14 BULLETIN 854, U. S. DEPARTMENT OF AGRICULTURE. 



ACTUAL DEPTH OF FLOW. 



The depths of flow in the tile lines tested were measured by means 

 of the 12 piezometer tubes distributed along the flume, as previously 

 described. 



As first laid, the tile at the lower end of the experimental line dis- 

 charged into the open air with a free drop of several inches. This 

 produced a backwater curve of the drop-off type which extended 

 back for a considerable distance into the tile line, decreasing the 

 depth of the water near the lower end. For the steeper slopes this 

 effect was much extended, and, indeed, in extreme cases reached 

 throughout the length of the experimental line. Such a condition 

 was objectionable for two reasons: first, the hydraulic gradient under 

 such circumstances would be represented by the slope of the water 

 surface, which was then somewhat greater than the grade of the tile ; 

 second, since the condition was not one of uniform flow, it would 

 become necessary to take account of the change of velocity at differ- 

 ent points in the tile, with the corresponding changes in velocity head, 

 hi determining the head consumed in overcoming friction. Since these 

 additional complications were unnecessary and objectionable, the 

 drop-off curve was eliminated by installing a low, movable dam 

 (shown in PI. VIII, fig. 1) just below the lower end of the tile line. 

 By adjusting the height of this dam, the water surface at the outlet 

 could be maintained in close agreement with any desired depth 

 throughout the experimental line. 



The water entered the upper end of the tile through a conical 

 entrance pipe (PI. Ill, figs. 1 and 2) designed to give an entrance 

 velocity approximating that of the steady, uniform flow in the tile 

 line. But it was found impracticable to adjust the entrance velocity 

 exactly to that of the line, with the result that the upper 50 feet 

 of tile were required to bring the velocity to the condition of uniform 

 flow, and the piezometers at the upper end would not always agree 

 with the others along the tile. With this exception, the readings of 

 depth in the various piezometers along the tile line could generally 

 be brought into satisfactory agreement. 



With the tile only partly full, there were occasional quite erratic 

 readings on some piezometers. These indicated unusual disturb- 

 ances within the tile line. When through the warped or elliptical 

 shape of the tile the joints do not fit closely, a. portion of one tile 

 at the joint may project inward in such a way as to present a square 

 obstruction against the edge of the moving stream of water. Violent 

 impact of the water against such an obstruction produces a marked 

 disturbance of the stream, and is indicated by extensive ripples and 

 foam on the water surface which may persist for several feet down- 

 stream. Several such cases were carefully examined by uncovering 

 the tile and inspecting the water surface within, as well as by measur- 



