Storm sewers shall be not less than twelve inches in diameter for those sections receiving overland run-off. Those sections transporting sump pump discharge only may be reduced to a minimum of eight inches in diameter, depending on capacity requirements, provided that minimum conditions for slope and depth are met. The minimum allowable velocity in the pipe shall be two and one-half feet per second with the pipe running full. The maximum velocity in the pipe shall be twelve feet per second. The following are minimum slopes for various pipe sizes and acceptable applications for each:
Inner Dia- Minimum Slope in Inches
meter of Pipe Slope of Fall per
(in.) (percent) 10 Ft. Run Application
4 1.28 1 17/32 Building storm water
6 0.96 1 5/32 drain only
8 0.64 3/4 Sewers carrying dis-
10 0.44 17/32 charges from building storm water drains only
12 0.36 7/16 Sewers carrying both
15 0.24 9/32 discharges from storm
18 0.20 1/4 water drains and over-
21 0.16 3/16 land run-off
24 0.12 5/32
30 0.10 1/8
36 0.08 3/32
42 0.06 3/32
48 0.05 1/16
Where building storm water drains require pipe sizes other than four or six inches, any of the larger sizes in the above table may be used. However, in no case shall the slope used be less than the minimum given in the above table for that size of pipe.