TABLE 21. Minimum Slope of Sewers*
Sewer Minimum slope, ft/
diameter, 1OO ft (m/30.5 m)
in (mm) of length
4(102) 1.20(0.366)
6(152) 0.60(0.183)
8(203) 0.40(0.122)
10(254) 0.29(0.088)
12 (305) 0.22 (0.067)
15(381) 0.15(0.046)
18(457) 0.12(0.037)
20(505) 0.10(0.030)
24(610) 0.08(0.024)*Based on the Manning formula with n = 0.13 and the sewer flow-
ing either full or half full.pipe size is 18 in (457.2 mm) and the flow velocity is 5.75 ft/s (1.75 m/s). Likewise, sec-
tion 7-8 must handle 14.4 ft^3 /s (0.41 mVs). The required pipe size is 21 in (533 mm), and
the flow velocity in the pipe is 6.35 ft/s (1.94 m/s). Section 9-10 of the main sewer han-
dles 19.2 ft^3 /s (0.54 m^3 /s), and must be 24 in (609.6 mm) in diameter. The velocity in this
section of the sewer pipe will be 6.9 ft/s (2.1 m/s). The last section of the main sewer han-
dles the total flow, or 24 fWs (0.7 m
3
/s). Its size must be 27 in (686 mm), Fig. 12, al-
though a 24-in (610.0-mm) pipe would suffice if the slope at point B could be increased to
0.012.
Related Calculations. Most new sewers built today are the separate type, i.e.,
one sewer for sanitary service and another sewer for storm service. Sanitary sewers are
usually installed first because they are generally smaller than storm sewers and cost less.
Combined sewers handle both sanitary and storm flows and are used where expensive ex-
cavation for underground sewers is necessary. Many older cities have combined sewers.
To size a combined sewer, compute the sum of the maximum sanitary and stormwater
flow for each section of the sewer. Then use the method given in this procedure after hav-
ing assumed a value for n in the Manning formula and for the slope of the sewer main.
Where a continuous slope cannot be provided for a sewer main, a pumping station to
lift the sewage must be installed. Most cities require one or more pumping stations be-
cause the terrain does not permit an unrestricted slope for the sewer mains. Motor-driven
centrifugal pumps are generally used to handle sewage. For unscreened sewage, the suc-
tion inlet of the pump should not be less than 3 in (76 mm) in diameter.