Since the fluid must adhere to the solid surface, the actual velocity distribution is not uniform. The velocity is maximum on the free surface and becomes zero on the wetted perimeter where the wall shear stress is developed.

Chezy and Manning Equations

The fundamental relations used to determine the uniform flow rate in open channel are semi-empirical and are governed by Chezy and Manning equations. Consider a control volume flow with volume flow rate of and weight in an open channel as shown in Fig. 2. Since it is uniform depth flow, so it can be shown from continuity equation that . Then, -momentum equation for the control volume can be written as,
                                                  (1)