Nov. 8. 191S Use of Current Meters in Irrigation Canals 223 



secured and the cross section of the canal in order to give the discharge. 

 In such measurements there are two principal sources of error: (i) The 

 cross-sectional area is difficult to obtain except in flumes or lined canals 

 of uniform cross sections and (2) mistakes may be made in choosing a 

 coefficient to be used in reducing the maximum surface velocities as 

 obtained from the floats to the mean for the whole canal. 



The following results relate to the proper coefficient to be used to 

 reduce surface-float velocities to the mean velocity for the whole cross 

 section. The average errors discussed are those arising from the de- 

 terminations of float velocities and the choice of coefficients and do not 

 include errors in determining the canal cross sections. For the other pur- 

 poses of these experiments the areas of the canal sections were carefully 

 determined. In the usual field use of float methods there may be a con- 

 siderable error introduced due to errors in the approximate determinations 

 of canal cross sections of variable dimensions, which would give larger 

 probable errors for the discharge than would result from the probable 

 error due to the choice of the coefficient to use with the velocity of the 

 float alone. 



Various formulas have been derived for the relation of the surface 

 velocity to the mean velocity. These have been derived both for the 

 relation of the surface velocity to the mean velocity in any single vertical 

 in the section and for the relation of the maximum surface velocity to 

 the mean velocity of the whole channel. Ganguillet and Kutter ^ give 

 a formula, deduced by Bazin, in which the ratio of the maximum to 

 mean velocities in a channel are made to vary with 



RS 



As this term is equal to the C in Chezy's formula, a table is given for 

 the value of the ratio for dift"erent values of C In this formula Kutter 

 substitutes the values of n and R from his general formula and gives a 

 table for the values of the ratio of mean to maximum velocity, depending 

 on R and n. The formula derived by Bazin, which forms the basis of 

 this table, was based on 61 series of gagings. 



In the canal experiments discussed in this paper in which float measure- 

 ments were made several small floats would be started simultaneously 

 at scattered points in the portion of the channel having the highest 

 velocities. The time of the most rapid float was used to compute the 

 maximum surface velocity. This gives lower coefficients than would be 

 obtained by the use of the average of all floats. Small floats such as 

 twigs or chips were used which would have both a small submergence 

 and a small exposed surface above the water. It was found that there 

 was little difference in the velocities of the floats thrown into the main 

 threads of the canals unless some became caught in noticeable side 



1 Ganguillet. E., and Kutter. W. R. Op. cit. 



