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 herein. The following formula and values shall be used for calculating all stream flow and runoff for the policies and regulations established herein unless otherwise specified herein:
A. Runoff from drainage areas of five hundred (500) 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 100 acres, and by latest government quadrangle maps for larger areas. |
I | = | Percent of imperviousness of the area; may vary between 70 percent and 95 percent. 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 time of concentration and latest government records for the area. |
B. Runoff from drainage areas greater than five hundred (500) acres will be determined by use of accepted engineering principles as approved by the director.
C. The size of closed storm sewers, open channels, culverts, and bridges shall be determined by using the Manning formula which may be modified for use with runoff determined by the rational formula to:
Q | = | 1.486 r 2/3 s 1/2 A | |
n | |||
Q | = | Discharge in cubic feet per second. | |
A | = | Cross sectional area of water in conduit in square feet. | |
r | = | Hydraulic radius of water in conduit. | |
s | = | Mean slope of hydraulic gradient in feet of vertical rise per foot of horizontal distance. | |
n | = | Roughness coefficient. | |
Other design methods may be required to maintain hydraulic grade line below the ground and/or gutter elevation to eliminate flooding of the inlets or manholes. (Ord. 1969, 2011)