

The calculator uses Manning’s uniform-flow equation and compares the resulting velocity with selected minimum and maximum assumptions.

V = (1/n) × R(2/3) × √(S)

  • Where V is the mean velocity in m/s at the entered depth
  • R is the hydraulic radius in meters (wetted area divided by wetted perimeter)
  • n is Manning’s coefficient of roughness
  • S is the dimensionless energy slope (entered percent divided by 100)
  • Q = V × A is flow capacity at the specified depth
  • Boundary shear Ï„ = 9810 × R × S in Pa, using water unit weight 9810 N/m³. Particle characteristics are not modeled.

What is a Self Cleansing Velocity?

Self Cleansing Velocity refers to the minimum velocity required in a fluid system to prevent the accumulation of sediment or debris.

It is a critical parameter in determining the efficiency and functionality of pipelines, channels, and open channels.

When fluid flows at a certain velocity, it possesses enough kinetic energy to carry away particles or sediment that may settle on the system’s surfaces.

This self-cleansing action prevents the accumulation of materials, which can lead to clogs, blockages, or reduced capacity. A velocity comparison alone does not ensure continuous, unobstructed flow; sediment properties, loading and flow duration also matter.

By maintaining an adequate self-cleansing velocity, pipelines and channels can operate optimally.

How to Calculate Self Cleansing Velocity?

Example Problem:

The following SI example shows a Manning velocity estimate at a stated hydraulic radius.

First, determine the variables in the equation above:

R = 1 m (a 4 m diameter circular pipe at the just-full reference)

n = .01

Slope = 1%

S = 0.01

Use the same stated depth for the capacity and velocity check.

Finally, using the formula above, calculate the self-cleaning velocity:

V = (1/n) × R(2/3) × √(S)

V = (1/.01) × 1(2/3) × √(.01)

V = 10 m/s. This exceeds the original adjustable 10 ft/s maximum assumption; it is an arithmetic example, not an approved design.