ABSTRACT
The flow of a liquid with a free surface is much more complicated than flow in pipes and other closed conduits. For liquids, flow may take place when the uppermost boundary is the free surface of the liquid itself. The state of the boundary surfaces, too, varies much more widely — from smooth timber, for instance, to the rough and uneven beds of rivers. The flow in an open channel may be uniform or non-uniform, steady or unsteady. The choice of a suitable friction factor for an open channel is likely to be much more uncertain than a similar choice for a pipe. The actual velocity distribution in an open channel is influenced both by the solid boundaries and by the free surface. The great importance of the critical conditions is that they separate two distinct types of flow: that in which the velocity is less than the critical value and that in which the velocity exceeds the critical value.
