Environmental Engineering FOURTH EDITION

162 ENvIRONIVlENTAL ENGINEERING

Hydraulic elements 2, L, L, and - r

vtull Qfun 4"ll %Ill

Figure 8-8. Hydraulic elements chart for open channel flow.

and, since D = 8 in.,

d = (0.33)(8in.) = 2.64in.

• = 0.64,
  V

21

and since v, the velocity when flowing full, is 1.54 ft /s,

21 = (0.64)(1.54ft/S) = 0.98ft/S.

If the desired slope between manholes is not sufficient to maintain adequate veloc-
ities at minimum flow, additional slope must be provided or a larger pipe diameter
must be used. On the other hand, if the slope of the pipe is too steep, it must be
reduced. Often a drop manhole is used, which is, in effect, a method of wasting energy
(see Fig. 8-4).
Standard American convention requires that a head loss of 0.1 ft (0.06 m) be
allowed for the head loss through each manhole. The invert elevation of the pipe
leaving the manhole is thus at least 0.1 ft lower than the invert elevation of the pipe
coming in.