288 EXPERIMENT STATIOT^ RECORD. 



"The method followed in determining the slope was to drive small finishing wire 

 nails into the tops of submerged oak stakes at each end of the section to he tested. 

 It was not always possible to have the top of the nail coincide exactly with the sur- 

 face of the water, but this difference introduced no error in the results, provided the 

 heads of both nails occupied th6 same relative ]i(isition to the surface of the water. 

 In the case of pulsations or slight waves caused by winds, the tops of both nails were 

 even with the highest or lowest water surfaces." 



From the data reported the author draws the following conclusions: 

 "(1) Sections of canals inearth, although carefully built of a trapezoidal form, 

 with the l)otti)m width horizontal, soon change to segments resembling those of an 

 ■ elliijse. 



"(2) In all large or medium sized canals in earth, berms are necessary in order 

 to prevent a portion of the excavated material from rolling into the canal. 



" (3) The carrying capacity o1" new irrigation canals and ditches during the first 

 season of their operation are less than in suljsequent seasons, providing the same 

 conditions are maintained. 



"(4) The coetficient of friction in canals well lined with sediment in good order 

 and long in use is less than has been usually supjiosed. 



" (5) The frictional resistance of coarse materials, such as gravel, pebbles, or cobble 

 rock, depends to a large extent on whether such material is well packed or loose. 



"(6) That a rough channel exerts a greater influence in retarding the flow of a 

 small ditch than the same degree of roughness exerts on the large canal or river. 



"(7) In the past, canal builders have to a great extent ovei'looked the injurious 

 effects of the growth of aquatic plants. 



" (8) The effect of water jalants in checking the flow and lessening the capacity of 

 irrigation canals may be much grtater than a rough uneven channel. 



"(9) In parts of the arid West where such vegetation grows abundantly the canals 

 should be Imilt in such a way as to prevent its growth, or, if tins is impracticable, to 

 facilitate its removal." 



"On account of the dissimilarity l)etween the physical conditions of the channel 

 from which the present values of (n) have been derived and the ditches and canals 

 of irrigated America, the writer has attempted to assign values for (n) which would 

 be more in accordance with the conditions which now exist in the Rocky Mountain 

 States. Future experiments in which the details are more accurately con(iu(;ted may, 

 however, modify the values of (n) as here given." The proposed values for the 

 coefficient of friction (n) for different kinds of irrigation ditches and canals are as 

 follows : 



"n=.0175 for canals in earth in excellent condition, well coated with sediment, reg- 

 ular in cross section and free from vegetation, loose pebbles, and cobble rock. 

 "n=.020 for canals in earth in good condition, lined with ^^•ell-packed gravel ])artly 



covered with sediment and free from vegetation. 

 "n=.0225 for canals in earth in fair condition, the wetted surface being lined with 

 sediment with an occasional patch of low-water jdants, or composed of loose 

 gravel without vegetation. 

 "n=.0250 for canals in earth in average condition having few sharp bends and lieing 

 fairly uniform in cross section; the water slopes and bottom being lined with 

 sediment and low water plants, or composed of loose gravel and fragments of 

 rock less than 2 inches in diameter and free from vegetation. 

 " n=.0275 for canals in earth below the average in grade, alignment and cro.ss section; 

 having indentations on the sides, the edges in places partially filled with earth 

 and gravel, and the lining composed of coarse gravel anil cobble rock unpacked. 

 This value would also apply to a smooth, regular surface if the channel were 

 partially filled with aquatic plants. 



