THE FLOW OF WATER IN IRRIGATION CHANNELS. 39 



No. 173, Expt. S-80, central maia canal, California Development Co., California. 

 This test was made on a long reach, of straight canal. The banks were nearly vertical 

 as left by cleaning of silt with a bucket dredge. The bottom is very hard and quite 

 regular, despite this method of cleaning. The velocity was retarded for about 1 foot 

 from each bank by a growth of tules. The silt-laden waters form slick banks. Plate 

 XII, figure 3, shows the reach tested and the portable rating car in action. If freed 

 from growth at all times, which is impracticable in this region, the value of n would be 

 under 0.020. For small canals in this region see Nos. 162 and 215. Coefficient 

 71=0.0221. 



No. 174, Expt. H-20, Billings Land & Irrigation Co., Montana. This canal was 

 originally constructed in varied strata having an earth surface underlaid with a stratum 

 of gravel, while the bed of the canal was in Benton shale. This has now been covered 

 in places with graded gravel. In general the upper end of the reach tested had a 

 smaller sectional area, consequently a higher velocity, and the gravel was scoured 

 clean, while the lower end of the reach had a lower velocity, and the gravel influence 

 had been reduced by the deposit of silt. Coefficient ri,=0.0221. 



No. 175, Expt. S-84, Salt Kiver Valley Canal, Salt River project. United States 

 Reclamation Service, Arizona. This test was made on a straight reach of canal origi- 

 nally constructed ii\ graded gravel underlying silty loam soil. The high velocity 

 encountered (mean 3.12 feet per second) scoured the bed of the canal, exposing hard- 

 packed small gravel, while near the sides a slick deposit of silt formed a sm-face with 

 but little retarding action on the water. The fringe of grass and small roots at the ex- 

 treme edges (PI. XIII, fig. 1), influenced but a very small portion of the flow. Coeffi- 

 cient n=0.0222. 



No. 178, Expt. H-2, Big Ditch near Billings, Mont. This test was made on a canal 

 originally excavated in Billings loam, which tends to be clayey. The bed of the canal 

 is in the original earth with a slight deposit of sand which undercuts beneath the feet 

 in wading, showing that the mean velocity, 2.09 feet per second, was almost sufficient 

 to cause scouring of sand deposit. Fine mud has been deposited at the sides where the 

 velocities are low. Coefficient w=0.0225. 



No. 179, Expt. 11-38, Bitter Root Valley Irrigation Co., Montana. The first half 

 of this reach is on tangent, while the second half is on a 20° curve around a gravelly 

 point. On the tangent the bed of the canal is covered with fine sand in serrations from 

 1 to 2 feet longitudinally with the canal and about 6 inches deep. In the second half 

 of the reach the sand covers the middle portion of the bed, while gravel up to cobble 

 size forms the edges. The value of n found, 0.0226, is lower than ia to be expected on 

 this type of channel. 



No. 180, Expt. S^O, Lateral No. 10, Orland project. United States Reclamation 

 Service, California. Many of the conditions holding for this test are clearly shown in 

 Plato XIII, figure 2. The gravel, most of which is under hen's-egg size, is well com- 

 pa^-ted in the bed of the ditch, while a few scattered jjatches of moss have a retarding 

 influence. There were about two patches, each 5 feet in diameter, in each 100 feet of 

 length down the ditch. Coefiicient n=0.0228. 



No. 184, Expt. S-39, main canal. South Orland project. United States Reclamation 

 Service, California. This reach of earth channel (PI. IV, fig. 3) lies just above the 

 lined section tested in No. 51. It ia straight, originally constructed in a yellow clay 

 which i« very slick when wet. A darker deposit of silt now covers much of this clay. 

 A value of n of about 0.017 might be expected but for patches of moss and water grasses 

 fK:cuj>ying about 20 7>er cent of the bottom of the channel. This influence brings about 

 a vahje of 0.0231 for n. 



No. J86, P^xpt. S-30, River Branch Canal, Sacramento Valley Irrigation (!o., Cali- 

 fornia. Thi.H rlitrh waH originally exf-avated in Sacranuinto nilty clay loam, which 

 brealcH info very hard small clods (I'l. XIII, fig. ',',). The; bed of lh<! ditch was very 

 alick and hard. A few scattered soft lumps of mud and a fringe of graas retarded the 



