The author proposes a redefinition of critical flow conditions that become universal for pressurised flows, free-surface flow conditions in any shape of channel cross-sectional areas. In fact, the Darcy-Weisbach formula gives the head loss over the length in either a circular pipe or open channel in the same way. The crucial obstacle in the use of the Moody diagram is that one needs to know a priori the relative roughness of the channel, so in fact the information on surface ...
