12 BULLETIN 194, U. S. DEPARTMENT OF AGKICULTUEE. 



as it would if the lower nail were set first. Enough cross sections 

 were taken to determine the mean water area and length of wetted 

 perimeter throughout the reach. Nails were set with their tops flush 

 with the water surface at each end of the reach. Levels were then 

 run between these nails as bench marks. Temperatures of air and 

 water were taken. After all these data had been taken the chief of 

 party was familiar enough with the conditions to write an intelligent 

 description covering the features influencing the flow of water 

 throughout the reach tested. 



Throughout the descriptive matter in this publication station is 

 at the upper end of the reach, and integral stations represent 100-foot 

 intervals below station 0. 



Canal reach. — 'Actual conditions encountered in the operation of 

 irrigation systems and under which canals must carry water were 

 those desired, as it was believed the designer must take these into 

 consideration in developing the hydraulic elements of his proposed 

 canal. Other things being equal, a reach of canal 1,000 feet long 

 was chosen with quite uniform flow, not only throughout the reach 

 but also up and down stream so far as conditions might influence 

 results. These reaches were chosen on tangents, on curves, and on 

 both, in order to determine the influence of curves on the retardation 

 factor. 



Discharge measurements. — Small discharges were, when practica- 

 ble, measured with a CipoUetti weir, under standard conditions. 

 Discharges of more than 4 second-feet were measured with the current 

 meter. The surface widths of ditches less than 10 feet in width were 

 divided by vertical fines one-half foot apart. Those of canals more 

 than 10 feet in width were divided into approximately 20 verticals, 

 the sections thus formed near the banks being narrower than those 

 toward the middle, for the reason that the velocity changes more 

 rapidly near the banks. The average velocity in the verticals was 

 determined, as a rule, by the multiple-point method, interpreted 

 through vertical velocity curves. In any one vertical the meter was 

 held at points 0.2, 0.4, 0.6, and 0.8 of the total depth below the surface 

 of the water. In addition to these points, at approximately every 

 other vertical, the meter was held at points 0.1 or 0.2 foot (depending 

 on the surface velocity and consequent roughness) below the surface, 

 and just clearing the bottom. These points gave the necessary 

 information to plat correctly the vertical velocity curves between 

 the surface of the water and the point 0.2 of the depth below the 

 surface and between the bottom of the channel and the point 0.8 of the 

 depth below the surface. In addition to all of these points, in many 

 verticals the meter was held at a point 0.3 of the depth below the 

 surface for the purpose of developing the point representing approxi- 

 mately the maximum velocity in the vertical. The velocity at this 



