Nov. 8. 191S Use of Current Meters in Irrigation Canals 231 



velocity alone. The lined sections were subdivided into two classes: 

 Those with relatively steep sides and those following the flatter slopes 

 more usual to earth canals. There is no marked difference in the results 

 of the two types. These measurements indicate, as was to be expected, 

 that the points of mean velocity are farther from the water edges in 

 sections with sloping sides than in the vertical-sided flumes and occur 

 between one-fifth and one-fourth of the width of the water surface from 

 the edges. The average and maximum errors are not large. 



For earth canals results are given for both velocities and for total dis- 

 charges. The results for such sections are more variable. The velocity 

 at from one-fifth to one-fourth of the width from the edges will average 

 to give results close to the actual velocity for the whole section, but indi- 

 vidual gagings may vary from the mean by over 15 per cent. The 

 results for the total discharge are more consistent than those for velocity 

 alone. The error in the cross-sectional area, due to using the two measure- 

 ments of depth to give the mean depths, tends to balance some of the 

 errors in velocity. For all measurements the determination of the 

 depth and velocity at points one-fifth of the width of the water surface 

 from the sides gives average results from i to 3 per cent too high. Any 

 single gaging will average to give errors of 5 to 7, and they may be as 

 high as 1 5 per cent. 



These results indicate that under favorable conditions two index 

 verticals can be found in canals, the velocity at which will agree with 

 the average for the whole cross section. These points are from one-fifth 

 to one-sixth of the width of the canal from the sides in sections with 

 vertical sides and from one-fourth to one-fifth for other types. In sec- 

 tions with vertical sides, such as flumes, and in earth sections the depths 

 at these index verticals will also be quite close to the average depth in 

 the whole section, so that the index points can be used also to determine 

 the total discharge. In definite sections with sloping sides, such as con- 

 crete-lined canals, it is preferable to use known relations of depth and 

 area and use the index points for the determination of velocities only. 



Such short-cut methods would not generally be desirable at permanent 

 rating stations. They might be useful for approximate measurements 

 where time was an important factor, or as checks on the division of water 

 in canal at large turnouts. Such gagings could be made of the canal 

 above and below, and also of the turnout. Where other means of meas- 

 uring or controlHng the device are not available, such rapid methods 

 might be of value. 



SUMMARY 



Comparisons of various methods of current-meter gaging of irrigation 

 canals are made with measurements in which the velocities at from 70 

 to 1 20 points were taken. Canals of various types of cross section having 

 discharges of from 2 to 2,600 second-feet and velocities of from 0.5 to 

 8.0 feet per second were included. 



