Minimum Sanitary Sewer Slopes

By Thomas Gail Haws, P.E.


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INTRODUCTION

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:

MINIMUM SANITARY SEWER SLOPE TABLE 1 (Standard Arizona Practice)
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


MINIMUM SANITARY SEWER SLOPE TABLE 2

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

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