Page 455 - Civil Engineering Formulas

STORMWATER, WASTEWATER, AND ENVIRONMENTAL PROTECTION 381

TABLE 13.3 Manning Formula Conveyance Factor
Pipe Pipe
n
diameter, cross-sectional
in (mm) area, sq ft (m ) 0.011 0.013 0.015 0.017
2
6 (152) 0.196 (0.02) 6.62 5.60 4.85 4.28
8 (203) 0.349 (0.03) 14.32 12.12 10.50 9.27
10 (254) 0.545 (0.05) 25.80 21.83 18.92 16.70
12 (305) 0.785 (0.07) 42.15 35.66 30.91 27.27
15 (381) 1.227 (0.11) 76.46 64.70 56.07 49.48
18 (457) 1.767 (0.16) 124.2 105.1 91.04 80.33
21 (533) 2.405 (0.22) 187.1 158.3 137.2 121.1

To size the main and lateral sewers, use the Manning formula and the appro-
priate conveyance factor from Table 13.2.
When the conveyance factor C is used, the Manning formula becomes
f
Q C f S 1/2 (13.80)
3
where Q flow rate through the pipe, ft /s; C conveyance factor corre-
f
sponding to a specific n value listed in Table 13.3; S pipe slope or hydraulic
gradient, ft/ft.
Next, compute the flow rate for each lateral sewer discharging into the main
sewer using
LS (SS )/(percent lateral sewer flow, expressed as a whole number)
fr
fr
(13.81)
In this formula, SS is expressed in cfs, as computed earlier.
fr
Compute the lateral sewer pipe diameter using the Manning formula, as given
2.5
above. Next, compute the value of d LS for each lateral sewer pipe in the system.
Then take the sum of all lateral pipes, or
2.5
2.5
d LS1 d LS2 .... (13.82)

LS
The Σ LS should be less than d 2.5 for the main sewer. If the Σ LS is greater than
the d 2.5 for the main sewer, the diameter of the main sewer should be increased
until its d 2.5 is greater than Σ .
LS
Next, compute the sewer size with infiltration from the groundwater. Infil-
tration is the groundwater that enters a sewer. The quantity and rate of infiltra-
tion depend on the character of the soil in which the sewer is laid, the relative
position of the groundwater level and the sewer, the diameter and length of the
sewer, and the material and care with the sewer is constructed. With tile and
other joined sewers, infiltration depends largely on the type of joint used in the
pipes. In large concrete or brick sewers, the infiltration depends on the type of
waterproofing applied.