Volume 17, Numéro 4, Pages 35-42

Technical Note - Manning’s Roughness Coefficient In A Trapezoidal-shaped Channel

Authors : Achour Bachir . Amara Lyes .


The study proposes to establish the relation which governs the Manning’s n roughness coefficient in a trapezoidal-shaped channel. It appears that n depends on the depth of the flow, the bottom width of the channel, the slope of the channel, the absolute roughness characterizing the state of the internal wall of the channel, the side slope, the acceleration due to gravity, and the kinematic viscosity of the flowing liquid. The coefficient n is therefore not constant for a given trapezoidal canal, the bottom width of which as well as the side slope are known. It does not depend solely on the material constituting the channel as suggested in the literature. Considering the cases of the smooth and rougher trapezoidal canal, with a given side slope, the curves of variation of n have been drawn. The defined shear Reynolds number as a function of the bottom width, the slope of the channel and the kinematic viscosity plays an important role in the variation of n up to a lower limit value of the relative roughness beyond which it has no longer influence. The rough turbulent flow regime is then achieved. For shallow depths, the coefficient n takes on large values, which is in mathematical accordance with Manning's equation.


Manning's coefficient ; relative depth ; trapezoidal channel ; free surface flow ; open channel flow