THE PLOW OF WATEE IN IKEIGATION CHANNELS. 15 



OFFICE EQUIPMENT AND METHODS. 



Multiplication, division, and addition were performed on mechan- 

 ical devices. Areas included within vertical velocity curves were 

 determined \\'ith planimeters and checked by arithmetical ordinates. 

 Vertical velocity curves were drawn through platted points by using 

 xylonite parabolas. The wetted perimeters were determined by 

 stepping aromid cross sections, plotted to such a scale that the length 

 of the perimeter could be read as closely as the measurement in the 

 field would warrant. Checking was done by means of slide rules and 

 plotted curves showing the values of n for various other hydrauUc 

 elements. 



CORRECTION FOR VELOCITY HEADS. 



In order to bring out the influence of the correction for the differ- 

 ence in velocity heads when there is a difference in areas at the ends 

 of the reach, a concrete example is given: 



Test Xo. 175, made on the Salt River Valley Canal, Arizona. The 

 various items of the data were as follows: 



Example showing correction for change in velocity heads. 



Length of reach feet. . 900. 



Assumed elevation of water surface at station do 90. 000 



Elevation of water surface at station 9 do " 89. 319 



Fall in water surface do 00. 681 



Area at station square feet. . 38. 610 



Area at station 3 do 40. 41 



Area at station 6 do 45. 09 



Area at station 9 do 43. 35 



Discharge, found by current meter second-feet. . 131. 33 



Mean area throughout reach, A square feet. . 42. 16 



Mean velocity throughout reach, V feet per second . . 3. 115 



Mean velocity at station do ... . 3. 40 



Mean velocity at station 9 do 3. 03 



Velocity head for velocity at station foot. . . 1797 



Velocity head for velocity at station 9 do 1427 



Difference in velocity heads do 0370 



This difference added to fall, =00.681 + 0.0370= . . .do 718 



Slope, 8, without correction for velocity heads 00075667 



Slope, 8, corrected for velocity heads 00079778 



Mean wetted perimeter throughout reach feet.. 19. 52 



Mean hydraulic radius throughout reach do 2. 158 



Using the above data and solving equation 4, page 4, the value of 

 0.0217 is found forn if the slope as found by the level, uncorrected for 

 velocity licads, is used. If the corrected slope is used, a value of 

 0.0223 for n is found. The change due to this correction is academic 

 rather than prartical for the lower velocities, })ut in such as are found 

 in some flurn(!s and concrete sections the change is very material and 

 should not bo disregarded. 



