46 BULLETIN 126, U. S. DEPARTMENT OF AGRICULTURE. 



Perhaps the most important conclusion to be drawn from the 76 

 measurements of the 18 canals summarized in the table is that the 

 so-called coefficient of friction n is on an average larger than has 

 usually been assumed by engineers. The results show that only in 

 rare cases, where conditions are more or less ideal, is one justified in 

 assuming a value as low as 0.012 for n. The Ridenbaugh Canal of 

 the Nampa and Meridian irrigation district of Idaho, shown in Plate 

 XVII, figure 2 (p. 80), belongs to this class. The flow in this concrete- 

 lined canal has been measured by at least five engineers and their 

 average result as regards the value of n is a trifle below 0.012. In the 

 results given in the table, one also finds a value of n as high as 0.0197 

 and five others greater than 0.018. Again, in the concrete-lined canals 

 of southern California a coating consisting of a vegetable and mineral 

 accumulation was found adhering to the perimeters. The effect of 

 this coating on the flow of water is seen hi the high coefficient of 

 friction for these channels as given in Table III. This coating may 

 be observed hi Plate I, figure 1, which shows a portion of the Santa 

 Ana Canal near Orange, Cal. 



The following approximated values for n may serve as a guide to 

 those who are required to estimate, prior to construction and opera- 

 tion, the discharge of lined canals. 



I. n = 0.012 for concrete-lined canals having a smooth sidewalk 

 finish, clean bottom, no moss, uniform cross section, well-formed 

 joints, long tangents, flat spiral curves, no perceptible undulations 

 on the surface of the water, and in general the best construction and 

 the best conditions obtainable hi practice. 



II. n = 0.013 for concrete-lined canals having conditions slightly 

 better than those of Type III and not so good as those of Type I. 



III. n =0.014 for concrete-lined canals having an unplastered or 

 rough troweled surface, clean bottom, uniform cross section, well- 

 formed joints, medium curvature, no spirals, slight surface undula- 

 tions, no aquatic vegetation, and in general good construction and 

 favorable conditions. 



IV. n = 0.015 for concrete-lined canals having conditions similar to 

 those of Type III, but with greater curvature and some debris or 

 other retarding influences. 



V. ?i = 0.016 for concrete-lined canals of average workmanship and 

 medium conditions, having a rough surface, imperfect joints, and 

 sharp curves; also for canals of smooth lining and good workman- 

 ship, but having one or more unfavorable conditions, such as sand 

 and gravel in the bottom or projecting joints which decrease the 

 velocity of water. 



VI. % = 0.017 for concrete-lined canals roughly coated, but other- 

 wise in medium condition. 



