The conduct dimensioning, that it is to say, computing the aspect ratio, is also one of the main objectives of our study. Computing the non dimensional aspect ratio and the height need knowledge of the other parameters governing the flow, that is to say, the friction factor, the diameter, the discharge, the energy slope of conduit and the kinematic viscosity of the flowing liquid. As shown by the relationships of Colebrook-White and Darcy-Weisbach. The calculation of the aspect ratio is not easy. These relationships require some transformations and rearrangements to permit respond to our objectives.
This work has been interested in the uniform flow in a trapezoidal shaped channel that is frequently used in irrigation and agricultural drainage. The channel dimensioning necessitates iterations for calculate the friction factor which gives us a major error. We base on the method of rough reference model we calculate the linear dimension of the channel is passing through stages of calculations. The formulas founded are direct and explicit. The steps of calculations are simplified and illustrated by an example of calculation.