The Manning’s n coefficient represents the friction applied to the flow by the inner wall of a channel or a pipe. For a correct design of hydraulic systems, the designer should have an appropriate value of this coefficient. The present study aims to establish the proper Manning’s n relationship including all the parameters that affect the flow in a partially filled circular pipe, such as the relative roughness, the slope of the energy grade line, and the kinematic viscosity. A new dimensionless parameter, acting as a Reynolds number, is introduced reflecting the relative effect of friction forces versus viscous forces. The study highlights the significant role of this parameter in the variation of the Manning’s n coefficient with the relative flow depth.


Manning’s coefficient, Darcy-Weisbach, Partially filled pipe, Dimensionless diagram.

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