VELOCITY AND STATIC HEADS 



21 



same minimum opening RS would do as well. The only difference 

 would be that the rising surface of the water would not be a plane but a 

 curved surface. 



So far we have been discussing a hypothetical condition of flow, and 

 have assumed that there is no friction or loss of energy by impact. How 

 far does this represent or approximate conditions as they may exist in 

 actual flow of water? This is an important matter to determine, and 

 fortunately our knowledge of hydraulic phenomena enables us to go a 

 certain distance in answering this question. In the first place, it is to be 

 remembered that of the three distinct methods for converting velocity 



^■^xjZ), Elevation \P 



UKII ,1 I II,, ,,,,1,1,1 , , . y ,,, ,;i;ii;i((tl"" ^- ^ 



Plan S 



Fig. 203. Diagram Illustrating Flow in an Open Conduit of Variable Width. 



Bottom level, no friction or impact. The variation in width is that necessary to raise the water 

 surface on a uniform slope. 



head into static head, described above and illustrated in Figs. 201, 202, 

 and 203, any two, or all three, may operate together without conflict. 

 Surface friction in absorbing energy, in one sense, is comparable to a 

 rising slope of the conduit bottom, similar to that of the left-hand portion 

 of Fig. 202. Hence, if the amount of friction is known, or is susceptible 

 of estimate, it may be allowed for by combining the method of Fig. 202 

 with the other processes. 



Impact is not so easily disposed of. In most hydraulic operations it is 

 desired to avoid the occurrence of impact entirely. If it occurs, it is 

 likely to introduce at once an element of extreme variability and uncer- 

 tainty, not susceptible of accurate prediction or calculation except in 

 certain cases. Hence the present question may be considered to be, 

 with how much assurance, or with what certainty, may we assume that 

 impact may be avoided in velocity transformations? 



Impact does not occur in a flowing stream in which the velocity is 



