Collection of Wastewater 159able 8-1. Manning Roughness Coefficient n
Type of channel, closed conduits Roughness coefficient nCast iron
Concrete, straight
Concrete, with bends
Concrete, unfinished
Clay, vitrified
Corrugated metal
Brickwork
Sanitary sewers coated with slime0.013
0.011
0.013
0.014
0.012
0.024
0.013
0.013For circular conduits flowing full, the Manning formula may be rearranged aswhere Q = discharge (cfs) and D = pipe diameter (inside, inches). Instead of making
this calculation, a nomograph that represents a graphical solution of the Manning
equation may be used. Figures 8-6 and 8-7 are such nomograph in English and metric
units, respectively.
Note that the Manning equation (Eq. (8.3)) calculates velocities in sewers flowing
full, and thus uses the maximum expected flow in the sewer. For flows less than the
maximum, the velocity is calculated using the hydraulic elements graph shown as
Fig. 8-8. In Fig. 8-8, the hydraulic radius (area per wetted perimeter) is calculated for
various ratios of the depth when partially full (d) to the depth when completely full
(0, the inside diameter of the pipe). The velocity vis calculated using Eq. (8.3) for any
depth d, and the discharge is obtained from multiplying the velocity times the area.
Experimental evidence has shown that the friction factor also varies slightly with the
depth, as shown in Fig. 8-8, but this is often ignored in the calculations.
The maximum flow in a given sewer does not occur when the sewer is full but
when dlD is about 0.95, because the additional flow area gained by increasing the dlD
from 0.95 to 1.00 is very small compared with the large additional pipe surface area
that the flaw experiences.Em 8.2. From Example 8.1, it was found that the flow carried by an 8-in. cast
iron pipe flowing full at a grade of MOO is 0.54 cfs with a velocity of 1.54 ft/s. What
will be the velocity in the pipe when the flow is only 0.1 cfs?
q 0.1
_- - = 0.185.
Q - 0.54
From the hydraulic elements chart
d- 0.33
D