Nov. 8,1915 
Use of Current Meters in Irrigation Canals 
225 
From these experiments it appears that the coefficient is constant for 
different values of n for cross-section areas over about 35 square feet. 
The rate of variation of the coefficient is greatest for the smaller chan¬ 
nels. The observations for cross-sectional areas over 100 square feet 
were too few in number to give dependable averages for canals of larger 
size, but both these results and Bazin's formula indicate that the coeffi¬ 
cient is practically constant*for such larger cross sections. 
Similar curves were also obtained based on the value of the coefficient 
and the discharges. These were similar in form and indicate that the 
velocity within the limits of the experiments did not materially affect 
the ratio of maximum surface to mean velocity. These values are not 
given, as the coefficients based on canal areas are more convenient to use. 
The results were further classified by the shape of the channel. Ap¬ 
parently the coefficient does not vary with the form of cross section, as 
the coefficient from the curves agrees fairly well with the observations 
when the proper values of n are used, whether the canal is rectangular 
or irregular or whether the section is deep or shallow relative to its width. 
The average variation of the observed coefficients from the curves was 
0.045. The average of all observations agreed with the curves, the plus 
variations equaling those of minus sign. Expressed as a percentage, the 
average variation was 6. For any single observation the observed value 
of the velocity coefficient is as likely to differ from the mean curve by 
less than 0.045 a $ it is to differ by more than this amount. For the 
average values of the coefficient this amounts to a variation of 6 per 
cent. In 17 of the 92 experiments, or 18.5 per cent of the total number, 
the observed value differed by over 10 per cent from the curves. 
The more usual practice where such methods of measurements of 
velocities by floats are made is to use some general value of the coefficient, 
usually 0.80 or 0.85. These experiments, as well as the observations 
given by Kutter, clearly indicate that the coefficient varies quite ma¬ 
terially for different-sized canals and for different values of n. These 
results give values for the coefficients which are less than 0.80 for all 
values of n over 0.016, becoming ^s low as 0.60 for small canals having 
high values of n. 
The value of n for any given canal is, of course, uncertain to some 
extent. The coefficient varies most rapidly with the lower values of n . 
An error of 0.002 in selecting the value of n makes a difference of 5 per 
cent in the value of the correct coefficient to be used for low values of n , 
and less than 2 per cent for the higher values. 
The coefficients to be used should be selected from the cross-section 
area and the value of n. The character of the canals corresponding to 
the different values of n given in Table IV can be secured from the 
general list following: 
