THE FLOW OF WATER IN IRRIGATION CHANNELS. 47 



estimates of two men will separate. The lines of levels run by the 

 two may develop slightly different critical points in the canal bank 

 at which the canal would overflow. The depth of freeboard between 

 the capacity water surface and the lower bank profile will probably 

 differ. The controlling cross sections will not necessarily agree. 



If the canal is dry at the time of inspection, then the solution of 

 the problem as to the capacity is further comphcated by the fact that 

 no test can be made to determine the value of n, and the judgment alone 

 must be rehed on as to the correct value. The surface slope is also 

 indeterminate, and a grade line through the controlling points of the 

 profile of the canal bed must be assumed as the slope. This value 

 also will be different in two estimates. 



In the courts of law of the Western States many attorneys and 

 judges have come to regard as the last word a statement to the effect 

 that the capacity of a canal has been computed by Kutter's formula. 

 While the algebra of the formula is an exact science and exactly 

 the same answer will be obtained by several parties if the same hy- 

 drauhc data, including the value of n, are assumed, the fact remains 

 that the assumption of the value of n may be largely a matter of 

 judgment, and the results of two men should not be discredited 

 merely for the reason that they disagree sHghtly. 



RECOMMENDATIONS FOR VALUES OF n FOR DIFFERENT KINDS OF 



CHANNELS. 



Either in the design of canals or the determination of the carrying 

 capacity the material forming the perimeter divides into the following 

 general classes: 1, concrete hning; 2, wooden flumes; 3, metal flumes; 

 4, masonry Uning; 5, earth canals; 6, cobble-bottom canals; and 7, 

 sidehiU cuts with retaining walls. There are a few special cases 

 that do not come under any one of the classes mentioned, but a proper 

 comparison can be made by assuming parallel conditions. 



The following values are apphcable for velocities up to about 5 

 feet per second and with hydrauhc radii up to about 2 feet. If velo- 

 cities and radii are greatly to exceed the above figures, slightly lower 

 values of n should bo used. 



VALUES OF n FOR CONCRETE LINING. 



I. 71=0.012 for the highest grade of material and workmanship 

 and exceptionlly good conditions. The surface of the lining to bo 

 a.s smooth to tlie hand as a troweled sidewalk. Tho expansion joints 

 to be so well covered that they practically fulfill the same condition. 

 Tho climate and water to bo such that moss does not accumulate to 

 any great extent, Tho water to be j)ractically free from slii fling 

 material, Tho alignment to bo compos(!d of long tangents joiired 

 by spiraled curves, while tlio interior of tho chaimel must bo of uniform 

 dimensions, tru<! fo griulc. f hronglioul, tlici cross section. 



