Manning’s Roughness Coefficient

Manning's Roughness Coefficient is a dimensionless coefficient describing the internal roughness of a drainage pipe. This value must be carefully chosen to suit the design conditions. It is dependent on mostly the same issues as the Colebrook-White roughness coefficient, primarily the pipe material and condition. Care must be taken that a conservative approach is taken for most drainage designs, as the pipe must function after 50 years of service so laboratory values for pipe materials will not be suitable for general drainage design. Some typical design values and a range for new pipes and damaged pipes are included below. Generally a conservative design value of 0.014 is often used for new drainage pipes as sliming and deterioration tend to affect all pipe materials in a similar manner.

Manning Coefficient for Partial Flow Conditions

There is some evidence that the Manning coefficient should be altered for partial flow conditions. It has been proposed to use the following equation to calculate the appropriate Manning Coefficient for partial flow conditions, where ID is the internal diameter and h is the pipe fill height.

This effect can be seen in the below chart (orange line).