68 BULLETIN 194, U. S. DEPARTMENT OF AGRICULTURE. 



Use of charts. Assume that it is desired to construct in well- 

 made concrete a channel that will carry 1,000 second-feet of water 

 at a mean velocity of 10 feet per second. Allow as a safety factor 

 an overload of 5 per cent, making the designed capacity 1,050 second- 

 feet. 'Therefore the area of such a channel must be 105 square feet. 

 To test the possibilities of a rectangular cross section refer to figure 

 4, Any point on the "area" line representing a value of 105 will 

 show values of depth, width, and hydraulic radius corresponding to 

 this area, but the most economic channel — that is, the one having 

 the least area for the greatest hydraulic radius — is at the point of 

 intersection with the broken line. Therefore it is desirable to ap- 

 proach as nearly as possible to a channel 14.5 feet wide by 7.2 feet 

 deep. A further study of the chart shows the hydraulic radius of 

 such a channel to have a value of about 3.6 feet. Assume that the 

 character of the lining, curvature, and so on should give a value of 

 n of 0.012. Turning to figure 6, follow a line parallel to the guide 

 lines from the point of intersection of n equals 0.012 ajid E, equals 

 3.6 feet. This line intersects the velocity line equal to 10 feet per 

 second on a slope line equal to 0.00122 feet per foot. But suppose 

 the topography of the land to be such that a slope of 0.00122 is not 

 obtainable or is not desirable. Suppose the best location for a canal 

 is on a slope of 0.0015 feet per foot. The intersection of 10 feet 

 per second velocity line with 0.0015 feet per foot slope line is down 

 the guide lines from the intersection of n line of 0.012 and hydraulic 

 radius line of 3.1. With this value for hydraulic radius go back to 

 figure 4, and the intersection of area line 105 square feet and hydrau- 

 lic radius line 3.1 shows the water section of the necessary channel 

 to be 25.5 feet wide and 4.1 feet deep, while the same intersection on 

 figure 5 gives a trapezoidal channel 19 feet wide on the bottom and 

 4.2 feet deep. 



Figures 6, 7, and 8 may be used for the general solution of problems 

 involving Kutter's formula. Given any three of the variables — 

 slope, radius, n and velocity, and the fourth may be determined as 

 accurately as is warranted. 



VARIATIONS OF n IN THE SAME CHANNEL. 



It is well known that the same channel does not necessarily have 

 the same value of n throughout the season. Vegetable growths, 

 especially moss, may so change the value of n from early spring to 

 the middle of summer that the channel may carry but 75 per cent 

 of its rated capacity for the same depth of water in the chamiel. 

 The writer made a series of current-meter measurements on a promi- 

 nent canal near North Yakima, Wash., during the month of July, 

 1914. The canal was rated and had carried more than 70 second- 

 feet when clean in the spring, but in July it carried but 62 second- 



