CANAL CONSTRUCTION. 4? 



be as wide at the top as the length of the diann-u-r 

 of the pipe, and one-half diameter deep from the 

 irnUT to any point of the sides or bottom. A ditch 

 of this form offers less friction surface in proportion 

 to its cross sectional area than any other form and 

 also keeps the depth of the water in the ditch nearly 

 half its width. The diameter of a pipe we will say 

 is four feet. Its circumference would be therefore 

 ;>. U1G multiplied by four, equal to 12.5664 feet ; when 

 it is halved lengthwise, half the circumference would 

 equal 6.2832 feet. 



To get the greatest velocity and quantity of water 

 to flow in a rectangular canal it should be of such form 

 as to cause the water in it to flow exactly one-half as 

 deep as wide, because the velocity of flow in such a canal 

 is proportional to the square root of the hydraulic mean 

 depth, and the hydraulic mean depth is at its maximum 

 when the breadth of the water is just twice its depth. 

 Fanning says that the variation of velocity, with varying 

 depth, is nearly as the variation of the square root of 

 the hydraulic mean depth. 



Grades and Slopes. The grade is one of the im- 

 portant things to be considered in canal construction. 

 Ditches running from twenty to over one hundred miles 

 have widths from twenty to eighty feet, some being 

 built with and some without bermes the grades ranging 

 from one foot to seven feet a mile. The steeper grades 

 are not common and are for short distances only. The 

 average grades for main ditches, carrying from two to 

 six feet of water, are from one and one-half to two and 

 three-fourths feet a mile. Such low grades will answer 

 only for the larger ditches carrying large volumes of 

 water, and where the ratio of volume to resistance or 

 friction on the sides is large. In smaller distributing 

 ditches, where the volume is smaller and the resistance 

 proportionately much greater, a steeper grade must be 



