The dimensioning of the channels and especially the computation of normal depth plays a significant role in the practice of hydraulic engineer. The classical methods usually used are graphical or iterative to determine the linear dimensions of a pipe or a channel. They also consider, unreasonably, Chezy’s coefficient or Manning’s roughness coefficient as a given data of the problem, despite the fact that these coefficients depend on the linear dimensions sought.

This problem can be easily solved by the application of a new method called the rough model method or simply the RMM. In this study, this new analytical method is presented and applied for the calculation of the linear dimensions of an open channel with horizontal bottom and circular sides. These linear dimensions are: the normal depth of flow, the width of the base of the channel and the diameter of circular parts of the channel. The method takes into account, in particular, the effect of the absolute roughness which is a readily measurable parameter in practice. In a first step, the method is applied to a referential rough model in order to establish the relationships that govern its hydraulic characteristics. In a second step, these equations are used to easily deduce the required linear dimensions by introducing a non-dimensional correction factor. A practical example is considered to better explain the advocated method and to appreciate its simplicity and efficiency.


Linear dimensions, Open channel,, Uniform flow, Discharge, Slope, Turbulent flow, Rough model method.

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