The maximum condition of rainfall for a 50-year frequency shall be used for the purpose of determining all runoff for the sizing of drainage channels and structures, unless otherwise specified in this chapter. The following formulas and value shall be used for calculating all stream flow and runoff for the policies and regulations established in this chapter:
A. Runoff from drainage areas of nine hundred (900) acres or less shall be determined by the rational formula:
Q = AIR
Where:
Q = Flow in cubic feet per second.
A = Area to be drained in acres, determined by field surveys for areas less than one hundred (100) acres, and by the latest government quadrangle maps for larger areas.
I = Percent of imperviousness of the area. Percent of imperviousness may vary between seventy percent (70%) and ninety five percent (95%). Coefficients lower than these indicated values may be used; however, in such instances, individual block calculations will be submitted with plans.
R = Rate of rainfall over entire drainage area in inches per hour, based on the time for concentration and the latest government records for the area.
B. Runoff from drainage areas greater than nine hundred (900) acres will be determined by use of accepted engineering principles.
C. The size of closed storm sewers, open channels, culverts and bridges shall be determined using the Manning formula which may be modified for use with runoff determined by the rational formula to:
Q = (1.5r) (2/3 s) (1/2 A)
n
Where:
Q = Discharge in cubic feet per second.
A = Cross sectional area of water in conduit square feet.
r = Hydraulic radius of water in conduit.
s = Mean slope of hydraulic gradient in feet of vertical rise per feet of horizontal distance.
n = Roughness coefficient, based on condition and type of material of conduit lining, but not less than 0.013.
(Prior code §7-4)