36 BULLETIN 194^ U. S. DEPARTMENT OF AGEICULTURE. 



EARTH CHANNELS. 



No. 119, Expt. S-2, Farmers' (Tristate) Canal, Nebraska. This test and also No. 

 120 are on long, straight reaches of a large canal, constructed in Brule clay. An 

 engineer connected with the original construction states that in the original design 

 the value of n was estimated as 0.025, but, as shown in Plate IX, figure 2, the canal is 

 running to but partial capacity, and the mean velocity is almost sufficient to scour 

 the material. It had one riffle midway of its length, due to old bridge approaches" 

 jutting into the canal. The values of n in Nos. 118, 119, and 120 are comparable, as 

 the Interstate Canal is in the neighborhood of the Tristate. A fringe of grass extends 

 along the edge but retards only a very small part of the flow. Tlie bottom is extremely 

 even, smooth, and hard, and with the addition of a coating of sediment from the murky 

 waters of the North Platte River, appears to be very efficient. Gentle curves adjoin 

 both upper and lower ends of the reach. For further notes see No. 120. Coefficient 

 n=0.0130. 



No. 120, Expt. S-1, Farmers' (Tristate) Canal, Nebraska. This reach (PI. IX, 

 figs. 2 and 3) was perfectly clean cut throughout its length, and in the opinion of the 

 wi-iter gives the better value of n than reach in No. 119. In both tests the fall is 

 slight, and the mean value of the results of several tests with the level was accepted. 

 The reach is a tangent between two gentle curves. The bottom was as described in 

 No. 119. Coefficient 7i=0.0164. 



No. 126, Expt. S-83, Maricopa Canal, Salt River project. United States Reclama- 

 tion Service. This reach was part of a long stretch of canal with a clean, sandy bot- 

 tom and a slight fringe of grass along the edge, but the influence of the latter was 

 practicably negligible (PI. X, fig. 1). The slope is very gentle, so that a very slight 

 error in the levels would appreciably affect the value of n. The measuremect with 

 current meter was made above the section tested, where the velocity was greater. 

 Coefficient n=0.0166. 



No. 127, Expt. S-3, Winter Creek Ditch, Nebraska. This test was made on a long, 

 straight reach of ditch, with very clean, hard bottom, in a cemented material. A 

 fringe of grass bordered both edges. A stiff wind was blowing directly down stream 

 during the test, and a value of n of 0.0180 is probably better for tliis canal under normal 

 conditions than that actually found by the measurement made. Coefficient n= 

 0.0170. 



No. 130, Expt. H-7, Billings Land & Irrigation Co.'s Canal, Montana. This test 

 was made on a straight reach of canal excavated in clay loam soil. The little grass 

 at the edges is of slight consequence, as the bottom is slick, though roughed by cutting 

 in places. The mean velocity, 2.45 feet per second, was about the limit, as cutting 

 was taking place where the bottom was not protected with a deposit of gravel. A 

 downstream wind probably reduces the value of n from about 0.018, ma,king it quite 

 comparable with No. 127 above. Coefficient n=0.0174. 



No. 132, Expt. H-5b, Cove Ditch, Montana. This test is on the same ditch, with 

 same channel conditions as No. 133. Tliis reach is all curve, the first 300 feet being 

 on a 30° curve, while the ensuing 300 feet is on a reverse 30° curve. "Wliile the slopes 

 and cross sections are quite different, the value of n for tliis test and for No. 133 agree 

 quite closely. Coefficient n=0. 0180. 



No. 133, Expt. H-5a, Cove Ditch, Montana. Tliis reach is part tangent (station 

 to station 2) and part on a 20° curve (stations 2 to 4). The ditch is 6 years old. 

 Originally excavated in sandy loam soil, the bottom is now covered with a silt deposit. 

 A fringe of grass retards the velocity at the edge, but not the main flow. Coefficient 

 n=0.0186. 



No. 134, Expt. H-19, Billings Land & Irrigation Co's. main canal, Montana. This 

 reach of canal follows a gentle hillside contour, although practically straight. There 

 is a little sand and fine gravel scattered over a general bottom of clean soil. The 



