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)		 Pipe Diameter
(mm)		 Minimum Slope
S={V/[1.486/n*(D/4)^(2/3)]}^2
(m/m or ft/ft) (mm/m) (percent)
4 100 0.0084 8.4 0.84
6 150 0.0049 4.9 0.49
8 200 0.0034 3.4 0.34
10 250 0.0025 2.5 0.25
12 300 0.0019 1.9 0.19
15 380 0.0014 1.4 0.14
18 450 0.0011 1.1 0.11
21 530 0.00092 0.92 0.092
24 600 0.00077 0.77 0.077
30 760 0.00057 0.57 0.057
36 900 0.00045 0.45 0.045
42 1050 0.00037 0.37 0.037
48 1200 0.00031 0.31 0.031
54 1350 0.00026 0.26 0.026
60 1500 0.00023 0.23 0.023
66 1650 0.00020 0.20 0.020
72 1800 0.00018 0.18 0.018
84 2100 0.00015 0.15 0.015
96 2400 0.00012 0.12 0.012


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

A note on slope units. Minimum slope is a ratio, so it has no units and the same number applies whether you work in feet or in meters: a slope of 0.0034 is 0.0034 ft/ft and 0.0034 m/m alike. To read it in millimeters per meter, multiply by 1000 (0.0034 becomes 3.4 mm/m); to read it as a percent, multiply by 100 (0.0034 becomes 0.34%). Table 1 gives all three forms directly.