FLOW OF WATER IX DREDGED DRAINAGE DITCHES. 



the coefficient G is first obtained from Chezy's formula, 



V= C^RS 



and then the value of n by solving for n in Kutter's formula. In 

 solving Chezy's formula for C the following terms must be known: 



V, the mean velocity along the slope course. 



B, the mean hydraulic radius along the slope course. 



S, the mean slope of the water surface along the slope course. 



As has been explained, the data required for computing these 

 values were obtained by field measurements. The mean velocity, 

 V, was determined by dividing the discharge by the mean cross- 

 sectional area of the channel along the slope course. The discharge 

 was computed from the soundings and velocity measurements made 

 at the gauging section. The method of computing the discharge 

 was as follows: The mean velocity in the vertical at each velocity 

 measuring point was obtained by taking one-sixth of the sum of the 

 velocity at the surface, four times the velocity at mid depth, and the 

 velocity at the bottom, or by taking the mean of the velocities 

 obtained at 0.2 and 0.8 depths. The mean velocity for each section 

 between the verticals was taken as the average of the mean velocities 

 for the verticals on either side of the section, and the discharge for 

 each section was obtained by multiplying the mean velocity for the 

 section by the area of the section. The total discharge of the stream 

 was obtained by taking the sum of the discharges for all sections 

 across the stream. 



The cross-sectional measurements were platted on cross-section 

 paper, on a scale of 5 or 10 feet to the inch, depending upon the size 

 of the channel. The areas and wetted perimeters for each cross section 

 were obtained by means of a planimeter and map measurer. The 

 mean cross-sectional area, which was used in determining the mean 

 velocity for the course, was obtained by taking the average of all the 

 cross-sectional areas along the course. 



For some of the channels the mean hydraulic radius for the course 

 was obtained by taking the average of all the hydraulic radii as com- 

 puted for each cross section; for others it was obtained by dividing 

 the mean cross-sectional area by the average of all the wetted per. 

 imeters as determined for each cross section along the course. There 

 was no appreciable difference in the results obtained by the use of the 

 two methods unless the channel was subject to large variations in 

 cross section, in which case the first method was used. 



On all of the channels the slope of the water surface was practically 

 uniform from the upper to the lower ends of the slope courses, and 

 the differences in the velocities at the upper and lower ends of the 

 courses were so small that the application of a correction for the differ- 



