CHAPTER 1 

 INTRODUCTION 



The hydraulics of steady flow in open channels is a small but 

 important part of the rapidly developing science of hydraulics. 

 Ordinarily treated but briefly in texts on hydraulics, it has become a 

 necessary part of the hydraulic engineer's equipment. The construc- 

 tion of canals, flood channels, and other open channel works of both 

 large and small magnitude has proceeded rapidly in recent years. 

 A knowledge of the hydraulics of open channel flow is essential if 

 economical and safe designs are to be obtained. 



A clear conception of the meaning of the many terms used in the study 

 of the flow of water in open channels is of utmost importance for prog- 

 ress in understanding the subject. A larger number of fundamental 

 variables is necessary than in the study of flow in closed conduits or 

 pipes. It is therefore possible to devise more derived variables. Many 

 of these, such as velocity head, total head, and hydraulic radius, are 

 essential. The use of too many derived variables, however, is con- 

 fusing. Only those which have proven significant, and have been 

 generally accepted, will be used in this book. 



Any elongated depression through which water flows may be termed 

 a channel. The flow is said to be open channel flow if the water has a 

 free surface. Thus the flow in a pipe flowing part full is open channel 

 flow. The sides and bottom of the channel are considered to be imper- 

 vious. If the cross section of the channel does not change along its 

 length, and the channel is straight in alignment and on constant grade, 

 it is said to be a uniform channel. Natural watercourses are never 

 truly uniform, but if exceptionally regular, they may be considered to 

 be uniform for some purposes. The grade of a uniform channel is its 

 slope, referred to the horizontal. 



A transverse cross section taken at right angles to the axis of the 

 channel is usually called, briefly, a section. If the channel is full just to 

 the point of overflowing, the area of the cross section is the " area flow- 

 ing full." Areas at lesser depths are referred to the particular depth or 

 stage for definiteness. Thus " area at 10-foot depth " means the cross- 

 sectional area of flow when the depth is 10 feet. It is obvious that the 

 area, a single numerical measure, is insufficient to characterize com- 



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