For all areas not otherwise designated in a floodway or floodway-fringe area, the developer shall be responsible for having an engineer, registered in the State, prepare a drainage assessment of all of the area of the proposed subdivision and all areas affected by runoff resulting from development of the proposed subdivision in accordance with the following provisions:
A. The one hundred (100) year maximum flood shall be used as the basis for the sizing of all drainage channels, bridges, and other structures, unless otherwise specified in this section;
B. The calculation of all runoff shall be based on saturated urbanization of the drainage basin for minor tributaries and surface drainage, as reflected in the comprehensive plan, and shall be based on the maximum degree of urbanization, as reflected in the comprehensive plan, for the drainage basin of a major tributary or river; and
C. The calculation of stream flow and runoff characteristics of the subdivision shall be carried out in consultation with the City Engineer, and the methodology and formulas used shall result in quantities which would be not less than those derived from the application of the following formulas and values:
1. Runoff from all drainage areas shall be not less than that determined by the Rational Formula:
Q = AIR
Q Cubic feet per second.
A Area to be drained, in acres, determined by field surveys for areas less than six hundred forty (640) acres and by latest government quadrangle maps for larger areas.
I Percent of imperviousness of the area. This may vary between forty percent (40%) and ninety-five percent (95%), but no value of less than fifty percent (50%) may be used for saturated urban development.
R Rate of rainfall over entire drainage area in inches per hour, based on time of concentration and latest government records for the area.
2. The size of closed storm sewers, open channels, culverts, and bridges shall be not less than that determined by using the Manning Formula:
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, based on condition and type of material of conduit lining, but not less than 0.013.
(Code 1991, § 12-498)