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 structural property should be known in advance. In practical applications this is not possible. The author proposes how to tackle this problem effectively and seriously with diagrams presented in the book. The book is factual, hard-hitting and definitely provides new, never-before-published material.\nprof. dr hab. Pawel Rowinski\nInstitute of Geophysics Polish Academy of Sciences\nVice-President of Polish Academy of Sciences – PAN