Storm sewer sizes and slopes shall be designed in accordance with the following formula:
Q = AIR
Wherein:
Q = Quantity of run-off in cubic feet per second
A = Drainage area in acres
I = Rainfall intensity in inches per hour
R = Run-off factor or co-efficient of imperviousness in % -·: 100
(a) Drainage Area. The area contributing to the particular pipe in question shall be measured in acres.
(b) Rainfall Intensity. The rainfall intensity shall be based on Gumbels Frequency Curves as printed on Page 282 of the Concrete Pipe Handbook, as updated, revised and/or supplemented. A five-year storm shall be used with a minimum inlet time of fifteen minutes at the upper catch basin.
(c) Run-Off. Run-off values shall be based on the following table:
Type of Surface Value of R
Watertight surfaces such
as roofs and pavements 0.80
Built-up residential areas 0.35
Parks 0.15
Farm land 0.20