THE FLOW OF WATER IN CONCRETE PIPE. 91 
from the top, these men spaded it away from the inside form, so that 
the inner surface of the conduit is very smooth. (See PI. IX, fig. 2.) 
Simultaneously with the hydraulic experiments upon the tunnels 
and cement-lined pressure pipes described on pages 80 to 83 , inclusive, 
measurements were made on three reaches of the cut-and-cover 
flow lines. For nearly half the runs nonparallel flow was indicated. 
That is to say, the slope of the water surface was either greater or 
less than the slope of the invert. The computations were not com- 
Eleted for such runs, but for all runs where the flow was uniform the 
ydraulic elements are given in Table 11, page 70. The quantity 
was measured by Venturi meter, as described for the pressure con- 
duits. The areas and wetted perimeters of the water sections were 
determined by measurements from arch intrados to water surface. 
The velocities were computed from the continuity equation V=-4* 
The coefficients of retardation indicate that quite high efficiencies 
are perfectly feasible in conduits of great size if care is taken to work 
the concrete mix into close contact with smooth forms and if care is 
exercised in moving the forms so that no offsets or shoulders are 
developed. A glance at the items in Table 11, page 70, shows the 
same indication toward incipient fouling that was found in the ex- 
periments on the pressure tunnels and pipes. The sequence of tests 
is shown by the order of observation numbers. For any given reach 
of conduit the retardation factors indicate a rougher surface from 
day to day when the water is first turned into a conduit. 
