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By Thomas Gail Haws, P.E.
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Minimum sanitary sewer slopes are established to provide a minimum cleansing velocity (usually 2 or 3 feet per second) at full flow or half-full flow. (The physics of hydraulic flow are such that a sloped pipe will carry flow by gravity half full at the same velocity as full.) Table 1 below gives minimum sewer slopes to provide 2 feet per second velocity assuming a Manning pipe roughness (n) of 0.013. For larger diameter pipes, considerations other than minimum cleansing velocity (such as construction tolerances and potential ground subsidence) may govern the minimum slope selection. Table 2 is a more generalized table with a range of common values:
Pipe Diameter (inches) | Minimum Slope S={V/[1.486/n*(D/4)^(2/3)]}^2 |
---|---|
4 | 0.0084 |
6 | 0.0049 |
8 | 0.0034 |
10 | 0.0025 |
12 | 0.0019 |
15 | 0.0014 |
18 | 0.0011 |
21 | 0.00092 |
24 | 0.00077 |
30 | 0.00057 |
36 | 0.00045 |
42 | 0.00037 |
48 | 0.00031 |
54 | 0.00026 |
60 | 0.00023 |
66 | 0.00020 |
72 | 0.00018 |
84 | 0.00015 |
96 | 0.00012 |
PIPE SIZE |
MINIMUM SLOPE | ||||||
---|---|---|---|---|---|---|---|
(in) | (mm) | FOR 2.0 ft/s (0.6 m/s) | FOR 2.5 ft/s (0.75 m/s) | ||||
Manning's n --> | 0.010 | 0.013 | 0.015 | 0.010 | 0.013 | 0.015 | |
8 | 200 | 0.0020 | 0.0034 | 0.0045 | 0.0031 | 0.0052 | 0.0070 |
10 | 250 | 0.0015 | 0.0025 | 0.0033 | 0.0023 | 0.0037 | 0.0052 |
12 | 300 | 0.0011 | 0.0019 | 0.0026 | 0.0018 | 0.0030 | 0.0040 |
15 | 380 | 0.00085 | 0.0014 | 0.0019 | 0.0013 | 0.0022 | 0.0030 |
18 | 450 | 0.00067 | 0.0011 | 0.0015 | 0.0010 | 0.0017 | 0.0023 |
24 | 600 | 0.00045 | 0.00077 | 0.0010 | 0.00071 | 0.0012 | 0.0016 |
30 | 760 | 0.0003 | 0.00056 | 0.00074 | 0.00052 | 0.00088 | 0.0012 |