THE FLOW OF WATEE IIST IRRIGATIOlSr CHANNELS. 45 



irregular; from stations 4 to 6, more uniform in widtti but rougli on rock side; from 

 stations 6 to 7, rock wall is rough and width variable; from stations 7 to 8, rock wall 

 quite smooth, bottom clean or little gravel, the width uniform. Coefficient ?i=0.0228. 



No. 264, Expt. S-17a, Logan, Hyde Park and Smithfield Canal, Utah. This test 

 made on a reach 337 feet long between an earth section and the reach in No. 266 

 below (PI. XX, fig. 1). The excavation is on a steep hillside. The upper bank is 

 mostly of willow roots, while the lower bank is a well-made concrete wall. The 

 bottom is covered with coarse gravel. This reach is nearly straight •with bends at both 

 ends. This is classed as B rating by reason of these bends. Coefficient n=0.0256. 



No. 265, Expt. H-26, Hedge Canal, Montana. This reach is excavated in rock cut 

 with concrete floor and a rubble masonry lower wall, faced with 3 inches of concrete, 

 deposited against wood forms. The bottom is mostly covered with sand and rav- 

 ellings of small rock. The upper bank is rough rock excavated quite true to cross 

 section. The alignment is practically straight except for one sharp curve between 

 stations 0+80 and 1+50. The value of n is higher than in No. 262, which has a 

 smoother section. Coefficient n=0.0269. 



No. 266, Expt. S-17b, Logan, Hyde Park and Smithfield Canal, Utah. Just below 

 the reach described in No. 264 the canal enters the section covered in this test. The 

 same concrete wall formed the lower bank, and the bottom was about the same, but 

 the upper bank is a rough vertical rock cut. The difference in the value of n is about 

 what is to be expected. Coefficient n=0.0278. 



MISCELLANEOUS SECTIONS. 



Nos. 267 and 268, Expts. S-74 and S-73, lower canal. Riverside Water Co., Cali- 

 fornia. These tests made on a straight reach of canal in a sandy soil with a shifting 

 sand bottom and a wood lining on the lower side (PI. XX, fig. 2). The canal in test 

 No. 267 is in the shade of a dense row of trees and is free from moss accumulations. 

 Coefficient n=0.0249. 



The canal in test No. 268 is in the sun and moss has accumulated on the wood 

 lining. Coefficient n=0.0291. 



In both tests the water was retarded by a rank growth of grass for about 1 foot from 

 the bank opposite the wood lining. The difference in the values of n is directly due 

 to the moss which grows in sunlight and does not in shade. 



No. 269, Expt. S-17C, Logan, Hyde Park and Smithfield Canal, Utah. This reach 

 is fairly straight, excavated in rough rock. The bottom is strewn with coarse gravel 

 (PI. XX, fig. 3). Coefficient n=0.0298. 



THE USE OF VALUES OF n. 



The engineer is required to exercise his judgment as to the value 

 of n in two general ways: 



1. The value to ho substituted in the formuhx in the design of a 

 channel or in the changing of a channel from one containing material 

 to another. 



2. The value that is to he used wIkmi the engincuM- is called upon 

 to (h'termin(! the maximum carrying capacity of a canal idrc^ady 

 ConMtruct(!d hut at th(5 tim*; of ins[)(H',tion carrying no water or hut a 

 small portion of its capacity. 



In tli(! (h^sign of chanruils it lias hc^^n ciistoniai-y to nllow lit(-l(^ or 

 no factor of safety. In the o[)inion of th(^ writer; it is as necessary to 

 allow for a sliglit overload in a canal as in any oth(u- conservative 

 const ruction, 'i'his should lie Jicconiplishcd hy choosing as n(Mirly 



